Organosulfur compounds

Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfur is essential for life. Of the 20 common amino acids, two (cysteine and methionine) are organosulfur compounds, and the antibiotics penicillin and sulfa drugs both contain sulfur. While sulfur-containing antibiotics save many lives, sulfur mustard is a deadly chemical warfare agent. Fossil fuels, coal, petroleum, and natural gas, which are derived from ancient organisms, necessarily contain organosulfur compounds, the removal of which is a major focus of oil refineries.
Sulfur shares the chalcogen group with oxygen, selenium, and tellurium, and it is expected that organosulfur compounds have similarities with carbon–oxygen, carbon–selenium, and carbon–tellurium compounds.
A classical chemical test for the detection of sulfur compounds is the Carius halogen method.
Classes.
Organosulfur compounds can be classified according to the sulfur-containing functional groups, which are listed (approximately) in decreasing order of their occurrence.
Sulfides.
Sulfides, formerly known as thioethers, are characterized by C−S−C bonds Relative to C−C bonds, C−S bonds are both longer, because sulfur atoms are larger than carbon atoms, and about 10% weaker. Representative bond lengths in sulfur compounds are 183 pm for the S−C single bond in methanethiol and 173 pm in thiophene. The C−S bond dissociation energy for thiomethane is 89 kcal/mol (370 kJ/mol) compared to methane's 100 kcal/mol (420 kJ/mol) and when hydrogen is replaced by a methyl group the energy decreases to 73 kcal/mol (305 kJ/mol). The single carbon to oxygen bond is shorter than that of the C−C bond. The bond dissociation energies for dimethyl sulfide and dimethyl ether are respectively 73 and 77 kcal/mol (305 and 322 kJ/mol).
Sulfides are typically prepared by alkylation of thiols. They can also be prepared via the Pummerer rearrangement. In one named reaction called the Ferrario reaction phenyl ether is converted to "phenoxathiin" by action of elemental sulfur and aluminium chloride.
Thioacetals and thioketals feature C−S−C−S−C bond sequence. They represent a subclass of sulfides. The thioacetals are useful in "umpolung" of carbonyl groups. Thioacetals and thioketals can also be used to protect a carbonyl group in organic syntheses.
The above classes of sulfur compounds also exist in saturated and unsaturated heterocyclic structures, often in combination with other heteroatoms, as illustrated by thiiranes, thiirenes, thietanes, thietes, dithietanes, thiolanes, thianes, dithianes, thiepanes, thiepines, thiazoles, isothiazoles, and thiophenes, among others. The latter three compounds represent a special class of sulfur-containing heterocycles that are aromatic. The resonance stabilization of thiophene is 29 kcal/mol (121 kJ/mol) compared to 20 kcal/mol (84 kJ/mol) for the oxygen analogue furan. The reason for this difference is the higher electronegativity for oxygen drawing away electrons to itself at the expense of the aromatic ring current. Yet as an aromatic substituent the thio group is less electron-releasing than the alkoxy group. Dibenzothiophenes (see drawing), tricyclic heterocycles consisting of two benzene rings fused to a central thiophene ring, occurs widely in heavier fractions of petroleum.
Thiols, disulfides, polysulfides.
Thiol groups contain the functionality R−SH. Thiols are structurally similar to the alcohol group, but these functionalities are very different in their chemical properties. Thiols are more nucleophilic, more acidic, and more readily oxidized. This acidity can differ by 5 p"K"a units.
The difference in electronegativity between sulfur (2.58) and hydrogen (2.20) is small and therefore hydrogen bonding in thiols is not prominent. Aliphatic thiols form monolayers on gold, which are topical in nanotechnology.
Certain aromatic thiols can be accessed through a Herz reaction.
Disulfides R−S−S−R with a covalent sulfur to sulfur bond are important for crosslinking: in biochemistry for the folding and stability of some proteins and in polymer chemistry for the crosslinking of rubber.
Longer sulfur chains are also known, such as in the natural product varacin which contains an unusual pentathiepin ring (5-sulfur chain cyclised onto a benzene ring).
Thioesters.
Thioesters have general structure R−C(O)−S−R. They are related to regular esters (R−C(O)−O−R) but are more susceptible to hydrolysis and related reactions. Thioesters are prominent in biochemistry, especially in fatty acid synthesis.
Sulfoxides, sulfones and thiosulfinates.
A sulfoxide, R−S(O)−R, is the "S"-oxide of a sulfide ("sulfide oxide"), a sulfone, R−S(O)2−R, is the "S","S"-dioxide of a sulfide, a thiosulfinate, R−S(O)−S−R, is the "S"-oxide of a disulfide, and a thiosulfonate, R−S(O)2−S−R, is the "S","S"-dioxide of a disulfide. All of these compounds are well known with extensive chemistry, e.g., dimethyl sulfoxide, dimethyl sulfone, and allicin (see drawing).
Sulfimides, sulfoximides, sulfonediimines.
Sulfimides (also called a sulfilimines) are sulfur–nitrogen compounds of structure R2S=NR′, the nitrogen analog of sulfoxides. They are of interest in part due to their pharmacological properties. When two different R groups are attached to sulfur, sulfimides are chiral. Sulfimides form stable α-carbanions.
Sulfoximides (also called sulfoximines) are tetracoordinate sulfur–nitrogen compounds, isoelectronic with sulfones, in which one oxygen atom of the sulfone is replaced by a substituted nitrogen atom, e.g., R2S(O)=NR′. When two different R groups are attached to sulfur, sulfoximides are chiral. Much of the interest in this class of compounds is derived from the discovery that methionine sulfoximide (methionine sulfoximine) is an inhibitor of glutamine synthetase.
Sulfonediimines (also called sulfodiimines, sulfodiimides or sulfonediimides) are tetracoordinate sulfur–nitrogen compounds, isoelectronic with sulfones, in which both oxygen atoms of the sulfone are replaced by a substituted nitrogen atom, e.g., R2S(=NR′)2. They are of interest because of their biological activity and as building blocks for heterocycle synthesis.
"S"-Nitrosothiols.
"S"-Nitrosothiols, also known as thionitrites, are compounds containing a nitroso group attached to the sulfur atom of a thiol, e.g. R−S−N=O. They have received considerable attention in biochemistry because they serve as donors of the nitrosonium ion, NO+, and nitric oxide, NO, which may serve as signaling molecules in living systems, especially related to vasodilation.
Sulfur halides.
A wide range of organosulfur compounds are known which contain one or more halogen atom ("X" in the chemical formulas that follow) bonded to a single sulfur atom, e.g.: sulfenyl halides, RSX; sulfinyl halides, RS(O)X; sulfonyl halides, RSO2X; alkyl and arylsulfur trichlorides, RSCl3 and trifluorides, RSF3; and alkyl and arylsulfur pentafluorides, RSF5. Less well known are dialkylsulfur tetrahalides, mainly represented by the tetrafluorides, e.g., R2SF4.
Thioketones, thioaldehydes, and related compounds.
Compounds with double bonds between carbon and sulfur are relatively uncommon, but include the important compounds carbon disulfide, carbonyl sulfide, and thiophosgene. Thioketones (RC(=S)R′) are uncommon with alkyl substituents, but one example is thiobenzophenone. Thioaldehydes are rarer still, reflecting their lack of steric protection ("thioformaldehyde" exists as a cyclic trimer). Thioamides, with the formula R1C(=S)N(R2)R3 are more common. They are typically prepared by the reaction of amides with Lawesson's reagent. Isothiocyanates, with formula R−N=C=S, are found naturally. Vegetable foods with characteristic flavors due to isothiocyanates include wasabi, horseradish, mustard, radish, Brussels sprouts, watercress, nasturtiums, and capers.
"S"-Oxides and "S","S"-dioxides of thiocarbonyl compounds.
The "S"-oxides of thiocarbonyl compounds are known as thiocarbonyl "S"-oxides or sulfines, R2C=S=O, and thiocarbonyl "S","S"-dioxides or sulfenes, R2C=SO2. These compounds are well known with extensive chemistry, e.g., see "syn"-propanethial-"S"-oxide and sulfene.
Triple bonds between carbon and sulfur.
Triple bonds between sulfur and carbon in sulfaalkynes are rare and can be found in carbon monosulfide (CS) and have been suggested for the compounds F3CCSF3 and F5SCSF3. The compound HCSOH is also represented as having a formal triple bond.
Thiocarboxylic acids and thioamides.
Thiocarboxylic acids (RC(O)SH) and dithiocarboxylic acids (RC(S)SH) are well known. They are structurally similar to carboxylic acids but more acidic. Thioamides are analogous to amides.
Sulfonic, sulfinic and sulfenic acids, esters, amides, and related compounds.
Sulfonic acids have functionality R−S(=O)2−OH. They are strong acids that are typically soluble in organic solvents. Sulfonic acids like trifluoromethanesulfonic acid is a frequently used reagent in organic chemistry. Sulfinic acids have functionality R−S(O)−OH while sulfenic acids have functionality R−S−OH. In the series sulfonic—sulfinic—sulfenic acids, both the acid strength and stability diminish in that order. Sulfonamides, sulfinamides and sulfenamides, with formulas R−SO2NR′2, R−S(O)NR′2, and R−SNR′2, respectively, each have a rich chemistry. For example, sulfa drugs are sulfonamides derived from aromatic sulfonation. Chiral sulfinamides are used in asymmetric synthesis, while sulfenamides are used extensively in the vulcanization process to assist cross-linking. Thiocyanates, R−S−CN, are related to sulfenyl halides and esters in terms of reactivity.
Sulfonium, oxosulfonium and related salts.
A sulfonium ion is a positively charged ion featuring three organic substituents attached to sulfur, with the formula [R3S]+. Together with their negatively charged counterpart, the anion, the compounds are called sulfonium salts. An oxosulfonium ion is a positively charged ion featuring three organic substituents and an oxygen attached to sulfur, with the formula [R3S=O]+. Together with their negatively charged counterpart, the anion, the compounds are called oxosulfonium salts. Related species include alkoxysulfonium and chlorosulfonium ions, [R2SOR]+ and [R2SCl]+, respectively.
Sulfonium, oxosulfonium and thiocarbonyl ylides.
Deprotonation of sulfonium and oxosulfonium salts affords ylides, of structure R2S+−C−−R′2 and R2S(O)+−C−−R′2. While sulfonium ylides, for instance in the Johnson–Corey–Chaykovsky reaction used to synthesize epoxides, are sometimes drawn with a C=S double bond, e.g., R2S=CR′2, the ylidic carbon–sulfur bond is highly polarized and is better described as being ionic. Sulfonium ylides are key intermediates in the synthetically useful Stevens rearrangement. Thiocarbonyl ylides (RR′C=S+−C−−RR′) can form by ring-opening of thiiranes, photocyclization of aryl vinyl sulfides, as well as by other processes.
Sulfuranes and persulfuranes.
Sulfuranes are relatively specialized functional group that are tetravalent, hypervalent sulfur compounds, with the formula SR4 and likewise persulfuranes are hexavalent SR6. All-carbon hexavalent complexes have been known for the heavier representatives of the chalcogen group, for instance the compound hexamethylpertellurane (Te(Me)6) was discovered in 1990 by reaction of tetramethyltellurium with xenon difluoride to TeF2(CH3)4 followed by reaction with dimethylzinc. The sulfur analogue hexamethylpersulfurane (S(CH3)6) has been predicted to be stable but has not been synthesized yet.
The first ever all-carbon persulfurane actually synthesized in a laboratory has two methyl and two biphenyl ligands:
It is prepared from the corresponding sulfurane 1 with xenon difluoride / boron trifluoride in acetonitrile to the sulfuranyl dication 2 followed by reaction with methyllithium in tetrahydrofuran to (a stable) persulfurane 3 as the cis isomer. X-ray diffraction shows C−S bond lengths ranging between 189 and 193 pm (longer than the standard bond length) with the central sulfur atom in a distorted octahedral molecular geometry.
Computer simulation suggests that these bonds are very polar with the negative charges residing on carbon.
Naturally occurring organosulfur compounds.
Not all organosulfur compounds are foul-smelling pollutants. Penicillin and cephalosporin are life-saving antibiotics, derived from fungi. Gliotoxin is a sulfur-containing mycotoxin produced by several species of fungi under investigation as an antiviral agent. Compounds like allicin and ajoene are responsible for the odor of garlic, and lenthionine contributes to the flavor of shiitake mushrooms. Volatile organosulfur compounds also contribute subtle flavor characteristics to wine, nuts, cheddar cheese, chocolate, coffee, and tropical fruit flavors. Many of these natural products also have important medicinal properties such as preventing platelet aggregation or fighting cancer.
In pollution.
Most organic sulfur compounds in the environment are naturally occurring, as a consequence of the fact that sulfur is essential for life and two amino acids (cysteine and methionine) contain this element.
Some organosulfur compounds in the environment, are generated as minor by-products of industrial processes such as the manufacture of plastics and tires.
Selected smell-producing processes are organosulfur compounds produced by the coking of coal designed to drive out sulfurous compounds and other volatile impurities in order to produce 'clean carbon' (coke), which is primarily used for steel production.
In fossil fuels.
Odours occur as well in chemical processing of coal or crude oil into precursor chemicals (feedstocks) for downstream industrial uses (e.g. plastics or pharmaceutical production) and the ubiquitous needs of petroleum distillation for gasolines, diesel, and other grades of fuel oils production.
Organosulfur compounds might be understood as aromatic contaminants that need to be removed from natural gas before commercial uses, from exhaust stacks and exhaust vents before discharge. In this latter context, organosulfur compounds may be said to account for the pollutants in sulfurous acid rain, or equivalently, said to be pollutants within most common fossil fuels, especially coal.
The most common organosulfur compound present in all petroleum fractions is thiophene (C4H4S), a cyclic and aromatic liquid. In addition, the heavy fractions of oil contain benzothiophene (C8H6S, thianaphtene) and dibenzothiophene. Most of the last compounds are solids and smell like naphthalene. Many methylated, dimethyl, diethyl benzothiophene derivatives are present in diesel and fuel oils which make fuel oils very difficult to clean.
All these heterocyclic sulfides account for 200–500 ppm of natural fuel, the heavily substituted dibenzothiophenes remain after HDS and account for 10–20 ppm. These molecules are also found in coals and they must be eliminated before consumption.
Reduced molybdenum together with nickel is currently used to eliminate thiophenes from petroleum (HDS) due to its great affinity towards sulfur.
In addition tungsten together with nickel and cobalt is used for hydrodesulfurization (HDS) in large refineries. The adsorption mechanism of thiophene to transition metals is proposed to occur through the π system, where the organosulfur compound lies almost parallel to the metal surface.
Many researchers focus their efforts in optimizing the oxidation state of the transition metals for HDS, like Cu(I) and Ag(II) which together with Pd(0) have proven to be more specific for π bonding with thiophenes of all kinds.
Basis for odor.
Humans and other animals have an exquisitely sensitive sense of smell toward the odor of low-valent organosulfur compounds such as thiols, sulfides, and disulfides. Malodorous volatile thiols are protein-degradation products found in putrid food, so sensitive identification of these compounds is crucial to avoiding intoxication. Low-valent volatile sulfur compounds are also found in areas where oxygen levels in the air are low, posing a risk of suffocation. It has been found that copper is required for the highly sensitive detection of certain volatile thiols and related organosulfur compounds by olfactory receptors in mice. Whether humans, too, require copper for sensitive detection of thiols is not yet known.


