Good post don Felix!

Fun fact: different types of magma form different types of volcanoes. Magma's types are closely related to plate tectonic activity, so we can tell a lot about the underground from volcanoes on top.

For example, look at the west coast of the US and the Andes in South America. They form due to subduction of tectonic plates and generate stratovolcanoes. Mt St Helens is one such volcano.