The Crust: Earth’s Fascinating Layers, Part 1

Have you ever wondered what is beneath your feet? Not just the grass, but the miles and miles of soil, sediment, and rock located under the surface of the earth? If you stand in your backyard and dig a hole, what would you see? Plant roots? Soil? Yes, and much more too!

Below the plants, soils, and sediments, you may find rock depending on where you live. The soil and rock may be wet or dry but what you will see in your backyard is only a little bit of what is underground.

Geology You Cannot See

According to geophysicist Lars Stixrude of the Carnegie Institution of Washington, the deepest rock sample was collected from about nine miles below the land surface, but the center of the Earth is almost 3,975 away from the land surface.

So, how do we know what is underground? Geologists perform experiments with seismic waves to determine the materials that make up the layers of our Earth. Seismic waves are waves of energy that travel through soil and rock. Geologists use fancy equipment to measure how fast and how far the waves travel, which tells them what types of materials are underground.

Layer Number 1 – The Earths Crust

Now, 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?

Dig through the surface grass and soil and you will find a layer of sediment over the 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 a variety of rock types that 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 (mountains and valleys) has minerals that are lighter in color and weight.

The Earth’s crust is made up of igneous, sedimentary, and metamorphic rocks. Most of the rocks in the crust are igneous rocks that formed from the cooling of magma inside and outside of volcanoes. Granite and basalt are two of the most common igneous rocks on Earth. Sedimentary rocks, formed in the oceans, lakes, and deserts when sediments like clay, silt, and sand, accumulate and are cemented together are the second type of rocks on Earth. Sandstone and shale are common sedimentary rocks. Another type of rock in the crust is metamorphic. These are rocks that have undergone a change from one rock type to another when they were exposed to high heat and/or high pressure below the land surface. Common metamorphic rocks are slate, which was formerly sedimentary shale, marble which was formerly sedimentary limestone, and gneiss which was formerly igneous granite.

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.

Tectonic Plates

The Earth’s crust consists of a series of tectonic 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 (the second layer) of the Earth. 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. Think of the plates like a cracked eggshell floating on top of the egg whites (the asthenosphere).

Back to our trip underground. Are you still digging?

Uh Oh! You just ran into the Moho! You have reached the bottom of the crustal plates. The Mohorovicic discontinuity (geologists call it 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, which is called the mantle. This less than two-mile thick zone was discovered and named after a geophysicist from Croatia named Andrija Mohorovicic in 1909.

Click the link below to read more about the Mantle in part 2 of our series!

Up next: Part 2 of our Earth’s Layers series – 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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