Camas Valley Formation

The Camas Valley Formation is a geologic formation in Oregon. It preserves fossils dating back to the Paleogene period.


Mons (planetary nomenclature)

Mons (plural: montes , from the Latin word for "mountain") is a mountain on a celestial body. The term is used in planetary nomenclature: it is a part of the international names of such features. It is capitalized and usually stands after the proper given name, but stands before it in the case of lunar mountains (for example, there is a Martian mountain Arsia Mons and a lunar mountain Mons Argaeus).
The term "tholus" ("dome") is used for names of smaller (especially domical) uplands, and the term "colles" ("hills") in names of groups of still smaller knobs. Peculiar round mountains found on Venus get names with the term "farrum".
Nature of montes.
The term "mons", like other terms of planetary nomenclature, describes only the external view of the feature, but not its origin or geological structure. It is used for mountains of any origin, and objects in this class are very diverse. Usually they are results of tectonic, impact or volcanic processes. Examples of such mountains are the Maxwell Montes on Venus, the Montes Apenninus on the Moon and Olympus Mons on Mars, respectively. More unusual origins are also possible. For example, the Geryon Montes of Mars are an erosional remnant of a former plateau within Ius Chasma, part of the Valles Marineris canyon system.
During impact events, mountains can appear not only on the border of the resulting crater, but also in its center. Classical central peaks generally are not named, although some peaks within craters are named: for example, Aeolis Mons within Gale crater on Mars or the Scheria Montes within Odysseus crater on Tethys.
Names of montes.
The term "mons" was firstly used for extraterrestrial mountains by Michael van Langren in 1645. He assigned names to some mountains on the Moon, but none of these names remain in use. Two years later Johannes Hevelius proposed the first names of extraterrestrial mountains still used. The lunar Alps and Apennines still bear names given by Hevelius, and five of his other mountain names nowadays refer to other features (in some cases his "mountains" later turned out to be bright rays from craters).
The International Astronomical Union approved the first names of extraterrestrial mountains (together with many other traditional lunar names) in 1935, although without a generic Latin term, which was restored in 1961. As of May 2015, 262 extraterrestrial mountains and mountain systems have official names: 117 on Venus, 50 on Mars, 48 on the Moon, 23 on Io, 13 on Titan, 9 on Iapetus, 1 on Mercury and 1 on Tethys. They are named differently on different celestial bodies:


Gabbro

Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite.
Etymology.
The term "gabbro" was used in the 1760s to name a set of rock types that were found in the ophiolites of the Apennine Mountains in Italy. It was named after Gabbro, a hamlet near Rosignano Marittimo in Tuscany. Then, in 1809, the German geologist Christian Leopold von Buch used the term more restrictively in his description of these Italian ophiolitic rocks. He assigned the name "gabbro" to rocks that geologists nowadays would more strictly call "metagabbro" (metamorphosed gabbro).
Petrology.
Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, and minor amounts of amphibole and olivine.
The pyroxene content is mostly clinopyroxene, generally augite, but small amounts of orthopyroxene may also be present. If the amount of orthopyroxene is more than 95% of the total pyroxene content (5% or less clinopyroxene content), then the rock is termed norite. On the other hand, gabbro has more than 95% of its pyroxenes in the form of the monoclinic clinopyroxene/s. Intermediate rocks are termed gabbro-norite. The calcium-rich plagioclase feldspar (labradorite-bytownite) and pyroxene content vary between 10–90% in gabbro. If more than 90% plagioclase is present, then the rock is an anorthosite. If, on the other hand, the rock contains more than 90% pyroxenes (often both are present), it is termed pyroxenite. Gabbro may also contain small amounts of olivine ("olivine gabbro" if substantial amount of olivine is present), amphibole and biotite. The quartz content in gabbro is less than 5% of total volume.
'Quartz gabbros' or monzogabbros are also known to occur, for example the "cizlakite" at Pohorje in northeastern Slovenia, and are probably derived from magma that was over-saturated with silica. Essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of the feldspathoid minerals nepheline, cancrinite, and sodalite as accessory minerals rather than quartz. (Silica saturation of a rock can be evaluated by normative mineralogy). Gabbros contain minor amounts, typically a few percent, of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel.
Gabbro is generally coarse-grained, with crystals in the size range of 1 mm or larger. Finer-grained equivalents of gabbro are called diabase (also known as dolerite), although the term "microgabbro" is often used when extra descriptiveness is desired. Gabbro may be extremely coarse-grained to pegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse-grained gabbro, some may exhibit acicular crystal habits.
Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclase oikocrysts have grown earlier than the groundmass minerals.
Distribution.
Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and plagioclase, or as part of a layered intrusion as a cumulate formed by settling of pyroxene and plagioclase. Cumulate gabbros are more properly termed "pyroxene-plagioclase adcumulate".
Gabbro is an essential part of the oceanic crust, and can be found in many ophiolite complexes as parts of zones III and IV (sheeted dyke zone to massive gabbro zone). Long belts of gabbroic intrusions are typically formed at proto-rift zones and around ancient rift zone margins, intruding into the rift flanks. Mantle plume hypotheses may rely on identifying mafic and ultramafic intrusions and coeval basalt volcanism.
Nearly all gabbros are found in plutonic bodies, and the term (as the International Union of Geological Sciences recommends) is normally restricted just to plutonic rocks, although gabbro may be found as a coarse-grained interior facies of certain thick lavas.
Gabbro in the United States.
Located in the Mocksville Complex in Mocksville, North Carolina is the Farmington Gabbro rock unit. The gabbro contains minerals such as plagioclase feldspar, hornblende, pyroxenes, olivine, biotite with small amounts of apatite, magnetite, ilmenite, hematite, pyrite, pyrrhotite, and hercynitic spinel.
Uses.
Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.
Orbicular varieties of gabbro can be used as ornamental facing stones, paving stones and it is also known by the trade name of "black granite". It is also used for kitchen countertops.


