Field analyses of groundwater samples are needed (unless the water table is deep and inaccessible). The analyses should include pH, dissolved oxygen (DO), ORP, specific conductance, temperature and salinity. The samples should be laboratory analyzed by GC/FID. We have found that analyses for *n*-alkanes by GC/MS are often inaccurate. The GC/FID analyses must be conducted so that sufficient separation exists between peaks. For example, the *n*-C17 alkane and pristane elute very close to each other. To enhance peak separation, a run time of about 40 minutes is recommended. In some cases, some of the samples should be analyzed for aromatics such as benzene, toluene, ethylbenzene and *o, m, p*- xylenes and base/neutral extractable compounds (B/Ns), also targeting C1- and C2 naphthalenes, alkyl-benzenes, alkyl-*cyclo*-hexanes and dibenzothiophenes. It is further recommended that a sample of fresh fuel be collected from each site for comparison purposes. However, it is possible that the fresh oil may be significantly different from the spilled oil.

### **12. Evaluating the age range**

566 Earth Sciences

The volume of petroleum in the environment will have an impact on weathering. Samples collected within a large pool of separate phase may not exhibit any weathering many years after a release. Accordingly, samples collected within a highly contaminated location may

The UST condition can be assessed by: the number, size and location of corrosion holes. Corrosion often takes considerable time to develop. Metallurgists are commonly consulted to provide opinions on the UST release ages. However, soil conditions should be evaluated

Stray electrical currents may have a significant impact on unprotected steel USTs. In particular, central air-conditioning units, which normally run on underground 220-volt currents, are often the culprits with newer buildings or homes. Unfortunately, USTs are commonly installed adjacent to these units and leakage can often initiate within 5 years.

There may be additional site-specific factors in addition to the five listed. For example, USTs are often abandoned in-place, possibly by a previous owner. The abandonment date might represent when a previous owner suspected leakage. Ecosystem responses may also need to be evaluated. For example, contamination may induce stressed vegetation or impacts to water bodies. The time needed to produce such impacts may be significant. Investigators need to evaluate these factors and determine if they are sufficiently important to consider in

Oudijk et al. (2006) and Oudijk (2009b) provided guidelines for age-date sampling. Samples can be either impacted soil or separate phase, although soil samples are preferred. Samples as distant as possible from the source are needed to assess the maximum release age; however, a sufficient quantity of petroleum must be present to perform laboratory analyses.

An important decision is the sampling locations. It is assumed that locations distant from the source represent older ages. However, downgradient locations may be more susceptible to excessive weathering. Furthermore, petroleum in samples collected from within separatephase pools may not weather as much as locations proximate or outside the pool. Accordingly, an understanding of sampling locations with respect to accumulations of separate phase is needed. As discussed by Wade (2001) and Oudijk et al. (2006), age dating

Field analyses of groundwater samples are needed (unless the water table is deep and inaccessible). The analyses should include pH, dissolved oxygen (DO), ORP, specific conductance, temperature and salinity. The samples should be laboratory analyzed by GC/FID. We have found that analyses for *n*-alkanes by GC/MS are often inaccurate. The GC/FID analyses must be conducted so that sufficient separation exists between peaks. For example, the *n*-C17 alkane and pristane elute very close to each other. To enhance peak separation, a run time of about 40 minutes is recommended. In some cases, some of the samples should be analyzed for aromatics such as benzene, toluene, ethylbenzene and *o, m, p*-

**11. Recommended sampling and laboratory analyses**

A hydrocarbon concentration of greater than 1,000 mg/kg is recommended.

not provide productive evidence.

to determine their connectivity.

**10.5 Other considerations**

based on one sample is unwise.

the matrix.

**10.4 UST condition**

To evaluate the age, a Kaplan Stage is selected for each sample, whether soil or separate phase. Based on known environmental conditions, a weathering-potential regime is then chosen for the site. A matrix, comparing the Kaplan Stages to the weathering-potential regimes, is provided detailing potential release ages (Table 7). Compared to Kaplan et al. (1997), the Kaplan Stages on Table 7 were modified to include additional parameters. These potential age ranges are based on:

The matrix was constructed with the following assumptions:

 the very-aggressive column represents a marine-spill situation. The *n*-alkanes degrade quickly in such environments. There are reports that *n*-alkanes may persist for two or three years (Colwell, 1978), but they will, in general, be gone within 4 years (and often much earlier). With regard to *n*-alkane depletion, time frames of weeks or months are more common than years (de Souza & Triguis, 2004). Accordingly, <4 years can be placed into the matrix for Stage 6 in an aggressive environment.

the very-weak column represents spills in an Arctic or Antarctic climate.  Sexstone et al. (1978b) reported that biodegradation in tundra soil could be slow "with no major preferential utilization of classes of hydrocarbons during the period of exposure". The longer-chain *n*alkanes (>C10) are commonly solid at temperatures less than 10o C (Whyte et al., 1998). Kershaw & Kershaw (1986) found significant depletion at surface locations from a 35-year

Age Dating of Middle-Distillate Fuels Released to the Subsurface Environment 569

 time frames of known or possible pollution events, such as appearance of petroleum in waterways, furnace failures, excessive oil usage or abandonments. This process may

With the date of petroleum appearance, such as when oil began seeping into a creek, the upper and lower ages can be constrained. For example, if it is known that an UST was installed in 1980 and the furnace began to take in water in 2004, we can conclude that UST

If heating oil was released, the history of the heating system should be considered. The dates of repairs and service calls should be reviewed. The fuel company is often called when a furnace fails and causes for any failures should be determined. The date of any on-site work that included excavating should also be identified. Gardening is a common cause for

Reports should be obtained on the UST condition. If one pin-sized hole is present, it is probable that the leak began within the last 1 to 2 years. However, if 25 coin-sized holes are

Calculations of plume extent are important in assessing the reasonableness of estimated age ranges. A large and extensive plume is more indicative of an older release. However, care is needed in making such conclusions because aggressive biodegradation can greatly reduce plume sizes. If monitoring wells are available, groundwater migration rates may be calculated

Lastly, underground power lines should be assessed. Residential USTs are often adjacent to air-conditioning units and associated power lines can quickly instigate leakage. At a site in New Jersey, a power line produced a sequence of holes on the top and bottom of an

Table 7 provides an age range, for example, of 4 to 8 years. This range could be interpreted as 6 years ±2 years. It is stressed that the values on Table 7 are only potential ranges and might contain significant error, if they are not verified through alternative means. Each release is different and on-site petroleum weathering may not necessarily fall within the

An additional geochemical method to critique the age-date estimate is to establish the sulphur content of the oil. In many countries, the government limits sulphur contents in onroad diesel fuel (Table 8). By determining the sulphur content, the age of the diesel fuel can

Error is the difference between an observed or calculated value and the true value. Usually, we do not know the true value, but can approximate error ranges from earlier experiments or theoretical predictions (Bevington & Robinson, 1992). The purpose of calculating error

All geochemical data collected to assess some type of natural phenomenon contain error caused by sampling and analysis (Miesch, 1967). Subsurface modeling is plagued by uncertainties that are both epistemic (reducible through observations) and aleatory (irreducible because of inherent stochasticity) (Srinivasan et al., 2007). The amount of error

and time frames for plume movement assessed and compared to geochemical data.

found, it is more likely that the leak began 10 years ago or later.

underlying UST. The power line had only existed for 2 years.

ranges is to evaluate the confidence of our results.

entail a review of fuel bills to determine when excessive usage began; state of the UST, such as the number and sizes of corrosion holes and pitting; extent and magnitude of petroleum-impacted soil and groundwater, and miscellaneous factors such as high-voltage underground electrical lines.

failure occurred between 1980 and 2004.

severing feed and return lines.

confines of this technique.

often be constrained.

**14. Range of error**

old spill in the Canadian Northwest Territories, but with depth, up to 80% of the oil persisted. Collins et al. (1994) found only marginal depletion of *n*-alkanes in subsurface soils from a 12-year-old crude-oil spill in a permafrost region of Alaska. Gore et al. (1999) and Kerry (1993) also found minimal subsurface biodegradation in a similar Antarctic environment. A very weak environment is where microbes are dormant, cannot come in contact with hydrocarbons or have been removed because of toxicity. Accordingly, >60 years can be placed into the matrix for Stage 6 in a very-weak environment;

The matrix should only be used to provide potential ages and should not be the sole factor for an age-date opinion. The matrix is a method to lead the investigator towards the correct age, but it is not the final say.

### **13. Critiquing the matrix age range**

Once a potential age range is obtained from the matrix, it must be compared to several factors to assess its reliability. Other factors that may impact the age are:

age of the UST;

With the date of petroleum appearance, such as when oil began seeping into a creek, the upper and lower ages can be constrained. For example, if it is known that an UST was installed in 1980 and the furnace began to take in water in 2004, we can conclude that UST failure occurred between 1980 and 2004.

If heating oil was released, the history of the heating system should be considered. The dates of repairs and service calls should be reviewed. The fuel company is often called when a furnace fails and causes for any failures should be determined. The date of any on-site work that included excavating should also be identified. Gardening is a common cause for severing feed and return lines.

Reports should be obtained on the UST condition. If one pin-sized hole is present, it is probable that the leak began within the last 1 to 2 years. However, if 25 coin-sized holes are found, it is more likely that the leak began 10 years ago or later.

Calculations of plume extent are important in assessing the reasonableness of estimated age ranges. A large and extensive plume is more indicative of an older release. However, care is needed in making such conclusions because aggressive biodegradation can greatly reduce plume sizes. If monitoring wells are available, groundwater migration rates may be calculated and time frames for plume movement assessed and compared to geochemical data.

Lastly, underground power lines should be assessed. Residential USTs are often adjacent to air-conditioning units and associated power lines can quickly instigate leakage. At a site in New Jersey, a power line produced a sequence of holes on the top and bottom of an underlying UST. The power line had only existed for 2 years.

Table 7 provides an age range, for example, of 4 to 8 years. This range could be interpreted as 6 years ±2 years. It is stressed that the values on Table 7 are only potential ranges and might contain significant error, if they are not verified through alternative means. Each release is different and on-site petroleum weathering may not necessarily fall within the confines of this technique.

