The Earth’s Interior Layers – Part 1, The Crust

Have you ever wondered what is beneath your feet? I am not talking about the grass and soil, but the material located miles below the land surface. If you stand in your backyard and dig a hole, what would you see? You cannot dig too far, in fact, according to geophysicist Lars Stixrude of the Carnegie Institution of Washington, the deepest sample is from about nine miles below the land surface so geologists use seismic data to determine the materials that make up the layers of our Earth. But imagine for a minute that you could dig straight to the center of the Earth – what would you encounter along your journey of roughly 3,975 miles?

The Crust
Dig through the surface grass and soil and you will find a layer of sediment over rock. You are in the Earth’s crust. Look around. In this layer you may find top sediments like sand, silt and clay and then variety of rock types are composed of minerals rich in aluminum, calcium, iron, magnesium, oxygen, potassium, silicon, and sodium. On the surface, the crust has many geologic features such as mountains, valleys and ocean floors. The thinner ocean crust is made of heavy, dark minerals while the thicker continental crust has minerals that are lighter in color and weight.

The types of sediment and rock that you encounter as you move through the crust will depend on your location on Earth. If you are in the mountains you will likely encounter rock quickly as you dig while beach locations will have more accumulated sediment at the near surface. Keep digging, because the crust is up to 25 miles thick in some areas. While this may seem thick at first, you are now in the thinnest layer of the Earth. In the crust, the rocks are cool and brittle making them relatively easy to break, or fracture.

The Earth’s crust has a series of plates that slowly move around the planet. These plates are formed by the lithosphere, which consists of the crust and the upper layer of the mantle below. The plates are approximately 50 miles thick and glide around the earth on the asthenosphere, which is a fluid part of the mantle immediately below the lithosphere.

Uh Oh! You just ran into the Moho! You have reached the bottom of the crustal plates. The Mohorovičič discontinuity (or the Moho for short) is the transition zone at the base of the crust. In this small zone, the seismic waves speed up; this indicates that the rocks are becoming denser in the next layer called the mantle. This less than two mile thick zone was discovered and named after a geophysicist from Croatia named Andrija Mohorovičič in 1909.

Up next: The Mantle


Images Courtesy USGS
Crystal at the Center of the Earth; Ronald Cohen and Lars Stixrude; Carnegie Institute of Washington
Earth’s Interior; J. Louie; University of Nevada Reno; 1996
Structure of the Interior of the Earth; Lisa Gardiner; National Earth Science Teacher Association; 2010


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