Capote Mountain Tuff

The Capote Mountain Tuff is a geologic group in Texas. It preserves fossils dating back to the Paleogene period.


Amsden Formation

The Amsden Formation is a geologic formation in Montana. It preserves fossils dating back to the Carboniferous period.


Grande Greve Formation

The Grande Greve Formation is a geologic formation in Quebec. It preserves fossils dating back to the Devonian period.


LaPorte City Formation

The LaPorte City Formation is a geologic formation in Iowa. It preserves fossils dating back to the Silurian period.


Blaire Van Valkenburgh

Blaire Van Valkenburgh is an American paleontologist and holds the Donald R. Dickey Chair in Vertebrate Biology in the Department of Ecology and Evolutionary Biology at University of California Los Angeles. She is a former president of Society of Vertebrate Paleontology.
Early life and education.
Van Valkenburgh was born in Washington, D.C. and raised in Alexandria, Virginia. Her mother, Lois Van Valkenburgh, was a civil rights activist. From 1964 to 1970, she attended T. C. Williams High School.
Van Valkenburgh received a bachelor's degree from Stockton State College in New Jersey in 1974, a Ph.D. in Vertebrate Paleobiology from Johns Hopkins University in 1984, where she worked with Robert Bakker, after which she worked as a postdoctoral fellow with Alan Walker at Johns Hopkins before moving to UCLA in 1986.
Career.
Van Valkenburgh is a paleontologist and holds the Donald R. Dickey Chair in Vertebrate Biology in the Department of Ecology and Evolutionary Biology at University of California Los Angeles. She has served as chair of the department and as associate dean of academic programs in the life sciences at UCLA.The focus of her research is the paleobiology and paleoecology of Carnivora. Her contributions include quantification of guild structure in fossil carnivore communities and the study of iterative evolution in carnivore feeding adaptations.
Van Valkenburgh served as president of the Society of Vertebrate Paleontology from 2008 to 2010 and as associate editor of the Journal of Vertebrate Paleontology from 2011 to 2017.


Park City Formation

The Park City Formation is a geologic formation in Montana and Utah. It preserves fossils dating back to the Permian period.


Richard T. Liddicoat

Richard T. Liddicoat, Jr. (March 2, 1918– July 23, 2002) was an American gemologist. Liddicoat was an educator in gemology, who also made contributions in the area of diamond quality grading and gem identification. Liddicoat was the Chairman of the Board of Governors at Gemological Institute of America (GIA).
Early life.
On March 2, 1918, Liddicoat was born as Richard Thomas Liddicoat, Jr. in Kearsarge, Michigan to Richard T. and Carmen Beryl (Williams) Liddicoat. Both of his grandfathers were miners from Cornwall.
Education.
In 1939, Liddicoat earned a bachelor's degree in geology from University of Michigan. In 1940, Liddicoat earned a master's degree in mineralogy from University of Michigan. During World War II, he attended the California Institute of Technology where he obtained a Master's degree in Meteorology in 1944.
Career.
On June 28, 1940, Liddicoat joins the staff of GIA as an Assistant Director of Education. About a year later, in Fall 1941, Liddiocoat helped develop the Diamolite, and also published, alongside Shipley, his first article for Gems & Gemology; "A Solution to Diamond Color Grading Problems".
After serving in the United States Navy during WWII, he returned to GIA as Director of Research in February 1946. 18 months later, he published his first edition of the Handbook of Gem Identification. In the next year, he was named Assistant Director of GIA, and in 1949, named director of GIA New York and GIA Gem Trade Laboratory.
In April 1952, he was also named executive director of GIA, and then later assumes editorship of Gems & Gemology. Exactly a year later, April 1953, he officially introduces the GIA diamond grading system as part of a new educational class in New York. Successfully, the GIA Gem Trade Laboratory issues its first Diamond Grading Reports. That led him to co-author the first edition of The Diamond Dictionary, in 1960. That also led him to devise the "rapid sight" system for estimating diamond cutting quality. He also later co-authors the first edition of The Jeweler's Manual.
In the new decade, in 1970, Liddicoat launches a series of extension education classes in Israel taught by Glenn Nord, making GIA's first global outreach. Further expanding that outreach, in Japan, he launches the first GIA gemology courses in the following year. He then receives the 1976 American Gem Society's Robert M. Shipley Award, and creates the GIA Research Department under the leadership of Dr. D. Vincent Manson.
In Spring 1981, he published his first issue of Gems & Gemology in an expanded and redesigned format. The next year, he creates the GIA Alumni Association under Robert Earnest. Chairs GIA's first International Gemological Symposium in Los Angeles. In 1983, he steps down as President of GIA; named chairman of the GIA Board of Governors, and named Honorary Member of AGTA. He is then named 1984 Man of the Year by the Consolidated Jewelers Association of New York, also being named a Founding Organizer of ICA.
Achievements.
The following year, Liddicoat received Modern Jeweler Magazine's Lifetime Achievement Award, also receiving the Morris B. Zale Lifetime Achievement Award, and becoming the first Honorary Lifetime Member of the Gem Testing Laboratory of Great Britain. In 1989, he also was honored with the dedication of the Richard T. Liddicoat Gemological Library and Information Center at GIA.
He was then named into the National Home Study Council's Hall of Fame in 1991, and in the same decade named GIA Chairman of the Board for Life in 1992, and received GIA League of Honor Lifetime Achievement Award.
In June 2000, a life-size bronze statue based on Liddicoat was created by staff member Michael Clary, placed at the entrance of GIA Carlsbad. The next year, he receives the AGS Lifetime Achievement Award in July
Personal.
Liddicoat's wife was Mary Imogene Hibbard, who died in 1995.
On July 23, 2002, Richard T. Liddicoat died at his home in Santa Monica, California of cancer.
Notable contributions.
Richard T. Liddicoat was GIA's driving force for 60 years. Following the example set by GIA's founder, Robert M. Shipley, Liddicoat positioned GIA as a leading international educational institution, grading laboratory, manufacturer of gemological instruments, and research center. Those who had the privilege of knowing and working with him have been able to expand the frontiers of gemology by “standing on the shoulders” of this giant.
Architect of the international diamond grading system.
In 1953, Liddicoat introduced the GIA diamond grading system - a practical approach to grading the quality of colorless to light yellow polished diamonds on the basis of color, clarity, and cut. A central feature was the D-to-Z color grading system for faceted colorless to light yellow diamonds—the vast majority of diamonds seen in the trade.
Beginning in 1953, GIA instructors taught this grading system to hundreds of students so they could evaluate their own diamonds. Subsequently, many of these students asked GIA to set up a procedure whereby they could submit their grading worksheets, and eventually the diamonds themselves, for an independent assessment. And so, GIA issued the first GIA Diamond Grading Report in 1955.
Expanding GIA's Reach through EducationIn 1942, as Director of Education, Liddicoat developed an intensive one-week lab class that was launched at the American Gem Society (AGS) Conclaves in Philadelphia and Chicago. The class gave jewelers the opportunity to work with equipment in a classroom setting under the guidance of GIA instructors.
The number of international students grew under Liddicoat's leadership. In 1970, he arranged for instructor Glenn Nord to teach in Israel. The following year, Kenzo Yamamoto and Yoshiko Doi began teaching GIA classes in Japan (since then, nearly 3,000 graduate gemologists have been trained at GIA's Japanese affiliate). From these early beginnings, GIA has established 13 schools in 14 countries.
Creating gem testing techniques and affordable instruments.
Liddicoat presided over four decades of innovation in gem instrument development at GIA, and established gem instruments as one of the Institute's highest priorities. In addition to his early work with Shipley to develop the Diamolite, Liddicoat's scientific background enabled him to foster the development of instruments such as the prism spectroscope and the ProportionScope. His objective throughout instrument development at GIA was to provide jewelers with practical tools that could help them in the day-to-day operation of identifying gemstones.
Reaching out to the world through books, publications, and a library.
Liddicoat believed in disseminating information to students and the gem and jewelry trade alike. His first article in Gems & Gemology, co-authored with Robert M. Shipley in 1941, was “A solution to diamond color grading problems.” (He later admitted that it was an overly ambitious title.)
Liddicoat became editor of Gems & Gemology in 1952, and eventually editor-in-chief until his death in 2002. He led a procession of notable mineralogists, gemologists, and other scientists who contributed groundbreaking articles to the journal. Over the years, he would write more than 40 articles, editorials, and news items for G&G..
Liddicoat also participated in writing three important books for gemologists. The first edition of Richard Liddicoat's Handbook of Gem Identification, published in 1947, presented “simple and often conclusive” tests to identify gems. Now in its 12th edition (4th printing), it is one of the most widely read textbooks in gemology. The Diamond Dictionary, first published in 1960 and now in its 3rd edition (1993), provides a standard reference for diamond terminology. The Jewelers' Manual, a handy reference guide to gemology and jewelry for the working jeweler, was first published in 1964 and is now in its 3rd edition (1989).
Liddicoat's contributions to spreading gem and jewelry knowledge were immortalized with the creation of the Richard T. Liddicoat Gemological Library and Information Center in 1989. Although Shipley had begun a library in the very first days of GIA, Liddicoat was the one who envisioned a world-class, state-of-the-art library that would serve not only GIA staff and students, but also jewelers, gemologists, and consumers worldwide. Today, the Liddicoat Library is the largest gemology and jewelry library in the world. At almost , it houses over 30,000 volumes and journals as well as extensive collections of photos, videotapes, and other media resources.
Protecting the public through research and gem identification.
When Robert M. Shipley set up his Los Angeles laboratory in the early 1930s, the Institute's resources were devoted to identifying gems and to documenting the properties and techniques that would aid in their identification. Late in 1948, Shipley assigned Liddicoat to New York to lead the expansion of that new branch. As director of GIA New York and the GIA Gem Trade Laboratory, Liddicoat was responsible for integrating the facilities of the two labs. The early identification work was typically a group effort among staff members who became known as the “Liddicoat brain trust.”
By the 1970s, the GIA Gem Trade Laboratory staff was examining large numbers of gemstones, including the grading of diamonds and the identification of colored stones and pearls. Sensing the need for a group of scientists who would focus on the many emerging technical challenges in gemology, Liddicoat formally established the current GIA Research Department in 1976.


