Alaska, Volcanic Eruption

- If the erupted magma contains 52–63% silica, the lava is of intermediate composition.
- These "andesitic" volcanoes generally only occur above subduction zones (e.g. Mount Merapi in Indonesia).
- Andesitic lava is typically formed at convergent boundary margins of tectonic plates, by several processes:
+ Hydration melting of peridotite and fractional crystallization