Hot Springs and Geysers
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| Geothermal waters and examples of their manifestation in the landscape. |
Hot springs, springs that emit water ranging in temperature from about 30° to 104°C, are found in two geologic settings. First, they occur where very deep groundwater, heated in warm bedrock at depth, flows up to the ground surface. This water brings heat with it as it rises. Such hot springs form in places where faults or fractures provide a high-permeability conduit for deep water, or where the water emitted in a discharge region followed a trajectory that first carried it deep into the crust. Second, hot springs develop in geothermal regions, places where volcanism currently takes place or has occurred recently, so that magma and/or very hot rock resides close to the Earth’s surface (figure above a). Hot groundwater dissolves minerals from rock that it passes through because water becomes a more effective solvent when hot, so people use the water emitted at hot springs as relaxing mineral baths (figure above b). Natural pools of geothermal water may become brightly coloured the gaudy greens, blues, and oranges of these pools come from thermophyllic (heat-loving) bacteria and archaea that thrive in hot water and metabolize the sulphur containing minerals dissolved in the groundwater (figure above c).
Numerous distinctive geologic features form in geothermal regions as a result of the eruption of hot water. In places where the hot water rises into soils rich in volcanic ash and clay, a viscous slurry forms and fills bubbling mud pots. Bubbles of steam rising through the slurry cause it to splatter about in goopy drops. Where geothermal waters spill out of natural springs and then cool, dissolved minerals in the water precipitate, forming colourful mounds or terraces of travertine and other chemical sedimentary rocks (figure above d).
Under special circumstances, geothermal water emerges from the ground in a geyser (from the Icelandic spring, Geysir, and the word for gush), a fountain of steam and hot water that erupts episodically from a vent in the ground (figure above e). To understand why a geyser erupts, we first need a picture of its underground plumbing. Beneath a geyser lies a network of irregular fractures in very hot rock; groundwater sinks and fills these fractures. Heat transfers from the rock to the groundwater and makes the water’s temperature rise. Since the boiling point of water (the temperature at which water vaporizes) increases with increasing pressure, hot groundwater at depth can remain in liquid form even if its temperature has become greater than the boiling point of water at the Earth’s surface. When such “superheated” groundwater begins to rise through a conduit toward the surface, pressure in it decreases until eventually some of the water transforms into steam. The resulting expansion causes water higher up to spill out of the conduit at the ground surface. When this spill happens, pressure in the conduit, from the weight of overlying water, suddenly decreases. A sudden drop in pressure causes the super-hot water at depth to turn into steam instantly, and this steam quickly rises, ejecting all the water and steam above it out of the conduit in a geyser eruption. Once the conduit empties, the eruption ceases, and the conduit fills once again with water that gradually heats up, starting the eruptive cycle all over again.
Figures credited to Stephen Marshak.