Reef Systems

Reefs are wave-resistant, framework-supported carbonate or organic mounds generally built by carbonate-secreting organisms, or in some usages the term may be used for any shallow ridge of rock lying near the surface of the water. Reefs contain a plethora of organisms that together build a wave-resistant structure to just below the low-tide level in the ocean waters and provide shelter for fish and other organisms. The spaces between the framework are typically filled by skeletal debris, which together with the framework become cemented together to form a wave-resistant feature that shelters the shelf from high-energy waves. Reef organisms (presently consisting mainly of zooxanthellae) can survive only in the photic zone, so reef growth is restricted to the upper 328 feet (100 m) of the seawater.

Reefs are built by a wide variety of organisms, today including red algae, mollusks, sponges, and cnidarians (including corals). The colonial sclerac-tinia corals are presently the principal reef builders, producing a calcareous external skeleton characterized by radial partitions known as septa. Inside the skeleton are soft-bodied animals called polyps, containing symbiotic algae essential for the life cycle of the coral and the building of the reef structure. The polyps contain calcium bicarbonate that is broken down into calcium carbonate, carbon dioxide, and water. The calcium carbonate is secreted to the reef building its structure, whereas the algae photosyn-thesize the carbon dioxide, producing food for the polyps.

There are several different types of reefs, classified by their morphology and relationship to nearby landmasses. Fringing reefs grow along and fringe the coast of a landmass and are often discontinuous. They typically have a steep outer slope, an algal ridge crest, and a flat, sand-filled channel between the reef and the main shoreline. Barrier reefs form at greater distances from the shore than fringing reefs, and are generally broader and more continuous than fringing reefs. They are among the largest biological structures on the planet—for instance, the Great Barrier Reef of Australia is 1,430 miles (2,300 km) long. A deep, wide lagoon typically separates barrier reefs from the mainland. All of these reefs show a zonation from a high-energy side on the outside or windward side of the reef, grow fast, and have a smooth outer boundary. In contrast, the opposite side of the reef receives little wave energy and may be irregular and poorly developed, or grade into a lagoon. Many reefs also show a vertical zonation in the types of organisms present, from deepwater to shallow levels near the sea surface.

Atolls or atoll reefs form circular-, elliptical-, or semicircular-shaped islands made of coral that rise from deep water; atolls surround central lagoons, typically with no internal landmass. Some atolls do have small central islands, and these, as well as parts of the outer circular reef, are in some cases covered by forests. Most atolls range in diameter from half a mile to more than 80 miles (1-130 km), and are most common in the western and central Pacific Ocean basin and in the Indian Ocean. The outer margin of the semicircular reef on atolls is the most active site of coral growth, since it receives the most nutrients from upwelling waters on the margin of the atoll. On many atolls coral growth on the outer margin is so intense that the corals form an overhanging ledge from which many blocks of coral break off during storms, forming a huge pile of broken reef debris at the base of the atoll called talus slope. Volcanic rocks, some of which lie more than half a mile (1 km) below current sea level, underlay atolls. since corals can grow only in very shallow water fewer than 65 feet (20 m) deep, the volcanic islands must have formed near sea level, grown coral, and subsided over time, with the corals growing at the rate that the volcanic islands were sinking.

Charles Darwin proposed such an origin for atolls in 1842 based on his expeditions on the HMS Beagle from 1831 to 1836. He suggested that volcanic islands were first formed with their peaks exposed above sea level. At this stage coral reefs were established as fringing reef complexes around the volcanic island. He suggested that with time the volcanic islands subsided and were eroded, but that the growth of the coral reefs was able to keep up with the subsidence. In this way, as the volcanic islands sank below sea level, the coral reefs continued to grow and eventually formed a ring circling the location of the former volcanic island. When Darwin proposed this theory in 1842, he did not know that ancient, eroded volcanic mountains underlay the atolls he studied. More than 100 years later, drilling confirmed his prediction that volcanic rocks would be found beneath the coralline rocks on several atolls.

With the advent of plate tectonics in the 1970s the cause of the subsidence of the volcanoes became apparent. When oceanic crust is created at midocean ridges, it is typically about 1.7 miles (2.7 km) below sea level. With time, as the oceanic crust moves away from the midocean ridges, it cools and contracts, sinking to about 2.5 miles (4 km) below sea level. In many places on the seafloor small volcanoes form on the oceanic crust a short time after the main part of the crust formed at the midocean ridge. These volcanoes may stick above sea level a few hundred meters. As the oceanic crust moves away from the midocean ridges, these volcanoes subside below sea level. If the volcanoes happen to be in the tropics where corals can grow, and if the rate of subsidence is slow enough for the growth of coral to keep up with subsidence, then atolls may form where the volcanic island used to be. If corals do not grow or cannot keep up with subsidence, then the island subsides below sea level and the top of the island gets scoured by wave erosion, forming a flat-topped mountain that continues to subside below sea level. These flat-topped mountains are known as guyots, many of which were mapped during exploration of the seafloor associated with military operations of World War II.

Reefs are extremely sensitive and diverse environments and cannot tolerate large changes in temperature, pollution, turbidity, or water depth. Reefs have also been subject to mining, destruction for navigation and even sites of testing nuclear bombs in the Pacific. Thus human-induced and natural changes in the shoreline environment pose a significant threat to the reef environment.

