Geysers and hot springs

Geysers are a type of hot spring that periodically erupts steam and hot water. They are the surface expressions of vast underground circulation systems, where constituents from underground rocks are dissolved in the hot fluids, carried to the surface, and deposited. The world's active thermal areas are natural laboratories where ore-forming processes can be observed firsthand.

Locations of Hot Springs and Geysers

The term geyser derives from an old Icelandic word, gjose, which means “to erupt.” Great Geyser, a spouting hot spring in southwestern Iceland, is the namesake of similar features worldwide. A true geyser is a clear, boiling spring that periodically erupts steam and hot water mixtures. Geysers are among the rarest and most spectacular natural phenomena and are found in only a few regions. Notable geyser areas occur in Iceland, Chile, the North Island of New Zealand, Japan, and Kamchatka. The most famous area is Yellowstone National Park in Wyoming, which contains more than ten thousand thermal features—more than in all the rest of the world. The three hundred geysers of Yellowstone account for 60 percent of the world's geysers.

How They Work

The source of heat that drives hydrothermal systems is magmasolid but still-hot rock five to ten kilometers beneath the Earth's surface. Nearly all the world's major hydrothermal systems are found close to active or potentially active volcanoes. Although some hydrothermal features, such as crater lakes on active volcanoes, are heated by steam and gas that evolved directly from small, shallow magma bodies, most hydrothermal phenomena are the surface expressions of immense underground convection cells of hot water and are indirectly linked to their magmatic heat source. Heat from magma or hot rock is conducted into the surrounding rocks and from there into groundwater that circulates through the rocks along fractures or through permeable strata. Although surface hot springs occur only within local areas, their underground circulation systems are tens of kilometers across and extend several kilometers deep.

The water in hot springs begins as rain and snowfall, which percolates several kilometers down into the Earth's crust through permeable volcanic rocks and sediments. The normal geothermal gradient (the rate at which temperature increases with depth in the Earth) of the continental crust is about 20 degrees Celsius per kilometer, but in hydrothermal regions like Yellowstone National Park, the geothermal gradient is ten to thirty times that value. The water becomes heated, and because hot water is more buoyant than cold water that continually displaces it underground, a huge subterranean convection system is established. The hot water rises to the surface, usually along faults or through connected pores in the rock. Depending on the permeability of the rocks, the volume of water, and the amount of heat, the complete cycle from snowflake to hot spring may take centuries or millennia.

Chemical Composition

The chemical composition of hot-spring waters is quite variable. Although some dissolved constituents may come directly from magma, laboratory experiments and chemical analyses of spring waters have shown that most dissolved matter comes from the underlying rocks. Thus, the compositions of hot-spring waters are strongly controlled by the compositions of the rocks through which they circulate, and such rocks are usually volcanic in origin. Rhyolite (a silica-rich type of volcanic rock) is almost universally found beneath the world's major hydrothermal regions, including Yellowstone. Rhyolite is the source of silica that forms the mineral deposits that line underground channels and are deposited on the surface.

Hot-Spring Classification

There are three major classes of hot springs based on their fluid types (liquid- or vapor-dominated), fluid compositions (acidic, neutral, or alkaline), and surficial deposits (sinter, mud, or travertine).

The first type is chloride springs, including geysers, which discharge clear water at or near its boiling point and is neutral or slightly alkaline. The most important dissolved constituents in the water are several parts per thousand of sodium chloride (common salt) and up to a few hundred parts per million of silica (hydrous silicon dioxide). The chloride is very soluble and remains in solution, but the silica readily comes out of solution during cooling of the water, and siliceous sinter is deposited on the surface. A type of common opal, sinter takes many forms including nodular masses, terraces, cones, and round “geyser eggs” that form only in the turbulent pools adjacent to geyser vents. Sinter is white or pale pink when first deposited but turns gray with age. Hot-spring formations are quite delicate, and when deposition ceases, they commonly break down into tiny white chips. Also carried in neutral-chloride waters are smaller amounts of potassium and lithium salts, boric acid, fluorides, and sulfates, together with traces (parts per million or billion) of heavy metals such as copper, lead, zinc, silver, gold, manganese, and thallium. The associated steam is mostly water vapor, with a small amount of carbon dioxide, and traces of other gases such as hydrogen sulfide, hydrogen, nitrogen, methane, ammonia, and hydrogen fluoride.