An additional geochemical method to critique the age-date estimate is to establish the sulphur content of the oil. In many countries, the government limits sulphur contents in onroad diesel fuel (Table 8). By determining the sulphur content, the age of the diesel fuel can often be constrained.

### **14. Range of error**

568 Earth Sciences

old spill in the Canadian Northwest Territories, but with depth, up to 80% of the oil persisted. Collins et al. (1994) found only marginal depletion of *n*-alkanes in subsurface soils from a 12-year-old crude-oil spill in a permafrost region of Alaska. Gore et al. (1999) and Kerry (1993) also found minimal subsurface biodegradation in a similar Antarctic environment. A very weak environment is where microbes are dormant, cannot come in contact with hydrocarbons or have been removed because of toxicity. Accordingly, >60

 under moderate conditions, *n*-alkanes are normally removed from subsurface soils in about 20 years (Christensen & Larsen, 1993; Kaplan, 2003). Accordingly, 20 to 24 years

 degradation follows a clear sequential pattern as depicted by Kaplan et al. (1996) and Table 4. This sequential pattern is normally the case. However, there are cases where different compounds of the same class degrade at significantly different rates. For example, Olson et al. (1999) found that components within the aliphatic fraction of diesel fuel degraded at different rates, although the aliphatic fraction, as a whole,

 removal of *n*-alkanes tends to be linear, instead of exponential (Christensen & Larsen, 1993; Hurst & Schmidt, 2005; Galperin & Kaplan, 2008c). Therefore, age ranges are extrapolated in a linear manner from Stage 6 to Stage 1 (zero-order kinetics). Chapelle (2001) explains that there is often a lag time between introduction of a contaminant to a soil or groundwater system and acclimation of microbes. However, in the first days or weeks after release and acclimation, once that acclimation has occurred, biodegradation rates may be significant. D́ez et al. (2007) found that biodegradation rates of heavy oils can be slow, even in a marine environment. Accordingly, it can be assumed that as oil ages and becomes more viscous, the rate of biological consumption decreases. Colwell (1978) found that rates could initially be logarithmic for marine spills and later linear. Bonroy et al. (2007) reports that biodegradation rates will vary seasonally. Walker et al. (1976) found that the degradation rate of the alkane fraction of a crude oil was linear, whereas the rate for aromatics varied. Ostendorf et al. (2007) found that *n*-alkane degradation rates in unsaturated soil followed zero-order kinetics, whereas aromatics followed first-order. Bjorklof et al. (2008) found that petroleum degradation rates can be linear, but they are mass-transfer dependent. Therefore, rates will more likely be linear in a permeable soil where the petroleum can dissolve more easily. It is assumed that rates averaged across the years are linear, although it is understood that this

age ranges can then be extrapolated between the very-aggressive and very-weak

The matrix should only be used to provide potential ages and should not be the sole factor for an age-date opinion. The matrix is a method to lead the investigator towards the correct

Once a potential age range is obtained from the matrix, it must be compared to several

factors to assess its reliability. Other factors that may impact the age are:

years can be placed into the matrix for Stage 6 in a very-weak environment;

can be placed into the matrix for Stage 6 in a moderate environment;

degraded quicker than the aromatic fraction;

assumption may not apply everywhere, and

age, but it is not the final say.

age of the UST;

**13. Critiquing the matrix age range**

environments and the moderate environments.

Error is the difference between an observed or calculated value and the true value. Usually, we do not know the true value, but can approximate error ranges from earlier experiments or theoretical predictions (Bevington & Robinson, 1992). The purpose of calculating error ranges is to evaluate the confidence of our results.

All geochemical data collected to assess some type of natural phenomenon contain error caused by sampling and analysis (Miesch, 1967). Subsurface modeling is plagued by uncertainties that are both epistemic (reducible through observations) and aleatory (irreducible because of inherent stochasticity) (Srinivasan et al., 2007). The amount of error

Age Dating of Middle-Distillate Fuels Released to the Subsurface Environment 571

The limitations of the methods described herein need to be fully understood by its users and should be explained whenever an age-dating opinion is provided. As stated by Morgan &

*"Fundamental differences exist between forensic geoscience and its sister disciplines, fundamental enough to make the unwary geoscientist succumb to philosophical and practical pitfalls which will not only endanger the outline of their report, but may well indeed provide false-negative or falsepositive results leading to contrary or inaccurate conclusions. In the law, such outcomes have*

The need to apply age-dating methods cautiously cannot be understated. It must be noted that the methods described herein may not be applicable to all spill sites. It is certainly possibly that site-specific circumstances could exist to preclude these methods from consideration. The investigator will need to carefully consider each site individually and the possibility exists that other methods, such as tree-ring studies, isotopic analyses or groundwater migration rate calculations, may be more applicable or financially viable.

Because of costs associated with cleanups, many cases come to litigation. Age dating is often central to the litigation. For a plaintiff or defendant to be successful, a legally defensible agedating method is needed. The C&L method, which is solely dependent on chemistry, has been strongly attacked inside and outside the courtroom. Hence, a revised method is needed

Each pollution site has specific characteristics that must be evaluated. Investigators need a thorough understanding of the geologic and hydrologic conditions, in addition to the nature of the release. Investigators also need to combine knowledge of chemistry, microbiology, and site-specific history to provide an opinion on the release time frame. Because scientific processes associated with releases are too complex to model through a simple formula, a qualitative or semi-quantitative technique is needed. The methods described herein are an

Aichberger, H., Loibner, A. P., Celis, R., Braun, R., Ottner, F. & Rost, H. 2006. Assessment of

Alimi, H. 2002. Invited commentary of the Christensen and Larsen technique. *Environ.*

Atlas, R. M. 1981. Microbial degradation of petroleum hydrocarbons: An environmental

Atlas, R. M. & Bartha, R. 1992. Hydrocarbon biodegradation and oil spill bioremediation.

Balouet, J-C., Oudijk, G., Smith, K. T., Petrisor, I., Grudd, H. & Stocklassa, B. 2007. Applied

in a shallow sand aquifer. *Ground Water Monitor Remed*. 7:64–71.

dendroecology and environmental forensics. Characterizing and age dating environmental releases: Fundamentals and case studies. *Environ. Forensics* 8:1–17. Barker, J. F., Patrick, G. C. & Major, D. 1987. Natural attenuation of aromatic hydrocarbons

factors governing biodegradability of PAHs in three soils aged under field

Bull (2007, p. 56),

**15. Conclusions**

and proposed here.

**16. References**

effort to develop such an approach.

*Forensics* 3:5.

conditions. *Soil Sediment Contam*. 15:73–85.

perspective. *Microbiol. Rev*. 45:180–209.

*Adv. Microbial Ecol*. 12:287–315.

*devastating and untenable consequences".*

a Mandated year was 2006 in California and 2010 in Alaska.

Table 8. Maximum allowable sulphur concentrations allowable in motor diesel fuel in selected countries

often cannot be found in textbooks or with a formula. It is commonly based on a 'rule-ofthumb' or experience. Nevertheless, some calculations can be performed to provide insight into potential error ranges.

To assess error ranges in our calculations, we can use the C&L method as an example. C&L uses the *n*−C17/pristane ratio as its basis and, to calculate the ratio, an analysis with a gas chromatograph is needed. This analysis generally has a precision of about 5%. Furthermore, the method assumes that *n*−C17/pristane ratios for unweathered oil range from 2.0 to 2.2. Hence, the starting point has an error of about 10%. Therefore, errors are summed and the resulting error is about 15%. This exercise assumes that the C&L method perfectly reflects weathering processes in the subsurface. This, we know to be untrue. Consequently, we can assume that the method's error is at least 20% or ±2 years

In their article, Christensen & Larsen (1993) cite an error range of ±1 year or about 10%. Hurst & Schmidt (2005), employing a similar method, cite a minimum error of 15% or ±1.5 years. These cited error ranges seem optimistic and there are several references in the scientific literature arguing that the range is greater (Stout et al., 2002; Oudijk, 2007).

Because the age-dating method described herein is semi-quantitative, it may be difficult to assign an error range. Furthermore, there have not yet been experiments to test these techniques and evaluate what these error ranges may be. Nevertheless, one should assume that the range, under the best of scenarios, may be ±2 years and, under many circumstances, it may only provide a 5-year age window. In other circumstances, the method may only be able to constrain the age, wherein an example of an opinion might be 'less than 5 years' or 'greater than 10 years.'

One important limitation to this method is the presence of multiple, overlapping releases. It may be possible to distinguish the releases, but applying an age to each may be exceedingly difficult.

The limitations of the methods described herein need to be fully understood by its users and should be explained whenever an age-dating opinion is provided. As stated by Morgan & Bull (2007, p. 56),

*"Fundamental differences exist between forensic geoscience and its sister disciplines, fundamental enough to make the unwary geoscientist succumb to philosophical and practical pitfalls which will not only endanger the outline of their report, but may well indeed provide false-negative or falsepositive results leading to contrary or inaccurate conclusions. In the law, such outcomes have devastating and untenable consequences".*

The need to apply age-dating methods cautiously cannot be understated. It must be noted that the methods described herein may not be applicable to all spill sites. It is certainly possibly that site-specific circumstances could exist to preclude these methods from consideration. The investigator will need to carefully consider each site individually and the possibility exists that other methods, such as tree-ring studies, isotopic analyses or groundwater migration rate calculations, may be more applicable or financially viable.

