But how are volcanic eruptions formed?

It's been a couple of yrs. ago since I last took Geology & Physical Geography & so, there may be some inaccuracies with my descriptions. However, reading back my books will take so much time & so, I'll simply write this out of memory since I really liked both science courses that I still remember the basics.

Anyhow, volcanoes erupt when a "shift" occurs within the plates several kilometers under the earth known as "the subduction zone" whereby continental [granitic] plates collide with oceanic [basaltic] plates.

And since granite is lighter than basalt, basalt then sinks whereas granite rises. As the old saying goes, "heavy sinks, light rises."