See also coral; deltas; estuary; hurricanes; ocean basin; sea-level rise.

FURTHER READING

Beatley, Timothy, David J. Brower, and Anna K. A. Schwab. Introduction to Coastal Management. Washington, D.C.: Island Press, 1994. Davis, R., and D. Fitzgerald. Beaches and Coasts. Malden,

Mass.: Blackwell, 2004. Dean, C. Against the Tide: The Battle for America's Beaches. New York: Columbia University Press, 1999.

Dolan, Robert, Paul J. Godfrey, and William E. Odum. "Man's Impact on the Barrier Islands of North Carolina." American Scientist 61 (1973): 152-162. Kaufman, W., and Orrin H. Pilkey Jr. The Beaches are Moving. Durham, N.C.: Duke University Press, 1983. King, C. A. M. Beaches and Coasts. London: Edward Arnold, 1961.

Komar, Paul D., ed. CRC Handbook of Coastal Processes and Erosion. Boca Raton, Fla.: CRC Press, 1983. Longshore, David. Encyclopedia of Hurricanes, Typhoons, and Cyclones, New Edition. New York: Facts on File, 2008.

Nordstrom, K. F., N. P. Psuty, and R. W. G. Carter. Coastal Dunes: Form and Process. New York: John Wiley & Sons, 1990. Pilkey, O. H., and W. J. Neal. Coastal Geologic Hazards. In The Geology of North America, Volume 1-2, The Atlantic Continental Margin, edited by R. E. Sheridan and J. A. Grow. Boulder, Colo.: Geological Society of America, 1988.

U.S. Army Corps of Engineers Engineer Research and Development Center home page. Available online. URL: http://www.erdc.usace.army.mil/. Updated August 22, 2008. U.s. Army Corps of Engineers home page. Available online. URL: http://www.usace.army.mil/. Updated September 17, 2008.

Williams, Jeffress, Kurt A. Dodd, and Kathleen K. Gohn. Coasts in Crisis. Reston, Va. United States Geological Survey, Circular 1075, 1990.

benthic, benthos The benthic environment includes the ocean floor and the benthos are those organisms that dwell on or near the seafloor. Bottom-dwelling benthos organisms include large plants that grow in shallow water, as well as animals that dwell on the seafloor at all depths.

Many of the sediments on the deep seafloor are derived from erosion of the continents and carried to the deep sea by turbidity currents, carried by wind (e.g., volcanic ash), or released from floating ice.

Other sediments, known as deep-sea oozes, include pelagic sediments derived from marine organic activity. When small organisms such as diatoms die in the ocean, their shells sink to the bottom and over time can make significant accumulations. Calcareous ooze occurs at low to middle latitudes where warm water favors the growth of carbonate-secreting organisms. Calcareous oozes are not found in water more than 2.5-3 miles (4-5 km) deep because this water is under such high pressure that it contains a lot of dissolved Co2, which dissolves carbonate shells. The depth below which all calcium-bearing shells and tests dissolve is known as the calcium carbonate compensation depth. Siliceous ooze is produced by organisms that use silicon to make their shell structure.

The benthic world is amazingly diverse, yet parts of the deep seafloor are less explored than the surface of the Moon. organisms that live in the benthic community generally use one or more of three main strategies for living. Some attach themselves to anchored surfaces and get food by filtering it from the seawa-ter. other organisms move freely about on the ocean

bottom and get their food by predation. Still others burrow or bury themselves in the ocean bottom sediments and obtain nourishment by digesting and extracting nutrients from the benthic sediments. All the benthic organisms must compete for living space and food, with other factors including light levels, temperature, salinity, and the nature of the bottom controlling the distribution and diversity of some organisms. Species diversification is related to the stability of the benthic environment. Areas that experience large variations in temperature, salinity, and water agitation tend to have low species diversification, but may have large numbers of a few different types of organisms. In contrast, stable environments tend to show much greater diversity, with a larger number of species present.

There are a large number of different benthic environments. Rocky shore environments in the intertidal zone have a wide range of conditions from alternately wet and dry to always submerged, with wave agitation and predation being important factors. These rocky shore environments tend to show a distinct zonation in benthos, with some organisms inhabiting one narrow niche and other organisms in others. Barnacles and other organisms that can firmly attach themselves to the bottom do well in wave-agitated environments, whereas certain types of algae prefer areas from slightly above the low-tide line to about 33 feet (10 m) depth. The area around the low-tide mark tends to be inhabited by abundant organisms, including snails, starfish, crabs, mussels, sea anemones, urchins, and hydroids. Tide pools are highly variable environments that host specialized plants and animals including crustaceans, worms, starfish, snails, and seaweed. The subtidal environment may host lobster, worms, mollusks, and even octopus. Kelp, brown benthic algae, inhabit the sub-tidal zone in subtropical to subpolar waters and can grow down to a depth of about 130 feet (40 m), often forming thick underwater forests that may extend along a coast for many kilometers.

