Hydrothermal Processes
Hydrothermal Processes
Introduction
Hydrothermal processes concern the subsurface movements of hot water. (“Thermos” means heat and “hydros” means water.) The heat is usually supplied by upwellings of magma from Earth's mantle, and the water comes from precipitation that percolates down from the surface. Ocean water can also come into contact with the magma that rises continuously from the mantle to form new oceanic crust along the mid-ocean ridges. Two metals, calcium and magnesium, are transported in large quantities by hydro-thermal processes at the sea floor and are important to the carbon dioxide balance of the ocean and thus of the atmosphere.
Historical Background and Scientific Foundations
Some manifestations of hydrothermal processes include geysers, fumaroles, and hot springs. These are generally found in regions of recent volcanic activity. At such locations, surface water may work its way down through rocks to high-temperature regions near a magma reservoir below Earth's surface. The water then becomes heated, therefore less dense, and rises back to the surface through fissures and cracks. Geysers such as Old Faithful Geyser in Yellowstone National Park in Wyoming erupt when a large amount of hot water fills an underground cavity, a portion of which is transformed into steam that escapes in a powerful jet out of the ground.
Fumaroles emit mixtures of steam and other gases. Hydrogen sulfide, one of the gases normally released from fumaroles, oxidizes to sulfuric acid and native sulfur at the surface. These chemicals account for the brightly colored rocks found in many thermal areas.
Hot springs are natural discharges of groundwater with elevated temperatures. They occur in thermal areas where the surface of Earth intersects the water table (the uppermost level of water-saturated rock). The temperature and rate of discharge of a hot spring are determined by the rate at which water circulates through the system of underground channels supplying the spring, the amount of heat supplied at depth, and how much the heated water is diluted by cool groundwater near the surface. Hot springs found in volcanic areas may have water temperatures near boiling.
Widespread hydrothermal activity along ocean ridges provides a chemical link between sea-floor processes and atmospheric carbon dioxide (CO2). Cold bottom waters can penetrate to depths of several kilometers below the sea floor through cracks in fresh ridge-crest basalt. Once this water is heated by and reacts chemically with the deeper hot basalt at temperatures above 572°F (300°C), it rises to the surface through hot springs on the ocean floor. The chemical reactions that occur during this process include the removal of magnesium and sulfate and the enrichment of calcium, potassium, and several other elements in the seawater.
The chemical exchange of calcium for magnesium is of particular importance because calcium reacts with bicarbonate (HCO3–) in the ocean to form CO2. Thus, the only significant process that balances calcium inputs to ocean water results in the release of CO2, which eventually finds its way into the atmosphere. Researchers estimate that hydrothermal vents presently account for 14–22% of all CO2 entering the atmosphere from natural sources.
Impacts and Issues
In the 1980s, Robert M. Owen and David K. Rea of the University of Michigan found evidence that increased hydrothermal activity on the seafloor may have been responsible for a period of increased CO2 atmospheric levels and subsequent global warming that occurred 50 million years ago. The two oceanographers hypothesized that tectonic activity (processes where sections of Earth's crust come into contact with one another) during the Eocene epoch caused heightened hydrothermal activity. This, in turn, caused a global greenhouse effect, which may provide the only historical analog to the anthropogenic (human-caused) global warming currently occurring.
In order to determine levels of past hydrothermal activity, Owen and Rea measured the concentrations of iron and silica, two chemicals commonly found in hot-spring waters, in sediment and rock core samples taken in the East Pacific. This data, along with geological data for the Eocene period gathered by other researchers, showed iron levels six times greater and silica levels up to 20 times greater than present ones. This indicated that hydrothermal processes in the ocean were indeed greatly heightened during the Eocene.
The Eocene was marked by a pronounced climate change comparable to what is projected for the world within the next century. The temperature increased 9°F (5°C) above that of the previous epoch. Data from that period also show that the air was humid, there was reduced atmospheric circulation, and an amplified warming occurred at the poles.
WORDS TO KNOW
EOCENE EPOCH: Geological period from 55.8 million years ago to 33.9 million years ago. Global climate was much warmer than today during most of the Eocene, with tropical conditions extending up into today's temperate latitude. The start of the Eocene was marked by the Paleocene-Eocene Thermal Maximum, a sudden rise in global temperature lasting only about 200,000 years that caused the extinction of many species and cleared the way for the evolution of modern mammals.
FUMAROLE: Opening in the ground that emits volcanic gases and steam. A gas commonly emitted is carbon dioxide.
