1. adj. [Geology]

Pertaining to hot fluids, particularly hot water, or the activity of hot water, or precipitates thereof. Hydrothermal alteration can change the mineralogy of rock, producing different minerals, including quartz, calcite and chlorite. Hydrothermal activity is commonly associated with hot water that accompanies, or is heated by, magma.

See: hydrothermal alteration,  igneous,  magma,  mineral