Underlying Yellowstone National Park in Wyoming is a thermal vent that runs from Earth's mantle to its crust. Some 600,000 years ago, this "hot spot" fueled one of the largest explosive eruptions in recent history. It has been 70,000 years since Yellowstone's crater-shaped volcano last erupted, but the park's 10,000 geothermal features, all heated by underground lava reservoirs, demonstrate that the volcano remains active.

In regions of current or recent volcanism, as cold rainwater seeps downward, it is heated by rocks that overlie magma chambers deep in the ground. Because water is subjected to greater pressure at these depths, its boiling point is higher than normal. As a result, instead of boiling, the underground water becomes superheated. The superheated water then rises toward the surface where it feeds into hot pools of water or escapes through narrow cracks in the ground.

Because minerals dissolve more readily in hot water than cold, hot springs often contain high mineral concentrations. When groundwater reaches the surface and cools, some minerals come out of solution and are deposited. Stepped terraces, like those present at Yellowstone's Mammoth Springs, are made of travertine, a mineral that is usually white when deposited but may be red, brown, or yellow if iron compounds or other minerals are present.

Hot springs that erupt intermittently with a great deal of steam and boiling water are called geysers. As superheated groundwater rises, the pressure decreases and the water turns to steam. The steam, in turn, propels the water that lies above it toward the surface. The eruption continues until most of the water and steam are emptied from the vent and storage chambers, after which groundwater begins to collect again and the process repeats. The period between geyser eruptions depends on the time it takes for the underground system to recharge.

Two other common geothermal formations in Yellowstone are mud pots and fumaroles, or steam vents. A mud pot forms when the rock surrounding a hot spring is chemically weathered by volcanic gases dissolved in the water. The weathered rock mixes with the heated water to form boiling, liquid clay. Although they are usually gray, some mud pots are called paint pots because certain undissolved minerals color the clay. Fumaroles lack an abundant groundwater source, and are thus vents from which only steam and other volcanic gases escape.

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