Sandy and muddy bottom benthic environments often form at the edges of deltas, sandy beaches, marshes, and estuaries. Many of the world's temperate to tropical coastlines have salt marshes in the intertidal zone and beds of sea grasses growing just below the low-tide line. Surface-dwelling organisms in these environments are known as epifauna, whereas organisms that bury themselves in the bottom sand and mud are called infauna. Many of these organisms obtain nourishment either by filtering seawater that they pump through their digestive system or by selecting edible particles from the sea-floor. Deposit-feeding bivalves such as clams inhabit the area below the low-tide mark, whereas other deposit feeders may inhabit the intertidal zone. Other organisms that inhabit these environments include shrimp, snails, oysters, tube-building crustaceans, and hydroids.

Coral reefs are special benthic environments that require warm water greater than 64.4°F (18°C) to survive. Colonial animals secrete calcareous skeletons, placing new active layers on top of the skeletons of dead organisms, and thus build the reef structure. Encrusting red algae, as well as green and red algae, produce the calcareous cement of the coral reefs. The reef hosts a huge variety and number of other organisms, some growing in symbiotic relationships with the reef builders, others seeking shelter or food among the complex reef. Upwelling waters and currents bring nutrients to the reef. The currents release more nutrients produced by the reef organisms. Some of the world's most spectacular coral reefs include the Great Barrier Reef, off the northeast coast of Australia, reefs along the Red Sea and Indonesia, and reefs in the Caribbean and south Florida.

unique forms of life were recently discovered deep in the ocean near hot vents located along the midocean ridge system. The organisms that live in these benthic environments are unusual in that they get their energy from chemosynthesis of sulfides exhaled by hot hydrothermal vents, and not from photosynthesis and sunlight. The organisms that live around these vents include tube worms, sulfate-reducing chemosynthetic bacteria, crabs, giant clams, mussels, and fish. The tube worms grow to enormous size, some being 10 feet (3 m) long and 0.8-1.2 inches (2-3 cm) wide. Some of the bacteria that live near these vents include the most heat-tolerant (thermophyllic) organisms recognized on the planet, living at temperatures of up to 235°F (113°C). They are thought to be some of the most primitive organisms known, being both chemosynthetic and ther-mophilic, and may be related to some of the oldest life-forms that inhabited the Earth.

The deep seafloor away from the midocean ridges and hot vents is also inhabited by many of the main groups of animals that inhabit the shallower continental shelves. The number of organisms on the deep seafloor is few, however, and the animals tend to be much smaller than those at shallower levels. Some deepwater benthos similar to the hot-vent communities have recently been discovered living near cold vents above accretionary prisms at subduction zones, near hydrocarbon vents on continental shelves, and around decaying whale carcasses.

See also beaches and shorelines; black smoker chimneys; continental margin.

binary star systems Most stars are parts of systems that include two or more stars that rotate in

NASA image captured by Chandra X-Ray Observatory of two white dwarf stars in binary star system J0806 (UPI Photo/NASA/Landov)

orbit around each other. When the system consists of two stars, it is known as a binary star system. Larger groups of stars are known as multiple star systems, or star clusters. optical doubles are stars that appear to be binaries but are actually not related and just appear to be close in their visible configuration. In binary systems the two stars rotate about their common center of mass (the center of mass of both stars combined) and are held in place by the mutual gravitation attraction between them.

Binary star systems are classified according to how they appear to astronomers on Earth. simple visual binaries are systems in which the two stars are far enough apart to be visibly distinct when viewed through a telescope from Earth, and each star is bright enough to be monitored separately from the other. In other cases the binary system may be too far, or the stars too close or small, to be visibly distinct from Earth, but the rotation of the stars around each other can be detected spectroscopically by observing shifts in the frequency and wavelength of a wave for an observer moving relative to the source of the waves, known as Doppler shifts, as each star alternately moves toward and away from the observer on Earth as the stars rotate around each other. The Doppler shift is recorded as a shift toward the blue end of the spectrum as the star moves toward the observer, and a redshift as the star moves away. Binary systems that can be detected only by using these spectroscopic Doppler shifts are known as spectroscopic binaries, and they are of two main types. Double-line spectroscopic binaries contain two distinct sets of spectral lines, one for each star, that shift back and forth from blue to redshifts as the star moves alternately toward and away from the observer. In these systems both stars are large and bright enough to be distinguished spectroscopically. In other systems one star may be too small or faint to be distinguished from the other, and the result is a single-line system in which one set of spectroscopic lines is observed to shift back and forth, caused by the stars rotating around each other even though they are too close to be resolved individually.

A rare class of binary star systems is known as eclipsing binaries. In these systems the orbital plane of the binary system is aligned nearly head-on with the line of sight from Earth, so as each star passes in front (in the line of sight) of the other, it blocks the light coming from the blocked star, and the amount of light observed from Earth alternately changes as each star passes periodically in front of the other. Observations of eclipsing binaries can yield information about each star's mass, orbits, orbital periods, radii, and luminosity or brightness.

The range in the orbital periods of binary star systems is very large, spanning from hours to centuries. Knowledge of the orbital periods, plus the distance to the binary system, can be used to determine additional physical properties of the binary system, such as the combined mass of the stars. If the distance of each star from the center of mass of the system can be measured, then the individual masses of each star can also be determined. Calculations based on observations of binary star systems have formed the basis for most of what is known about the masses of stars in the solar system.

See also astronomy; astrophysics; Einstein, Albert; electromagnetic spectrum; universe.

