Hydrothermal essentially means “hot water.”
Hydrothermal rocks are those rocks whose minerals crystallized from hot water or whose minerals have been altered by hot water passing through them. Thus, these rocks are distinct from metamorphic rocks, which are created by solid‐state mineral transformations. In fact, many hydrothermal rocks (such as those that form from hot springs and geysers or crystallize as veins in cracks in other rocks) actually build up in layers, much as sedimentary rocks do.
Veins result when hot water moves through cracks in the bedrock of the crust. The water leaches elements from the rocks it passes through. Various minerals are precipitated on the sides of the crack as the temperatures decrease. The shape and orientation of the minerals depends on the temperature, pressure, and rate of flow. When all the available space in the crack has been filled with mineral deposits, the crack is sealed and the vein is complete.
The water involved in hydrothermal processes is usually either seawater that is moving downward through oceanic crust near midoceanic ridges or meteoric water. Meteoric water is water that is derived from the atmosphere as rain or snow and that moves down into the bedrock from the earth's surface. Water trapped in the original sediments during deposition and lithification (connate water) can also be included in hydrothermal reactions but is not a major source of hydrothermal fluid. Magmatic water derived from magmas is also a minor component.
The water is heated to very high temperatures as it moves deeper into the crust. It eventually rises again, often removing elements from the rocks it passes through and carrying them in solution. As the hot water rises toward the surface, it begins to cool. This temperature drop induces a number of chemical reactions, and hydrothermal minerals are precipitated.
Metasomatism is the process by which hot‐water solutions carrying ions from an outside source move through a rock mass via fractures or pore space. Some of the rock mass is usually dissolved away, and the ions introduced by the water are incorporated into the new minerals that precipitate. Unlike metamorphism, metasomatism can significantly change the overall chemistry of the parent rock. Elements commonly added during metasomatism are iron, sodium, potassium, oxygen, and silica. Easily soluble elements, such as calcium and magnesium from limestones, are often dissolved and carried away, creating more room for new chemical reactions.