Scientists have only understood the composition of the Earth for about the past 100 years. It wasn’t until the modern seismograph was invented that they had the tools to explore the different layers of the Earth. (A seismograph is a device used by scientists to study earthquakes.)
The outermost layer of the Earth is called the crust. Scientists have known all about the crust for much longer than the other layers because it is where humans live. The crust is made up of rocks covered by soil. The rocks of the crust are made up of mostly oxygen and silicon. There are other elements in the rocks as well including aluminum, iron, and calcium. The crust is by far the thinnest layer of the Earth. Under the oceans, it is only five to ten kilometers deep. Continental crust is between thirty and seventy kilometers deep.
The crust is not one complete layer like the skin of an apple. Instead, the crust is made up of about twelve crustal plates. These plates float on the liquid mantle below and are constantly moving.
Millions of years ago, there was one supercontinent named Pangaea. Over time, the crustal plates moved apart to form the seven continents known today. This isn’t the first time this change has happened. Scientists estimate there have been at least three supercontinents formed and broken apart in the Earth’s history. Another supercontinent will form someday far in the future. Currently, North America and Asia are moving closer together at a rate of about 2.5 centimeters a year.
The mantle is the layer below the crust. It is made up of incredibly hot liquid magma. Magma is melted rocks. Magma reaches the surface as lava erupting out of volcanoes.
The mantle is the largest layer of the Earth. It is almost three thousand kilometers deep and makes up most of the mass of the Earth. Humans have never been to the mantle, but a group of scientists in the middle of the Pacific Ocean are trying to change that. They are planning on drilling a hole into the thinnest part of the crust under the ocean. This complicated endeavor will involve six-mile-long drilling pipes and drill bits that have to be changed every fifty hours. It is estimated that the project could reach the mantle in the early 2020s.
The Outer Core
Below the mantle is the outer core. This layer is about 2,250 kilometers thick and is filled with liquid iron and nickel. The iron and nickel are liquid because they have melted in the intense heat of the outer core.
The outer core was discovered by seismologist Inge Lehmann from Denmark. A seismologist is a person who studies earthquakes. By looking at the way earthquake waves moved through the Earth, Lehmann was able to determine that the outer core was liquid. Seismic waves move much more slowly through liquids than solid rocks. They are also reflected back to the source of the wave instead of moving through in a straight line as they do through solid materials.
The Inner Core
Inside the outer core is the inner core. Around 1,200 kilometers thick, the inner core is even hotter than the outer core, but the iron and nickel within it are solid because of the high pressure exerted on them by the outer layers of the Earth.
Lehmann was able to predict this based on the fast, straight seismic waves she observed passing through the inner core. How did Lehmann track the speed of seismic waves in the center of the Earth without being there? By measuring the amount of time it took waves to move from one seismic station to another, she could determine how fast the waves were traveling. Comparing results from stations all around the world created a picture of what the center of the Earth looked like.
Scientists continue to study and debate the different layers of the Earth today. The more technology improves, the more information scientists can gather about the layers of the Earth. Someday, scientists may even be able to travel to the outer and inner core.