Because volcanoes are vents for magma, it makes sense that volcanoes would be formed above underground magma chambers. If you recall, magma is molten rock that has been heated because of high temperatures and pressures beneath the Earth’s crust. This pressure mostly occurs where the tectonic plates meet and subduct. Look at the map of tectonic plates in Figure 8.2.

Figure 8.2: Tectonic plates. Some of the largest tectonic plates meet along the coasts of Asia and North America. Compare this map to the map in Figure 8.3.

So, volcanic activity tends to occur along subduction plate boundaries, where one plate slides underneath another. The edges of the Pacific Plate make up a long subduction boundary. There are a huge number of earthquakes along these boundaries, because these are regions where the plates are colliding. For the same reason, the majority of the volcanic activity on the Earth also occurs along these convergent boundaries. This is called the Pacific Ring of Fire where over 75% of the world’s volcanoes are found. The Cascade Range of volcanoes runs through southwestern Canada and the Pacific Northwest of the United States. These volcanoes are the result of subduction of the Juan de Fuca plate beneath the North American plate. (Figure 8.3).

Figure 8.3: The Pacific Ring of Fire is where the majority of the volcanic activity on the Earth occurs.

Of course, this is not the only area where volcanoes occur. Beneath the ocean, there are also divergent boundaries, where tectonic plates are pulling away from each other. As the plates pull away from each other, they create a deep canyon or fissure on the sea floor through which molten rock escapes. Mid-ocean ridges, like the Mid-Atlantic ridge, form here as lava flows out through the fissure (Figure 8.4). Submarine volcanoes can also form on the ocean floor. At times these volcanoes can grow to create islands above the water’s surface. This explains how Surtsey, a small island of Iceland formed (Figure 8.5).

Figure 8.5: A volcanic eruption in Iceland.