5.3: Layers of Earth

Layers of the Earth

In order to understand the details of plate tectonics, it is essential to first understand the layers of the earth. Firsthand information about what is below the surface is very limited; most of what we know is pieced together from hypothetical models, and analyzing seismic wave data and meteorite materials. The Earth can be divided into layers based on chemical composition and physical characteristics.

The core can be broken into two: the inner and outer core. The inner core is a solid iron-nickel alloy with a radius of about 760 mi. The inner core’s temperature is believed to be the same as the sun’s surface (approximately 5,430 °C). The transition between the inner and outer core is approximately 3,200 mi beneath the Earth’s surface.

The outer core is also iron and nickel, but it’s liquid. The liquid outer core flows, producing the Earth’s magnetic field.

The next layer is called the mantle, the thickest layer on the Earth. The mantle is a plastic solid, which can flow very slowly. The heat from the Earth’s core causes the mantle to convect, like water over a stove but much slower, and it is the mantle’s convection that is the driving force of plate tectonics.

The Earth’s surface layer is called the crust, and it makes up only 1% of Earth’s mass. The crust is subdivided into two components: oceanic and continental crust. Referring to the image below, note that the oceanic crust is only about 3 miles thick but is slightly denser than the continental crust. Most of this oceanic rock is called basalt, a dark, dense rock.

Continental crust is much thicker than oceanic crust (averages between 20 to 25 miles thick) but is slightly less dense than oceanic crust. The main type of rock on continents is called granite. So if these two types of crust collide, what do you think would happen to the oceanic crust? As a whole, notice that the crust is lighter than the mantle. It is sometimes said that the crust “floats” on the mantle like an iceberg in water, which is not too far from the truth and is called isostasy. Finally, the crust is the coldest, most rigid, and brittle layer with many folds and fractures.

Two additional layers in the Earth are very important to plate tectonics. The asthenosphere is located in the upper mantle at a depth between ~ 50 and 124 miles below the surface. It is a mostly solid but weak layer that flows like silly putty. Above the asthenosphere is the lithosphere. The lithosphere consists of the uppermost mantle and crust (both oceanic and continental). The lithosphere makes up the tectonic plates.

What moves the plates?

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