How does a sudden drop in pressure cause water to vaporize?

In the context of geothermal energy, the process where water vaporizes due to a sudden drop in pressure is known as "flashing."1

This phenomenon occurs because the boiling point of a liquid is directly dependent on the ambient pressure exerted upon it. At any given temperature, a liquid has a specific vapor pressure; boiling occurs when this vapor pressure equals the surrounding pressure.23

When high-temperature, pressurized water is extracted from underground and moved into a lower-pressure environment—such as a flash tank—the surrounding pressure drops below the water's current vapor pressure. Because the water is already at a high temperature, this reduction in pressure causes the liquid to instantly reach its boiling point, forcing it to vaporize into steam. This principle is fundamental to flash steam power plants, which utilize this rapid phase change to drive turbines for electricity generation.123