A plate boundary is a location where two tectonic plates meet. There are three different types of tectonic plate boundaries, which are defined by the relative motion of each plate. The three types of plate boundaries are divergent, convergent, and transform. These plate boundaries can occur both within continents and under oceans.
Tectonic plates are huge pieces of Earth’s lithosphere, or outer shell, that shift on top of the Earth’s mantle, or the rocky inner layer of Earth. Plates can be made up of oceanic lithosphere or continental lithosphere. In total, Earth’s lithosphere is composed of seven major tectonic plates, and additional minor plates. Each plate shifts somewhere between 0 and 10 mm annually. The boundaries where plates meet are sites of transformation. Depending on the type of boundary, the interactions of tectonic plates can cause earthquakes, volcanic activity, or mountain-building.
When two tectonic plates slowly move away from each other it creates a divergent boundary. As this happens, magma rises from the Earth’s mantle into the widening gap between the plates. When it reaches the surface it forms solid rock, creating new crust material. Divergent plate boundaries often occur under the ocean. As the magma cools underwater it forms basalt, which makes up the majority of the oceanic crust. Examples of divergent faults include the Mid-Atlantic Ridge, a "crack" in the Atlantic Ocean where molten rock rises upward, and the boundary between the African and Arabian Plates in the Red Sea.
At a convergent boundary two plates move towards each other. As the plates slowly collide, one or both of them buckle upwards to create ripples in the surface of the Earth’s crust. This is the process by which mountain ranges are formed. Occasionally one plate forces the other down to form an underwater sea trench. Strong earthquakes and the volcano formation are common as plates clash at a convergent boundary. Examples of convergent boundaries include the boundary between the Eurasian and Indian plates in the Himalayan Mountains and the boundary between the Nacza and American plates in South America.
The third type of plate boundary is called a transform boundary. Here, two plates slide past one another. As this happens, rocks and sediment located on the boundary are crushed between the plates, which results in an undersea canyon or a linear fault valley. As with the other two types of boundaries, earthquakes are common as the two plates grind past each other. Any natural or human-made structures that stretch over a transform boundary are slowly moved in two different directions along with the plates. This is the only plate boundary where no magma has a chance to rise to the surface of the Earth’s crust, meaning that no new earth is formed along transform boundaries. An example of a transform boundary is the border between the Pacific and Australian Plates, which is in New Zealand.
These three types of plate boundaries were identified as part of the theory of plate tectonics, which was advanced and developed in the 1950s through the 1970s. Before plate tectonics was identified, there was no unified theory of geography, and scientists were unable to account for or describe the movement of tectonic plates.