Geophysics is the study of what's inside the Earth. Scientists study surface rocks, observe the planet's movements and analyze its magnetic fields, gravity and internal heat flow, all to learn more about the planet's interior. The Earth is made up of distinct structural or compositional layers -- the terms can be used interchangeably -- each of which has its own properties.
The crust is the very outer layer of Earth. When you walk on dirt or in a field, what you're walking on is the Earth's crust. The crust is mainly made up of alumino-silicates. The continental crust, which makes up dry land, is between 35 and 70 kilometers thick (22 to 44 miles), whereas the oceanic crust, which makes up the sea floor, is between 5 and 10 kilometers (3.1 and 6.2 miles) thick.
The mantle is divided into two sections, the upper and the lower mantle. The lower mantle is identified by having a higher density than the upper. Both sections together are 2,900 kilometers (1,800 miles) thick and make up 80 percent of the Earth's volume. It is predominantly composed of ferro-magnesium silicates. The mantle isn't molten, but it nears melting point, once you get 100 to 200 kilometers below the planet's surface. The mantle contains convective material, which circulates heat and may be what causes movement in the tectonic plates.
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The outer core is 2,300 kilometers (1,400 miles) thick. It's made up of molten, liquid metals -- iron and nickel -- and minor sulfur. Geophysicists believe that the outer core is responsible for controlling the Earth's magnetic field. Although scientists can't actually analyze material from the outer core firsthand, they can make the assumption that it's liquid based on the behavior of shear and compressional waves when they're passed through it.
The Inner Core
Like the outer core, this compositional layer of the Earth is made up of metal. Unlike the outer core, however, it's solid metal. The inner core is composed almost completely of iron, but around 10 percent of it is thought to be sulfur, nickel or oxygen. It's 1,200 kilometers (750 miles) thick, which makes the two parts of the core combined over half of the diameter of the Earth.