Plate tectonics is the fundamental theory of Earth Science. It explains why mountains form where they do, why certain areas are more prone to earthquakes than others, and why the continents are constantly shifting. Plate tectonics is the theory that the Earth's lithosphere, which is the rigid outermost layer of the planet, is divided into several tectonic plates that are in constant motion. The plates move around the Earth's surface, and interact with each other in various ways. When two plates collide, one plate may be forced beneath the other in a process called subduction. This can lead to the formation of volcanoes, mountain ranges, and earthquakes. When two plates move away from each other, new oceanic crust is created in the gap between them. This process is called seafloor spreading. Plate tectonics is a complex and dynamic theory that is constantly being refined as scientists learn more about the Earth's interior.