Every day, we walk or drive around some part of the Earth’s surface.  We see trees, grass, bushes and flowers sticking up.  We know that subways run underground and that our water wells get water from underground.  But, what do we really know about what is under our feet?  Probably very little.  Fortunately, scientists have been studying the structure of the Earth and we can learn a lot by studying what they have discovered.

Scientists have described the internal or inside structure of the Earth.  The problem that the scientists have been faced with is that they cannot make direct observations.  They must gather lots of information, analyze the data and analyze it, scientists sometimes change their ideas or theories and rewrite the books.

How can scientists collect information about something they cannot see or observe directly?  The two main ways scientist collect information about the Earth is by drilling into it and by using seismic information.  The first method they use involves drilling into the Earth.  They can bring out sections from holes as deep as 8 kilometers into the Earth.  The problem with this method is the Earth is about 6,370 kilometers deep!  Another way that scientists have collected data is by using seismographic equipment.  These tools measure the seismic or shockwaves produced by earthquakes.  Analyzing this data has given scientists valuable insight into the structure of our planet. 

On Russia's Kola Peninsula, near the Norwegian border at about the same latitude as Prudhoe Bay, the Russians have been drilling a well since 1970. It is now over 12 kilometers or 40,000 feet deep, making it the deepest hole on earth (the previous record holder was the Bertha Rogers well in Oklahoma -- a gas well stopped at 32,000 feet when it struck molten sulfur

Once the information is collected and the data is analyzed, the theories are developed.  From these theories, scientists are able to create models.  Just as the theories may change when new information is learned, the models may change as well.

Currently, scientists believe the Earth is made of four layers; the crust, the mantle, the outer core, and the inner core.  Using this model, scientists have been able to explain many geological events such as earthquakes, volcanoes, the movement of continents, etc.  With this model, scientists hope to develop a better understanding of the past of our planet so they might better understand the current events and better predict the future of our home.

The Earth’s Crust

Think of an egg and its parts.  The yolk is in the center, then there is the white of the egg, and it is all surrounded by a thin shell.  The Earth can be compared to an egg.  It also has three parts; the core is in the center, then there is the mantle, and it is all surrounded by the crust.

As far as scientists know now, all life exists either on the surface or within the first few meters of the crust.

Just like a loaf of bread, the outer layer of the Earth is called the crust.  We live on the crust, all of our water, lakes, oceans, oil wells, coal mines, and everything we get from the ground comes out of the crust.  We have never been able to dig a hole through the crust.  The crust is made up of different kinds of rock, but the two main types are granite and basalt.  Basalt is much stronger than granite.  The upper crust is made primarily of granite covered by sedimentary rocks.  Sedimentary rocks are formed when materials on the Earth’s surface are worn away and then deposited in layers that form several different kinds of sedimentary rock.  Beneath the granite we find basalt.  Basalt makes up the bottom of the crust.  Ocean crust is made only of basalt.  Scientists do however, believe that the ocean floor may have as much as a half mile deep layer of sediment on it.

There are approximately 90 different chemical substances, or elements, that have been identified in the Earth’s crust.  Five elements make up more than 90 percent of the crust; oxygen, silicon, aluminum, iron, and calcium.  Sodium, potassium, and magnesium are among the most commonly found elements in the remaining ten percent of the crust.

The elements and compounds found within the crust provide many of the energy resources we depend on for survival and comfort, including gas, coal, and oil.  They also provide the metals we commonly use, such as gold, iron, and lead.  Minerals that have found important places in our lives are also provided by the Earth’s crust; diamonds, quart, and graphite. 

Scientists refer to the crust as the lithosphere, the solid portion of the Earth.  The word litho comes from the Greek word meaning rock or stone.  The scientists believe that Earth’s crust, or lithosphere, is broken into several segments or plates.  They also believe that these crustal plates move, or drift, on hot molten material that is beneath the crust. 

Thick crust or thin?

The thin, outer layer of the Earth’s surface can be divided into two kinds of crust: continental crust and ocean crust.

As you might expect, continental crust is found under the landforms or continents.  This crust is usually about 35 kilometers (22 miles) thick; however, it may be up to 70 kilometers (43 miles) thick in mountainous areas.

This type of crust may be subdivided into two layers.  The top part of  continental crust is called the sial.  It is made of granite-like rock.  Granite is a light-colored igneous rock.

The second layer of crust is called the sima, and it is made of basalt-like rock.  Basalt is a dark colored igneous rock that is denser than granite.

