Metamorphism that occurs when hot waters, laden with minerals and chemically active, intersect surrounding country rock. It may occur at low temperature and pressure, typically yielding pegmatites, serpentinite, and soapstone (see talc), often as an after-effect of igneous activity. Oceanic hydrothermal metamorphism occurs at rift centres, where magma oozes on to the ocean floor to form pillow lava.