This week, the Mount Agung volcano on the island of Bali in Indonesia erupted for the first time in more than 50 years. Residents are fleeing to neighbouring islands, tourists are scrambling to find flights home and the tiny island is on edge. Volcanologists are trying to gather as much data as possible to help planners, residents and first responders best prepare for any possible further eruptions.
What makes a volcano erupt?
Volcanoes erupt because of a change in pressure in the magma near the Earth’s crust. The magma is molten rock that forms deep under the Earth’s surface, about 30 to 50 kilometres down, where the crust meets the Earth’s mantle. It is under extreme pressure and temperature so the rocks can melt. That pressure needs to be relieved at routine intervals and takes the path of least resistance – that path is through fissures between the tectonic plates of the Earth’s crust.
Canadian volcanologist Julie Roberge from the Instituto Technica Nationale in Mexico City explains it this way: “So an analogy for this is a bottle of soda. So, you know, if you leave it on the table, it has a fixed pressure and there’s gas dissolved in your Coke, Sprite or whatever. But when you move the bottle, what happens is that these bubbles are coming out and they want to come out of the bottle. A volcano is the exact same thing. So what happened to your bottle of soda, if you leave the cap on, versus if you take the cap off, is the difference between an eruption where the lava will just flow… versus it will explode like Agung.”
So what drives the lava upwards and out of the cone of the volcano is gas pressure that needs to vent.
What causes the increased gas pressure buildup?
In the case of Mount Agung, it sits in the so-called Ring of Fire that rims Japan and most of Asia, including Indonesia. This area of the world is a subduction zone. The oceanic plate is diving below the continental tectonic plate and that creates a weak point in the Earth’s crust.
When one tectonic plate, the oceanic plate of the eastern Pacific, goes down below the continental plate, the water rises and that lowers the melting temperature of the rock above. This forms magma and the magma is less dense than the surrounding rock. It rises like the wax in a lava lamp and the pressure builds… and it needs to be relieved. So, ultimately, it’s…