Blow or Flow?

Volcanoes are immense, powerful forces of nature.  There are many myths and legends about their fiery nature. Cataclysmic eruptions can lead to climate change and may have even been a factor in the extinction of the dinosaurs. However, not all eruptions are so violent. The question is, what causes a volcano to have a “gentle” lava flow or an explosive eruption?
There are two major types of volcanoes: shield volcanoes and stratovolcanoes. They are different in appearance and behaviour. This is due to the chemistry of their magma, which in turn is largely down to their tectonic setting.
WASP plate tectonics poster
Shield volcanoes, so named as they look like a warrior’s shield (from the side), occur most commonly at hot spots and divergent plate boundaries. The magma here is mainly derived from the mantle and has a mafic composition (higher in magnesium and iron and most importantly lower in silica). This gives it low viscosity, so it is relatively runny.
Mt Fuji – a classic stratovolcano with steep sides. (Wikipedia Commons, accessed 20/04/2020), accessed 20/4/2020)
Stratovolcanoes are steep sided and probably more what you imagine as your classic “volcano” shape. They are most commonly found at subduction zones.  Here the magma is typically formed by crustal material melting, which gives it a more siliceous composition (higher silica content). Its chemistry causes it to have high viscosity.

The link between chemistry and shape
Imagine (or even try) pouring a spoon of oil onto a plate, then pour another on top, and then another. The oil has low viscosity, so it spreads out over a large distance. This is similar to what happens with lava from a shield volcano. Lava that erupts moves down the flanks of the volcano and away, causing it to have gentle slopes and a wide base. Contrasting to this, a stratovolcano has higher viscosity lava. In this instance imagine (or try) taking a spoon of margarine and then putting another on top, and then another. They would pile up creating a steep sided mound. This is what happens in a stratovolcano.  The lava does not flow far thus creating a steep sided volcano with a narrow base (ash and debris also typically pile up on the sides, but more on that later).
Try experimenting with different viscosity liquids to see how the shape of a “volcano” changes.

The link between chemistry and explosivity
The lower viscosity magma in a shield volcano allows gases to escape more easily (analogy: use a straw to blow through some tomato sauce). This means shield volcanoes generally have effusive eruptions. Whereas the higher viscosity magma in a stratovolcano traps gas more readily (analogy: use a straw to try and blow air through plasticine). When the gas can’t escape pressure builds up and builds up until….
It Blows!  The higher the gas content, the more explosive the eruption will be. 

Or take a more in depth look with our Volcanic Hazards STEM Project.

Although people typically think of lava when contemplating volcanic hazards, lava flows are generally slower than walking pace.  A highly explosive stratovolcano on the other hand, is much more dangerous and can have global consequences. Luckily, volcanoes are well monitored, and they generally give warning that they are going to erupt, making it easier for volcanologists to prepare local communities and alert authorities.
Here are some more experiments you can do just about anywhere, to investigate lava viscosity: