In chapter 7, we introduced the inner workings of the earth: the composition of the earth, plate tectonics and the rock cycle. In this chapter we are going to expand upon those topics and discuss in detail the mechanisms which shape the lithosphere.
The location of volcanoes are concentrated around the Pacific rim, the east African rift, and the Atlantic ridge – especially on Iceland. There are also some stray volcanoes in the middle of the Pacific and Atlantic oceans. The majority of the earth’s surface is devoid of volcanic activity. Why do volcanoes cluster in these regions?
Volcanism is directly related to the theory of plate tectonics. Volcanoes are found only at particular types of plate boundaries and rarely in the middle of ocean plates. In the following discussion we will summarize the three circumstances which explain the formation of nearly every volcano on earth. First, though, let’s go over the two types of volcanoes.
In an earlier chapter, we described two types of earth crust: The thick, relatively buoyant and light weight silica rich continental crust, and the heavy, thin ocean crust made of iron and magnesium-silicate rocks. Two types of magma correspond to these two types of crust (liquid rock is called magma when it is below the surface of the earth – above the surface it is called lava). Volcanoes on the ocean floor are made of the same material that makes up the ocean floor – iron or magnesium silicate minerals. This magma is very runny, or not viscous. It forms broad shield volcanoes such as the island of Hawaii which erupts large quantities of lava. On the other hand, volcanoes that form from silica rich magma tend to be steep and tall, such as Mount St. Helens in Washington. Silica rich magma is also very thick, or viscous, and tends to cause very violent, ash-rich eruptions without a lot of lava.
Volcanism Where Plates Collide
There are two types of plate collisions: Ocean-continent, and ocean-ocean. The plate that is subducted melts partway down (partial melting), and the melted material rises up through the crust and forms a volcano. The melted material contains bits of the sea floor that were pulled down with it and water, so it is more silica rich and buoyant than pure ocean crust. As the magma rises it melts additional crust around it, making it even more silica rich – especially when it rises through continental crust. As discussed above, this silica rich magma is much more viscous than its mafic (magma that is rich in iron and magnesium, and has less than 50% silica) cousin. The rising magma often forms plugs in the neck of the volcano, leading to pressure buildup within the magma chamber, and eventually a highly explosive eruption. Oftentimes, the magma does not get out of the chamber, and ends up cooling slowly just below the surface, forming granite or some other type of intrusive igneous rock.
Where plates collide, two types of volcanoes are formed. When an ocean plate collides with a continental plate, volcanic mountains are formed on the continental plate. Examples of active volcanic mountains today are the Cascade Range in Washington, and the Andes in South America. Sometimes, following a plug of silica-rich magma in a volcano is a volume of more runny mafic magma from deep in the earth. When this mafic magma is erupted, it spreads rapidly, covering large areas. The flood basalts of the Colombian Plateau in Oregon-Washington covered the region 12 million years ago.
Where two ocean plates collide, volcanoes are formed in an ‘island arc’. Island arcs are characterized by their linear structure, mirroring the plate boundary. Japan, the Philippines, and New Zealand are examples of island arcs.
Volcanism Where Plates Diverge
Earlier, we noted that there was a series of volcanoes in eastern Africa, and we also noted several near the mid-Atlantic ridge, and on the island of Iceland. These are examples of volcanoes which form where plates diverge. The driving force behind plates moving apart is the upwelling of hot magma in the mantle which spreads laterally, driving plate motion. Sometimes, some of that hot magma melts the crust that overlies it, causing the crust to thin, and volcanoes form.
The east African rift is a zone of crustal thinning and movement, where the African continent is in the process of splitting in two. This is an example of volcanism at the formation of a plate boundary. Here, the continental crust is much more thin that normal because of an upwelling of hot magma from deep in the earth’s crust. This is the beginning of a new plate boundary. The volcanoes of the east African rift are a combination of the silica rich, explosive type, and the silica-poor mafic type.
The island of Iceland sits atop the mid-Atlantic ridge, and is an example of a series of volcanoes which form where ocean plates diverge. The volcanoes that comprise Iceland are all mafic, so they have formed an island with shallowly sloping mountains or shield volcanoes. Shield volcanoes are large, gently sloping summits formed from successive eruptions of mafic lava.
Hot Spot Volcanoes in the Middle of Ocean Plates
Sometimes, for reasons that are still not well understood, an upwelling of hot magma forms a ‘hot spot’ in the middle of an ocean plate. The hot spot creates a shield volcano. Often, the hot spot will remain in the same place, while plate moves over it, creating a series of islands. One example of this is the Hawaiian island chain. The oldest island, Niihau is located northwest of the youngest island, Hawaii. The island chain documents the motion of the Pacific plate as it passed over the hotspot.