Question: How do the tectonic plates under (I think) a volcano make it erupt?

Natasha Dowey answered on 13 Mar 2019:

So- for volcanoes to erupt you need one of a few scenarios.
First, we need to talk about plate tectonics. Tectonic plates are the massive chunks of crust the planet is divided into. Think of the planet as an egg, the outer crust is like the shell. Imagine cracking that shell so that cracks form all other the egg, forming ‘continents’… this is a bit like the Earth.
-In some places, the plates of the crust are moving apart. They move apart because hot magma is rising from below the crust, and as it rises, it forces the crust apart and forms volcanoes. This is happening right now along the East African Rift, and underwater in the middle of the Atlantic (Icelandic volcanoes).
-But then, because plates are moving apart, somewhere plates will be forced together. And when crust rocks are forced together, they either crumple up, forming mountains, or one plate is forced underneath the other and melts, forming new hot magma that rises up into the crust and creates volcanoes!
-In some places, like Hawaii, there are zones where the crust is a bit weak, and a hot plume of magma lies underneath punching a hole through, forming volcanoes right in the middle of a plate- no boundaries needed.

Once the hot magma is in the crust, it rises buoyantly, because it is less dense than the rocks around it. At some point it may stop and form a system of magma chambers, if the density difference becomes less. The actual eruption is all to do with pressure. If you think of a bottle of coke thats been shaken up, it contains gas, but we don’t see the bubbles until we unscrew the top. Then suddenly all these bubbles form and froth out of the bottle. Thats a bit like a magma chamber. Magma contains gas, but it is under really high pressure in the chamber. If the pressure gets high enough, and a hole opens at the top of the volcano, suddenly the whole systems depressurises and the magma erupts. Different types of magma do this differently- where there are really sticky, thick magmas, rich in silica, it is hard for the bubbles to form and the magma can depressurise really explosively into particles of ash and pumice. If the lava is runny, the bubbles form easily and the magma may erupt as a lava flow, like a river- seen at Hawaii.