Mission Valley Formation

The Mission Valley Formation is a marine sandstone geologic formation in the Mission Valley region of southwestern San Diego County in Southern California.
Geology.
The formation's sandstone characteristics are: soft and friable, light olive gray, fine to medium grained, and composed mostly of quartz and potassium feldspar. The Mission Valley Formation thins from the west to the east, with a maximum thickness of .
It overlies the Stadium Conglomerate formation, and underlies the Pomerado Conglomerate formation.
Fossils.
It preserves fossils dating back to the Paleogene period of the Cenozoic Era. The formation's rocks can contain a molluscan fauna in the western and central exposures and a land-mammal fauna in the eastern exposures.


Volcanism on Io

Volcanism on Io, a moon of Jupiter, is represented by the presence of volcanoes, volcanic pits and lava flows on the moon's surface. Its volcanic activity was discovered in 1979 by "Voyager 1" imaging scientist Linda Morabito. Observations of Io by passing spacecraft (the "Voyagers", "Galileo", "Cassini", and "New Horizons") and Earth-based astronomers have revealed more than 150 active volcanoes. Up to 400 such volcanoes are predicted to exist based on these observations. Io's volcanism makes the satellite one of only four known currently volcanically active worlds in the Solar System (the other three being Earth, Saturn's moon Enceladus, and Neptune's moon Triton).
First predicted shortly before the "Voyager 1" flyby, the heat source for Io's volcanism comes from tidal heating produced by its forced orbital eccentricity. This differs from Earth's internal heating, which is derived primarily from radioactive isotope decay and primordial heat of accretion. Io's eccentric orbit leads to a slight difference in Jupiter's gravitational pull on the satellite between its closest and farthest points on its orbit, causing a varying tidal bulge. This variation in the shape of Io causes frictional heating in its interior. Without this tidal heating, Io might have been similar to the Moon, a world of similar size and mass, geologically dead and covered with numerous impact craters.
Io's volcanism has led to the formation of hundreds of volcanic centres and extensive lava formations, making it the most volcanically active body in the Solar System. Three different types of volcanic eruptions have been identified, differing in duration, intensity, lava effusion rate, and whether the eruption occurs within a volcanic pit (known as a "patera"). Lava flows on Io, tens or hundreds of kilometres long, have primarily basaltic composition, similar to lavas seen on Earth at shield volcanoes such as Kīlauea in Hawaii. Although most of the lava on Io is made of basalt, a few lava flows consisting of sulfur and sulfur dioxide have been seen. In addition, eruption temperatures as high as were detected, which can be explained by the eruption of high-temperature ultramafic silicate lavas.
As a result of the presence of significant quantities of sulfurous materials in Io's crust and on its surface, some eruptions propel sulfur, sulfur dioxide gas, and pyroclastic material up to into space, producing large, umbrella-shaped volcanic plumes. This material paints the surrounding terrain in red, black, and/or white, and provides material for Io's patchy atmosphere and Jupiter's extensive magnetosphere. Spacecraft that have flown by Io since 1979 have observed numerous surface changes as a result of Io's volcanic activity.
Discovery.
Before the "Voyager 1" encounter with Io on March 5, 1979, Io was thought to be a dead world much like the Moon. The discovery of a cloud of sodium surrounding Io led to theories that the satellite would be covered in evaporites.
Hints of discoveries to come arose from Earth-based infrared observations taken in the 1970s. An anomalously high thermal flux, compared to the other Galilean satellites, was discovered during measurements taken at an infrared wavelength of 10 μm while Io was in Jupiter's shadow. At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede. These results were considerably different from measurements taken at wavelengths of 20 μm, which suggested that Io had similar surface properties to the other Galilean satellites. Robert Nelson and Bruce Hapke attempted to explain these features in Io's spectrum by suggesting fumarolic activity as a mechanism for producing short-chain sulphur allotropes on Io's surface. It has since been determined that the greater flux at shorter wavelengths was due to the combined flux from Io's volcanoes and solar heating, whereas solar heating provides a much greater fraction of the flux at longer wavelengths. A sharp increase in Io's thermal emission at 5 μm was observed on February 20, 1978 by Witteborn, "et al." The group considered volcanic activity at the time, in which case the data was fit into a region on Io in size at . However, the authors considered that hypothesis unlikely, and instead focused on emission from Io's interaction with Jupiter's magnetosphere.
Shortly before the "Voyager 1" encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal "Science" predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by its slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.
"Voyager 1"'s first images of Io revealed a lack of impact craters, suggesting a very young surface. Craters are used by geologists to estimate the age of a planetary surface; the number of impact structures increase with the age of the planetary surface. Instead, "Voyager 1" observed a multi-coloured surface, pockmarked with irregular-shaped depressions, which lacked the raised rims characteristic of impact craters. "Voyager 1" also observed flow features formed by low-viscosity fluid and tall, isolated mountains that did not resemble terrestrial volcanoes. The surface observed suggested that, just as Peale and colleagues had theorized, Io was heavily modified by volcanism.
On March 8, 1979, three days after passing Jupiter, "Voyager 1" took images of Jupiter's moons to help mission controllers determine the spacecraft's exact location, a process called optical navigation. Although processing images of Io to enhance the visibility of background stars, navigation engineer Linda Morabito found a tall cloud along its limb. At first, she suspected the cloud to be a moon behind Io, but no suitably sized body would have been in that location. The feature was determined to be a plume generated by active volcanism at a dark depression later named Pele. Following this discovery, eight other plumes were located in "Voyager" images of Io. These plumes were later named after mythological deities associated with fire, volcanoes, or mayhem: Loki (two separate plumes), Prometheus, Volund, Amirani, Maui, Marduk, and Masubi. Thermal emission from multiple sources, indicative of cooling lava, were also found. Surface changes were observed when images acquired by "Voyager 2" were compared to those taken four months previously by "Voyager 1", including new plume deposits at Aten Patera and Surt.
Heat source.
Io's main source of internal heat comes from the tidal forces generated by Jupiter's gravitational pull. This external heating differs from the internal heat source for volcanism on Earth, which is a result of radioactive isotope decay and residual heat from accretion. In the Earth, these internal heat sources drive mantle convection, which in turn causes volcanism through plate tectonics.
The tidal heating of Io is dependent on its distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. Its Laplace orbital resonance with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from circularizing its orbit. The eccentricity leads to vertical differences in Io's tidal bulge of as much as as Jupiter's gravitational pull varies between the periapsis and apoapsis points in Io's orbit. This varying tidal pull also produces friction in Io's interior, enough to cause significant tidal heating and melting. Unlike Earth, where most of its internal heat is released by conduction through the crust, on Io internal heat is released via volcanic activity and generates the satellite's high heat flow (global total: 0.6–1.6 W). Models of its orbit suggest that the amount of tidal heating within Io changes with time, and that the current heat flow is not representative of the long-term average. The observed release of heat from Io's interior is greater than estimates for the amount presently generated from tidal heating, suggesting that Io is cooling after a period of greater flexing.
Composition.
Analysis of "Voyager" images led scientists to believe that the lava flows on Io were composed mostly of various forms of molten elemental sulfur. The colouration of the flows was found to be similar to its various allotropes. Differences in the lava colour and brightness are a function of the temperature of polyatomic sulfur and the packing and bonding of its atoms. An analysis of the flows that radiate out from Ra Patera revealed differently colored materials, all associated with liquid sulfur, at different distances from the vent: dark albedo material close to the vent at , red material in the central part of each flow at , and orange material at the farthest ends of each flow at . This colour pattern corresponds to flows radiating out from a central vent, cooling as the lava travels away from it. In addition, temperature measurements of thermal emission at Loki Patera taken by "Voyager 1"'s Infrared Interferometer Spectrometer and Radiometer (IRIS) instrument were consistent with sulfur volcanism. However, the IRIS instrument was not capable of detecting wavelengths that are indicative of higher temperatures. This meant that temperatures consistent with silicate volcanism were not discovered by "Voyager". Despite this, "Voyager" scientists deduced that silicates must play a role in Io's youthful appearance, from its high density and the need for silicates to support the steep slopes along patera walls. The contradiction between the structural evidence and the spectral and temperature data following the "Voyager" flybys led to a debate in the planetary science community regarding the composition of Io's lava flows, whether they were composed of silicate or sulfurous materials.
Earth-based infrared studies in the 1980s and 1990s shifted the paradigm from one of primarily sulfur volcanism to one where silicate volcanism dominates, and sulfur acts in a secondary role. In 1986, measurements of a bright eruption on Io's leading hemisphere revealed temperatures of at least . This is higher than the boiling point of sulfur (), indicating a silicate composition for at least some of Io's lava flows. Similar temperatures were also observed at the Surt eruption in 1979 between the two "Voyager" encounters, and at the eruption observed by Witteborn and colleagues in 1978. In addition, modeling of silicate lava flows on Io suggested that they cooled rapidly, causing their thermal emission to be dominated by lower temperature components, such as solidified flows, as opposed to the small areas covered by still molten lava near the actual eruption temperature.
Silicate volcanism, involving basaltic lava with mafic to ultramafic (magnesium-rich) compositions, was confirmed by the "Galileo" spacecraft in the 1990s and 2000s from temperature measurements of Io's numerous hot spots, locations where thermal emission is detected, and from spectral measurements of Io's dark material. Temperature measurements from "Galileo"'s Solid-State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) revealed numerous hot spots with high-temperature components ranging from at least to a maximum of , like at the Pillan Patera eruption in 1997. Initial estimates during the course of the "Galileo" mission suggesting eruption temperatures approaching have since proven to be overestimates because the wrong thermal models were used to calculate the temperatures. Spectral observations of Io's dark material suggested the presence of orthopyroxenes, such as enstatite, magnesium-rich silicate minerals common in mafic and ultramafic basalt. This dark material is seen in volcanic pits, fresh lava flows, and pyroclastic deposits surrounding recent, explosive volcanic eruptions. Based on the measured temperature of the lava and the spectral measurements, some of the lava may be analogous to terrestrial komatiites. Compressional superheating, which could increase the temperature of magma during ascent to the surface during an eruption, may also be a factor in some of the higher temperature eruptions.
