12.2 Continued: Volcanoes

As we’ve learned plate interactions may result in the formation of a volcano or create an earthquake. We will look at both in some detail.   There are three types of volcanoes that we will examine: composite volcanoes, shield volcanoes, and rift eruptions.

Composite volcanoes:   When you hear the word “volcano” you get a mental image. That mental image is probably a composite volcano. Composite volcanoes are formed when layer after layer of ash and lava build up. The key to a composite volcano being formed is the thickness of magma: in composite volcanoes the magma is very thick.

The thick lava traps gas underneath it The thick lava traps gas underneath it. When the pressure of gas gets too much, there is an explosive volcanic eruption.   Composite volcanoes are usually found in clusters/rows called volcano belts. This is because they usually form along subduction zones as a result of convergent plates.

Note: In Canada, most of the composite volcanoes do not have that classic cone shape. This is due to erosion from glaciers.

Shield volcanoes:   The largest volcanoes on earth are shield volcanoes. The magma creating the shield volcano is much thinner than the magma that creates composite volcanoes. Because of this gas does not get trapped as much and pressure does not build up quite as much. This means less explosive eruptions.

The thinner magma also explains the shape of a shield volcano The thinner magma also explains the shape of a shield volcano. The magma can flow very freely and as a result spreads out much more quickly than the thicker magma found in composite volcanoes. They are called shield volcanoes because of this spread out shape (looks like a shield on the ground when looked at from above).

Shield volcanoes don’t usually form at plate boundaries Shield volcanoes don’t usually form at plate boundaries. Instead they tend to form near those areas of the lithosphere that are very thin (hotspots). That means a lot of them form in the ocean. A good example of this is the Hawaiian Island chain. There are a few examples of shield volcanoes in land, including the Anahim Belt in B.C., and more famously Yellowstone National Park in the United States.

Rift eruptions   Rift eruptions usually occur where there is plate divergence, like an ocean ridge or rift valley. A rift eruption is considered a subsection of shield volcanes. However, unlike composite or shield volcanoes that have a lot of gas trapped in a small area, rift eruptions have a lot of surface area and as a result are not as powerful. They do, however, tend to release enormous amounts of lava.

The Mid-Atlantic Ridge is a great example of an area you would find rift eruptions.