The Earth is made up of layers, like an onion. The layers are: the crust, the mantle, the outer core, and the inner core. The mantle is divided into two sections: the lithosphere, a plastic layer that also contains the crust, and the asthenosphere, where convection currents occur. Convection currents are caused by temperature differences between hot lower magma and cooler surface magma. Colder magma is pushed to the hot center by hot magma rising and taking its place, due to minute density differences caused by temperature differential. The swirling convection currents can also cause the tectonic plates that make up the crust to shift as they ride on the mantle.

A German named Alfred Wegener founded the idea of Continental Drift. His idea was that the continents were all a single 'supercontinent' 300 million years ago. He based his theory on a plant whose seeds were too large to be spread easily to the continents he found it on, a fossil of a freshwater reptile found in both South America and Africa, the fact that the continents seem to fit together like jigsaw puzzle pieces, and evidence of tropical plants in Greenland, and glacial deposits near the equator. However, his theory was ignored by most because he although he could prove that the continents had moved, he couldn't figure out how they had moved.

Harry Hess first proposed the idea of Sea-floor Spreading in 1960. New sonar had discovered that the floor of the ocean was not flat, like many thought, but ridged. with mountains and valleys. Hess deduced that this was due to the sea floor spreading, from weak points where magma seeped from the sea floor, creating new sea floor and causing formations like seamounts, or underwater mountains, and other things. A piece of evidence that supported this was that rock samples taken from areas the same distance away from opposite sides of the Mid-Ocean Ridge, a volcanic underwater mountain range stretching through the middle of the Atlantic ocean, were the same age. Additionally, samples taken from opposite sides of the ocean, equal distances from the Mid-Ocean Ridge, were still the same age, but older than the ones from near the Ridge. Magnetic stripes of iron were also locked in place in the rock by iron in the magma that created the new sea floor. The iron stripes' locations were always mirrored on the other side of the Mid-Ocean Ridge, proving that they had formed at the same times and then had been pushed outward. The volcanic Mid-Ocean Ridge, the ages of rocks, and the magnetic iron strips proved that the sea floor was indeed spreading. This theory was important because it proved that Alfred Wegener's theory of Continental Drift actually worked.

The modern Theory of Plate Tectonics combines Continental Drift and Seafloor Spreading. It states that the lithosphere is divided into tectonic plates, which move around because of convection currents in the mantle. Between these plates, sometimes magma from the mantle comes up, forming new rock, and sometimes plates collide and go under each other and are melted in the mantle. In this way, the continents can move and change, but the size of the Earth stays the same.

There are three types of stress that tectonic plates are exposed to: Compression, tension, and shear. Compression happens when two plates push into each other, tension happens when two plates move away from each other, and shear happens when two plates move in opposite directions, grinding against each other's edges.