In some ways, Earth resembles a giant jigsaw puzzle. That is because its outer surface is composed of about 20 tectonic plates, enormous sections of Earth’s crust that roughly fit together and meet at places called plate boundaries.

Plate boundaries are important because they are often associated with earthquakes and volcanoes. When Earth’s tectonic plates grind past one another, enormous amounts of energy can be released in the form of earthquakes. Volcanoes are also often found near plate boundaries because molten rock from deep within Earth—called magma—can travel upward at these intersections between plates.

There are many different types of plate boundaries. For example, sections of Earth’s crust can come together and collide (a “convergent” plate boundary), spread apart (a “divergent” plate boundary), or slide past one another (a “transform” plate boundary). Each of these types of plate boundaries is associated with different geological features.

Typically, a convergent plate boundary—such as the one between the Indian Plate and the Eurasian Plate—forms towering mountain ranges, like the Himalaya, as Earth’s crust is crumpled and pushed upward. In some cases, however, a convergent plate boundary can result in one tectonic plate diving underneath another. This process, called “subduction,” involves an older, denser tectonic plate being forced deep into the planet underneath a younger, less-dense tectonic plate. When this process occurs in the ocean, an trench">ocean trench can form. These trenches are some of the deepest places in the ocean, and they are often the sites of strong earthquakes.

When subduction occurs, a chain of volcanoes often develops near the convergent plate boundary. One such chain of volcanoes can be found on the western coast of the United States, spanning across the states of California, Oregon, and Washington.

A divergent plate boundary often forms a mountain chain known as a ridge. This feature forms as magma escapes into the space between the spreading tectonic plates. One example of a ridge is the Mid-Atlantic Ridge, an undersea chain of mountains that formed as two pairs of tectonic plates spread apart: the North American Plate and the Eurasian Plate in the north, and the South American Plate and the African Plate in the south. Because ocean ridges are found underwater, often at great depths, they can be hard to study. In fact, scientists know more about the surfaces of some of the other planets in our solar system than they do about ocean ridges.

A transform plate boundary occurs when two plates slide past each other, horizontally. A well-known transform plate boundary is the San Andreas Fault, which is responsible for many of California’s earthquakes.

A single tectonic plate can have multiple types of plate boundaries with the other plates that surround it. For instance, the Pacific Plate, one of Earth’s largest tectonic plates, includes convergent, divergent, and transform plate boundaries.