FURTHER READING

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Upper Saddle River, N.J.: Addison-Wesley, 2007.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West, 1991.

biosphere The biosphere encompasses the part of the Earth that is inhabited by life, and includes parts of the lithosphere, hydrosphere, and atmosphere. Life evolved more than 3.8 billion years ago, and has played an important role in determining the planet's climate and insuring that it does not venture out of the narrow window of parameters that allow life to continue. In this way the biosphere functions as a self-regulating system that interacts with chemical, erosional, depositional, tectonic, atmospheric, and oceanic processes on the Earth.

Most of the Earth's biosphere depends on photosynthesis for its primary source of energy, driven ultimately by energy from the Sun. Plants and many bacteria use photosynthesis as their primary metabolic strategy, whereas other microorganisms and animals rely on photosynthetic organisms as food for their energy, and thus use solar energy indirectly. Most of the organisms that rely on solar energy live, by necessity, in the upper parts of the oceans (hydrosphere), lithosphere, and lower atmosphere. Bacteria are the dominant form of life on Earth (comprising about 5 x 1030 cells), and also live in the greatest range of environmental conditions. Some of the important environmental parameters for bacteria include temperature, between -41 to 235°F (-5 to

113°C), pH levels from 0 to 11, pressures between a near vacuum and 1,000 times atmospheric pressure, and supersaturated salt solutions to distilled water.

Bacteria and other life-forms exist with diminished abundance to several miles (kilometers) or more beneath the Earth's surface, deep in the oceans, and some bacterial cells and fungal spores are found in the upper atmosphere. The lack of nutrients and the lethal levels of solar radiation above the shielding effects of atmospheric ozone limit life in the upper atmosphere.

Soils and sediments in the lithosphere contain abundant microorganisms and invertebrates at shallow levels. Bacteria exist at much deeper levels and are being found in deeper and deeper environments as exploration continues. Bacteria are known to exist to about two miles (3.5 km) in pore spaces and cracks in rocks, and deeper in aquifers, oil reservoirs, and salt and mineral mines. Deep microorganisms do not rely on photosynthesis, but rather use other geo-chemical or geothermal energy to drive their metabolic activity.

The hydrosphere and especially the oceans teem with life, particularly in the near-surface photic zone environment where sunlight penetrates. At greater depths below the photic zone most life is still driven by energy from the Sun, as organisms rely primarily on food provided by dead organisms that filter down from above. In the benthic environment of the seafloor there may be as many as 10 billion (1010) bacteria per milliliter of sediment. Bacteria also exist beneath the level that oxygen can penetrate, but the bacteria at these depths are anaerobic, primarily sul-fate-reducing varieties. Bacteria are known to exist to greater than 2,789 feet (850 m) beneath the seafloor.

In 1977 a new environment for a remarkable group of organisms was discovered on East Pacific Rise and observed directly in 1979 by geologist Peter Lonsdale and his team from Woods Hole oceano-graphic Institute in Massachusetts using the deep-sea submarine ALVIN. The organisms survive on the seafloor along the midocean ridge system, where hot hydrothermal vents spew heated nutrient-rich waters into the benthic realm. In these environments seawa-ter circulates into the ocean crust where it is heated near oceanic magma chambers. This seawater reacts with the crust and leaches chemical components from the lithosphere, then rises along cracks or conduits to form hot black and white smoker chimneys that spew the nutrient-rich waters at temperatures of up to 662°F (350°C). Life has been detected in these vents at temperatures of up to 235°F (113°C). The vents are rich in methane, hydrogen sulfide, and dissolved reduced metals such as iron that provide a chemical energy source for primitive bacteria. Some of the bacteria around these vents are sulfate-reducing che-

mosynthetic thermophyllic organisms, living at high temperatures using only chemical energy and therefore exist independently of photosynthesis. These and other bacteria are locally so great in abundance that they provide the basic food source for other organisms, including spectacular worm communities, crabs, giant clams, and even fish.

See also atmosphere; benthic, benthos; black smoker chimneys; supercontinent cycles.

FURTHER READING

Raven, Peter, and Linda Berg. Environment. New York: John Wiley & Sons, 2008.

black holes The final stage of stellar evolution for stars with a large mass may be a black hole, a superdense collection of matter that has collapsed from a giant star or stars, and has such a strong gravity field that nothing can escape from it, not even light. Black holes are known by physicists as a singularity, a point with zero radius and infinite density. These dense but invisible objects form when a star has at least three solar masses left in its core after it has completed burning its nuclear fuel. This stage of stellar evolution is typically marked by the star experiencing a supernova explosion, after which, if enough mass is left over, the star's nucleus collapses to a small point and warps space-time, forming a black hole. Black holes have such a strong gravitational field that they apparently draw material into them that is never to be seen again.

Black holes are one of the possible end states of old stars. Stars that have a low total mass (less than 1.4 solar masses) end their evolution as a white dwarf, whereas stars with masses between 1.4 and three solar masses may end their life cycle as a small dense mass known as a neutron star. When the mass of the dying star is greater than three solar masses, the star collapses as the nuclear fuel runs out. The gravitational attraction of the mass is so great that electrons and even neutrons cannot support the core against its own gravity, so it continues to collapse into what is called a singularity. There is no force known in nature that is strong enough to resist the gravitational attraction of a collapsing star once the pressure is so great that the neutrons degenerate and collapse. The force is so strong that not even light can escape from inside a black hole, hence the name.

