Hydraulic Gradient The slope on the water table which promotes horizontal fluid flow in permeable rock units.

Hydrothermal Eruption Rare explosive discharge of hot water and steam that is powerful enough to excavate a crater and lift rock fragments and its pulverized equivalents into the air to...

Illite A potassium-bearing mica-like clay mineral that is a product of hydrothermal alteration. It commonly replaces feldspars and other alumino-silicate minerals and it forms above ~220°C....