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Geothermal Features

What Are They?

Geysers are probably the most well known geothermal feature, but hot springs and fumaroles (gas or steam vents) can be equally impressive.

Geysers

At the surface of a geyser, we usually see a water-filled pool. Beneath this pool, a pipe travels underground for tens of metres. After an eruption, water flows from cracks in the rock into the empty pipe – superheated water at the bottom and cooler water at the top. The hot water heats the cooler water. As the water heats, dissolved gas (such as carbon dioxide) forms bubbles which expand and reduce the pressure in the water. When the pressure of the superheated water is lowered, it suddenly turns to steam and an eruption occurs.

During the eruption, the pipe empties and the cycle starts again. It is this cycle that means that geysers often erupt with a regular height, frequency and strength.

Constant Geyser in Norris Geyser Basin at Yellowstone.
Steam can also be seen rising from the pools surrounding the gesyer.
Photo © Lyall Anderson, National Museums of Scotland.

Hot & Cool Springs

Hot springs are similar to the more common cold water springs in that the groundwater flows slowly to the surface from the rocks below. As with geysers, these waters can have dissolved minerals and gases within them. The temperature of hot springs is lower than that for geysers, and so they can be found further from the volcanic activity than geysers. This is because the temperature of the groundwater lowers with increasing distance from the volcano.

Cool springs are like those found all over the world, only they can also be rich in dissolved silica and so are also capable of creating sinter. At Mammoth hot springs in Yellowstone, travertine sinters form in addition to the silica sinters. The travertine is formed when the water is rich in carbonates instead of silica.

Fumaroles

Fumaroles are vents at the surface of the ground where gas or steam escapes. Carbon dioxide and hydrogen sulphide are two common gases to be emitted from fumaroles. They too come from the volcanic related fluids, but are not circulating in the groundwaters. The acidic gases often cause decomposition of the rock through which they pass. The resultant sticky mud isn’t very pleasant!

At this fumarole at Yellowstone, gases escape from the vent in the ground. The air around the vent is therefore quite poor and so vegetation finds it hard to grow.
Photo © Dr Lyall Anderson, National Museums of Scotland.