The concept of a black hole as an infinitesimally small singularity with infinite mass is difficult to comprehend, in part because it is not adequately explained by the classical Newtonian laws of physics, or gravitational theory. To understand fully the workings of black holes it is necessary to move into the realm of quantum mechanics and Albert Einstein's theory of relativity. A quantitative treatment of relativity is beyond the scope of this book, but many aspects of Einstein's theories can be understood qualitatively. To understand how black holes work, it is necessary to know that nothing can travel faster than the speed of light, and that the gravitational force acts on everything, including electromagnetic radiation, or light.

To understand black holes, it is necessary to understand the concept of escape velocity. Objects on Earth must move 6.8 miles (11 km) per second to escape the pull of the Earth's gravitational attraction and move into open space. Escape velocity ve for any planetary or stellar object is proportional to the square root of the mass—of the body being escaped from divided by the square root of its radius r, which is the distance from the center of the body being escaped from and the location where the moving object is escaping. This can be written as

where G is the gravitational constant, 6.67428 x 10-11m3kgs-1. This relationship means that for objects denser and smaller than the Earth but with the same mass, the escape velocity would need to be faster for any object to escape its gravitational field. As a massive object such as a huge star begins to collapse, therefore, the escape velocity required for anything to leave its gravitational field rises rapidly as the star shrinks from a large radius to a small object a fraction of its original size. If the star shrinks to a quarter of its original size, the escape velocity doubles—and as a star experiences a rapid collapse after a supernova, the escape velocity rises to such extremely high values that it is virtually impossible for any object to escape the star's gravitational pull. If an object the size of the Earth were to collapse to about 1/3 inch (1 cm), the escape velocity would be 186,000 miles per second (300,000 km/sec), the speed of light. From Einstein's theory of relativity, which states that even light is attracted by gravity, it becomes clear that at some point during the collapse of massive stars even light will no longer be able to escape the gravitational pull of the body, and the collapsed star will become dark forever. Since some large stars collapse to a size smaller than an elementary particle, the escape velocity becomes infinitely high, and the gravitational attraction becomes stronger and stronger. At this point the black hole can pull objects in, but nothing can ever escape. That is the meaning of the term black hole. The only way to detect a black hole is by its immense gravitational field, which can deflect light as it bends toward the huge gravitational pull. Astronomers use sophisticated measurements to v e r tell when a star moves optically behind a black hole, and they can measure the deflection of the light. This allows for the determination of some of the physical properties (like mass, charge, and angular momentum) of the black hole.

Every object with a specific mass has a critical radius at which the escape velocity equals the speed of light. When the object is compressed to that radius, nothing can escape its gravitational pull—not even light—and the object becomes invisible. This critical radius is known as the schwarzschild radius, named after the German physicist Karl schwarzschild, who first described this phenomenon. The sun has a schwarzschild radius of about 9.8 feet (3 m), but stars with the mass of the sun do not usually collapse to become black holes since they are too small. The smallest stellar objects that form black holes have about three solar masses, and the schwarzschild radius for these stars is about 5.6 miles (9 km).

Another concept useful for understanding black holes is that of the event horizon, which is the surface of an imaginary sphere with a radius equal to the schwarzschild radius, centered on a collapsing star. The event horizon is an imaginary surface, but can be thought of as the surface of the black hole, since beyond the event horizon, no event that happens can ever be heard, seen, or detected by any known means. The event horizon does not represent the size of the material that collapsed to form the black hole. since this material should theoretically collapse to a tiny singularity, it merely represents the radius past which the gravitational pull of the dense black hole at the center of the sphere is so strong that nothing can escape once inside that radius.

Black holes are said to warp the space-time continuum in the way we understand it from a classical Newtonian mechanical way. According to Einstein's theory of relativity, all matter tends to warp space in its vicinity, and objects respond to this warp by changing their direction of movement as they approach other objects. Newtonian physics would describe this as a gravitational pull, whereas relativity theory suggests that the objects are just following the curved space that was distorted by the nearby massive object. The more massive the objects, the more they curve the space. In the case of black holes the warping of space is extreme because of the huge mass in the black hole, and at the event horizon, space is actually folded over upon itself, such that objects that cross the event horizon disappear from space forever.

As material falls into a black hole, the gravitational stresses are so great that they distort and tear apart objects as they plunge toward the event horizon. These objects become heated and emit radiation, so the regions surrounding black holes are sometimes emitters of strong radiation. once the material crosses the event horizon, however, nothing can escape, not light, not radiation, and the mass is never seen or heard from again.

The gravity fields of black holes are so strong that it is virtually impossible to get close to one without being physically torn apart by the strong gravity, and the difference in the strength of the gravity from one end of any approaching object (or person) and the other end. Nonetheless it is interesting and informative to discuss what it might be like to approach, and even enter, a black hole. The first thing an outside observer of an object approaching a black hole would notice is that light, and other electromagnetic radiation coming from the object, shows a redshift (toward longer wavelengths) that increases as the object gets closer to the event horizon. This is not a Doppler shift caused by the motion of the object, as the object near the black hole would exhibit the redshift even if it were motionless with respect to the observer. This redshift is a quantum mechanical effect known as a gravitational redshift. Einstein's general theory of relativity shows that as photons (light) try to escape a strong gravitational field, they have to use up some of their energy. Photons are light, and they always move at the speed of light, so this loss of energy is equated with a decrease in frequency, or a lengthening of the wavelength of the light (or other electromagnetic radiation) coming from the object approaching the black hole. The distant observer measures this as a redshift. Interestingly, an observer on the object emitting the radiation would see no redshift, and the radiation (light) would have the same energy and wavelength as when it was emitted. These gravitational redshifts have been measured on light coming from many dense objects in the universe, and objects even the size of Earth and the sun have detectable gravitational redshifts. The largest, by far, are from black holes.

