How is the strength of a volcanic eruption related to the magma in the volcano?

Question

How is the strength of a volcanic eruption related to the magma in the volcano?

A. The more magma the volcano contains, the more slowly it will erupt.

B. If the magma is more viscous, the eruption is stronger.

C. The strength of the eruption is determined by the depth of the magma below the volcano.

D. They are not related.

it's B right?

Answer 1

dunno but i think it's something to do with the pressure from underground gases

Answer 2

Yes, it is B.

By strength of eruption I presume you mean how explosive it is. This is related to the viscosity of the magma, but what causes the eruption to be explosive is actually the gas dissolved in the magma.

A magma contains gas dissolved in it, like a bottle of coke. When the top is on the coke bottle, the drink is at a high pressure, so the gas cannot escape. This is analogous to magma deep underground, with loads of rock above it providing a high pressure.

When you take the top off a coke bottle, the pressure is released, and the gas can escape. In the same way, when a magma gets near to the surface, the pressure of the overlying rock becomes less, and so the bubbles come out of solution.

This is where viscosity becomes important.

If the magma is basaltic (like in Hawaii) then it is not viscous, and the bubbles of gas can easily rise through the magma to the surface and escape (like in coke).

However, if the magma is andesitic (like at Mt St Helens) then it is very viscous, and the bubbles of gas are trapped inside the liquid (think of a bottle of washing up liquid or shower gel - any bubbles take much longer to rise).

This causes a problem as the gases have a much larger volume when they are bubbles than when they are dissolved. As they cannot escape, this causes the pressure to build up inside the magma.

If the lava is erupted at the surface, then it will be a violent as rapid expansion of gas causes it to be thrown into the air.

If the lava does not erupt, then the pressure build up reaches  a critical point and the volcano can explode.