There is little doubt anymore that earthquakes and volcanoes are linked, in more ways than one, but it can be an interesting and worthwhile exercise to explore just how they are connected.
Most volcanoes are formed when one tectonic plate subducts or slides beneath another. When this happens, the subducting plate begins to melt, and the molten rock tries to work it’s way to the surface, melting more rock along the way, and usually creating great pockets of magma. When this magma works its’ way through fissures in the rock, and finally vents out of the Earth in the form of lava, it is a volcanic eruption.
There are two main reasons that earthquakes are often linked to volcanic eruptions. First, when magma starts moving from the magma chamber into the fissures in the rock, as it forces fissures further apart, it can initiate and earthquake swarm. This is so common in fact that earthquake swarms around a mountain are often used as a precursor of an eminent eruption.
The second reason is basically the other way around. Most earthquakes occur at plate boundaries or faults, which is usually also fairly near the active volcanoes. Many of these faults move past each other, and as they do, they usually “stick” to each other from time to time. As the pressure builds, eventually the two plates break free of each other, releasing the strain. This is an earthquake. The energy released in an earthquake can be so small that people aren’t even aware that an earthquake has occurred, or so strong that man made structures crumble or are flattened, and even rivers can be diverted or offset as the ground takes up its new position.
Such a quake will often actually cause the fissures through which magma can flow, or it can cause massive landslides on a volcanic mountain that allow the magma access to the surface, regardless of what actually triggered the earthquake. This is what happened in the case of Mount St. Helens. Looking at slow motion footage of the eruption, moments before the main eruption, there was a quake that caused thousands of tons of rock face to slide down the mountain, exposing the vents that had been covered. This has the same effect as uncorking a shaken champagne bottle, only on a vastly greater and intense scale. In the case of St. Helens, as soon as the fissures were exposed, the volcanic gases came out of suspension in the magma, expanding and resulting in an enormous explosion that blasted a cubic mile of pulverized rock, dirt, and ash miles into the atmosphere, and literally knocking over an enormous tract of forest.
Earthquakes, it seems, can both be created by volcanoes, and can act as a catalyst for volcanic eruption. The dangers of either can be severe, but the dangers of both, put together, can be catastrophic.