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.

Indonesia Bali Volcano

Volcanoes erupt because of a change in pressure in the magma near the Earth's crust. (Firdia Lisnawati/AP)

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 small earthquakes and natural shifts in the tectonic plates that are creating this pressurized chamber that needs to vent.

INDONESIA VOLCANO Mount Agung illuminated at sunset

Mount Agung sits in the so-called 'Ring of Fire' that rims Japan and most of Asia, including Indonesia.

How do volcanologists know when a volcano is ready to erupt?

The first indication is always the seismic activity. These are often murmurs of activity deep below the surface. They aren't felt as earthquakes because they are too deep or too subtle, but seismologists watch every shift and groan of the tectonic plates with interest. 

Some of the seismic monitoring is global scale. One earthquake over in Mexico, for instance, means the rest of the tectonic plates need to ease the strain of the new position and they shift — often in ways we don't completely understand of course, but it's an indicator that seismologists need to be alert. Other seismic data is obtained right at the site of the volcano. When it comes to Mount Agung in Bali, there have been up to 600 seismic events per day as the volcano prepares for something. That's why the country and the island are on high alert.

Not only that — most active volcanoes with nearby populations are monitored heavily with GPS sensors providing a topographical map of the volcano to look for any deformations.

What do the deformations tell a scientist about the volcano?

The whole point of a volcano is that the gas pressure is building inside and it isn't venting enough to accommodate the extra volume of gasses. So it can deform the mountain that houses the volcanic crater, even by tiny degrees. A GPS-based topographical map (as we find on Mount Agung) can tell if there are any shifts or bulges or changes in the height of the crater that could suggest that magma is moving inside and preparing for something.

When Mount St. Helens erupted there was a 100 metre change in the height of the crater days before. That made it pretty obvious that something big was happening. Mount Agung isn't having the same obvious signals, but there is off-gassing, movement and the general seismic activity that is placing everyone on high alert.

And I should say, Mount Agung is no stranger to devastating eruptions. The last time it erupted was 1963 with a volcanic explosivity index rating of 5. It's a logarithmic scale like the Richter scale and that's big. Even Mount St. Helens was about a 4 to 5.  So this volcano could be very, very dangerous.

Why is it so difficult to predict volcanic activity?

Because no one wants to get too close!  And because these shifts and movements of the Earth are happening 30 kilometres or more below the surface of the Earth. It's hard enough to predict the weather on the surface let alone what is happening deep below.  And because each volcano seems to have its own personality, studying one eruption doesn't necessarily help with a volcano on the other side of the world. Historical data is the best. For instance, the 1963 eruption of Mount Agung was pretty well documented and yet conditions are bound to be different this time. Last time Agung erupted, it had a period of unrest, of seismic signals and venting of steam, and then had one big eruption. But it wasn't done. Two months later it rumbled again and erupted again. 

Will that happen this time? Science can't tell until the seismologists get their hands on what is happening down there at that fault line. There's no reliable data beyond that 1963 eruption that could say that this is just part of what Agung likes to do.

What's in store for Mount Agung?

If there is a big event — I mean the volcano really blows — that could create a category 5 strength eruption. That is catastrophic in and of itself but that may only really explode about a cubic kilometre of material. As Natural Resources Canada volcanologist Melanie Kelman explains, it's not the eruption per se that is dangerous, so long as you aren't standing on the slopes. It's the aftermath: "There's a couple hazards that are more of an issue further away. One is lahars, which are volcanic debris flows and mud flows. And lahar is actually an Indonesian word because they have so many of them in Indonesia. Because those travel down streams and can travel a long way, they can travel maybe up to 100 kilometres from the source volcano, so they can cause problems much further away."

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One of the post-eruption hazards is lahars, which are volcanic debris flows and mud flows. (Andri Tambunan/Getty)

There's a huge risk to the entire island of Bali. The whole place is only 150 kilometres by about 110 kilometres at the largest points.  All of the 4 million inhabitants will be affected by the volcano, some by the relatively minor annoyance of cleaning ash up. Others will have to deal with mudslides and destroyed homes.

All we can do now is hope Agung will go back to sleep and release the gas and pressure it needs to.