Most people have by now noticed that Gunung Agung has stopped erupting. And to understand why that has happened, and what will happen soon, we need to look at what happened during the eruption. We also need to look at what is currently happening.
When a volcano is showing no visible signs of activity, we have to turn to instrumentation to understand what is going on where we can’t see anything.
But, before that, we must look at what a volcano is at its most fundamental level.
Volcanoes as energy transfer systems
Most people tend to think of volcanoes as outlets of magma, but the magma is just a way of transporting energy. In mantle plume volcanoes this energy has travelled from the bowels of Earth, at spread centre volcanoes the energy is transferred from the mantle, and in subduction volcanoes that energy is caused mainly by friction from the subducting slab grinding against the thicker upper crustal part.
Now, many will say that it is lava that is squirting out. Well, yes it is lava that is squirting out, but the greatest difference between lava and your average garden rock is heat. And heat is a form of energy.
So, any volcano will be a basic energy transfer system. But, the magma type does have effects of how the volcano will erupt. The energy transfer affects when, size and type of eruption in combination with the type of magma involved and the involved volatiles residing in the magma.
These combinations are affecting the behaviour of every single volcano on the planet. Sometimes two close by volcanoes erupt in completely different patterns and ways due to this.
Gunung Agung as an energy transfer system
Agung is a subduction volcano of a fairly ordinary type. At depth magma accumulates at a point called the accretion zone, it is where melted subducted rock pools complete with its molecular components and energy in the form of heat.
This pool of magma will take on characteristics of the subducting plate. If it is water rich the magma will be water rich, if it is rich in metals and sulphuric compounds the ensuing magma will also be that.
Due to the heat the melt will weigh less than the rock above, so it will be buoyant and try to migrate upwards like a hot-air balloon.
In the end that balloon of hot magma will enter the magma chamber of Gunung Agung and it will start to expand, both from the heat increase and the volume of the fresh magma. There will also be considerable exsolution of volatiles as the magma nears the top of the magmatic system.
In the case of Agung the magma is rich in silicates, water and sulphur. The magma at Agung is of a highly silicic type called andesite. That means that the magma is sticky and prone to being able to contain substantial amounts of water and gas before fracturing. That makes it potentially very explosive as it comes out.
What has happened at Agung?
Before any visible signs happened at Agung, scientists started to pick up small earthquakes and see a slight inflation of the volcano indicating that new magma was entering the system. In this case they knew that Agung was reactivating about a year prior to the current activity.
This made it possible to increase the monitoring and start to produce more detailed evacuation maps. At the same time local authorities started to consult with the scientists to prepare for what would potentially come. As activity increased over time these plans became more detailed and the local population was informed about evacuation plans.
As things progressed even further an exclusion zone was put into place and people where evacuated from the mountain.
After that the volcano started doing what volcanoes do best, and that is erupting. The opening stage was a hydrothermally driven phreatic detonation that lasted for a short while. After that the volcano returned to a more restful look.
A phreatic detonation is when increased heat from below rapidly heats water inside the mountain causing steam explosions. This often clears out an upper conduit through the mountain that the magma can follow at a later stage.
During this interlude the seismic signals and inflation continued and increased until the mountain finally erupted by ejecting fine grained tephra and prodigious amounts of sulphurous gases. This eruption phase has been calculated to have been a small VEI-3 with 0.012km3 of ejecta.
Here comes what is rather unusual. A second vent opened up after a couple of days, and pure steam came gushing out next to the vent ejecting the tephra. This can be seen on the video above. Normally all water has been boiled off from a volcanic system prior to the magmatic phase, but here there must have been a rather powerful aquifer or water body next to the volcano that took to long time to heat up to an explosive level.
In the end the shear volume of water quenched the andesite in the conduit and a glassy plug formed that corked up the eruption completely. This is among other things measurable on sulphur gas output that has diminished quite a lot from during the eruption phase.
What is happening now?
After the eruption there have been almost daily episodes of violent tremor caused by ever more increasing pressure inside the volcano. This pressure is caused by the energy in the form of heat causing the old magma in the system to expand further as it heats up, and it is also caused by more magma entering the system.
This tremor has been very large compared to other similar systems, so we do know that the rate of pressure increase is high. There has been a lot of volcanic type earthquakes also, and that is another sign of increasing pressure.
What will happen?
The shear intensity and duration of the tremor episodes tell a story that it is in the long run impossible for the quenched plug to hold back the lava. It also tells that the plug is very solid and sitting very firmly in the barrel of Agung.
This is potentially bad news. Either the plug will start to slide up and form a dome or a spine before being blown away. Or, the plug is so firm that it will shatter inside the conduit and come out explosively during an explosive phase.
And the general rule is that the longer time we must wait, the larger the ensuing eruption will be.
Currently the seismic signals have quietened down, this is normal prior to an eruptive phase. But, it is also possible that we will see several more phases of intensified seismic unrest prior to the next eruption.
If I was to hazard a guess here, it would be that we will see a more intense eruptive phase at any time, ranging from in a few hours to a few weeks. But, my opinion may change as we get more data.
One thing I am thankful for is that it is the Indonesian authorities that oversee this. The local scientists and authorities are very good at these things. I would even go so far as to say that they are the best in the field currently.
EDIT: Correctly attributing the photographer of the featured image, thank you to Emilio Kuzma-Floyd for pointing out our oversight, and providing this GIF for usage