Not all volcanoes are alike. Volcanoes arise from different geological processes. For example, the volcanoes in SF Fig. 7.8 may look similar in shape, but they have very different properties. Mount St. Helens and Mount Fuji (SF Fig. 7.8 A and C) are both located on subduction zones. They are known as composite volcanoes because the lava produced is a composite, or mix, of continental and oceanic crust. Composite volcanoes rarely erupt, but when they do, they erupt explosively.

Mauna Kea (SF Fig. 7.8 B) is over a hot spot, which is a spot on the earth’s surface where a hot magma source is very shallow. The Hawaiian Islands, which are located on the middle of the Pacific plate, formed as the plate moved over this hotspot. Mauna Kea is a dormant volcano. Kīlauea, another volcano on the island of Hawai‘i, is currently active. It is one of the longest continuously erupting volcanoes and usually produces a slow, gentle flow of lava. Iceland, which has a number of volcanoes (e.g., SF Fig. 7.8 D), is a very unusual volcanic landmass because it is formed where the mid-ocean rift actually rises above the surface of the water. Eyjafjallajökull and Mt. Saint Helens are composite volcanoes, which are typically steep and produce explosive eruptions. In SF Fig 7.8, you see an eruption of Eyjafjallajökull (SF Fig 7.8D) and the aftermath of an explosive eruption on Mt. Saint Helens (SF Fig 7.8 A).