Although temperature measurements of Io's volcanoes settled the sulfur-versus-silicates debate that persisted between the "Voyager" and "Galileo" missions at Jupiter, sulfur and sulfur dioxide still play a significant role in the phenomena observed on Io. Both materials have been detected in the plumes generated at Io's volcanoes, with sulfur being a primary constituent of Pele-type plumes. Bright flows have been identified on Io, at Tsũi Goab Fluctus, Emakong Patera, and Balder Patera for example, that are suggestive of effusive sulfur or sulfur dioxide volcanism.
Eruption styles.
Observations of Io by spacecraft and Earth-based astronomers have led to the identification of differences in the types of eruptions seen on the satellite. The three main types identified include "intra-patera", "flow-dominated", and "explosion-dominated" eruptions. They differ in terms of duration, energy released, brightness temperature (determined from infrared imaging), type of lava flow, and whether it is confined within volcanic pits.
Intra-patera eruptions.
"Intra-patera" eruptions occur within volcanic depressions known as "paterae", which generally have flat floors bounded by steep walls. Paterae resemble terrestrial calderas, but it is unknown whether they form when an empty magma chamber collapses, like their terrestrial cousins. One hypothesis suggests that they are produced through the exhumation of volcanic sills, with the overlying material either being blasted out or integrated into the sill. Some paterae display evidence for multiple collapses, similar to the calderas atop Olympus Mons on Mars or Kīlauea on Earth, suggesting that they may occasionally form like volcanic calderas. Because the formation mechanism is still uncertain, the general term for these features uses the Latin descriptor term employed by the International Astronomical Union in naming them, "paterae". Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of shield volcanoes and are larger, with an average diameter of . Patera depths have been measured for only a few paterae and typically exceed 1 km. The largest volcanic depression on Io is Loki Patera at across. Whatever the formation mechanism, the morphology and distribution of many paterae suggest that they are structurally controlled, with at least half bounded by faults or mountains.
This eruption style can take the form of either lava flows, spreading across the floor of the paterae, or lava lakes. Except for observations by Galileo during its seven close flybys, it can be difficult to tell the difference between a lava lake and a lava flow eruption on a patera floor, due to inadequate resolution and similar thermal emission characteristics. Intra-patera lava flow eruptions, such as the Gish Bar Patera eruption in 2001, can be just as voluminous as those seen spreading out across the Ionian plains. Flow-like features have also been observed within a number of paterae, like Camaxtli Patera, suggesting that lava flows periodically resurface their floors.
Ionian lava lakes are depressions partially filled with molten lava covered by a thin solidified crust. These lava lakes are directly connected to a magma reservoir lying below. Observations of thermal emission at several Ionian lava lakes reveal glowing molten rock along the patera margin, caused by the lake's crust breaking up along the edge of the patera. Over time, because the solidified lava is denser than the still-molten magma below, this crust can founder, triggering an increase in thermal emission at the volcano. For some lava lakes, like the one at Pele, this occurs continuously, making Pele one of the brightest emitters of heat in the near-infrared spectrum on Io. At other sites, such as at Loki Patera, this can occur episodically. During an overturning episode at these more quiescent lava lakes, a wave of foundering crust spreads out across the patera at the rate of about per day, with new crust forming behind it until the entire lake has been resurfaced. Another eruption would only begin once the new crust has cooled and thickened enough for it to no longer be buoyant over the molten lava. During an overturning episode, Loki can emit up to ten times more heat than when its crust is stable.
Flow-dominated eruptions (Promethean Volcanism).
"Flow-dominated" eruptions are long-lived events that build up extensive, compound lava flows. The extent of these flows makes them a major terrain type on Io. In this style of eruption, magma emerges onto the surface from vents on the floor of paterae, vents surrounding paterae, or from fissures on the plains, producing inflated, compound lava flows similar to those seen at Kīlauea in Hawaii. Images from the "Galileo" spacecraft revealed that many of Io's major flows, like those at Prometheus and Amirani, are produced by the build-up of small breakouts of lava on top of older flows. Flow-dominated eruptions differ from explosion-dominated eruptions by their longevity and their lower energy output per unit of time. Lava erupts at a generally steady rate, and flow-dominated eruptions can last for years or decades.
Active flow fields more than long have been observed on Io at Amirani and Masubi. A relatively inactive flow field named Lei-Kung Fluctus covers more than , an area slightly larger than Nicaragua. The thickness of flow fields was not determined by Galileo, but the individual breakouts on their surface are likely to be thick. In many cases, active lava breakouts flow out onto the surface at locations tens to hundreds of kilometres from the source vent, with low amounts of thermal emission observed between it and the breakout. This suggests that lava flows through lava tubes from the source vent to the breakout.
Although these eruptions generally have a steady eruption rate, larger outbreaks of lava have been observed at many flow-dominated eruption sites. For example, the leading edge of the Prometheus flow field moved between observations by "Voyager" in 1979 and "Galileo" in 1996. Although generally dwarfed by explosion-dominated eruptions, the average flow rate at these compound flow fields is much greater than what is observed at similar contemporary lava flows on Earth. Average surface coverage rates of per second were observed at Prometheus and Amirani during the "Galileo" mission, compared to per second at Kīlauea.
Explosion-dominated eruptions (Pillanian Volcanism).
"Explosion-dominated" eruptions are the most pronounced of Io's eruption styles. These eruptions, sometimes called "outburst" eruptions from their Earth-based detections, are characterized by their short duration (lasting only weeks or months), rapid onset, large volumetric flow rates, and high thermal emission. They lead to a short-lived, significant increase in Io's overall brightness in the near-infrared. The most powerful volcanic eruption observed on Io was an "outburst" eruption at Surt, observed by Earth-based astronomers on February 22, 2001.
Explosion-dominated eruptions occur when a body of magma (called a "dike") from deep within Io's partially molten mantle reaches the surface at a fissure. This results in a spectacular display of lava fountains. During the beginning of the outburst eruption, thermal emission is dominated by strong, 1–3 μm infrared radiation. It is produced by a large amount of exposed, fresh lava within the fountains at the eruption source vent. Outburst eruptions at Tvashtar in November 1999 and February 2007 centred on a long, tall lava "curtain" produced at a small patera nested within the larger Tvashtar Paterae complex.
The large amount of exposed molten lava at these lava fountains has provided researchers with their best opportunity to measure the actual temperatures of Ionian lavas. Temperatures suggestive of an ultramafic lava composition similar to Pre-Cambrian komatiites (about ) are dominant at such eruptions, though superheating of the magma during ascent to the surface cannot be ruled out as a factor in the high eruption temperatures.
Although the more explosive, lava-fountaining stage may last only a few days to a week, explosion-dominated eruptions can continue for weeks to months, producing large, voluminous silicate lava flows. A major eruption in 1997 from a fissure north-west of Pillan Patera produced more than of fresh lava over a - to -month period, and later flooded the floor of Pillan Patera. Observations by "Galileo" suggest lava coverage rates at Pillan between per second during the 1997 eruption. The Pillan flow was found to be thick, compared to the thick flows observed at the inflated fields at Prometheus and Amirani. Similar, rapidly emplaced lava flows were observed by "Galileo" at Thor in 2001. Such flow rates are similar to those seen at Iceland's Laki eruption in 1783 and in terrestrial flood basalt eruptions.
Explosion-dominated eruptions can produce dramatic (but often short-lived) surface changes around the eruption site, such as large pyroclastic and plume deposits produced as gas exsolves from lava fountains. The 1997 Pillan eruption produced a wide deposit of dark, silicate material and bright sulfur dioxide. The Tvashtar eruptions of 2000 and 2007 generated a tall plume that deposited a ring of red sulfur and sulfur dioxide wide. Despite the dramatic appearance of these features, without continuous resupply of material, the vent surroundings often revert to their pre-eruption appearance over a period of months (in the case of Grian Patera) or years (as at Pillan Patera).
Plumes.
The discovery of volcanic plumes at Pele and Loki in 1979 provided conclusive evidence that Io was geologically active. Generally, plumes form when volatiles like sulfur and sulfur dioxide are ejected skyward from Io's volcanoes at speeds reaching , creating umbrella-shaped clouds of gas and dust. Additional materials that might be found in the volcanic plumes include sodium, potassium, and chlorine. Although striking in appearance, volcanic plumes are relatively uncommon. Of the 150 or so active volcanoes observed on Io, plumes have only been observed at a couple of dozen of them. The limited area of Io's lava flows suggests that much of the resurfacing needed to erase Io's cratering record must come from plume deposits.
The most common type of volcanic plume on Io are dust plumes, or Prometheus-type plumes, produced when encroaching lava flows vaporize underlying sulfur dioxide frost, sending the material skyward. Examples of Prometheus-type plumes include Prometheus, Amirani, Zamama, and Masubi. These plumes are usually less than tall with eruption velocities around . Prometheus-type plumes are dust-rich, with a dense inner core and upper canopy shock zone, giving them an umbrella-like appearance. These plumes often form bright circular deposits, with a radius ranging between and consisting primarily of sulfur dioxide frost. Prometheus-type plumes are frequently seen at flow-dominated eruptions, helping make this plume type quite long-lived. Four out of the six Prometheus-type plumes observed by "Voyager 1" in 1979 were also observed throughout the "Galileo" mission and by "New Horizons" in 2007. Although the dust plume can be clearly seen in sunlit visible-light images of Io acquired by passing spacecraft, many Prometheus-type plumes have an outer halo of fainter, more gas-rich material reaching heights approaching that of the larger, Pele-type plumes.
Io's largest plumes, Pele-type plumes, are created when sulfur and sulfur dioxide gas exsolve from erupting magma at volcanic vents or lava lakes, carrying silicate pyroclastic material with them. The few Pele-type plumes that have been observed are usually associated with explosion-dominated eruptions, and are short-lived. The exception to this is Pele, which is associated with a long-lived active lava lake eruption, though the plume is thought to be intermittent. The higher vent temperatures and pressures associated with these plumes generate eruption speeds of up to , allowing them to reach heights of between . Pele-type plumes form red (from short-chain sulfur) and black (from silicate pyroclastics) surface deposits, including large -wide red rings, as seen at Pele. The erupted sulfurous components of Pele-type plumes are thought to be the result of an excess amount of sulfur in Io's crust and a decrease in sulfur solubility at greater depths in Io's lithosphere. They are generally fainter than Prometheus-type plumes as a result of the low dust content, causing some to be called stealth plumes. These plumes are sometimes only seen in images acquired while Io is in the shadow of Jupiter or those taken in ultraviolet. The little dust that is visible in sunlit images is generated when sulfur and sulfur dioxide condense as the gases reach the top of their ballistic trajectories. That is why these plumes lack the dense central column seen in Prometheus-type plumes, in which dust is generated at the plume source. Examples of Pele-type plumes have been observed at Pele, Tvashtar, and Grian.