The sima layer of the continental crust extends beyond the edges of the continents, under  the oceans, and forms the oceanic crust.  The oceanic crust is much thinner than continental crust.  It averages only 5-7 kilometers (3-4 miles) thickOcean crust is only made of the sima.  As we learned earlier, new oceanic crust is continually being formed along the mid-ocean ridge.

The Mantle

It’s egg time again.  Remember, the egg shell is the crust, so let’s see what the next part of the egg represents.  Whenever we peal an egg, the shell is actually stuck to a very thin layer of plastic-like stuff.  Well, the Earth has a very thin boundary or layer between the crust and the mantle called the Mohorovicic Discontinuity or Moho for short.  We will call it the Moho.  The Moho is sort of like a fence between the crust and the mantle.

Speaking of the mantle, it is the second layer of the Earth and it is the biggest.  The mantle makes up 80 percent of the Earth’s total volume and 68% of it’s MASS!!!  It is definitely the heaviest part of the Earth. 

The mantle appears to be composed of silicon, oxygen, aluminum, iron, and magnesium.  There seem to be greater percentages of iron deeper into the mantle.

Scientists have developed a model of the mantle that designates the first 100 kilometers as very rigid rock.  From that point to a depth of 250 kilometers, they believe the mantle is close to its melting point.  The rock has reached a point of plasticity, but it has not changed from a solid form to a true liquid.  It is almost like thick syrup.  It does not actually melt because of the extreme pressure exerted on it by the materials above, such as the continents and oceans. 

As you go deeper into the mantle the temperature gets hotter.  At the top is averages about 870 Celsius and near the bottom it reaches 2200 Celsius. 


The center of the Earth is like the yoke of the egg with the exception that scientists actually believe that the core is really two layers, an outer core and an inner core.

The outer core is believed to be 2,250 kilometers deep.  The temperature goes from 2,200 Celsius on the upper edge and gets hotter the deeper you go,  up to 5,000 Celsius.  Scientists believe the outer core is liquid, very dense melted iron and nickel.  This dense iron core is believed to help cause the Earth’s magnetic field.

The inner core is believed to be 1,300 kilometers in depth.  Temperatures are believed to be about 5,000 Celsius throughout.  They also believe that the inner core is solid iron and nickel.  It is solid because of the incredible pressure on it.  The pressure may be as much as 2,000,000 times stronger than the pressure on the surface. 



The Earth’s Structure Notes

1.  two ways scientists study the inside of the Earth are drilling and seismographic information
2. seismographic information comes from studying earthquakes.
3.  four layers of the Earth are the crust, mantle, outer core, and inner core
4.  depth of Earth from crust to inner core is 6,370 km or about 4,000 miles.
5. world’s deepest drill site is 12 kilometers or about 7 miles
6. models are made by scientists about the structure of the Earth
7. lithosphere is what scientists call the crust, the crunchy part.
8.  crust-the thin outer layer of the Earth, is composed of two layers. The top layer is granite and the bottom layer is basalt and is denser than the granite. 
9. ocean crust is thinner and is made of ONLY basalt, is about 5-7 km thick.
10. continental crust averages about 35 km thick.  Is thickest under mountains
11. temperature at top is 20C and about 870 C at the bottom.
12.  Mohorovicic Discontinuity is called the Moho.
13.  Moho is the boundary between the crust and the mantle.
14.  Adrija Mohorovicic was a Yugoslavian scientist who used seismic data to discover this area between the crust and the mantle.
15.  mantle is the second layer of the Earth.  Is about 2900 km thick.  Temperature is 870 C at the top and about 2200 C at the bottom.  The mantle is composed of same materials as the crust only has more iron.  The deeper you go the more iron content.  The deeper you go, the denser the mantle becomes.
16. the mantle also contains molten rock, called magma, which sometimes makes it to the surface as lava in volcanoes.  Earth’s heat and pressure causes the magma to flow.
17.. Earth’s core-is two parts, outer core and the inner core.  Both cores are made of iron and nickel.  Outer core is liquid and the inner core is solid. 

18. Outer core is about 2250 km thick and temperature goes from 2200 C to 5000 C.  Is liquid!!!
19. liquid outer core causes the Earth’s magnetic field.

20.  Inner core is about 1300 km thick, temperature is 5000 C.  Incredible pressure inside the Earth causes the inner core to be solid.  Pressure is 2 million times greater than here on the surface.

21. density of the Earth increases as we go deeper and deeper and temperature gets hotter.  The amount of iron also increases as we get deeper.