In this topic you will learn about the movement of Earth's crust.

During an earthquake or volcanic eruption people can actually see and feel Earth's crust move. Earth's solid, rocky surface containing the continents and ocean floor is called the crust. The crust can also move so slowly that its motion cannot be felt.

Many kinds of rocks tend to form in flat, horizontal layers. This is called original horizontality. Many layers now look twisted or tilted. This is a sign that the crust is moving gradually.

In 1912, a scientist, Alfred Wegener, wrote a book in which he listed evidence that Earth's continents had once fitted together into a huge supercontinent. Wegener claimed that the coastlines of the continents match by shape and also by matching rock types, glacial deposits, ancient life forms, and past climates.

Wegener called the huge supercontinent Pangaea. He reasoned that about 200 million years ago, Pangaea split into pieces that are today's continents. The continents then "drifted" apart to their present locations. His ideas became known as continental drift.

Scientists have found mountains, valleys, ridges, and canyons under the oceans. Huge cracks split the tops of the ridges. Parts of the sea floor plunge downward into deep valleys, or trenches. To explain these features, scientists suggest a model called sea-floor spreading, which states that new crust is forming at ridges in the sea floor, spreading apart the crust on either side of the ridges.

The crust cracks where the sea floor is spreading apart. Just below the ridges of the sea floor there is hot, melted rock called magma. Magma flows up through the cracks, cools, and hardens into new rock. This process pushes older rock farther away.

Scientists now use a new model to explain how the continents and sea floor move. This new model is called plate tectonics and it describes Earth's crust as broken into pieces, or plates, that move. Along with the crust, each plate contains material from a layer below the crust, called the mantle. Plates move around on the lower portion of the mantle.

Where plates collide, the sliding of a denser ocean plate under another plate is called subduction. This process continues as the plate sinks down into the mantle.

The movement of the plates can explain movements of Earth's crust. Each continent is part of a plate. At their boundaries, or edges, the plates move away from each other, collide, or slide past each other. This explains why the sea floor is spreading.