Garden City Formation

The Garden City Formation is a geologic formation in Idaho. It preserves fossils dating back to the Ordovician period.


Paraconformity


Harkless Formation

The Harkless Formation is a geologic formation in Nevada. It preserves fossils dating back to the Cambrian period.


Mulholland Formation

The Mulholland Formation is a Pliocene epoch geologic formation in the Berkeley Hills and San Leandro Hills of the East Bay region of the San Francisco Bay Area, California. It is found within Alameda County and Contra Costa County.
Geology.
It overlies the Bald Peak Basalt formation, and underlies the Pleistocene epoch Leona Rhyolite formation. It is composed of siltstone, sandstone, and conglomerates. It has fluviatile and lacustrine deposits.
Descending under the Bald Peak Basalt formation are the Pliocene epoch units of the Siesta Formation, Moraga Formation, and Orinda Formation. Below the Orinda are the local Miocene epoch units of the Monterey Formation Group: Tice Shale, Oursan Sandstone, Claremont Shale, and Sobrante Sandstone.
Fossils.
The Mulholland Formation preserves fossils dating back to the Neogene period.


Blue Fjord Formation

The Blue Fjord Formation is a geologic formation in Northwest Territories. It preserves fossils dating back to the Devonian period.


Anse Cascon Formation

The Anse Cascon Formation is a geologic formation in Quebec. It preserves fossils dating back to the Silurian period.


Almy Formation

The Almy Formation is a geologic formation in Wyoming. It preserves fossils dating back to the Paleogene period.


Roque Cinchado

The Roque Cinchado is a rock formation, regarded as emblematic of the island of Tenerife (Canary Islands, Spain). It lies within the Teide National Park (a World Heritage Site) in the municipality of La Orotava, near the volcano of the same name, in the heart of the island.
The Roque Cinchado is located about 1700 meters below the summit of Teide volcano and is a volcanic formation; it belongs to a lineup of large rock formations, remnants of the former summit of the island, known as "Roques García."
The Roque Cinchado is a 27-metre high rock pillar, composed mostly of volcaniclastic sedimentary rock layers Its upper sedimentary layers have been intruded by two sills of lava, which have made the upper part of the pillar more resistant to erosion.
The rock appeared on one thousand peseta bank notes, with Teide in the background.


Crenulation

In a geological context, crenulation or crenulation cleavage is a fabric formed in metamorphic rocks such as phyllite, schist and some gneiss by two or more stress directions causing the formation of the superimposed foliations.
Formation.
Crenulations form when an early planar fabric is overprinted by a later planar fabric. Crenulations form by recrystallisation of mica minerals during metamorphism. Micaceous minerals form planar surfaces known as foliations perpendicular to the principal stress fields.
If a rock is subjected to two separate deformations and the second deformation is at some other angle to the original, growth of new micas on the foliation planes will create a new foliation plane perpendicular to the plane of principal stress. The angular intersection of the two foliations causes a diagnostic texture called a "crenulation", which may involve folding of the earlier mica foliations by the later foliation.
Recognition.
Recognising a crenulation in a rock may require inspecting the rock with a hand-lens or petrographic microscope in thin section. Crenulations may be very cryptic, and there may be several recorded within a rock and especially, entrained within porphyroblasts.
Crenulations may manifest as kinking of previous foliation, such that the original foliation appears to be lined or inscribed by a later foliation.
In more advanced states, the later foliation will tend to form distinct foliation planes cross-cutting the earlier foliation, resulting in breaking, warping, and micro-scale folding of the earlier foliation into the new foliation.
When the crenulation foliation begins to dominate it may totally or almost completely wipe out the original foliation. This process occurs at different rates in rocks and beds of different lithology and chemical composition so that it is usually valuable to look at a variety of outcrops to gain a better appreciation of the effect of crenulation or discover the orientation or presence of earlier foliations.
Crenulation may also be the incipient foliation plane which precipitates shearing. In this case, it is often likely that the crenulation acts as a shear plane and it may be difficult to reconstruct earlier foliations and rock units across the crenulation foliation.
Analysis.
Crenulations, because they are the result of a second (or more) foliation, preserve important information on not only the stresses which formed the crenulation foliation, but the orientation of previous foliations.
Firstly, the crenulation must be analysed to determine the initial foliation, termed S1, and the overprinting subsequent foliation. The intersection of these two planes forms an intersection lineation. This intersection lineation, L1-2 may approximate the plunge of F2 interference folds.
The initial impact of a crenulation foliation may be cryptic, microscopic growth of new minerals at an angle to previous foliations. This may occur only in certain compositions of the rocks which favor growth of minerals under the P-T conditions at that time.
In more brittle conditions, especially in highly micaceous rocks, a crenulation may appear as "'kink bands", where S1 foliations are kinked by the S2 foliation so that the original minerals are broken or deformed. This may not result in new mineral growth.
Eventually, the crenulation foliation overprints the S1 foliation. In extreme cases, the S2 foliation will obliterate the previous foliation, especially in wet rocks which have compositions amenable to growth of minerals at that time. In this case, porphyroblasts may be the only means in which to observe earlier foliations, assuming they have inclusion trails of the S1 foliation.


Dust devil tracks

Many areas on Mars experience the passage of giant dust devils. These dust devils leave tracks on the surface of Mars because they disturb a thin coating of fine bright dust that covers most of the Martian surface. When a dust devil goes by it blows away the coating and exposes the underlying dark surface. Within a few weeks, the dark track assumes its former bright colour, either by being re-covered through wind action or due to surface oxidation through exposure to sunlight and the Martian atmosphere.
Formation and dynamics.
Dust devils occur when the sun warms up the air near a flat, dry surface. The warm air then rises quickly through the cooler air and begins spinning while moving ahead. This spinning, moving cell may pick up dust and sand and leave behind a clean surface.
Observation.
Dust devils on Mars have been photographed both from the ground and high overhead from orbit. They have even blown dust off the solar panels of two Rovers on Mars, thereby greatly extending their useful lifetime. The pattern of the tracks has been shown to change every few months. A study that combined data from the [[High Resolution Stereo Camera]] (HRSC) and the [[Mars Orbiter Camera]] (MOC) found that some large dust devils on Mars have a diameter of and last at least 26 minutes. Measurements from the Curiosity Rover confirms that the pressure drops when a dust devil passes nearby.
Changes through time.
A dust devil that was previously imaged in 2009, there was information found about two years later showing the tracks visible from the previous imaging are completely different from the old ones meaning there had been a dust storm to erase the old tracks.
Since it is not always too easy to observe these dust devil tracks on Mars tests have been done on Earth to further our understanding of these storms. Traditionally dust devils leave tracks that are darker, but satellite images have captured some cases where they are lighter than the traditional markings. In conclusion the marks were not darker than usual but the sand had been darkened by five minutes of rainfall during the previous night. Since there is no proof of water on Mars indefinitely it is believed that these lighter marks are being held together by electrostatic forces.
Images.
[[File:PIA24039-MarsCuriosityRover-DustDevil-20200809.gif|thumb|center|700px|Dust devil on Mars - viewed by the "Curiosity" rover - (August 9, 2020)]]
See also.
[[Category:Geology of Mars]]
[[Category:Surface features of Mars]]