Black holes distort the space-time continuum. Another strange quantum mechanical effect explained in Einstein's theory of general relativity is time dilation near massive objects such as black holes. A distant observer looking at a clock on the object approaching the black hole would notice that the clock (and time itself) moves progressively slower and slower as the object approaches the black hole's event horizon. When the object is at the event horizon, the clock (and time) would appear to stand still, and from the perspective of the outside observer, the object would be frozen at the event horizon forever, never entering past it. However, from the perspective of anyone on the object approaching the event horizon, there is no difference in the way time passes; each second seems like one second. The object and observer would simply pass through the event horizon and notice nothing different (assuming they could withstand the strong gravitational forces). Time dilation is difficult to understand but can be thought of in the same way that the redshift of electromagnetic radiation occurs. If time is considered to be measured, for instance, as the passage of a wavelength of light, each second corresponding to the passage of one wave crest, then as the wavelengths are increased by the gravitational redshift, the time is also gradually expanded until it appears to stop by the outside observer.

No one really knows what happens inside the event horizon of a black hole. The laws of physics do not adequately explain such dense small objects as singularities, and some new concepts are being investigated by physicists, such as a merging of the laws of quantum mechanics and general relativity into the field of quantum gravity—but these investigations are incomplete. There are many ideas, some approaching science fiction, that have been proposed for what may happen near the singularity at the center of the black hole's event horizon. Some models suggest that new states of matter are created; others have suggested that black holes may be gateways for matter and energy to enter other universes or to travel in time.

Black holes are difficult to detect, since they are invisible. Their huge gravitational field, however, and the energy released by matter outside the event horizon as it falls into the black hole may be detectable. There are several good candidates for possible black holes in the Milky Way Galaxy. The best may be a massive but invisible body in a binary star system known as Cygnus X-1. This possible black hole is orbiting with a supergiant star companion, and is known as a powerful X-ray source (presumably from the material approaching the event horizon). This binary star system has an orbital diameter of 12.4 million miles (20 million km) and an orbital period of 5.6 days, and the mass of the system is 30 times that of the sun. Calculations show that the invisible component of this binary system has a mass of 5-10 times that of the sun, enough to have formed a black hole. In this system it appears that hot gases are flowing from the supergiant star into the black hole companion, and this is the source of the X-ray radiation. Other calculations show that the invisible part of this binary star is small, less than 186,000 miles (300,000 km) across, and other calculations show that it is likely less than 186 miles (300 km) across. Thus Cygnus X-1 is one of the most likely candidates for a black hole in the Milky Way Galaxy. There are nearly a dozen other black hole candidates in the Milky Way Galaxy, and as the observational powers of physicists increase with new space-borne telescopes, more and more are being discovered. What is needed is a breakthrough in the field of quantum gravity to understand what may really happen underneath the event horizon.

See also astronomy; astrophysics; binary star systems; dwarfs (stars); Einstein, Albert; stellar evolution.

FURTHER READING

"Black Holes, Gravity's Relentless Pull." Support provided by the National Aeronautics and Space Administration (NASA). Available online. URL: http://hubble-site.org/explore_astronomy/black_holes/home.html. Accessed October 9, 2008. Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Upper Saddle River, N.J.: Addison-Wesley, 2007.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West, 1991.

black smoker chimneys Black smoker chimneys are hydrothermal vent systems that typically form near active magmatic systems along the mid-ocean ridge system, approximately 2 miles (3 km) below sea level. They were first discovered by deep submersibles exploring the oceanic ridge system near the Galápagos Islands in 1979, and many other examples have been documented since then, including a number along the mid-Atlantic ridge.

Black smokers are hydrothermal vent systems that form by seawater percolating into fractures in the seafloor rocks near the active spreading ridge, where the water gets heated to several hundred degrees Celsius. This hot pressurized water leaches minerals from the oceanic crust and extracts other elements from the nearby magma. The superheated water and brines then rise above the magma chamber in a hydrothermal circulation system and escape at vents on the seafloor, forming the black smoker hydrothermal vents. The vent fluids are typically rich in hydrogen sulfides (H2S), methane, and dissolved reduced metals, such as iron. The brines may escape at temperatures greater than 680°F (360°C), and when these hot brines come into contact with cold seawater, many of the metals and minerals in solution rise in plumes, since the hot fluids are more buoyant than the colder seawater. The plumes are typically about 0.6 miles (1 km) high and 25 miles (40 km) wide and can be detected by temperature and chemical anomalies, including the presence of primitive helium 3 isotopes derived from the mantle. These plumes may be rich in dissolved iron, manganese, copper, lead, zinc, cobalt, and cadmium, which

Black smoker chimney from the East Pacific Rise showing tube worms feeding at base of the chimney (Science Source/Photo Researchers, Inc.)

rain out of the plumes, concentrating these elements on the seafloor. Manganese remains suspended in the plumes for several weeks, whereas most of the other metals are precipitated as sulfides (e.g., pyrite, FeS2; chalcopyrite, CuFeS2; sphalerite, ZnS), oxides (e.g., hematite, Fe2O3), orthohydroxides (e.g., goethite, FeOOH), or hydroxides (e.g., limonite, Fe(OH)3). A group of related hydrothermal vents that form slightly farther from central black smoker vents, known as white smokers, typically have vent temperatures between 500 and 572°F (260-300°C).

