Question: why do volcanoes that have been asleep for ages suddenly erupt?

Kate Dobson answered on 13 Nov 2014:

Most volcanos give us some warning that they are going to erupt. If we look at the rocks that have been erupted in the past, we can get an idea of how frequently the volcano erupts and what it’s behaviour is likley to be. We know that some volcanoes will probably go off again, even though they haven’t erupted for hundreds of thousands of years (like Yellowstone), because we can see that there were similar periods of being “dormant” in the past.

The plumbing system beneath volcanoes can be very complicated. Magma can build up over 10s or 1000s of years, and then at some point, something changes and an eruption happens. The change can be caused by lots of different processes, but it generally involves pressure. If the magma chamber is full and you add more magma, something has to go somewhere!

Magmas has gas in it (usually water), it is those gases coming out of solution and expanding very quickly that drives most eruptions (like opening a shaken up bottle of coke). “Magmatic” eruptions happen when the pressure drops low enough as the magma moves towards the surface, and sometimes becasue a landslide all of a sudden moves the surface towards the magma (like in the Mt St Helens eruption). Thinking of a bottle of coke again, think what happens when you crack the lid…. sometimes though the gas can dribble out of the magma or the magma doesn’t get close enough to the surface and so there is no explosion or even no eruption.

Volcanos can also “wake up” becasue of water. Water and magma don@t mix very well. When water comes into contact with magma at between 800-1200C, it will turn into steam very very quickly. The expansion drives an eruption. There are two kinds of eruption involving water. In “hydrovolcanic” eruptions the water (the sea, a lake, a glacier, and the magma interact and the change to steam fragments the magma creating very large explosions of very fine ash. In “Phreatic” eruptions the water interacts with an existing body of magma but the magma doesn’t get blown to pieces. It is essentially an explosion of steam and rock, not steam and magma. The recent eruption in Japan was one of these phreatic eruptions.

Phreatic eruptions are generally harder to predict becasue the magma isn’t moving, and most of our monitoring methods pick up magma movement.