When you superimpose a map of active volcanoes in the world on a map of earthquakes during the past thirty years, you can see that they match perfectly. That is because most of the volcanism and most of the seismic activity on Earth are localised on the boundaries between tectonic plates. However, these two phenomena are not directly related.
The best example of the correlation between volcanism and seismically active zones is the “Ring of Fire” skirting the Pacific Ocean (link to the map of the Ring of Fire, content provided soon). All around the Pacific Ocean we find subduction zones (Philippines, Japan, Aleutian Islands, Alaska, South America). In subduction zones, the dense and hydrated oceanic lithosphere sinks in the mantle. The water contained in the oceanic crust lowers the melting point of the surrounding rocks, which creates magma. This hotter and less dense magma moves toward the surface, where it generates the intense volcanism that gave its name to the “Ring of Fire”.
But in the case of the “Ring of Fire”, earthquakes and volcanoes are not directly related. Of course earthquakes happen in these subduction zones, but they do not really cause eruptions.
Nonetheless, in very specific conditions, earthquakes and volcanic activity are indeed connected. Volcanic eruptions are observed sometimes after a big earthquake. Scientists have three plausible explanations for that:
- Seismic waves could weaken the top of the magma chamber and cause an eruption;
- Seismic waves could perturb the gases inside the magma chamber and cause an explosive eruption;
- The strain around the magma chamber could evolve after an earthquake, and cause an eruption.