Volcano

Most of the volcanoes on Earth are located along the boundaries between lithospheric plates, which can be convergent (subduction zones) or divergent (mid ocean ridges). The chain of volcanoes along the Pacific Rim, often referred to as the Ring of Fire, is an example of subduction zone volcanism. Iceland, in contrast, is a volcanic island straddling the Mid-Atlantic Ridge (a divergent plate boundary). Although they are not as numerous as plate boundary volcanoes, intraplate volcanoes can occur where plates pass over mantle hot spots or along continental rift zones where plates are being pulled apart. The Hawaiian Islands, for example, were formed as the Pacific Plate slowly passed over a magma-generating hot spot within the mantle.

Oceanic ridges are chains of volcanoes located along the boundary between two diverging oceanic plates. New oceanic crust is formed along the ridges as two oceanic plates move apart. Where the rate of plate formation is rapid, older crust is quickly pushed out of the way and only small volcanic vents form. Where the spreading rate is slower, volcanic eruptions may form large volcanoes.

Subduction zones are areas in which oceanic plates are overridden by continental plates, forcing the heavier oceanic crust deep enough to be melted and recycled as second generation magma. Because the size of Earth is constant, the amount of oceanic crust consumed by subduction zones must be approximately equal the amount produced along mid-ocean ridges. The volcanoes of the western coast of North and South America, Indonesia, the Philippines, Japan, Kamchatka, and Alaska all sit atop subduction zones and collectively comprise the Ring of Fire.

Some volcanoes form above hot spots far from the edges of tectonic plates, and are known as intraplate volcanoes. A hot spot is an upwelling of magma from far beneath the Earth's crust, caused by a disturbance at the boundary between the solid mantle and the liquid outer core of Earth's interior. Hot spots are, compared to mid-ocean ridges or subduction zones, relatively small and isolated features. Well known intraplate hot spot volcanoes include the Hawaiian Islands, the Jemez Mountains and Capulin volcanic field of New Mexico, and the volcanoes that produced volcanic rocks throughout the Yellowstone region of Wyoming.

Hot spots provide information about the rates and directions of movement of tectonic plates over geologic time scales. When magma from a hot spot rises from the lower mantle and through the lithospheric plate, a volcano forms on the surface. The plate continues to move over the stationary hot spot and eventually produces a chain of volcanoes that increase in age away from the hot spot and show the direction of plate movement. The Hawaiian Islands mark the track of a hot spot, over which the Pacific Plate has moved in a northwesterly direction, toward Japan. Likewise, the volcanic rocks of the Snake River Plain in southern Idaho recorded the progress of the North American plate as it moved over the Yellowstone hot spots.