Volcanos occur as a direct result of the earth’s interior being extremely hot.
And it’s so hot that the interior is actually consisted of molten rock, which we call magma.
And because it’s so hot, it’s able to convect. So, hotter magma actually rises up towards the earth’s surface; as it gets towards the surface it cools and it sinks back down towards the core again. On occasion, it will actually make it out onto the surface of the earth, and this is when a volcanic eruption will occur.
There are two main controls on what makes a volcano erupt explosively; one is how sticky the magma is, and the other is how much dissolved gas is actually in that magma or the molten rock. So, the most important element for determining how sticky magma is actually going to be is its silica content. Because silica can form bonds with other elements, and that makes long chains which make it very hard for the magma to actually flow. You can see that reflected in the rocks we have here. We have what would have been a stickier magma and its white in colour, as opposed a darker, what is called a more mafic magma, which would have been actually a lot runnier.
The second control on what makes a volcano erupt explosively is the amount of the gas that is dissolved inside the magma at depth. Carbon dioxide and water will be dissolved in the melt; they will start to form bubbles and as the magma gets closer and closer to the earth’s surface, gradually a pressure will build up as these bubbles join together and create a great pressure that eventually will fragment and you’ll get an explosive eruption at the earth’s surface.
So you can see in this, what would have been a runny basalt, you can still see the bubbles preserved here. If these gases can move through the magma and escape, you’ll get a lava flow as a result. And this here is an example of the lava flow, where it’s gone out easily through a basaltic or a runny magma.
And so we’ve got this nice ropey texture where the lava’s flowed across the earth’s surface. This rock here is an example of a stickier magma that’s lost its bubbles. And even though it’s lost its bubbles, you can still see that because it’s a sticky chemical composition, it’s got lots of silica; it’s flow-banded.
If the bubbles can’t actually all get out, you’ll get a moderately explosive eruption and the magma can actually get up into the air and it can cool and it’ll form a cone around the vent, and this is called a scoria cone. We’ve got lots of examples of this in Victoria, such as Mt Elephant.
If you increase the explosivity of the magma even more so, stickier and more volatiles and dissolved gas, you can actually form a larger eruption column and you’ll get what looks like a stratovolcano such as Mt Fuji in Japan.
Even greater volumes of volatiles or stickiness of magma, or a combination of both, and you can get ‘super volcanoes’, where it disperses its products so far away, that the profile of the volcano is very, very low, and sometimes so much material is actually ejected, that there can be a crater collapse and you’ll get a caldera, such as Taupo in New Zealand.
And just to further complicate things, sometimes the magma can interact with water, so lake water or ground water, in what’s termed phreatomagmatic behaviour. And here the magma’s so hot that it superheats the water into steam and that adds to how explosive the eruption actually is. And again, there are lots of examples in Victoria, such as Lake Purrumbete or Tower Hill.