### **15. Conclusions**

570 Earth Sciences

concentration allowable (mg/l)

Regulated as of

500 1994

500 1993

Maximum sulphur

Australia 10 2009 Canada 15 1997

China 2,000 2002 European Union 50 2005 New Zealand 10 2009 Singapore 50 2005 Taiwan 50 2007 USA 15 2007a

Table 8. Maximum allowable sulphur concentrations allowable in motor diesel fuel in

often cannot be found in textbooks or with a formula. It is commonly based on a 'rule-ofthumb' or experience. Nevertheless, some calculations can be performed to provide insight

To assess error ranges in our calculations, we can use the C&L method as an example. C&L uses the *n*−C17/pristane ratio as its basis and, to calculate the ratio, an analysis with a gas chromatograph is needed. This analysis generally has a precision of about 5%. Furthermore, the method assumes that *n*−C17/pristane ratios for unweathered oil range from 2.0 to 2.2. Hence, the starting point has an error of about 10%. Therefore, errors are summed and the resulting error is about 15%. This exercise assumes that the C&L method perfectly reflects weathering processes in the subsurface. This, we know to be untrue. Consequently, we can

In their article, Christensen & Larsen (1993) cite an error range of ±1 year or about 10%. Hurst & Schmidt (2005), employing a similar method, cite a minimum error of 15% or ±1.5 years. These cited error ranges seem optimistic and there are several references in the

Because the age-dating method described herein is semi-quantitative, it may be difficult to assign an error range. Furthermore, there have not yet been experiments to test these techniques and evaluate what these error ranges may be. Nevertheless, one should assume that the range, under the best of scenarios, may be ±2 years and, under many circumstances, it may only provide a 5-year age window. In other circumstances, the method may only be able to constrain the age, wherein an example of an opinion might be 'less than 5 years' or

One important limitation to this method is the presence of multiple, overlapping releases. It may be possible to distinguish the releases, but applying an age to each may be exceedingly

scientific literature arguing that the range is greater (Stout et al., 2002; Oudijk, 2007).

a Mandated year was 2006 in California and 2010 in Alaska.

assume that the method's error is at least 20% or ±2 years

selected countries

into potential error ranges.

'greater than 10 years.'

difficult.

Because of costs associated with cleanups, many cases come to litigation. Age dating is often central to the litigation. For a plaintiff or defendant to be successful, a legally defensible agedating method is needed. The C&L method, which is solely dependent on chemistry, has been strongly attacked inside and outside the courtroom. Hence, a revised method is needed and proposed here.

Each pollution site has specific characteristics that must be evaluated. Investigators need a thorough understanding of the geologic and hydrologic conditions, in addition to the nature of the release. Investigators also need to combine knowledge of chemistry, microbiology, and site-specific history to provide an opinion on the release time frame. Because scientific processes associated with releases are too complex to model through a simple formula, a qualitative or semi-quantitative technique is needed. The methods described herein are an effort to develop such an approach.

### **16. References**

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Christensen and Larsen method. In *The Annual International Conference on Soil, Sediment & Water.* Amherst, MA: Association of Environmental Health and Soils.

alkane biodegradation. *J. Chem. Technol. Biotechnol*. 64:41–48.

*Proceedings of the American Academy of Forensic Sciences* IX:101–102.

applicable? *The Chemist* March/April Issue, 9– 12.

microorganisms*. J. Serbian Chem. Soc*. 68:227–234.

*Appl. Environ. Microbiol*. 56:652– 656.

North Slope soils. *Arctic* 31:339–347.

5464.

1:191–206.

*em Petroleo e Gas.*

1986.

282.

Publishing Company.

**23**

*México*

**Geology and Geomorphology in Landscape**

**Ecological Analysis for Forest Conservation**

**and Hazard and Risk Assessment, Illustrated**

*1Centro de Ciencias de la Complejidad(C3), Departamento de Ecología y Recursos, Naturales, Facultad de Ciencias. Universidad Nacional Autónoma de México (UNAM)*

*3Instituto de Geografía, Universidad Nacional Autónoma de México (UNAM)*

María Concepción García-Aguirre1, Román Álvarez2 and Fernando Aceves3

The aim of landscape ecology is to understand both the effects of spatial patterns on ecological processes, and the development of those spatial patterns. It is considered holistic since it regards nature as a whole and it deals also with all human environment interactions. Application of landscape ecological principles for prioritizing rich species sites has the advantage of integrating spatial information, non-spatial information, and

Landscapes are complex systems constituted by a large number of heterogeneous components (with different geology, geomorphology, vegetation cover, ecological communities, land uses and so on) interacting in a non-linear way, that are hierarchically structured and scale-dependent (Wiens, 2009, Hall *et al*., 2004). Landscape description and structure has traditionally been done on the basis of landscape metrics, that is, basic measures of the amount of habitat and core habitat, the number of discrete patches and the perimeter to area ratio. However many of these metrics are generally poorly tested and require of rigorous validation if they are to serve as reliable indicators of habitat loss and fragmentation (McAlpine and Eyrie, 2002). There are strategies which can help to improve the reliability of landscape pattern analysis (Shao and Gu, 2008). Since landscape pattern is spatially correlated and scale dependent, often multiscale information is required

The study of causes, processes and consequences of land/cover change is one of the main research topics of landscape ecology. It is important to study processes and not merely spatial patterns considering cultures as a drivers of landscape change. Knowledge of temporal changes in landscape composition and structure, and their driving processes, can

provide insight into regional landscape dynamics (Luke, 2000; Burgui *et al*, 2004).

**1. Introduction**

(Wu, 2004).

horizontal relationships in space and time.

**with Mexican Case Histories**

*Ciudad Universitaria, C.P. Coyoacán, D.F. 2Instituto de Matemáticas Aplicadas y Sistemas, Universidad Nacional Autónoma de México (UNAM)*

## **Geology and Geomorphology in Landscape Ecological Analysis for Forest Conservation and Hazard and Risk Assessment, Illustrated with Mexican Case Histories**

María Concepción García-Aguirre1, Román Álvarez2 and Fernando Aceves3 *1Centro de Ciencias de la Complejidad(C3), Departamento de Ecología y Recursos, Naturales, Facultad de Ciencias. Universidad Nacional Autónoma de México (UNAM) Ciudad Universitaria, C.P. Coyoacán, D.F. 2Instituto de Matemáticas Aplicadas y Sistemas, Universidad Nacional Autónoma de México (UNAM) 3Instituto de Geografía, Universidad Nacional Autónoma de México (UNAM) México*

### **1. Introduction**

582 Earth Sciences

Zajic, J. E., Supplisson, B. & Volesky, B. 1974. Bacterial degradation and emulsification of no.

Zemo, D. 2007. Forensic tools for petroleum hydrocarbon releases. *SW Hydrol.*

Zibiske, L. M., and Risser, J. A. 1986. Effects of soil texture on respiration and metal solubility in heating oil-amended soils. *Bull. Environ. Contam. Toxicol*. 36:540–547.

Zytner, R G, Salb, A C. & Stiver, W H. 2006. Bioremediation of diesel fuel contaminated soil:

Comparison of individual compounds to complex mixtures. *Soil Sed. Contam*.

Zobell, C. E. 1946. Action of microorganisms on hydrocarbons. *Bacteriol. Rev*. 10:1–49.

6 fuel oil. *Environ. Sci. Technol. 8:664–668.*

July/August:26–35.

15:277–297.

The aim of landscape ecology is to understand both the effects of spatial patterns on ecological processes, and the development of those spatial patterns. It is considered holistic since it regards nature as a whole and it deals also with all human environment interactions. Application of landscape ecological principles for prioritizing rich species sites has the advantage of integrating spatial information, non-spatial information, and horizontal relationships in space and time.

Landscapes are complex systems constituted by a large number of heterogeneous components (with different geology, geomorphology, vegetation cover, ecological communities, land uses and so on) interacting in a non-linear way, that are hierarchically structured and scale-dependent (Wiens, 2009, Hall *et al*., 2004). Landscape description and structure has traditionally been done on the basis of landscape metrics, that is, basic measures of the amount of habitat and core habitat, the number of discrete patches and the perimeter to area ratio. However many of these metrics are generally poorly tested and require of rigorous validation if they are to serve as reliable indicators of habitat loss and fragmentation (McAlpine and Eyrie, 2002). There are strategies which can help to improve the reliability of landscape pattern analysis (Shao and Gu, 2008). Since landscape pattern is spatially correlated and scale dependent, often multiscale information is required (Wu, 2004).

The study of causes, processes and consequences of land/cover change is one of the main research topics of landscape ecology. It is important to study processes and not merely spatial patterns considering cultures as a drivers of landscape change. Knowledge of temporal changes in landscape composition and structure, and their driving processes, can provide insight into regional landscape dynamics (Luke, 2000; Burgui *et al*, 2004).

Geology and Geomorphology in Landscape Ecological Analysis for Forest

elevation, distance from shore, vegetation, and exposure**.**

**4. An outline of the geology of Mexico**

landscape (Staus *et al*., 2002).

years ago (Ortega *et al*. 1992).

(Anderson and Silver, 1981).

