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. A model is used to study parts of science where the real object cannot be studied. They use these models of the Earth to study how it works. 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 three layers; the crust, the mantle, and the 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 (the abyssal plain).
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.
Another name for the crust is 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, the lithosphere, 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. The upper part of continental crust is made up of granite and the lower part is basalt. Basalt is denser and stronger than granite. As we go deeper and deeper into the Earth, the temperature and the density increase. The temperature of the crust goes from 20 degrees Celsius at the surface to 870 degrees Celsius at the bottom.
Oceanic crust is much thinner than continental crust. It averages only 5-7 kilometers (3-4 miles) thick. Ocean crust is only made of basalt which is much denser and stronger than continental crust. Strange as it may seem, new oceanic crust is continually being formed deep in the ocean (at the mid-ocean ridge).
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. The mantle has a lot more iron than the crust and the amount of iron increases as you go deeper into the mantle.
The top part of the mantle is soft and gooey and is called the asthenosphere. The lithosphere sort of slides on top of the asthenosphere.
As you go deeper into the mantle the temperature gets hotter and denser. At the top, the temperature averages about 870 Celsius and near the bottom it reaches 2200 Celsius.
If you look at the picture below, you will see that it shows an upper mantle and a lower mantle. Some books refer to the asthenosphere as the upper mantle.
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.