Acid-sulfate systems constitute the second major class of hot springs, including mud pools, mud pots, and fumaroles (vents that emit only gases). Acid-sulfate hydrothermal systems are so hot that the water table boils underground. As a result, these systems are said to be vapor-dominated, as opposed to the other two types, which are liquid-dominated. Steam and carbon dioxide are the two major fumarole gases, but a trace of hydrogen sulfide gas plays a very important role. The hydrogen sulfide is readily oxidized to form sulfur dioxide gas (with its acrid smell), native sulfur, and sulfuric acid. The latter product accounts for the extreme acidity of acid-sulfate fluids, and the sulfuric acid vigorously leaches the surrounding rocks and soil. The main product of this chemical attack is kaolinite, a white clay mineral that accounts for features ranging from turbid pools to viscous mud volcanoes, depending on the proportions of mud and water. Tiny black grains of pyrite (iron sulfide) or graphite (elemental carbon) commonly form in the pools, giving the mud a gray color and forming a black surface scum. Mud pools can be further colored by yellow, orange, and red iron oxides. Yellow, needlelike crystals of native sulfur are a common sublimate around the mouths of fumaroles.

The third type of hot spring is less common than the others. Liquid-dominated alkaline springs occur only where hydrothermal fluids have passed through underground limestone formations and are rare in comparison to the other two types. Calcium carbonate, the major constituent of limestone, becomes dissolved in the water, is carried to the surface, and is ultimately deposited as mounds or terraces of travertine, a banded variety of hot-spring limestone. The water temperatures of alkaline springs are usually well below the boiling point of water, and any bubbling is caused by carbon dioxide gas, not steam.

Hydrothermal Environments

The physical and chemical differences between liquid-dominated chloride systems and vapor-dominated acid-sulfate systems are largely caused by their positions with respect to the water table. In hydrothermal areas, the water table stands higher than elsewhere because of the relative buoyancy of upwelling hot water. Chloride springs occur where the water table emerges at the surface and the water boils off into the atmosphere, leaving deposits of sinter. On higher ground where the water table is deeper, only steam and gases may reach the surface because the water table is boiling underground. Acidic fumaroles (dry gas vents) are, therefore, typical features.

Within and around thermal basins, the high soil temperatures and acid vapors often destroy plant roots, and pits filled with carbon dioxide are death traps to birds and small animals. Geysers and hot springs are also sustainers of life, however, as shown by the birds and mammals that congregate near the year-round open water and green vegetation of hydrothermal features. Certain species of plants, bacteria, and even entire food chains of colorful algae, flies, and rare spiders are wholly dependent on the supply of warm water.

The brilliant coloration of hot springs results from several factors. Brightly colored iron oxides and other metal compounds may form locally around hot springs, but most hot-spring deposits are rather dull. Hot springs sometimes show zones of colors that vary with water depth, a result of blue skylight blending with reflections from yellow sulfur crystals or algae that line the pool. Although the vivid surface coloration of hot springs partly results from mineral deposits, microorganisms are largely responsible. Certain species of algae and cyanobacteria (blue-green algae) thrive at temperatures up to about 75 degrees Celsius, and they live in the relatively cool outflow channels of hot springs. Different species have different colors and prefer different temperatures. Thus, green, brown, red, and yellow color patterns are formed in the variable temperatures of outflow channels.

Necessary Conditions

Geysers are short-lived and fragile features. Many have become inactive during historic times due to natural events and human activity. Of the ten major geyser areas that are truly outstanding, only three—in Yellowstone, Iceland, and the former Soviet Union—remain essentially undisturbed. Three have been destroyed by the nearby construction of dams, and four have been altered by nearby geothermal development. After the 1959 Hebgen Lake earthquake (magnitude 7.3) occurred near Yellowstone, hundreds of geysers and hot springs changed their activity patterns. Any disruption of the delicate plumbing systems by humankind or nature involving the extraction of fluids, injection of fluids, or perturbation of the water table will almost always yield unpredictable and possibly detrimental consequences to nearby thermal features.