Paleozoic

Precambrian

Conservation and Hazard and Risk Assessment, Illustrated with Mexican Case Histories 585

of different qualities (Falcucci, *et al*., 2007). Land cover change information through time, combined with thematic information can be stored and managed efficiently in a GIS since it relates different layers of spatial information. In addition, it is a powerful analysis tool that allows identification of spatial relationships among different maps, through connection of spatial data with its attributes (Belda and Melia, 2000, Baysnat *et al.,* 2000). Time series remote sensing provide researchers with a valuable tool for the dynamic analysis of

Environmental models implemented in computers have become important tools for designing management plans towards ecological and economic sustainability. Computers help to deal with the tremendous complexity reflected in the extensive temporal and spatial scales at which human and natural processes occur. Iverson and Prasad (2007) evaluated tsunami damage and built empirical vulnerability models of damage/no damage based on

The geology of Mexico is the result of multiple tectonic processes that have taken place along its geologic history. Current geologic configuration of Mexico is the consequence of continental block interaction with surrounding oceanic provinces. As a result, young sedimentary and volcanic outcrops are dominant. 80% of exposed units are placed on Cenozoic and Mesozoic eras (less than 250 million years), 13% correspond to the Paleozoic and only 7 % belong to the Precambrian, belonging to the Proterozoic (up to 2500 million

The oldest metamorphic Rocks in Mexico were found in Sonora State and belong to the Bamori Complex (Figure 1), it is conformed by muscovite schist, hornblende-amphibolites schist and quartzite, dated at 1755 20 million years (myr) by Anderson and Silver (1981). In Chihuahua State a metamorphic complex outcrops (metagranite, metadiorite, amphibolite, gneiss, metalimestone and quartzite), these rocks have an age between 1025 21 myr and 948 14 myr (Blount 1983). In southeast Mexico one finds dispersed outcrops from the Proterozoic composed of metamorphic rocks (augengneiss, orthogneis, marble, amphibolite and migmatite) that belong to the Oaxaqueño Complex, with an age between 1300 to 700 myr (SGM, 2007). Along the southern coast from Zihuatanejo, Guerrero to Puerto Ángel, Oaxaca emerges a group of paragneiss, pelitic schist, boitite schist, quartzite, marble, orthogneiss, amphibolite and migmatites that has been grouped inside the Xolapa complex (between 980 and 1300 myr; SGM, 2007). Intrusive rocks of the Paleo and Mesoproterozoic, emerge in Sonora state, showing small outcrops of granite, granodiorite, and in less proportion, diorite. Their ages vary between 1440 and 1140 myr

Sedimentary rocks of the Proterozoic outcrop in Sonora state like small patches of dolomite, limestone and sandstone. These deposits were dated as Neoproterozoic owing to the

Metamorphic rocks from this period are schist, marble and quartzite. They are located in the states of Baja California, Sinaloa, Sonora and Chihuahua, their ages fluctuate between

presence of conic stromatolite fossils of the Conophyton genus (SGM, 2007).

Landscape information is of the out most importance to develop appropriate policies for environmental planning and nature conservation (Gulink *et al*., 2000). An example of recognition of landscape ecology as an essential field of science for territorial planning is the project of the metropolitan region of Barcelona (Forman, 2004), in which environmental principles based on landscape ecology and sustainable use of resources and basic spatial models are applied, even when lacking quantitative regional analysis.

### **2. Geology and geomorphology as fundamental elements in landscape analysis**

Several terrain characteristics are important for soil scientists, geologists, and geographers, because of their strong influence in the capability of the land to support various plant or animal species, or for terrain evaluation. Geologic origin and structure can be estimated by air photo interpretation and satellite image analysis. Sedimentary (sandstone, shale, limestone) or igneous rocks can be differentiated using digital analysis (remote sensing and GIS). The recognition of strike and dip attitudes, land form types, drainage patterns and the orientation of highlights and shadows, as well as susceptibility to flooding, are based on geomorphology (Sabins, 1978, Lillesand and Keiffer, 1979, Verstappen, 1988). Geomorphological analysis is greatly improved by the use of aerial photographs and satellite images, since they provide a synoptic view of terrain and a relatively rapid description of geographic distribution of major landforms and dominant land cover. Terrain classification based on landforms, lithology and genesis (historical processes) can be further specified into biogeomorphic land units on the basis of geomorphologic processes, relative age, sediment, drainage, and land cover/use (García-Aguirre *et al*., 2010).

Ecological research provides ample evidence that topography can exert a significant influence on the processes shaping broad-scale landscape vegetation patterns. Unfortunately, the standard methods for landscape pattern analysis are not designed to include topography as a pattern shaping factor. Topography features may be derived from the digital elevation models (DEM) to obtain slope and aspect maps ( Dorner *et al*., 2002, Peiffer *et al*. 2003). A DEM and Landsat images were used to assess topographical complexity and evaluate changes in landscape composition and structure after fire (Viedma, 2008). Simultaneous analysis of maps of non-biotic elements (such as geology, geomorphology and topography) and biotic elements (land use/cover) allow to generate synthetic and systematic information of landscape in the form of biogeomorphic land unit maps (Zonneved, 1995).

### **3. Remote sensing and GIS in landscape analysis**

Remote sensing and GIS are essential tools for generation of landscape thematic information (Gulink *et al*., 2000) even when it is common to face problems during integration of different data sources in the GIS (Tinker *et al*., 1998). Advances in remote sensing technologies have provided practical means for land use/cover mapping, which is particularly important for landscape ecological studies. These tools can also efficiently identify and assess areas of landscape damage at different scales and help land managers to solve specific problems. However, it is a key consideration to evaluate the remote sensing data and methods used as well the scale and information needed, for instance, to correctly define the best resolution to use (Ludwing *et al*., 2007), as well as to find the best procedures to follow when linking data of different qualities (Falcucci, *et al*., 2007). Land cover change information through time, combined with thematic information can be stored and managed efficiently in a GIS since it relates different layers of spatial information. In addition, it is a powerful analysis tool that allows identification of spatial relationships among different maps, through connection of spatial data with its attributes (Belda and Melia, 2000, Baysnat *et al.,* 2000). Time series remote sensing provide researchers with a valuable tool for the dynamic analysis of landscape (Staus *et al*., 2002).

Environmental models implemented in computers have become important tools for designing management plans towards ecological and economic sustainability. Computers help to deal with the tremendous complexity reflected in the extensive temporal and spatial scales at which human and natural processes occur. Iverson and Prasad (2007) evaluated tsunami damage and built empirical vulnerability models of damage/no damage based on elevation, distance from shore, vegetation, and exposure**.**

### **4. An outline of the geology of Mexico**

The geology of Mexico is the result of multiple tectonic processes that have taken place along its geologic history. Current geologic configuration of Mexico is the consequence of continental block interaction with surrounding oceanic provinces. As a result, young sedimentary and volcanic outcrops are dominant. 80% of exposed units are placed on Cenozoic and Mesozoic eras (less than 250 million years), 13% correspond to the Paleozoic and only 7 % belong to the Precambrian, belonging to the Proterozoic (up to 2500 million years ago (Ortega *et al*. 1992).

### Precambrian

584 Earth Sciences

Landscape information is of the out most importance to develop appropriate policies for environmental planning and nature conservation (Gulink *et al*., 2000). An example of recognition of landscape ecology as an essential field of science for territorial planning is the project of the metropolitan region of Barcelona (Forman, 2004), in which environmental principles based on landscape ecology and sustainable use of resources and basic spatial

**2. Geology and geomorphology as fundamental elements in landscape**

Several terrain characteristics are important for soil scientists, geologists, and geographers, because of their strong influence in the capability of the land to support various plant or animal species, or for terrain evaluation. Geologic origin and structure can be estimated by air photo interpretation and satellite image analysis. Sedimentary (sandstone, shale, limestone) or igneous rocks can be differentiated using digital analysis (remote sensing and GIS). The recognition of strike and dip attitudes, land form types, drainage patterns and the orientation of highlights and shadows, as well as susceptibility to flooding, are based on geomorphology (Sabins, 1978, Lillesand and Keiffer, 1979, Verstappen, 1988). Geomorphological analysis is greatly improved by the use of aerial photographs and satellite images, since they provide a synoptic view of terrain and a relatively rapid description of geographic distribution of major landforms and dominant land cover. Terrain classification based on landforms, lithology and genesis (historical processes) can be further specified into biogeomorphic land units on the basis of geomorphologic processes, relative

Ecological research provides ample evidence that topography can exert a significant influence on the processes shaping broad-scale landscape vegetation patterns. Unfortunately, the standard methods for landscape pattern analysis are not designed to include topography as a pattern shaping factor. Topography features may be derived from the digital elevation models (DEM) to obtain slope and aspect maps ( Dorner *et al*., 2002, Peiffer *et al*. 2003). A DEM and Landsat images were used to assess topographical complexity and evaluate changes in landscape composition and structure after fire (Viedma, 2008). Simultaneous analysis of maps of non-biotic elements (such as geology, geomorphology and topography) and biotic elements (land use/cover) allow to generate synthetic and systematic information of landscape in the form of biogeomorphic land unit

Remote sensing and GIS are essential tools for generation of landscape thematic information (Gulink *et al*., 2000) even when it is common to face problems during integration of different data sources in the GIS (Tinker *et al*., 1998). Advances in remote sensing technologies have provided practical means for land use/cover mapping, which is particularly important for landscape ecological studies. These tools can also efficiently identify and assess areas of landscape damage at different scales and help land managers to solve specific problems. However, it is a key consideration to evaluate the remote sensing data and methods used as well the scale and information needed, for instance, to correctly define the best resolution to use (Ludwing *et al*., 2007), as well as to find the best procedures to follow when linking data

models are applied, even when lacking quantitative regional analysis.

age, sediment, drainage, and land cover/use (García-Aguirre *et al*., 2010).

**3. Remote sensing and GIS in landscape analysis**

**analysis**

maps (Zonneved, 1995).

The oldest metamorphic Rocks in Mexico were found in Sonora State and belong to the Bamori Complex (Figure 1), it is conformed by muscovite schist, hornblende-amphibolites schist and quartzite, dated at 1755 20 million years (myr) by Anderson and Silver (1981). In Chihuahua State a metamorphic complex outcrops (metagranite, metadiorite, amphibolite, gneiss, metalimestone and quartzite), these rocks have an age between 1025 21 myr and 948 14 myr (Blount 1983). In southeast Mexico one finds dispersed outcrops from the Proterozoic composed of metamorphic rocks (augengneiss, orthogneis, marble, amphibolite and migmatite) that belong to the Oaxaqueño Complex, with an age between 1300 to 700 myr (SGM, 2007). Along the southern coast from Zihuatanejo, Guerrero to Puerto Ángel, Oaxaca emerges a group of paragneiss, pelitic schist, boitite schist, quartzite, marble, orthogneiss, amphibolite and migmatites that has been grouped inside the Xolapa complex (between 980 and 1300 myr; SGM, 2007). Intrusive rocks of the Paleo and Mesoproterozoic, emerge in Sonora state, showing small outcrops of granite, granodiorite, and in less proportion, diorite. Their ages vary between 1440 and 1140 myr (Anderson and Silver, 1981).

Sedimentary rocks of the Proterozoic outcrop in Sonora state like small patches of dolomite, limestone and sandstone. These deposits were dated as Neoproterozoic owing to the presence of conic stromatolite fossils of the Conophyton genus (SGM, 2007).

