#shield volcano

Jagged

Eroded volcanic landscape. Approximately 15 million years ago this used to be a shield volcano (similar in structure to Hawai’i), but since its volcanism stopped the erosion has exposed the sides of the volcano. The most pronounced features are an ancient magma chambers. These would either feed magma to eruptions occurring on the surface or pulses of magma that did not make it to the top and have slowly crystallized at depth.

Volcanoes: How life and death hang in the balance

I’m sure some of you will have seen videos of Hawaiian Islanders filming their homes being engulfed by lava and bursting into flames from only a short distance away (http://bit.ly/1A2wNcw). More of you will know the tale of Pompeii and Herculaneum; two roman towns destroyed by lahars and pyroclastic flows in 79AD due to the eruption of Mt. Vesuvius. The question is: what makes the difference between fun home video and instant death? The answer lies in the amount of silica within the magma. Silica is important because it has a tetravalent structure, where each of the silica atoms is bonded to another 4 silica by covalent bonds. These bonds are very strong, so require a very high temperature or pressure to break them. When there is a high percentage of silica within magma it becomes very viscous as the tetravalent bonds prevent it from flowing easily. This is called a felsic or acidic magma and is a magma that is relatively enriched in silicon, oxygen, potassium and aluminium.

As magma rises towards the surface, gases that had been previously held in solution begin to exsolve (bubble) out. At depth, gases are held in solution with the rest of the magma but as the molten rock rises and pressure decreases the gas starts to become a separate phase. As these volatiles (gasses) are less dense than the surrounding rock, they try to rise to the surface quicker, separating from the magma. However, when there is a high silica content the molten rock is very viscous and some of the gas becomes trapped. If the top of the volcano is blocked by a plug (debris from the last eruption preventing lava from flowing out of the vent), then these trapped gases can generate an incredible amount of pressure as more and more gas laden magma becomes trapped within the volcano.

Eventually the pressure exceeds the confining strength of the plug and the volcano erupts violently. This is the reason Mount St. Helen blew off its side, the weakest part of the rock fails first and then all the stored magma and gas explodes outwards. This explains why the eruption at Vesuvius was so violent, and why stratovolcanoes tend to have a cone like geometry. The viscous lava can’t flow very far before it cools meaning the volcanoes build up steep sides with layers of lava and ash building up with each eruption.

So why are Hawaiian volcanoes shield shaped and produce a much less impressive ‘BOOM!’? Well these volcanoes contain a lot less silica within their magma, meaning the molten rock is a lot less sticky. These are known as mafic or basic magmas, molten rock that is relatively enriched in magnesium, iron, calcium and sodium. Not only does this mean that the lava can flow further before it cools (producing a low angle shield) but it also doesn’t trap as many volatiles so eruptions are far less explosive. Therefore the biggest risk to most inhabitants of the Hawaiian Islands are lava flows, and while these move quicker than their silica rich counterparts, they would still be easy to out powerwalk if necessary.

However, as with most geological phenomenon there is no hard and fast rule as to whether a volcano will erupt explosively or not. For example, Vesuvius has been known to erupt runnier lava, producing lava fountains and lakes rather than pyroclastic flows. While this is relatively rare for a volcano it just goes to show how unpredictable they can be and how a small change in chemistry can make a big difference!

So next time you are planning a holiday or thinking of moving house just consider whether that volcano looming over you is a sinister steep sided cone or a delicately dipping dome. It might make all the difference if it one day decides to wake up!

FOTO MOKU - Sept. 30, 2018 - An offer left at the edge of the Halemaumau crater on Hawaii Island.   Mauna Loa watches from behind.