Question: Mount St Helens did not erupt like an average volcano, why?

Kate Dobson answered on 12 Nov 2014:

It turns out that wasn’t that unusual, we just hadn’t observed that kind of eruption in detail before.

When mount St Helens erupted in 1980 there was a period of about 2 months before the eruption when the scientists monitoring the volcano knew something was happening and that an eruption could be imminent. However, the eruption happened because an earthquake triggered what we call a sector collapse, a large land slide (the largest ever recorded) that removed a lot of rock from one side of the volcano. Removing this rock took the weight away, and it was the drop in pressure that caused all the gas in the magma to form bubbles and expand (like opening the lid of a shaken bottle of coke) and the eruption to start. The eruption went sideways through the weakest point and happened so fast it overtook the landslide.