Cushetunk Mountain

Cushetunk Mountain (formerly Mount Ployden and Pickel's Mountain occasionally Mount Cushetunk or Coshanton) is a ring shaped mountain in the U.S. state of New Jersey, located in Readington and Clinton Township in Hunterdon County. The Lenape Native Americans called the mountain "Cushetunk" meaning "place of hogs" after the wild hogs found there. In the 1960s, the valley at the heart of the mountain was dammed and filled with water to create Round Valley Reservoir. Today, the mountain and its accompanying lake act as a vital water supply to Central New Jersey, while at the same time providing numerous recreational opportunities to the region.
Geography.
Cushetunk Mountain, located in Clinton Township, Readington Township, Franklin Township, New Jersey, and Raritan Township, New Jersey is a horseshoe shaped mountain with the tips of its two prongs ending in the west at the edge of the New Jersey Highlands. The mountain's U-shaped arc is approximately in diameter, with the more massive portion of the mountain occupying the southwest corner of the ridgeline. A deep man-made lake, Round Valley Reservoir, occupies the valley at the center of the horseshoe.
Cushetunk Mountain is occasionally referred to as a mountain range and includes Round Mountain, a peak located about south of Cushetunk Mountain in Readington. Round Mountain is linked to Cushetunk Mountain by a sheet of intruded diabase rock running beneath the surface.
Located on the mountains are the communities of Cushetunk, Lebanon and Lebanon Borough. In Clinton township the mountains Stretch from the south west part of the township to north central part. Lebanon covers the whole mountain (with the exception of Lebanon Borough an Cushetunk).
History.
Before the arrival of Europeans, Cushetunk Mountain was inhabited by Unami speaking Lenape, particularly the Musconetcongs who ranged between Cushetunk Mountain and Sourland Mountain to the south. An interesting claim put forward by Beauchamp Plantagenet, one of the first Europeans to explore the area around the mountain, states that a Native American king held his seat in a place resembling the valley formed by Cushetunk Mountain. The claim exists today as a legend, and no evidence has ever been found confirming Plantagenet's story of a ‘Raritan king’.
Two famous names from New Jersey's colonial history owned land on the north slope of Cushetunk Mountain in Potterstown (part of Readington and Clinton Township). One was John Stevens, a delegate to the Continental Congress, whose grandson founded Stevens Institute of Technology. The other was Lord Stirling, an American Revolutionary War General who was ranked 3rd or 4th behind George Washington.
Historically, wildlife at the mountain included apex predators that are no longer found in the region. A story from the colonial era involving Dr. John Rockhill, a member of the Society of Friends and supposedly Hunterdon's County's first physician, indicates that Cushetunk Mountain and neighboring Round Mountain were known to harbor wolves. It is also noted that settlers in nearby Flemington had to defend against these wolves during the winter.
Prior to the 1960s, when the valley surrounded by Cushetunk Mountain was dammed to form Round Valley Reservoir, the mountain was useful as a timber source. Farming use was limited due to the hard diabase rock in the mountain's soil.
Geology.
Like the nearby Palisades and Sourland Mountain, Cushetunk Mountain formed approximately 200 million years ago around the Triassic/Jurassic boundary as an intrusion of igneous rock, diabase, into local shale and sandstone. The intrusion occurred when the Newark Basin, an aborted rift zone that formed as Pangaea began to break up, was still volcanically active. Originally, it was thought that Cushetunk Mountain was formed well after the nearby Watchung Mountains, also of volcanic origin. The theory was that the deposition of sediment into the Newark Basin was still occurring after the Watchungs were extruded, which explained how Cushetunk Mountain was able to intrude into strata above the height of the Watchungs. However, it is now known that Cushetunk Mountain, as well as other intrusive bodies in the Newark Basin, are at least 10 million years older than the Watchungs, which implies that erosion and/or uplift must have occurred after Cushetunk Mountain was intruded in order to allow the Watchungs to form at the surface. Both Cushetunk Mountain and the Watchungs were elevated after millions of years of erosion stripped away the overlying sandstone and shale strata and exposed their durable trap rock masses to the surface.
Contrary to popular belief, the ring-like shape of the mountain does not represent a crater, particularly since the mountain was formed primarily within the Earth. Instead, the mountain's shape seems to be the result of an intruding sheet of magma becoming dramatically flexed as it penetrated local strata. Round Mountain, just to the south of Cushetunk Mountain, formed as the intruding sheet that created Cushetunk Mountain reached a syncline.
A unique feature of Cushetunk Mountain's geology is that the igneous intrusion forming the mountain partly overlaps Paleozoic limestone west of the Ramapo Fault. Occurring just south of Molasses Hill Rd in Clinton Township, the overlap is significant for two reasons. First, the overlap marks one of the only locations in the Newark Basin where an intrusion successfully crossed the Ramapo fault and effectively spilled over the western edge of the basin. Second, the trap rock comprising the intrusion at this location is fine grained and unlike the dense, coarse diabase common to Cushetunk Mountain. It is thought that this is because the intrusion erupted to the surface as it overran the Paleozoic limestone.
Ecology.
While most of the mountain ridges in New Jersey run generally north to south, Cushetunk Mountain primarily has an east-west ridge orientation because of the elongated north and south prongs of its horseshoe-like ridge. This produces significantly different microclimates between the north facing and south facing slopes of the mountain. On south facing slopes, the air temperature near the ground is consistently higher than on the north facing slopes, which do not receive as much sunlight. In terms of ecology this creates a noticeable impact on Cushetunk Mountain's vegetation.
Because the north slope of the mountain is cooler and shaded, moisture is more easily retained. This results in larger trees, as well as the growth of trees not seen on southern facing slopes. These trees include black birch, tulip tree, white ash, basswood, hickory, beech, and sugar maple. Shrubs are abundant in the understory of the northern slopes, as well as herbs typical of more northern forests, including wild ginger, wild sarsaparilla, black snakeroot, and columbine.
On the warmer, dryer southern slopes chestnut oak and red oak prevail, although the trees are also common to the northern slopes. In the understory, dogwood is dominant, and the diversity and number of shrubs is reduced. Grasses and sedges are the most prevalent ground cover.
Wildlife supported by Cushetunk Mountain includes a variety of woodland birds, including a nesting pair of bald eagles. Groundwater seeps, particularly on the northern slopes, provide habitat to amphibians, while outcrops of trap rock offer ideal environments for small reptiles.


Berthelsdorf Formation

The Berthelsdorf Formation is a geologic formation in Germany. It preserves fossils dating back to the Carboniferous period.


