In a hydrothermal system studied by Martin Hovland, why did the elongated salt rock mostly composed of gypsum form?

The formation of elongated salt rocks primarily composed of gypsum in a hydrothermal system can be attributed to the fact that some salts, which include gypsum, are less soluble as temperature and pressure increase. In hydrothermal systems, the conditions often involve elevated temperatures and pressures, which can directly influence the solubility of minerals.

As temperature and pressure rise, certain salts begin to precipitate out of solution because their solubility decreases under these conditions. Gypsum, which is calcium sulfate dihydrate, may initially be soluble, but as the conditions of the hydrothermal system change, it can reach a point where it becomes supersaturated, leading to crystallization. This supersaturation is primarily due to the combined effects of increasing temperature and pressure, which cause the relative solubility of gypsum to drop, prompting it to form solid deposits.

The specific conditions of the hydrothermal environment, including changes in temperature and salinity, play a crucial role in the crystallization process. While other options may explore aspects related to the solubility and crystallization of minerals, the key factor for the formation of gypsum in this context revolves around the behavior of salts under elevated conditions, making the explanation based on solubility with respect to temperature and pressure the most relevant.