Geysers require very specific physical and thermal conditions. There must be a potent source of heat, usually magma or hot but solidified rocks. There must be abundant underground water to form a deep convection system, and there must be a focused pathway (usually a major fault or fracture in the Earth's crust) for the hot fluids to rise toward the surface at temperatures close to boiling. Near the surface, there must be a shallow, cavernous storage system for the underground water and steam. The storage system must have a constricted surface opening to propel its fluids into the air. Finally, the storage system must be capable of recharging and reheating between eruptions.

When these conditions are not met, features other than true geysers develop. If there is no focusing pathway to the surface, fluids may remain underground. If the heat source is not hot enough, or if there is too much cool groundwater, then warm springs may result. If underground temperatures are very high but there is too little groundwater present, then dry steam vents (fumaroles) will form. If the surface opening is too large or if the shallow reservoir allows free circulation, then instabilities may not develop, and the hot spring may simply boil but not erupt; such vents are called perpetual spouters.

Geyser Eruptions

The causes of geyser eruptions are complex, and no single theory can explain the detailed activity of all geysers. The phenomenon, however, can be described generally in terms of the boiling behavior of water. Geysers are unstable, neutral-chloride hot springs that contain boiling or near-boiling water. The underground plumbing systems of geysers can be visualized as cavernous, vertical pipes that extend for tens to several hundred meters beneath the surface. On the surface, the water column is boiling at atmospheric pressure, and its temperature is, thus, about 100 degrees Celsius. At a depth of several hundred meters, the water is under considerable hydrostatic pressure and may have a temperature of several hundred degrees. When the water in any part of the system is heated close to its boiling point, then a very small drop in pressure will cause the water to “flash” explosively into steam.

One way to achieve the necessary pressure drop is to decrease the weight of the overlying water column, causing a slight drop in the hydrostatic pressure, which can happen as a consequence of boiling in the upper levels of the water column: Because steam bubbles are much lighter than water, any replacement of water by steam will result in a decrease of the column density, which will be immediately felt as a pressure drop at deeper levels. Many geyser pools begin to bubble and their water levels rise within a few moments of eruption, as the water is displaced by rising steam bubbles, some of which become trapped in the roofs of cavernous chambers. At some point in the plumbing system, flashing eventually occurs, and a mixture of steam and water is propelled out of the orifice. The eruption of fluids lowers the pressures in successively deeper levels of the system, and flashing occurs as a downward-propagating chain reaction. When all the eruptible fluids have been ejected from the reservoir, the eruption ceases. Among geysers that occur in groups, eruptions are often highly irregular as a result of interconnected plumbing systems that allow the shunting of hot fluids into one or another plumbing system.

The periodicity of geysers (the length of time between eruptions) depends on how long it takes the plumbing system to be recharged with hot water (which is largely controlled by the permeability of the surrounding rocks and the volume of the geyser reservoir), the time required for this water to reheat back to its boiling point, and the amount of fluid that was discharged during the previous event. Eruptions of long duration represent thorough evacuation of the reservoir. The time required for subsequent recharge will, thus, be lengthened, and the succeeding eruption will usually occur after a longer-than-normal period. Some irregularity is, therefore, typical of most geysers, including Yellowstone's Old Faithful, but the timing and vigor of an impending eruption can often be predicted quite accurately if the nature of the previous eruption is known.

Geyser eruptions can sometimes be induced by the addition of soap to the vent because the water's surface tension is lowered and frothing occurs more readily. This practice is illegal in virtually all protected natural geyser areas, as it can do irreparable harm to the orifice and plumbing system. Similarly, throwing coins or other objects into geyser pools is potentially harmful to the orifice or the plumbing.

Principal Terms

convection: the transfer of heat by mass movement, such as by the flow of hot water and steam

fumarole: a vent that emits only gases

geothermal gradient: the rate at which temperature increases with depth in the Earth

hydrostatic pressure: the pressure imposed by the weight of an overlying column of water

hydrothermal or geothermal: general terms that refer to natural systems of hot fluids that circulate underground

rhyolite: a type of silica-rich volcanic rock that is uncommon on the Earth but occurs almost universally beneath hydrothermal areas

sinter: a type of nonprecious opal that forms around chloride hot springs and geysers

sublimate: solid, crystalline material that is deposited directly from the vapor state; crystals of native sulfur around fumarole mouths are an example

travertine: a type of hot-spring limestone that is deposited from alkaline waters

water table: the level below which all rocks are saturated with water

Bibliography

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