On the seafloor along active spreading ridges the hydrothermal vent systems form mounds that are typically 164-656 feet (50-200 m) in diameter, and some are more than 66 feet (20 m) high. Clusters of black smoker chimneys several meters high may occupy the central area of mounds and deposit iron-copper sulfides. White smoker chimneys typically form in a zone around the central mound, depositing iron-zinc sulfides and iron oxides. Some mounds on the seafloor have been drilled to determine their internal structure. The Trans-Atlantic Geotraverse (TAG) hydrothermal mound on the mid-Atlantic ridge is capped by central chimneys made of pyrite, chalcopyrite, and anhydrite, overlying massive pyrite breccia, with anhydrite-pyrite and silica-pyrite-rich zones found a few to tens of meters below the surface. Below this the host basalts are highly silicified, then at greater depths form a network of chlori-tized breccia. White smoker chimneys made of pyrite (FeS2) and sphalerite (ZnS) rim the central mound. In addition to the sulfides, oxides, hydroxides, and orthohydroxides, including several percent copper and zinc, the TAG mound contains minor amounts of gold.

Seafloor hydrothermal mounds and particularly the black smoker chimneys host a spectacular community of unique life-forms, found only in these environments. Life-forms include primitive sulfate-reducing thermophilic bacteria, giant worms, giant clams, crabs, and fish, all living off the chemosyn-thetic metabolism made possible by the hydrothermal vent systems. Life at the black smokers draws energy from the internal energy of the Earth (not the Sun), via oxidation in a reducing environment. Some of the bacteria living at these vents are the most primitive organisms known on Earth, suggesting that early life may have resembled these chemosynthetic thermophilic organisms.

Black smoker chimneys and the entire hydrothermal mounds bear striking similarities to volcanogenic massive sulfide (VMS) deposits found in Paleozoic and older ophiolite and arc complexes including the Bay of Islands ophiolite in Newfoundland, the Troo-dos ophiolite in Cyprus, and the Semail ophiolite in Oman. Even older VMS deposits are common in Archean greenstone belts, and these are typically basalt or rhyolite-hosted chalcopyrite, pyrite, sphalerite, copper-zinc-gold deposits that many workers have suggested may be ancient seafloor hydrothermal vents. Interestingly, complete hydrothermal mounds with preserved black and white smoker chimneys have been reported recently from the 2.5 billion-year-old North China craton, in the same belt that the world's oldest well-preserved ophiolite is located.

The tectonic setting for the origin of life on the early Earth is quite controversial. Some favor environments in shallow pools, some favor deep ocean environments where the organisms could get energy from the chemicals coming out of seafloor hydrothermal vents. The discovery of black smoker types of hydrothermal vents in Archean ophiolite sequences is significant because the physical conditions at these midocean ridges more than 2.5 billion years ago would have permitted the inorganic synthesis of amino acids and other prebiotic organic molecules. Some scientists think that the locus of precipitation and synthesis for life might have been in small iron-sulfide globules, such as those that form around black smokers. Black smoker chimneys may provide a window into the past and the origin of life on Earth.

See also Asian geology; benthic, benthos; biosphere; greenstone belts; ophiolites.

FURTHER READING

Scott, Steven. "Minerals on Land, Minerals in the Sea."

Bowen, Norman Levi (1887-1956) Canadian Petrologist, Geologist Dr. Norman Levi Bowen was one of the most brilliant igneous petrologists of the 20th century. Although he was born in Ontario, Canada, he spent most of his productive research career at the Geophysical Laboratories of the Carnegie Institute in Washington, D.C. Bowen studied the relationships between plagioclase feldspars and iron-magnesium silicates in crystallizing and melting experiments. From these experiments he derived the continuous and discontinuous reaction series explaining the sequence of crystallization and melting of these minerals in magmas. He also showed how magmatic differentiation by fractional crystallization can result from a granitic melt from an originally basaltic magma through the gradual crystallization of mafic minerals, leaving the felsic melt behind. Similarly he showed how partial melting of one rock type can result in a melt with a different composition than the original rock, typically forming a more felsic melt than the original rock and leaving a more mafic residue (or restite) behind. Bowen also worked on reactions between rocks at high temperatures and pressures, and the role of water in magmas. In 1928 Bowen published his pioneering book, The Evolution of Igneous Rocks.