Gullies on Mars

Martian gullies are small, incised networks of narrow channels and their associated downslope sediment deposits, found on the planet of Mars. They are named for their resemblance to terrestrial gullies. First discovered on images from Mars Global Surveyor, they occur on steep slopes, especially on the walls of craters. Usually, each gully has a dendritic "alcove" at its head, a fan-shaped "apron" at its base, and a single thread of incised "channel" linking the two, giving the whole gully an hourglass shape. They are estimated to be relatively young because they have few, if any craters. A subclass of gullies is also found cut into the faces of sand dunes, that are themselves considered to be quite young. Linear dune gullies are now considered recurrent seasonal features.
Most gullies occur 30 degrees poleward in each hemisphere, with greater numbers in the southern hemisphere. Some studies have found that gullies occur on slopes that face all directions; others have found that the greater number of gullies are found on poleward facing slopes, especially from 30° to 44° S. Although thousands have been found, they appear to be restricted to only certain areas of the planet. In the northern hemisphere, they have been found in Arcadia Planitia, Tempe Terra, Acidalia Planitia, and Utopia Planitia. In the south, high concentrations are found on the northern edge of Argyre basin, in northern Noachis Terra, and along the walls of the Hellas outflow channels. A recent study examined 54,040 CTX images that covered 85% of the Martian surface found 4861 separate gullied landforms (e.g., individual craters, mounds, valleys, etc.), which totaled tens of thousands of individual gullies. It is estimated that CTX can resolve 95% of gullies.
This article gives a history of the discovery and research on gullies. As research progresses, the cause of Martian gullies has shifted from recent liquid water to pieces of dry ice moving down steep slopes, but research continues. On the basis of their form, aspects, positions, and location amongst and apparent interaction with features thought to be rich in water ice, many researchers think that the processes carving the gullies involve liquid water. When the volumes of the aprons are compared to the rest of the gully, it appears that there is much less volume in the apron; hence, much of the material may have contained water and ice that disappeared. However, this remains a topic of active research. Because the gullies are so young, this would suggest that liquid water has been present on Mars in its very recent geological past, with consequences for the potential habitability of the modern surface.
On July 10, 2014, NASA reported that gullies on the surface of Mars were mostly formed by the seasonal freezing of carbon dioxide (CO2), and not by that of liquid water as considered earlier.
Formation.
After being discovered, many hypotheses were put forward to explain the gullies. However, as in the usual progression of science, some ideas came to be more plausible than others when more observations were made, when other instruments were used, and when statistical analysis was employed. Even though some gullies resembled debris flows on Earth, it was found that many gullies were on slopes that were not steep enough for typical debris flows. Calculations showed that the pressure and temperatures were not right for liquid carbon dioxide. Moreover, the winding shape of the gullies suggested that the flows were slower than what would be produced in debris flows or eruptions of liquid carbon dioxide. Liquid carbon dioxide would explode out of the ground in the thin Martian atmosphere. Because the liquid carbon dioxide would throw material over 100 meters, the channels should be discontinuous, but they are not. Eventually, most hypotheses were narrowed to involve liquid water coming from an aquifer, from melting at the base of old glaciers (or snowpacks), or from the melting of ice in the ground when the climate was warmer.
Close-up images with HiRISE showed details that support the idea that a fluid was involved. Images show that channels were formed a number of times. Smaller channels were found in larger valleys, suggesting that after a valley formed another formed at a later time. Many cases showed channels took different paths at different times. Streamlined forms like teadrop-shaped islands were common in some channels. The following group of pictures of gullies illustrate some of the shapes that lead researchers to think that water was involved in creating at least some of the gullies.
However, more studies open up other possibilities; a study released in October 2010, proposes that some gullies, the ones on sand dunes, may be produced by a buildup of solid carbon dioxide during cold winter months.
On July 10, 2014, NASA reported that gullies on the surface of Mars were mostly formed by the seasonal freezing of carbon dioxide (CO2 ice or 'dry ice'), and not by that of liquid water as thought earlier.
The exact cause/causes of these gullies is still under debate. A study supported formation by the melting of ground ice or snowpack as the chief cause. Over 54,000 CTX images were examined which covered about 85% of the surface of the planet.
Aquifers.
Most of the gully alcove heads occur at the same level, just as one would expect if water came out of an aquifer. Various measurements and calculations show that liquid water could exist in aquifers at the usual depths where gullies begin. One variation of this model is that rising hot magma could have melted ice in the ground and caused water to flow in aquifers. Aquifers are layers that allow water to flow. They may consist of porous sandstone. The aquifer layer would be perched on top of another layer that prevents water from going down (in geological terms it would be called impermeable). Because water in an aquifer is prevented from going down, the only direction the trapped water can flow is horizontally. Eventually, water could flow out onto the surface when the aquifer reaches a break—like a crater wall. The resulting flow of water could erode the wall to create gullies. Aquifers are quite common on Earth. A good example is "Weeping Rock" in Zion National Park Utah. However, the idea that aquifers formed the gullies does not explain the ones found on isolated peaks, like knobs and the central peaks of craters. Also, a type of gully seems to be present on sand dunes. Aquifers need a wide collecting area which is not present on sand dunes or on isolated slopes. Even though most of the original gullies that were seen seemed to come from the same layer in the slope, some exceptions to this pattern have been found. Examples of gullies coming from different levels is shown below in the image of Lohse Crater and the image of gullies in Ross Crater.
Snowpacks.
Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust. This ice-rich mantle, a few yards thick, smooths the land, but in places it has a bumpy texture, resembling the surface of a basketball. The mantle may be like a glacier and under certain conditions the ice that is mixed in the mantle could melt and flow down the slopes and make gullies. Calculations show that a third of a mm of runoff can be produced each day for 50 days of each Martian year even under current conditions. Because there are few craters on this mantle, the mantle is thought to be relatively young. An excellent view of this mantle is shown below in the picture of the Ptolemaeus Crater Rim, as seen by HiRISE.
The ice-rich mantle may be the result of climate changes. Changes in Mars's orbit and tilt cause significant changes in the distribution of water ice from polar regions down to latitudes equivalent to Texas. During certain climate periods water vapor leaves polar ice and enters the atmosphere. The water comes back to ground at lower latitudes as deposits of frost or snow mixed generously with dust. The atmosphere of Mars contains a great deal of fine dust particles. Water vapor will condense on the particles, then fall down to the ground due to the additional weight of the water coating. When Mars is at its greatest tilt or obliquity, up to 2 cm of ice could be removed from the summer ice cap and be deposited at midlatitudes. This movement of water could last for several thousand years and create a snow layer of up to around 10 meters thick. When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice.
When the slopes, orientations, and elevations of thousands of gullies were compared, clear patterns emerged from the data. Measurements of altitudes and slopes of gullies support the idea that snowpacks or glaciers are associated with gullies. Steeper slopes have more shade which would preserve snow.
Higher elevations have far fewer gullies because ice would tend to sublimate more in the thin air of the higher altitude. For example, Thaumasia quadrangle is heavily cratered with many steep slopes. It is in the right latitude range, but its altitude is so high that there is not enough pressure to keep ice from sublimating (going directly from a solid to a gas); hence it does not have gullies. A large study done with several years worth of data from Mars Global Surveyor showed that there is a tendency for gullies to be on poleward facing slopes; these slopes have more shade that would keep snow from melting and allow large snowpacks to accumulate.
In general, it is now estimated that during periods of high obliquity, the ice caps will melt causing higher temperature, pressure, and moisture. The moisture will then accumulate as snow in midlatitudes, especially in the more shaded areas—pole facing, steep slopes. At a certain time of the year, sunlight will melt snow with the resulting water producing gullies.
Direct evidence for these snowpacks was recently discovered for the first time, showing that this mantle is indeed composed of <~1% dust and ice Changes observed within gullies over multiple Mars Years shows that dusty ice being exposed today is disappearing, and potentially melting to form channels within the mantle, and the rock underneath.
Melting of ground (pore) ice.
The third theory is that climate changes may be enough to allow ice deposited from atmospheric vapor into the ground to melt and thus form the gullies. During a warmer climate, the first few meters of ground could thaw and produce a "debris flow" similar to those on the dry and cold Greenland east coast. Since the gullies occur on steep slopes only a small decrease of the shear strength of the soil particles is needed to begin the flow. Small amounts of liquid water from melted ground ice could be enough to lead to erosion. However, it is likely that ice deposited in the pores of soil in the ground will diffuse back into the atmosphere rather than melt. Similar pore ice diffusion was also observed in-situ at the Phoenix landing site
Recent changes in gullies.
As soon as gullies were discovered, researchers began to image many gullies over and over, looking for possible changes. By 2006, some changes were found. Later, with further analysis it was determined that the changes could have occurred by dry granular flows rather than being driven by flowing water. With continued observations many more changes were found in Gasa Crater and others. Channels widened by 0.5 to 1 m; meter sized boulders moved; and hundreds of cubic meters of material moved. It was calculated that gullies could be formed under present conditions with as little as 1 event in 50–500 years. So, although today there is little liquid water, present geological/climatic processes could still form gullies. Large amounts of water or great changes in climate are not needed. However, some gullies in the past may have been aided by weather changes that involved larger amounts of water, perhaps from melted snow. With more repeated observations, more and more changes have been found; since the changes occur in the winter and spring, experts are tending to suspect that gullies were formed from carbon dioxide ice (dry ice). Recent studies describe using the High Resolution Imaging Science Experiment (HiRISE) camera on MRO to examine gullies at 356 sites, starting in 2006. Thirty-eight of the sites showed active gully formation. Before-and-after images demonstrated the timing of this activity coincided with seasonal carbon dioxide frost and temperatures that would not have allowed for liquid water. When dry ice frost changes to a gas, it may lubricate dry material to flow especially on steep slopes. In some years frost, perhaps as thick as 1 meter, triggers avalanches. This frost contains mostly dry ice, but also has tiny amounts of water ice.
Observations with HiRISE show widespread activity in southern hemisphere gullies, especially in those that appear fresh. Significant channel incision and large-scale mass movements have been seen. Sinuous channels which were thought to need liquid water for their formation have even been seen to form over just a few years when liquid water cannot exist. The timing of gully activity is seasonal and happens during the period when seasonal frost is present and defrosting.
These observations support a model in which currently active gully formation is driven mainly by seasonal CO2 frost. Simulations described in a 2015 conference, show that high pressure CO2 gas trapping in the subsurface can cause debris flows. The conditions that can lead to this are found in latitudes where gullies occur. This research was described in a later article entitled, "Formation of gullies on Mars by debris flows triggered by CO2 sublimation." In the model, CO2 ice accumulates in the cold winter. It piles up on a frozen permafrost layer that consists of ice-cemented dirt. When the higher intensity sunlight of spring begins, light penetrates the translucent dry ice layer, consequently warming the ground. The CO2 ice absorbs heat and sublimates—that is changes directly from a solid to a gas. This gas builds up pressure because it is trapped between the ice and the frozen ground. Eventually, pressure builds up enough to explode through the ice taking with it soil particles. The dirt particles mix with the pressurized gas and act as a fluid that can flow down the slope and carve gullies.
The major issue with the CO2 frost model is trying to explain the erosion of rocks. Although there is considerable evidence for CO2 frost transporting loose materials, it seems unlikely that sublimating CO2 gas can erode and weather rocks to form gullies. Instead, CO2 frost might only be able to modify pre-existing gullies.
Using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter researchers studied over 100 Martian gully sites and found no evidence for specific minerals being more likely to be associated with gullies, or with the formation of hydrated minerals that would have been made by recent liquid water. This research adds evidence that liquid water was not involved with gully formation. However, as described above, the amounts of liquid water thought to be generated in near-freezing conditions from melting snowpacks are unlikely to cause chemical weathering in the first place.
Some researchers believe that gully formation may involve both dry ice and liquid water.
How changing tilt affects the climate.
It is estimated that a few million years ago, the tilt of the axis of Mars was 45 degrees instead of its present 25 degrees. Its tilt, also called obliquity, varies greatly because its two tiny moons cannot stabilize it, like our relatively large moon does to the Earth. During such periods of high tilt, the summer rays of the sun strike the mid-latitude crater surfaces straight on, thus the surface remains dry.
Note that at high tilt, the ice caps at the poles disappear, the atmosphere thickness, and the moisture in the atmosphere goes up. These conditions cause snow and frost to appear on the surface. However, any snow that falls at night and during the cooler parts of the day disappears when the day warms.
Things are quite different as fall approaches, for the pole-facing slopes remain in the shade all day. Shade causes snow to accumulate through the fall and winter seasons.
In the spring at certain point, the ground will be warm enough and the air pressure high enough for liquid water to form at certain times of the day.
There may be sufficient water to produce gullies by erosion. Or, the water may soak into the ground, and later move down as a debris flow. Gullies on Earth formed by this process resemble Martian gullies.
The great changes in the tilt of Mars explain both the strong relationship of gullies to certain latitude bands and the fact that the vast majority of gullies exist on shady, pole-facing slopes. Models support the idea that pressure/temperature changes during high obliquity times are enough to allow liquid water to be stable in places where gullies are common.
Research published in January 2015 suggests that these seasonal changes could have happened within the last two million years (between 400,000 and two million years ago) creating conditions suitable for formation of the gullies through ice melt.
Associated features of gullies.
Some steep slopes show other features in addition to gullies. At the base of some gullies there may be curved ridges or depressions. These have been named "spatulate depressions." Along walls, like crater walls, ice often accumulates during certain phases of the Martian climate cycle. When the climate changes, this ice can sublimate into the thin Martian atmosphere. Sublimation is when a substance goes directly from a solid state to a gas state. Dry ice on Earth does this. So when the ice at the base of a steep wall sublimates, a spatulate depression results. Also, more ice from higher up the wall will tend to flow downward. This flow will stretch the surface rocky debris thereby forming transverse crevasses. Such formations have been termed "washboard terrain" because they resemble the old fashioned washboards. The parts of gullies and some associated features of gullies are shown below in a HiRISE images.
Images from around Mars.
The Phaethontis quadrangle is the location of many gullies that may be due to recent flowing water. Some are found in the Gorgonum Chaos and in many craters near the large craters Copernicus and Newton (Martian crater).
Gullies on dunes.
Gullies are found on some dunes. These are somewhat different than gullies in other places, like the walls of craters. Gullies on dunes seem to keep the same width for a long distance and often just end with a pit, instead of an apron. They are often just a few meters across with raised banks along the sides. Many of these gullies are found on dunes in Russell (Martian crater). In the winter dry ice accumulates on the dunes and then in the spring dark spots appear and dark-toned streaks grow downhill. After the dry ice is gone, new channels are visible. These gullies may be caused by blocks of dry ice moving down the steep slope or perhaps from dry ice starts the sand moving. In the thin atmosphere of mars, dry ice will expel carbon dioxide with vigor.