GEYSER: Hot spring that periodically sprays steam and hot water into the air. A geyser requires a pathway from the water table in contact with a geothermal heat source.
JURASSIC PERIOD: Unit of geological time from 200 million years ago to 145 million years ago, famous in popular culture for its large dinosaurs. Global average temperature and atmospheric carbon dioxide concentrations were both much higher during the Jurassic than today.
MAGMA: Molten rock deep within Earth that consists of liquids, gases, and particles of rocks and crystals. Magma underlies areas of volcanic activity and at Earth's surface is called lava.
PRECIPITATION: Moisture that falls from clouds. Although clouds appear to float in the sky, they are always falling, their water droplets slowly being pulled down by gravity. Because the water droplets are so small and light, it can take 21 days to fall 1,000 ft (305 m) and wind currents can easily interrupt their descent. Liquid water falls as rain or drizzle. All raindrops form around particles of salt or dust. (Some of this dust comes from tiny meteorites and even the tails of comets.) Water or ice droplets stick to these particles, then the drops attract more water and continue getting bigger until they are large enough to fall out of the cloud. Drizzle drops are smaller than raindrops. In many clouds, raindrops actually begin as tiny ice crystals that form when part or all of a cloud is below freezing. As the ice crystals fall inside the cloud, they may collide with water droplets that freeze onto them. The ice crystals continue to grow larger, until large enough to fall from the cloud. They pass through warm air, melt, and fall as raindrops.
TECTONIC: Relating to tectonics, the scientific study of the forces that shape planetary crusts (mountain ranges, continents, sea-beds, etc.).
UPWELLING: The vertical motion of water in the ocean by which subsurface water of lower temperature and greater density moves toward the surface of the ocean. Upwelling occurs most commonly among the western coastlines of continents, but may occur anywhere in the ocean. Upwelling results when winds blowing nearly parallel to a continental coastline transport the light surface water away from the coast. Subsurface water of greater density and lower temperature replaces the surface water and exerts a considerable influence on the weather of coastal regions. Carbon dioxide is transferred to the atmosphere in regions of upwelling.
WATER TABLE: Underground level or depth below which the ground is saturated with liquid water. Where the water table intersects the surface, water is found (e.g., lakes, springs, streams).
Other climate change episodes have also been linked to hydrothermal processes. Henrik Svensen et al. (2003) have hypothesized that large hydrothermal vent complexes identified in the Vøring and Møre basins in the northern Atlantic and onshore in the Karoo basin in South Africa may have released enough methane, an important greenhouse gas, to trigger global climate change and mass extinctions. Past hydrothermal activity in the northern Atlantic corresponds to the onset of global warming during the Eocene, whereas hydrothermal activity in the Karoo basin may explain a period of ocean anoxia (lack of oxygen) during the early Jurassic Period. Anoxic events such as this may have precipitated mass extinctions and are hypothesized to occur during periods of global warming. Finally, a cooling episode during the late Eocene in the Ross Sea of Antarctica has also been linked to hydrothermal systems.
See Also Carbon Dioxide (CO2); Geothermal Energy; Greenhouse Effect; Oceans and Seas.
BIBLIOGRAPHY
Periodicals
Dallai, Luigi, et al. “Fossil Hydrothermal Systems Tracking Eocene Climate Change in Antarctica.” Geology 29, no. 10 (October 2001): 931–934.
Owen, Robert M., and David K. Rea. “Sea-Floor Hydrothermal Activity Links Climate to Tectonics: The Eocene Carbon Dioxide Greenhouse.” Science 227, no. 4683 (January 11, 1985): 166–169.
Shackleton, Sir Nicholas J., and Anne Boersma. “The Climate of the Eocene Ocean.” Journal of the Geological Society London 138, no. 2 (April 1981): 153–157.
Svensen, Henrik, et al. “Global Climate Change Resulting from Voluminous Intrusive Basaltic Volcanism in Sedimentary Basins: The Methane Transport and Eruption Mechanisms.” American Geophysical Union, Fall Meeting 2003, abstract #V21C-0528, 2003.
Weisburd, S. “Hot Springs, Warm Climate and CO2.” Science News 127 (March 23, 1985): 20.
Web Sites
“Geothermal Energy and Hydrothermal Activity.” USGS Cascades Volcano Observatory. May 12, 2005. <http://vulcan.wr.usgs.gov/Glossary/ThermalActivity/description_thermal_activity.html> (accessed November 4, 2007).
Michele Chapman