N. L. Bowen is most famous for his works on the origin of igneous rocks, through the processes of magmatic differentiation by partial melting and magmatic differentiation by fractional crystallization. The phrase "magmatic differentiation by partial melting" refers to the process of forming magmas with differing compositions through the incomplete melting of rocks. For magmas formed in this way the composition of the magma depends on both the composition of the parent rock and the percentage of melt. If a rock melts completely, the magma has the same composition as the rock. However, rocks contain many different minerals, all of which melt at different temperatures. So if a rock is slowly heated, the resulting melt or magma will initially have the composition of the first mineral that melts, and then the first plus the second minerals that melt, and so on. If the rock melts completely, the magma will eventually end up with the same composition as the starting rock, but this does not always happen. oftentimes the rock only partially melts, so that the minerals with low melting temperatures contribute to the magma, whereas the minerals with high melting temperatures did not melt and are left as a residue (or restite). In this way the end magma can have a different composition than the rock from which it was derived.

Just as rocks partially melt to form different liquid compositions, magmas may solidify to different minerals at different times to form different solids (rocks). This process also results in the continuous change in the composition of the magma—if one mineral is removed, the resulting composition is different. If some process removes these solidified crystals that have been removed from the system of melts, a new magma composition results.

The removal of crystals from the melt system may occur by several processes, including the squeezing of melt away from the crystals or by sinking of dense crystals to the bottom of a magma chamber. These processes lead to magmatic differentiation by fractional crystallization, as first described by Bowen, who systematically documented how crystallization

Norman Levi Bowen (Queens University Archives)

of the first minerals changes the composition of the magma and leads to the formation of progressively more silicic rocks with decreasing temperature.

See also igneous rocks; petrology and petrography.

FURTHER READING

Bowen, Norman Levi. "Progressive Metamorphism of Siliceous Limestone and Dolomite." Journal of Geology 48, no. 3 (1940): 225-274.

-. "Recent High-Temperature Research on Silicates and Its Significance in Igneous Geology." American Journal of Science 33 (1937): 1-21. Bowen, Norman Levi, and John Frank Schairer. "The Problem of the Intrusion of Dunite in the Light of the Olivine Diagram." International Geological Congress 1 (1936): 391-396.

Brahe, Tycho (1546-1601) Danish Nobleman, Astronomer Tycho Brahe was born as Tyge Otte-sen Brahe on December 14, 1546, in Scania, a region of Denmark now part of Sweden. He was born to nobility, the son of Otte Brahe and Beate Bille, at his family's ancestral home, Knutstorp Castle. His father was a nobleman in the court of the Danish king, and he had an older and a younger sister, and a twin brother who died soon after birth. Tycho's uncle, Danish nobleman Jorgen Brahe, took him from his parents when he was two years old, and from then he lived at his uncle's home at Tosterup Castle. Brahe was educated at a Latin school from age six until he was 12. He enrolled at the University of Copenhagen in 1559 at age 13 and studied law but gradually became more interested in astronomy. One of the defining moments in his career was at Copenhagen, when he witnessed an eclipse on August 21, 1560, at the precise time that was predicted by his professors and astronomers. Tycho purchased a book, called an ephemeris, that gives tables and positions of astronomical objects in the sky at different times. He studied this and many other texts for several years until he came to realize in 1563 that most astronomical texts of the time disagreed with one another. He wrote that astronomy could not progress by the types of haphazard observations that he was reading and suggested that long-term systematic study of the heavens was needed. With the naked eye and the help of his sister Sophia he made many measurements of the stars and planets and improved many astronomical instruments. Brahe's work preceded the invention of the telescope, however, and later observations by German astronomer Johannes Kepler using the telescope proved to be more accurate than Brahe's.

In 1565 Brahe's uncle Jorgen Brahe died after a night of heavy drinking with Frederick II, king of

Denmark, when both men fell off a bridge into a river. Jorgen saved Frederick but caught pneumonia and later died. The next year Brahe and his friends were attending a dance at their professor's house and, after drinking considerably, he fought a duel with his fellow nobleman Manderup Parsbjerg in which Brahe lost part of his nose. This caused him to spend the rest of his life wearing prosthetic noses to hide his disfigurement. In 1571 Brahe's father died following a long illness, after which the son started building an astronomical observatory and alchemis-try laboratory at Herrevad Abbey near Ljungbyhed, Scania (present-day southwest Sweden).

At the age of 26 Tycho Brahe fell in love with a commoner, Kirsten Jorgensdatter, and they moved to Copenhagen and were married three years later. They had eight children, six of whom lived to adulthood, and the couple stayed together until Brahe died in 1601 at the age of 54. Brahe became quite wealthy and hosted many parties in his castle, at which his tame pet elk and dwarf Jepp, who acted as court jester, were usually present. Brahe thought the dwarf was clairvoyant and had him spend meals under the table. At one of Brahe's parties the elk consumed too much beer and fell down a flight of stairs and died. On October 10, 1601, Brahe became ill at a banquet and died 11 days later. His death was a mystery for years, many having thought he died of bacteria from drinking too much at the banquet. Since manners at the time did not allow one to get up and leave during a meal, his bladder was thought to have stretched and caused infection. But exhumation of his remains has shown that he more likely died of mercury poisoning, either accidentally by swallowing mercury-tainted medicine, or possibly by being murdered. The book Heavenly Intrigue: Johannes Kepler, Tycho Brahe, and the Murder Behind One of History's Greatest Scientific Discoveries by Joshua Gilder and AnneLee Gilder (2005) speculates that Johannes Kepler is likely to have poisoned Brahe, having the means, motive, and opportunity.

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