Final answer:
The formation of steam in a geyser involves boiling of groundwater due to geothermal heat under the Earth's surface, leading to pressure drop and a flash boil, causing the characteristic eruption of steam and water.
Step-by-step explanation:
The formation of steam from a geyser is most accurately described by C. boiling of groundwater because of heat under the Earth's surface. This process is a captivating illustration of thermodynamics where water deep within the underground channels of a geyser is subjected to high pressure and is heated to high temperatures due to the proximity of magma. The heat from the magma raises the temperature of the water, and since it is under pressure, it does not immediately boil despite exceeding its standard surface boiling point temperature.
When a pocket of water near the surface does reach its boiling point and is expelled, it causes a sudden drop in pressure, resulting in a state known as a flash boil. This sudden boiling effect then leads to the spectacular eruptive force of a geyser, projecting steam and water into the air. The processes involved in the formation of architectural features in caves, such as stalactites and stalagmites, or the various stages of the water cycle are different and not directly connected to the mechanism driving the geyser's steam production.