Question: what makes volcanos erupt

Rebecca Williams answered on 23 Jun 2014:

Hi Delia,

Funny you should ask that today – I have some local school children coming in this afternoon and we’re going to investigate just that question!

Volcanoes erupt because of bubbles! Watch this video of what happens when you put mentos sweets into diet coke https://www.youtube.com/watch?v=hKoB0MHVBvM The mentos cause the diet coke to release all of its gas as bubbles very very quickly making a coke foam. This creates a lot of pressure in the coke bottle – too much for the coke foam to just stay in the bottle. So, it comes bursting out the top in a diet coke eruption!

Well, the same thing happens with volcanoes. If the magma starts making bubbles too quickly, it creates a magma foam. This is less dense than the surrounding magma so it rises up. This makes more bubbles come out of the magma quickly, so the foam rises up. This also build pressure. It keeps rising, making more bubbles, making more pressure, until it forces its way out of the volcano as an eruption!

What causes the bubbles of gas to start coming out of the magma is more complicated. It might be because of at least 3 reasons:
1) the pressure in the magma chamber decreased (gas only stays in magma under pressure)
2) fresh magma came into the magma chamber – the extra heat might make bubbles form.
3) because the magma grew too many crystals – this also makes bubbles form