In this topic you will learn about how changes in Earth's crust affects the lives of many people.

Earth's crust is constantly moving. Sometimes it moves quickly enough to be seen and be felt. People who have been through an earthquake tell of seeing the ground move up and down. Earthquakes are related to cracks in the crust called faults. These faults may have formed from earlier earthquakes. Sometimes they form when an earthquake happens. During an earthquake, the crust on either side, or on both sides, of a fault is in motion.

Geologists are scientists who study Earth. They place sensitive devices all along faults. Scientists hope that records of tiny movements can be used to predict an earthquake.

The crust is Earth's hard surface. Under the crust is the mantle, Earth's thickest layer. The rock material here is solid. However, it can flow like a liquid—as putty can flow when you squeeze it between your hands. The rock material in the mantle is in motion. It rises and pushes against the bottom of the crust.

Tension and shear can build up the crust. Mountains can be formed as the crust pulls apart. Hot molten rock deep below the Earth's surface, called magma, rises upward. If magma reaches the surface, it may flow out as lava. Lava flows out or is hurled out when a volcano erupts. A volcano can build a new island.

While movements of the crust are building up Earth's surface, other forces are at work breaking it down. These forces are weathering and erosion. Weathering is the breaking down of materials of Earth's crust into smaller pieces. Erosion is the picking up and carrying away of the pieces. Weathering happens when the crust is exposed to water, air, and changes in temperature. Erosion is the carrying away of pieces of weathered rock by gravity, water, wind, and ice. The greatest agent of erosion is water. When water stops moving, it also stops carrying along bits and pieces of rock. The pieces of rock are dropped to the bottom of the stream, lake, or ocean. The dropping off of bits of eroded rock is called deposition. The Moon is Earth's nearest neighbor in space. The only weathering and erosion on the Moon are due to the impact of rocks from space hitting the Moon's surface. These rocks from space that strike the surface are meteorites.