Tectonic plates are pieces of the earth’s crust that vary in thickness and composition. There are three types of plate boundaries: transform, divergent, and convergent, which cause geological phenomena such as earthquakes and mountains. Examples include the San Andreas Fault, Mid-Atlantic Ridge, and Himalayas.
Tectonic plates are large, disconnected pieces of the earth’s crust, floating on the underlying mantle, composed of soft magma. Tectonic plates vary greatly in thickness depending on whether they are oceanic (about 5 km or 3 mi) or continental (about 50 km or 30 mi). Continental crust is generally made up of lighter rocks, while oceanic crust is made up of heavier rocks. This means that when an oceanic tectonic plate collides with a continental plate, it will surely slide under, or be subducted by, the continental plate.
There are three main types of tectonic plate boundaries, described in terms of how the tectonic plates move relative to each other. These include transform boundaries, where plates rotate parallel to each other, divergent boundaries, where plates move apart, and convergent boundaries, where plates press into and sometimes under each other. These plate boundaries cause faults, rift valleys, mid-ocean ridges, trenches, mountains, island arcs, subduction zones, volcanoes, and other geological phenomena.
An example of a transformation boundary between tectonic plates is the San Andreas Fault in California. Here, the Pacific Plate is moving northwest of the North American Plate. Transformation boundaries are often accompanied by earthquakes, caused when stress builds up between the plates and is released suddenly, within minutes or seconds. A primary earthquake can be followed by several aftershocks, smaller earthquakes that continue to release crustal stresses after the primary earthquake.
An example of a divergent boundary is the Mid-Atlantic Ridge in the middle of the Atlantic Ocean. Here, the crust is cracking at a rate of about 5 cm (2 in) per year. It is replaced by magma that rises from beneath the crust and solidifies rapidly, creating a series of ridges that give the Mid-Atlantic Ridge its name. The island of Iceland is an example of a portion of the ridge that rises above the water.
An example of a convergent border is the Himalayas, which include the world’s tallest mountain, Mt. Everest. Here, the Indian plate is pushing northward into the Eurasian plate, suffering under it and causing it to be uplifted. Because of this, the Himalayas are actually growing about an inch each year.
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