### Paleozoic

Metamorphic rocks from this period are schist, marble and quartzite. They are located in the states of Baja California, Sinaloa, Sonora and Chihuahua, their ages fluctuate between

Geology and Geomorphology in Landscape Ecological Analysis for Forest

Fig. 2. Mesozoic Lithologic Units

Conservation and Hazard and Risk Assessment, Illustrated with Mexican Case Histories 587

Outcrops from Triassic are scarce. In southwest Sonora there is calcareous sandstone, alternated with limonite. This rock contains fossils of gastropods, coral, bryozoans, sponges and ammonites from this period (González-León, 1980), and the same in northeast Mexico

Along the Lower Jurassic an alternate succession of shales and limestones are deposited in the states of San Luis Potosí, Querétaro and Hidalgo. Jurassic sediments in southern Mexico (Oaxaca and Guerrero states) is composed of a conglomerate and sandstone succession with quartz clasts (Cualac Formation). The Upper Cretaceous was identified with ammonites and radiolarian fossils. In south Baja California the lower Cretaceous is represented by

To the east, in Chihuahua and Nuevo León states the great sea transgression deposited thick beds of calcareous and siliceous rocks (Formations Taraises, Cupido, La Peña, La Virgen.). At the same time calcareous anhydrite and clayish successions were deposited in Tamaulipas, San Luis Potosí, Hidalgo, Queretaro, Puebla and Veracruz states. The most important formations are Lower and Upper Tamaulipas, Otates, El Abra, Tamabra, Tamasopo and Cuesta del Cura. The last Formation is composed of limestone and chert. In south and central Mexico sedimentary rocks appear in the states of Jalisco, Michoacán, Guerrero, México, Morelos and Oaxaca, where calcareous successions settled, and clayish components are constituted by conglomerates, sandstones and limonites, with interstratified limestone, marls, and gypsum in different facies. The most important formations are Zicapa, Tepexi de Rodríguez, Xochicalco, Morelos, Cuautla y Mexcala. These formations contain a wide variety of mollusca, gasteropods, ammonites, and milliolids.

were deposited at the Huizachal Formation conglomerate (SGM, 2007).

sandstones, limonites, shales and conglomerate successions.

Cambric and Carboniferous (Figure 1). Phyllites and schist with quartzite can be found at the southeast of the State of Chiapas (Late Mississippian). In Tamaulipas state metamorphic rocks appear in Huizachal-Peregrina Structure presenting mica-schist interstratified with green rocks, metaflints, serpentine, and metalimestones. In the Huizachal-Peregrina structure, near Ciudad Victoria, Tamaulipas, there are outcrops formed by mica-schist of low grade. The age for this unit was 330 myr (Stewart *et al*., 1999). Low grade metavolcanic sedimentary rocks from the Permic (SGM, 2007) exist north of Durango city. Outcrop schists, phyllites, quartzite's and metalavas are found northeast of Puebla. These rocks are from Early Permic (280 myr; Iriondo *et al*., 2003). There are outcrops of schists and quartzites in southeast Oaxaca. The age of these rocks ranges between 289±5 Ma and 219±6 Ma. (Grajales-Nishimura *et al*., 1999). In Zacatecas outcrops are found of a metamorphosed sedimentary succession, from Late Paleozoic, 260.2 ± 3 Myr (Díaz-Salgado, 2004).

Fig. 1. Distribution of metamorphic rocks of Precambrian and Paleozoic

The metamorphic rocks from Mesozoic (Figure 2) in the northeast of the country are phyllites and chlorite and biotite schist, amphibolite gneiss, metatonalite and metadiorite (De Cserna *et al*., 1962; 220 myr). A meta-volcanosedimentary succession was deposited from Late Jurassic to Early Cretaceous along the west of Mexico.

States of Michoacán, México and Guerrero present outcrops of metamorphic rocks of low grade, there are schist's, slates and quartzites. (Tejupilco Schist, Taxco Schist and Green Rock Taxco Viejo formations (De Cserna 1982) and probably belong to the Jurassic.

Fig. 2. Mesozoic Lithologic Units

586 Earth Sciences

Cambric and Carboniferous (Figure 1). Phyllites and schist with quartzite can be found at the southeast of the State of Chiapas (Late Mississippian). In Tamaulipas state metamorphic rocks appear in Huizachal-Peregrina Structure presenting mica-schist interstratified with green rocks, metaflints, serpentine, and metalimestones. In the Huizachal-Peregrina structure, near Ciudad Victoria, Tamaulipas, there are outcrops formed by mica-schist of low grade. The age for this unit was 330 myr (Stewart *et al*., 1999). Low grade metavolcanic sedimentary rocks from the Permic (SGM, 2007) exist north of Durango city. Outcrop schists, phyllites, quartzite's and metalavas are found northeast of Puebla. These rocks are from Early Permic (280 myr; Iriondo *et al*., 2003). There are outcrops of schists and quartzites in southeast Oaxaca. The age of these rocks ranges between 289±5 Ma and 219±6 Ma. (Grajales-Nishimura *et al*., 1999). In Zacatecas outcrops are found of a metamorphosed

sedimentary succession, from Late Paleozoic, 260.2 ± 3 Myr (Díaz-Salgado, 2004).

Fig. 1. Distribution of metamorphic rocks of Precambrian and Paleozoic

from Late Jurassic to Early Cretaceous along the west of Mexico.

The metamorphic rocks from Mesozoic (Figure 2) in the northeast of the country are phyllites and chlorite and biotite schist, amphibolite gneiss, metatonalite and metadiorite (De Cserna *et al*., 1962; 220 myr). A meta-volcanosedimentary succession was deposited

States of Michoacán, México and Guerrero present outcrops of metamorphic rocks of low grade, there are schist's, slates and quartzites. (Tejupilco Schist, Taxco Schist and Green

Rock Taxco Viejo formations (De Cserna 1982) and probably belong to the Jurassic.

Outcrops from Triassic are scarce. In southwest Sonora there is calcareous sandstone, alternated with limonite. This rock contains fossils of gastropods, coral, bryozoans, sponges and ammonites from this period (González-León, 1980), and the same in northeast Mexico were deposited at the Huizachal Formation conglomerate (SGM, 2007).

Along the Lower Jurassic an alternate succession of shales and limestones are deposited in the states of San Luis Potosí, Querétaro and Hidalgo. Jurassic sediments in southern Mexico (Oaxaca and Guerrero states) is composed of a conglomerate and sandstone succession with quartz clasts (Cualac Formation). The Upper Cretaceous was identified with ammonites and radiolarian fossils. In south Baja California the lower Cretaceous is represented by sandstones, limonites, shales and conglomerate successions.

To the east, in Chihuahua and Nuevo León states the great sea transgression deposited thick beds of calcareous and siliceous rocks (Formations Taraises, Cupido, La Peña, La Virgen.). At the same time calcareous anhydrite and clayish successions were deposited in Tamaulipas, San Luis Potosí, Hidalgo, Queretaro, Puebla and Veracruz states. The most important formations are Lower and Upper Tamaulipas, Otates, El Abra, Tamabra, Tamasopo and Cuesta del Cura. The last Formation is composed of limestone and chert. In south and central Mexico sedimentary rocks appear in the states of Jalisco, Michoacán, Guerrero, México, Morelos and Oaxaca, where calcareous successions settled, and clayish components are constituted by conglomerates, sandstones and limonites, with interstratified limestone, marls, and gypsum in different facies. The most important formations are Zicapa, Tepexi de Rodríguez, Xochicalco, Morelos, Cuautla y Mexcala. These formations contain a wide variety of mollusca, gasteropods, ammonites, and milliolids.

Geology and Geomorphology in Landscape Ecological Analysis for Forest

conglomerates.

**5. Case studies**

continental shelf carbonate sediments.

Conservation and Hazard and Risk Assessment, Illustrated with Mexican Case Histories 589

and sandstone and limestone are distributed in grabens and synclinal valleys in the Sierra Madre Oriental and Sierra Madre Occidental. In the Sierra de Chiapas, the Miocene differed in the deposition of the clay-calcareous successions, as well as in the thin horizons of the

On the other hand, in the regions of the rim of the Yucatan Platform the bar reefs and lagoons keep on developing carbonate sediments of limestone and dolomites. The Holocene deposits of coastal environments of the coast of the Gulf of Mexico in the states of Tamaulipas, Veracruz, Tabasco, Campeche, Yucatan, and Quintana Roo, are still in the process of sedimentation of silts, clays and marshy sand, flat dunes of coastal sand, and

The volcanic units of the Cenozoic are widely distributed in the Mexican territory, including the ignimbrites of the Sierra Madre Occidental and the Pliocenic-Quaternary sequences of the Transmexican Volcanic Belt (TMVB). The Sierra Madre Occidental is formed by an extensive volcanic plateau affected by grabens and normal faults. It spreads from Sonora to Guerrero states, although in the states of Jalisco, Michoacán and Guerrero it is fragmented and mixed with the Transmexican Volcanic Belt and the rocks of the Sierra Madre del Sur. The TransMexican Volcanic Belt (TMVB) extends from the Pacific Ocean up to the coast of the Gulf of Mexico along 920 km between parallels 19 and 20. It is formed by a large variety of volcanic rocks produced by a number of volcanic buildings, some of which constitute the main elevations in the country. Likewise, this activity has caused a big number of endorreic basins with the consistent development of lacustrine landscapes. The principal volcanoes of the TMVB are stratovolcanoes of variable dimensions, such as Pico de Orizaba,

Popocatépetl, Iztaccíhuatl, La Malinche, Nevado de Toluca and Nevado de Colima.

Nacional Nevado de Toluca from the biogeomorphic land units of the region.

summit in México, the Nevado de Toluca.

