Amber L. Madden-Nadeau
A volcano is defined as “a naturally occurring vent or fissure at the Earth’s surface through which erupt molten, solid, and gaseous materials”. Volcanoes come in many different shapes and sizes and have many different eruptive styles. They are most often found on the margins of the tectonic plates that comprise the Earth’s crust (the Earth’s structure is shown in Figure 1).
There are two types of plate boundary associated with volcanism. The first is a divergent or constructive margin, where two plates move apart and the drop in pressure this causes allows the mantle to melt. This melt is less dense than the surrounding solid mantle, so buoyantly rises and erupts onto the surface. An example of where we would find this sort of volcanism is at a Mid-Oceanic Ridge – such as the Mid-Atlantic Ridge – which is a continuous underwater mountain system formed at a divergent plate margin. The second type of plate margin that gives rise to volcanism is a destructive plate boundary. Here, two plates move towards each other, with the denser plate sinking or subducting beneath the less-dense plate. When this happens, the subducting plate releases its volatiles, such as water, into the surrounding mantle. This lowers the melting point of the mantle, allowing it to melt, rise buoyantly and, again, erupt onto the surface. An example of a volcano at a subduction zone would be Krakatau in Indonesia. You can see examples of these plate margins in Figure 2.
Volcanoes are sometimes found in the middle of plates, and this is termed intraplate or hot spot volcanism. This volcanism occurs when a hot column of mantle, termed a mantle plume, originates at the boundary between the Earth’s core and the mantle. As this column of mantle is hotter, it allows part of the mantle to melt, rising buoyantly, erupting onto the surface. Hot spot volcanism has resulted in the trail of Hawaiian Islands, as the plates have continued to move over the stationary mantle plume. These are shown in Figure 3.
Large, explosive eruptions often occur at subduction zones, whereas we associate the gentler, effusive eruptions with Mid-Oceanic Ridges or hotspot volcanism, such as that observed in Hawaii. But why do we observe differences in eruptive style?
The reason for this comes down to two basic properties of the magma itself. The first is the amount of volatiles it contains. This is because as the pressure builds in a magma chamber, these volatile gases expand and fragment the magma, which is the predominant cause of explosive volcanism. The second property is how much silica is in the magma. The greater the amount of silica, the higher the viscosity or less “runny” the magma is. The higher the viscosity, the more the magma will resist this fragmentation and the escape of volatiles. This means it requires the pressure to be a lot higher before the magma is fragmented, causing the eruption to be more explosive.
We see magmas with higher volatile contents generally at subduction zones, since volatiles are released by the down-going plate, as discussed earlier. Subduction-generated magmas also tend to have a higher silica content. As they travel through the crust to the surface, some of the crust melts and is incorporated into the magma. The longer travel time to the surface also allows the magma to stall in the crust and evolve to higher silica compositions as it crystallises.
Well, there you have it! You are now officially grounded in the different types of volcanism, the plate boundaries they are associated with and the basic controls of eruption style. Lecture over; class dismissed.
Cashman, K.V. & Sparks, R.S.J. 2013. How volcanoes work: A 25 year perspective. Geological Society of America Bulletin, 125, 664-690.
Perfit, M.R. & Davidson, J.P. 2000. Plate tectonics and volcanism. In: The Enclyclopedia of Volcanoes.
“I am a volcanologist and igneous petrologist in the first year of my DPhil in the Department of Earth Sciences studying controls on eruptive style using Krakatau Volcano, Indonesia as a case study.”