They occur predominantly in metamorphic and igneous rocks of the Earth's crust and upper mantle. Two metamorphic facies are defined by their characteristic amphiboles.

Where does hydration melting occur?

The volcanoes are a result of "hydration-induced" melting of the material in the mantle above the subducting material. The rocks in the oceanic crust are formed and spend a great deal of time in an environment rich in water.

Where does decompression melting occur?

Decompression melting often occurs at divergent boundaries, where tectonic plates separate. The rifting movement causes the buoyant magma below to rise and fill the space of lower pressure.