**5.1 Sierra de las Cruces**

Landscape studies in Mexico are numerous: Ochoa (2001) undertook the integration of geologic and geomorphic units, incorporating afterwards variables of climate, hydrology, vegetation and soil in the Tehuacán-Cuicatlán, Puebla, area. Casals-Carrasco *et al.,* (2000) performed a geomorphologic analysis in order to establish relationships among land cover, landforms and soils using interpretation of a stereoscopic pair of SPOT-PAN, and a TM false color composite image. Martínez (2002) elaborated environmental land unit maps of a sub watershed in Morelos State. García (1991) studied the influence of relief dynamics in landscape structure on the vegetation in Zapotitlán, Puebla watershed. García (1998) analized the east slope of Sierra de la Cruces, Monte Alto and Monte Bajo while, Garcia-Aguirre *et al.* (2007) related geology, landform and vegetation in the Ajusco volcano area in central Mexico. Aguilar (2007) performed an environmental diagnostic of the Parque

Two case studies will be described in detail. The first is related to the use of biogeomorphic land units of a region located nearby Mexico City as a basis for hydrology and vegetation analysis. The second case refers to the evaluation of risks and hazards of the fourth highest

The objectives of the first study are two-fold: to identify the most degraded areas of the region through the land unit analysis, and to study the relationship between forest loss and

runoff in the region (scale 1:250,000) through a conceptual and cartographic model.

A change in sedimentation takes place in the Upper Cretaceous marked by the suffocation of the platforms with a series of terrigenous successions that show the evolution of deltas and basins. These successions form big bundles of sandstone, conglomerate, limonite, marl, and shales.

The outcrops of intrusive rocks are scarce and dispersed along the territory. The most important deposits are found in southern Baja California (granites, Peridotite, utramafic sequence, and an ophiolite sequence (Kimbrough and Moore, 2003). A sequence of granite and diorite from Middle Jurassic is located in Puebla (Macizo de Teziutlán), dated between 163 ± 13 and 134 ± 11 Myr (Manjarrez and Hernández, 1989). In Guanajuato state outcrops a sequence of granite, diorite and tonalite from the Upper Jurassic outcrops, these deposits are related to the evolution of Mesozoic insular arcs from this period (SGM, 2007). Plutonic magmatism appears in southern Mexico, probably from Late Jurassic to Early Cretaceous, with a variable composition: granite, granodiorite and diorite. Some localities in which this plutonic intrusive appears are Tumbiscatio, near Zitácuaro city.

Cenozoic

A continental sedimentation of the Early Cenozoic marks the change of sedimentary rocks to volcanic sequences (Figure 3). The Red Conglomerates with intercalation of sandstones and limonites represent them in the Balsas, Red Conglomerate of Guanajuato, and Tehuacán formations. Transgressive events are reflected by the horizons of sandstone, lutite and conglomerates. In the Eocene-Oligocene, deposits of conglomerate and sandstone, limonite,

Fig. 3. Cenozoic Lithologic Units

and sandstone and limestone are distributed in grabens and synclinal valleys in the Sierra Madre Oriental and Sierra Madre Occidental. In the Sierra de Chiapas, the Miocene differed in the deposition of the clay-calcareous successions, as well as in the thin horizons of the conglomerates.

On the other hand, in the regions of the rim of the Yucatan Platform the bar reefs and lagoons keep on developing carbonate sediments of limestone and dolomites. The Holocene deposits of coastal environments of the coast of the Gulf of Mexico in the states of Tamaulipas, Veracruz, Tabasco, Campeche, Yucatan, and Quintana Roo, are still in the process of sedimentation of silts, clays and marshy sand, flat dunes of coastal sand, and continental shelf carbonate sediments.

The volcanic units of the Cenozoic are widely distributed in the Mexican territory, including the ignimbrites of the Sierra Madre Occidental and the Pliocenic-Quaternary sequences of the Transmexican Volcanic Belt (TMVB). The Sierra Madre Occidental is formed by an extensive volcanic plateau affected by grabens and normal faults. It spreads from Sonora to Guerrero states, although in the states of Jalisco, Michoacán and Guerrero it is fragmented and mixed with the Transmexican Volcanic Belt and the rocks of the Sierra Madre del Sur. The TransMexican Volcanic Belt (TMVB) extends from the Pacific Ocean up to the coast of the Gulf of Mexico along 920 km between parallels 19 and 20. It is formed by a large variety of volcanic rocks produced by a number of volcanic buildings, some of which constitute the main elevations in the country. Likewise, this activity has caused a big number of endorreic basins with the consistent development of lacustrine landscapes. The principal volcanoes of the TMVB are stratovolcanoes of variable dimensions, such as Pico de Orizaba, Popocatépetl, Iztaccíhuatl, La Malinche, Nevado de Toluca and Nevado de Colima.

### **5. Case studies**

588 Earth Sciences

A change in sedimentation takes place in the Upper Cretaceous marked by the suffocation of the platforms with a series of terrigenous successions that show the evolution of deltas and basins. These successions form big bundles of sandstone, conglomerate, limonite, marl,

The outcrops of intrusive rocks are scarce and dispersed along the territory. The most important deposits are found in southern Baja California (granites, Peridotite, utramafic sequence, and an ophiolite sequence (Kimbrough and Moore, 2003). A sequence of granite and diorite from Middle Jurassic is located in Puebla (Macizo de Teziutlán), dated between 163 ± 13 and 134 ± 11 Myr (Manjarrez and Hernández, 1989). In Guanajuato state outcrops a sequence of granite, diorite and tonalite from the Upper Jurassic outcrops, these deposits are related to the evolution of Mesozoic insular arcs from this period (SGM, 2007). Plutonic magmatism appears in southern Mexico, probably from Late Jurassic to Early Cretaceous, with a variable composition: granite, granodiorite and diorite. Some localities in which this

A continental sedimentation of the Early Cenozoic marks the change of sedimentary rocks to volcanic sequences (Figure 3). The Red Conglomerates with intercalation of sandstones and limonites represent them in the Balsas, Red Conglomerate of Guanajuato, and Tehuacán formations. Transgressive events are reflected by the horizons of sandstone, lutite and conglomerates. In the Eocene-Oligocene, deposits of conglomerate and sandstone, limonite,

plutonic intrusive appears are Tumbiscatio, near Zitácuaro city.

and shales.

Cenozoic

Fig. 3. Cenozoic Lithologic Units

Landscape studies in Mexico are numerous: Ochoa (2001) undertook the integration of geologic and geomorphic units, incorporating afterwards variables of climate, hydrology, vegetation and soil in the Tehuacán-Cuicatlán, Puebla, area. Casals-Carrasco *et al.,* (2000) performed a geomorphologic analysis in order to establish relationships among land cover, landforms and soils using interpretation of a stereoscopic pair of SPOT-PAN, and a TM false color composite image. Martínez (2002) elaborated environmental land unit maps of a sub watershed in Morelos State. García (1991) studied the influence of relief dynamics in landscape structure on the vegetation in Zapotitlán, Puebla watershed. García (1998) analized the east slope of Sierra de la Cruces, Monte Alto and Monte Bajo while, Garcia-Aguirre *et al.* (2007) related geology, landform and vegetation in the Ajusco volcano area in central Mexico. Aguilar (2007) performed an environmental diagnostic of the Parque Nacional Nevado de Toluca from the biogeomorphic land units of the region.

Two case studies will be described in detail. The first is related to the use of biogeomorphic land units of a region located nearby Mexico City as a basis for hydrology and vegetation analysis. The second case refers to the evaluation of risks and hazards of the fourth highest summit in México, the Nevado de Toluca.

### **5.1 Sierra de las Cruces**

The objectives of the first study are two-fold: to identify the most degraded areas of the region through the land unit analysis, and to study the relationship between forest loss and runoff in the region (scale 1:250,000) through a conceptual and cartographic model.

Geology and Geomorphology in Landscape Ecological Analysis for Forest

Fig. 5. Geologic map. Andesites and Basalt are dominant in this region

Fig. 6. Geomorphologic map. Dominant geoforms are modeled slopes and lava flows

Conservation and Hazard and Risk Assessment, Illustrated with Mexican Case Histories 591

Landscape analysis was performed to describe regional characteristics in an integral form; land cover features were overlaid with other landscape elements (geology, geomorphology, soils and climate) to obtain a land unit map (García-Aguirre, 2008).

Mountainous relief and flat plain may be appreciated in the shadow relief model derived from a DEM (figure 4). Chichinautzin region, towards the south of the zone, being an infiltration zone is very important from the hydrological view point. Refief of the east slope of the Sierra de las Cruces, Monte Alto and Monte Bajo is constituted by four geomorphic units: mountain, upper and lower piedmont, and hills. Notice the N-S orientation of this mountain range.

Fig. 4. Shadow relief model derived from the DEM. Study area is located inside the square. Sierra de Chichinautzin is located to the south, and Sierra Nevada toward the east

#### Land unit map

Remote sensing and GIS were linked to integrate geomorphological and geological information to find mayor associations among variables. Then, biogeomorphic land units were delineated on the basis of homogeneity of a dominant factor.

Figures 5 and 6 show the geology and geomorphology maps of the region. These maps were obtained by digitizing hardcopy maps of INEGI (1993) and reclassified using IDRISI (Eastman, 1997). Andesite and basalt are dominant in the area and in turn, andosols and lithosols (Figure 5). Lugo (1984) points the south of Cuenca de México as one of the zones of the country with higher concentration of young volcanoes, from the late Pleistocene and Holocene (Figure 6).

Landscape analysis was performed to describe regional characteristics in an integral form; land cover features were overlaid with other landscape elements (geology, geomorphology,

Mountainous relief and flat plain may be appreciated in the shadow relief model derived from a DEM (figure 4). Chichinautzin region, towards the south of the zone, being an infiltration zone is very important from the hydrological view point. Refief of the east slope of the Sierra de las Cruces, Monte Alto and Monte Bajo is constituted by four geomorphic units: mountain, upper and lower piedmont, and hills. Notice the N-S orientation of this

Fig. 4. Shadow relief model derived from the DEM. Study area is located inside the square.

Remote sensing and GIS were linked to integrate geomorphological and geological information to find mayor associations among variables. Then, biogeomorphic land units

Figures 5 and 6 show the geology and geomorphology maps of the region. These maps were obtained by digitizing hardcopy maps of INEGI (1993) and reclassified using IDRISI (Eastman, 1997). Andesite and basalt are dominant in the area and in turn, andosols and lithosols (Figure 5). Lugo (1984) points the south of Cuenca de México as one of the zones of the country with higher concentration of young volcanoes, from the late Pleistocene and

Sierra de Chichinautzin is located to the south, and Sierra Nevada toward the east

were delineated on the basis of homogeneity of a dominant factor.

soils and climate) to obtain a land unit map (García-Aguirre, 2008).

mountain range.

Land unit map

Holocene (Figure 6).

Fig. 5. Geologic map. Andesites and Basalt are dominant in this region

Fig. 6. Geomorphologic map. Dominant geoforms are modeled slopes and lava flows

Geology and Geomorphology in Landscape Ecological Analysis for Forest

**5.2 Nevado de Toluca geologic history and hazards**

Fig. 8. Nevado de Toluca, location map

Conservation and Hazard and Risk Assessment, Illustrated with Mexican Case Histories 593

Nevado de Toluca Volcano (NTV), located in central Mexico, is a large stratovolcano, with an explosive history. The area is one of the most important human developing centers (>2 million people) in Mexico and in the last 30 years large population growth and urban expansion have increased the potential risk in case of a reactivation of the volcano. NTV is the fourth highest summit in Mexico (4,665 masl) and it is a potentially dangerous large stratovolcano, that lies in the southeastern part of the Toluca Basin, some 70 km east of Mexico City (Figure 8). The NTV has been characterized by very explosive eruptions with

long periods of dormancy; the periods between eruptions are discussed below.

Geomorphologic map (Figure 6) shows modeled slopes and lava flows as dominant geoforms. The region has an extense footslope, in which half the area slopes down to the piedemont and the other half is mountainous terrain. Also extensive lava flows show the active quaternary volcanism in this zone.

Combination of biotic and non biotic features generated more than 100 units, that were reclassified into 48 units on the basis of its surface (only units with more than 500 ha), for map legibility (Figure 7). Alluvial and lacustrine units are mainly covered with pheozem and hystosol, agriculture and grasslands. The modeled slopes of andesites with andosol are subdivided into those with Abies and those with Pine forest. Towards the east (BGU39, BGU40, and BGU2), there are abundant lacustrine forms with agriculture, grasslands and human settlements. Basalt is abundant to the south with forest cover, agriculture and grassland (BGU29, BGU33). In the footslopes, mainly towards the north, there are units constituted by andesites, modeled slopes and cambisols, that are covered by oak forests, crops and grasslands. There are many holocenic volcanic structures over the mountainous area, with andosol and litosol, covered mainly by forests.

The regional vision provided by this study allowed to have a rapid overview of sites that should be preserved, such as basalt zones, that are nevertheless continuously invaded by human settlements. Results indicate that Sierra de Chichinautzin is the main recharging area, but the foot slopes of the Sierra de Las Cruces are also important infiltration and recharging zones as a result of the abundance of clastic volcanic forms therein.

Fig. 7. Land unit map (BGU=biogeomorphic land units)

### **5.2 Nevado de Toluca geologic history and hazards**

592 Earth Sciences

Geomorphologic map (Figure 6) shows modeled slopes and lava flows as dominant geoforms. The region has an extense footslope, in which half the area slopes down to the piedemont and the other half is mountainous terrain. Also extensive lava flows show the

Combination of biotic and non biotic features generated more than 100 units, that were reclassified into 48 units on the basis of its surface (only units with more than 500 ha), for map legibility (Figure 7). Alluvial and lacustrine units are mainly covered with pheozem and hystosol, agriculture and grasslands. The modeled slopes of andesites with andosol are subdivided into those with Abies and those with Pine forest. Towards the east (BGU39, BGU40, and BGU2), there are abundant lacustrine forms with agriculture, grasslands and human settlements. Basalt is abundant to the south with forest cover, agriculture and grassland (BGU29, BGU33). In the footslopes, mainly towards the north, there are units constituted by andesites, modeled slopes and cambisols, that are covered by oak forests, crops and grasslands. There are many holocenic volcanic structures over the mountainous

The regional vision provided by this study allowed to have a rapid overview of sites that should be preserved, such as basalt zones, that are nevertheless continuously invaded by human settlements. Results indicate that Sierra de Chichinautzin is the main recharging area, but the foot slopes of the Sierra de Las Cruces are also important infiltration and

recharging zones as a result of the abundance of clastic volcanic forms therein.

active quaternary volcanism in this zone.

area, with andosol and litosol, covered mainly by forests.

Fig. 7. Land unit map (BGU=biogeomorphic land units)

Nevado de Toluca Volcano (NTV), located in central Mexico, is a large stratovolcano, with an explosive history. The area is one of the most important human developing centers (>2 million people) in Mexico and in the last 30 years large population growth and urban expansion have increased the potential risk in case of a reactivation of the volcano. NTV is the fourth highest summit in Mexico (4,665 masl) and it is a potentially dangerous large stratovolcano, that lies in the southeastern part of the Toluca Basin, some 70 km east of Mexico City (Figure 8). The NTV has been characterized by very explosive eruptions with long periods of dormancy; the periods between eruptions are discussed below.

Fig. 8. Nevado de Toluca, location map

Geology and Geomorphology in Landscape Ecological Analysis for Forest

*et al*., 2007).

Conservation and Hazard and Risk Assessment, Illustrated with Mexican Case Histories 595

These secondary lahars are not related to volcanic activity and represent an increased hazard, because these materials are not consolidated. In torrential periods these materials can be removed by rain, triggering these secondary lahares, which can come up to the low zones affecting the populations who are settled in the mouths of the valleys as it happened to the people of Santa Cruz Pueblo Nuevo. To the north, the ravines are shallower (<30 m). The origin of the secondary lahars are uncompacted volcanic products (ash fall and, pyroclastic flows) mixed with water from the glacial melt and torrential rains (Aceves

*Ash fall hazards:* Ash and pumice fall deposits cover wide zones around the volcano. The most important deposits are the Upper and Lower Toluca Pumice. For the ash fall hazards map, the distribution and thickness of these deposits were considered as well as the present wind direction for the UTP. These isopach and isopleth maps show dispersal to the northeast. In the Toluca Basin, the maximum thickness measured for the UTP was 40 cm. In order to plot the isopach of 10 cm, map scales of 1:100,000, and 1:250,000 scale were used. The dominant wind direction was obtained from the National Meteorological Service and calculated for a height between 20 and 30 km (Fonseca 2003). The results were: east-northeast from November to

In conclusion, in the last 50,000 yr, NTV had eight vulcanian, and four Plinian eruptions, three large dome collapses, and one ultraPlinian eruption. Block and ash and pumice flows are the most common deposits. The eruptive history of NTV shows cataclysmic events of Plinian and utraPlinian type, beside Vulcanian eruptions, which represent a large hazard for the Toluca Basin. The regions that would be most affected by pyroclastic flows and lahars are: (1) Toluca, Lerma, Metepec, San Mateo Atenco, Santiago Tianguistenco and Capulhuac, located to the northeast of the volcano. These areas concentrate the major population and industrial centers. (2) Calimaya, Zacango, Tenango among several others located to the east of NTV are highly populated and important agricultural areas. (3) To the south, the flower producing centers:

March, west-northwest in April and west from May to October (Aceves et al., 2007).

Coatepec and Villa Guerrero, and the tourist towns of Ixtapan de la Sal and Tonatico.

large eruptions (VEI > 4) the affected zone would even include Mexico City.

Authors wishes to thank Verónica Aguilar for help in drawing of Figures 5, 6 and 7

**6. Acknowledgment**

Four volcanic hazards types were identified: pyroclastic flows (block and ash flows and pumice flows), lahars, debris avalanches and ash fall. The most destructive (based on energy and frequency) in the NTV are the block and ash flows and the pumice flows, both of them have reached distance of up to 35 km from the volcano summit. The principal affected areas are the northeast and south of the volcano, because these areas have major differences in altitude and present the major development of ravines. Lahars have been present in most of the eruptions. The deep and large valleys located to the east and south of the volcano are the most hazardous areas. The most active valleys are San Jerónimo, Chontalcuatlan, Grande, and El Zaguán Rivers. Debris avalanches present a hazard for areas to the east and south of the volcano, because of the active faults in the area and the instability caused by the difference in altitude, favoring the gravitational collapse of NTV. This type of event has occurred twice in the last 100,000 yrs, the deposits are located to the south of the volcano. The hazards from ash fall, arising from the dominant winds, are: from November to March, they would affect mainly the east and north-east sectors of the volcano, in April affectation would be to the north-west, and from May to October to the west. In case of small and medium eruptions (VEI = 1–3), the affected zone would be the Toluca Basin, but in case of

### Nevado de Toluca Volcanic hazards

*Pyroclastic flow Hazards*: These deposits are widely spread around the volcano, filling the stream valleys where several settlements are located (Figure 8). The pyroclastic flow deposits cover a minimum area of 630 km2 and assuming that they have an average thickness of 5 m, the approximate volume is 3.15 km3 (Macias *et al*. 1997). The maximum distance reached by these deposits is 32 km from the crater towards the south, in the Tizantes and Calderón stream valleys. The block and ash flows form massive units interstratified with surge horizons. The first dome collapse deposited the Zacango Block and Ash flow (37 kyr BP) composed of three massive units with associated surge horizons. The second dome collapse (28 kyr BP) deposited El Capulín Block and Ash Flow. These deposits are distributed around the volcano and cover approximately 630 km2 with a volume of 2.6 km3 (Aceves *et al*., 2007).

The pumice flows erupted by the NTV are: the Pink Pumice Flow (43 kyr BP); the White Pumice Flow (26 kyr BP). The MF2 (13.4 kyr BP) pumice flow is a gray ash flow enriched in pumice clasts (<2 cm) and charcoal (Aceves, *et al*. 2007). The Intermediate White Pumice (12.1 kyr BP) is composed of white ribbon pumice clasts and gray to reddish (altered) dacitic lithic clasts, interbedded with a surge (Cervantes 2001). The pumice flows are distributed around the volcano and cover more than 200 km2, with a volume of 0.2 km3. The pumice flows are related to the Plinian eruptions, many of which result in mudflows and include charcoal and variable amounts of pumice. Some are altered to paleosoils. One of the most pumice-rich flows belongs to the Upper Toluca Pumice Formation (UTP), dated 10,500 yr BP (Macias *et al*. 1997).

*Debris avalanches hazards*: The debris avalanches have been located towards the south of the NTV in the Meyuca, Calderón Chontalcuatlán and San Jerónimo river valleys. Two units compose the debris avalanches. The oldest unit (DAD1) is massive, with 35% blocks up to 2.5 m in diameter, in a heavy pink partially hardened sand sized matrix. The lithological composition is heterogeneous dacitic, andesitic and schist lithics, with''Jig saw'' blocks (Capra and Macias, 2000). DAD1 is around 10 m thick in Coatepec and continues towards the south to the Valley of the Chontalcuatlan River. The youngest avalanche (DAD2) is composed of two large cohesive debris flows: the Pilcaya (PDF) and the El Mogote (MDF) (Capra 2000). Thickness varies from 6 m in the proximal section to 40 m in the intermediate zone, extending out to a distance of 75 km from the crater. It covered an area of 220 km2 with a volume of 2.8 km3 (Aceves *et al*., 2007, Capra 2000).

*Lahar hazards:* Lahars at NTV are wide spread around the volcano filling new and old valleys. Lahars have rounded and subrounded dacite lithics (15–25 cm), small pumice fragments (<5 cm) fixed in a muddy-sand sized matrix. To the south, the lahar thickness is more than 30 m. The oldest lahars are made up of rounded and subrounded gray and red andesite blocks fixed in red clay sized matrix with scarce pumice fragments. The recent deposits contain subangular and subrounded blocks of gray and red dacite, with pumice fragments, some of which are hydrated, fixed in a siltclay sized matrix. The flow direction of these lahars was controlled by the topography, principally deep tectonic, glacial and fluvial valleys. In distal areas, the lahar deposits were transformed into fluvial mixing with the stream and river waters. To the east, the lahars contain more pumice fragments in a pale brown silt-clay matrix. There is a lahar with large hydrated pumice fragments (20–30 cm), in the Arroyo Grande channel. In this area, many secondary lahars exist, such as the one deposited in 1952 in the Ciénaga channel.

These secondary lahars are not related to volcanic activity and represent an increased hazard, because these materials are not consolidated. In torrential periods these materials can be removed by rain, triggering these secondary lahares, which can come up to the low zones affecting the populations who are settled in the mouths of the valleys as it happened to the people of Santa Cruz Pueblo Nuevo. To the north, the ravines are shallower (<30 m). The origin of the secondary lahars are uncompacted volcanic products (ash fall and, pyroclastic flows) mixed with water from the glacial melt and torrential rains (Aceves *et al*., 2007).

*Ash fall hazards:* Ash and pumice fall deposits cover wide zones around the volcano. The most important deposits are the Upper and Lower Toluca Pumice. For the ash fall hazards map, the distribution and thickness of these deposits were considered as well as the present wind direction for the UTP. These isopach and isopleth maps show dispersal to the northeast. In the Toluca Basin, the maximum thickness measured for the UTP was 40 cm. In order to plot the isopach of 10 cm, map scales of 1:100,000, and 1:250,000 scale were used. The dominant wind direction was obtained from the National Meteorological Service and calculated for a height between 20 and 30 km (Fonseca 2003). The results were: east-northeast from November to March, west-northwest in April and west from May to October (Aceves et al., 2007).

In conclusion, in the last 50,000 yr, NTV had eight vulcanian, and four Plinian eruptions, three large dome collapses, and one ultraPlinian eruption. Block and ash and pumice flows are the most common deposits. The eruptive history of NTV shows cataclysmic events of Plinian and utraPlinian type, beside Vulcanian eruptions, which represent a large hazard for the Toluca Basin. The regions that would be most affected by pyroclastic flows and lahars are: (1) Toluca, Lerma, Metepec, San Mateo Atenco, Santiago Tianguistenco and Capulhuac, located to the northeast of the volcano. These areas concentrate the major population and industrial centers. (2) Calimaya, Zacango, Tenango among several others located to the east of NTV are highly populated and important agricultural areas. (3) To the south, the flower producing centers: Coatepec and Villa Guerrero, and the tourist towns of Ixtapan de la Sal and Tonatico.

Four volcanic hazards types were identified: pyroclastic flows (block and ash flows and pumice flows), lahars, debris avalanches and ash fall. The most destructive (based on energy and frequency) in the NTV are the block and ash flows and the pumice flows, both of them have reached distance of up to 35 km from the volcano summit. The principal affected areas are the northeast and south of the volcano, because these areas have major differences in altitude and present the major development of ravines. Lahars have been present in most of the eruptions. The deep and large valleys located to the east and south of the volcano are the most hazardous areas. The most active valleys are San Jerónimo, Chontalcuatlan, Grande, and El Zaguán Rivers. Debris avalanches present a hazard for areas to the east and south of the volcano, because of the active faults in the area and the instability caused by the difference in altitude, favoring the gravitational collapse of NTV. This type of event has occurred twice in the last 100,000 yrs, the deposits are located to the south of the volcano. The hazards from ash fall, arising from the dominant winds, are: from November to March, they would affect mainly the east and north-east sectors of the volcano, in April affectation would be to the north-west, and from May to October to the west. In case of small and medium eruptions (VEI = 1–3), the affected zone would be the Toluca Basin, but in case of large eruptions (VEI > 4) the affected zone would even include Mexico City.

### **6. Acknowledgment**

594 Earth Sciences

*Pyroclastic flow Hazards*: These deposits are widely spread around the volcano, filling the stream valleys where several settlements are located (Figure 8). The pyroclastic flow deposits cover a minimum area of 630 km2 and assuming that they have an average thickness of 5 m, the approximate volume is 3.15 km3 (Macias *et al*. 1997). The maximum distance reached by these deposits is 32 km from the crater towards the south, in the Tizantes and Calderón stream valleys. The block and ash flows form massive units interstratified with surge horizons. The first dome collapse deposited the Zacango Block and Ash flow (37 kyr BP) composed of three massive units with associated surge horizons. The second dome collapse (28 kyr BP) deposited El Capulín Block and Ash Flow. These deposits are distributed around the volcano and cover approximately 630 km2 with a volume of 2.6

The pumice flows erupted by the NTV are: the Pink Pumice Flow (43 kyr BP); the White Pumice Flow (26 kyr BP). The MF2 (13.4 kyr BP) pumice flow is a gray ash flow enriched in pumice clasts (<2 cm) and charcoal (Aceves, *et al*. 2007). The Intermediate White Pumice (12.1 kyr BP) is composed of white ribbon pumice clasts and gray to reddish (altered) dacitic lithic clasts, interbedded with a surge (Cervantes 2001). The pumice flows are distributed around the volcano and cover more than 200 km2, with a volume of 0.2 km3. The pumice flows are related to the Plinian eruptions, many of which result in mudflows and include charcoal and variable amounts of pumice. Some are altered to paleosoils. One of the most pumice-rich flows belongs to the Upper Toluca Pumice Formation (UTP), dated 10,500 yr BP

*Debris avalanches hazards*: The debris avalanches have been located towards the south of the NTV in the Meyuca, Calderón Chontalcuatlán and San Jerónimo river valleys. Two units compose the debris avalanches. The oldest unit (DAD1) is massive, with 35% blocks up to 2.5 m in diameter, in a heavy pink partially hardened sand sized matrix. The lithological composition is heterogeneous dacitic, andesitic and schist lithics, with''Jig saw'' blocks (Capra and Macias, 2000). DAD1 is around 10 m thick in Coatepec and continues towards the south to the Valley of the Chontalcuatlan River. The youngest avalanche (DAD2) is composed of two large cohesive debris flows: the Pilcaya (PDF) and the El Mogote (MDF) (Capra 2000). Thickness varies from 6 m in the proximal section to 40 m in the intermediate zone, extending out to a distance of 75 km from the crater. It covered an area of 220 km2

*Lahar hazards:* Lahars at NTV are wide spread around the volcano filling new and old valleys. Lahars have rounded and subrounded dacite lithics (15–25 cm), small pumice fragments (<5 cm) fixed in a muddy-sand sized matrix. To the south, the lahar thickness is more than 30 m. The oldest lahars are made up of rounded and subrounded gray and red andesite blocks fixed in red clay sized matrix with scarce pumice fragments. The recent deposits contain subangular and subrounded blocks of gray and red dacite, with pumice fragments, some of which are hydrated, fixed in a siltclay sized matrix. The flow direction of these lahars was controlled by the topography, principally deep tectonic, glacial and fluvial valleys. In distal areas, the lahar deposits were transformed into fluvial mixing with the stream and river waters. To the east, the lahars contain more pumice fragments in a pale brown silt-clay matrix. There is a lahar with large hydrated pumice fragments (20–30 cm), in the Arroyo Grande channel. In this area, many secondary lahars exist, such as the one

with a volume of 2.8 km3 (Aceves *et al*., 2007, Capra 2000).

deposited in 1952 in the Ciénaga channel.

Nevado de Toluca Volcanic hazards

km3 (Aceves *et al*., 2007).

(Macias *et al*. 1997).

Authors wishes to thank Verónica Aguilar for help in drawing of Figures 5, 6 and 7

Geology and Geomorphology in Landscape Ecological Analysis for Forest

Nacional Autónoma de México, p 90

*del Instituto de Geografía*. No. 23: 53-70.

segmentation. *Landscape Ecol* 19:59–76.

Edition. John Wiley and Sons. 612 p.

202003\_Texto01.pdf.

suelo y vegetación escala 1:250,000. México.

(GIS). *Forest Ecology and Management* 239: 1-12.

Tuxtlas, Veracruz, México. *Geomorphology*. 121:245-256.

543 p.

Conservation and Hazard and Risk Assessment, Illustrated with Mexican Case Histories 597

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