Supernovas And The Formation Of The Heavy Elements

Supernovas are fundamentally important for life and the state of the universe, since nearly all of the elements heavier than carbon are formed in massive stars, and the elements heavier than bismuth 209 are all formed in supernova explosions. Only the elements hydrogen and helium are primordial in the universe, meaning that they have been in existence since the earliest moments of the universe. All of the other elements have been produced by nucleosynthesis, or the combination of large atomic nuclei from smaller ones, in stars and in more energetic events such as supernovas.

Heavy elements are created by successive nuclear fusion reactions, beginning with the fusion of two hydrogen atoms to form helium; then helium can fuse to carbon in some star cores. The temperature in very massive stars can be high enough to fuse carbon into magnesium, but it is very rare to synthesize any elements that require the fusion of two nuclei larger than carbon because the nuclear forces between the protons become prohibitively large with larger atomic nuclei. Production of heavier elements typically happens by a different process—the capture of a helium atom by a larger atomic nucleus—to produce heavier elements. In this way, a carbon 12 nucleus can collide with a helium 4 nucleus to produce oxygen 16, and oxygen 16 can collide with helium 4 to produce neon 20. The process of helium capture is thought to have produced many of the heavier elements in the universe, because a plot of the abundance of the elements shows that elements with nuclear masses of 4 units (helium), 12 units (carbon), 16 units (oxygen),

ID T3 C

Helium

Carbon ®0xy9en

Neon

Carbon ®0xy9en

Neon

Beryllium

Atomic number

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Plot of cosmic abundance of elements and their isotopes expressed relative to the abundance of hydrogen. The horizontal axis shows atomic number. Note how many of the common elements are located on peaks, with other elements being tens to hundreds of times less abundant. The elements on the peaks (e.g., iron) were produced in stellar nucleosynthesis.

20 units (neon), 24 units (magnesium), and 28 units (silicon) stand out as prominent peaks. The process continues in large-mass stars with sulfur 32, argon 36, calcium 40, titanium 44, chromium 48, iron 52, and nickel 56. However, nickel 56 is unstable and quickly decays into cobalt 56, then iron 56, which is a very stable nucleus (the most stable of all nuclei), consisting of 26 protons and 30 neutrons. This process therefore inevitably leads to the buildup of stable iron in the core of massive stars. Many other nuclear reactions occur in large evolved stars, but the plot of the relative abundance of the elements shows that helium capture is one of the most important in synthesizing the heavier elements.

A different process is needed to make elements heavier than iron. This process, called the slow, or s-process by astronomers, involves the capture and absorption of neutrons by other nuclei. Neutron capture occurs in the cores of large evolved stars, where the nuclei of iron atoms capture some of the neutrons produced as by-products of the nuclear reactions going on in the core. The process of adding neutrons to an atomic nucleus changes the isotope of that element to a heavier isotope, but it is still the same element. At some point, however, there are so many neutrons that the isotope decays radioactively to produce a nucleus of a new element. For instance, iron 56 will add neutrons and become iron 59, which will decay to cobalt 59. Cobalt 59 will then add neutrons to become cobalt 60, which decays to nickel 60, and the process goes on and on, producing successively heavier elements. It typically takes an atomic nucleus about a year to capture a neutron by this process so each unstable nucleus decays to the more stable form before the next neutron is added. The s-process is responsible for the synthesis of most heavy elements on Earth and in the solar system and universe, including the atoms in common things such as gold in jewelry, lead in batteries, and nearly all of the other heavy metals and elements.

Elements heavier than bismuth 209 can not be produced by the s-process since any nuclei heavier than bismuth 209 produced by neutron capture are unstable and immediately decay back to bismuth 209. Another mechanism, called the rapid or r-process is the only one known that can synthesize the heaviest elements such as thorium 232 and uranium 238, and this process occurs only in supernova explosions.

The violence of the first 15 minutes of a supernova explosion creates huge numbers of free neutrons so that the neutron capture rate of nuclei is so high that even unstable nuclei capture new neutrons before they can decay to more stable forms. The rapid bombardment of these nuclei with many neutrons in the first 15 minutes of the supernova creates all of the elements heavier than bismuth 209, explaining why these elements are so rare in the universe. This explains why the abundance of the heaviest elements (heavier than iron) is a billion times lower than the abundance of hydrogen and helium.

The early or primordial universe contained only hydrogen and helium, and all of the heavier elements were created in nucleosynthesis reactions inside stars or in supernova explosions. This model is supported by the observation that older globular clusters have more hydrogen and helium in them, and younger clusters are enriched in the heavier elements, having concentrated the remnants of novas and supernovas over time. Stars form when interstellar clouds are compressed by shock waves; then the stars evolve. Solar-sized stars evolve along the main sequence and end up as white dwarfs, and more massive stars end their lives in spectacular supernova explosions. Both processes spew heavy elements into interstellar space, where they may be captured in new interstellar clouds, and compressed into new stars by shock waves from supernova and other events.

See also astronomy; astrophysics; constellation; dwarfs (stars); nova; origin and evolution of the 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. Dibon-Smith, Richard. The Constellations Web Page. Available online. URL: http://www.dibonsmith.com/ index.htm. Last update November 8, 2007. Prialnik, Dina. "Novae." In Encyclopedia of Astronomy and Astrophysics, edited by Paul Murdin, 1,8461,856. London: Institute of Physics Publishing Ltd and Nature Publishing Group, 2001. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West Publishing Company, 1991.

telescopes The word telescope comes from the Greek tele (far) and skopein (to look or see), meaning far-seeing. The Greek mathematician Giovanni Demisiani coined the word in 1611 for a refracting instrument designed by Galileo Galilei, who modified an instrument built a few years earlier in 1608 in the Netherlands by two spectacle makers, Hans Lipper-shey and Zacharias Janssen. In 1616 the Italian Jesuit astronomer and physicist Niccolo Zucchi invented the first reflecting telescope, which Isaac Newton improved in 1668. Now the term telescope is used to describe a wide range of scientific instruments that observe remote objects by collecting electromagnetic radiation from them and enhancing this radiation by different processes in different types of telescopes. In the 20th century a wide range of types of telescopes were designed and constructed to collect and enhance radiation from a wide variety of wavelengths in the spectrum.

Many different types of telescopes exist, with the most common being optical telescopes. Optical telescopes are widely used in astronomy, and similar technology is also used in many other practical instruments such as in spotting telescopes, binoculars and monoculars, camera lenses, and theodolites for surveying instruments. optical telescopes collect and focus light from the visible part of the electromagnetic spectrum, whereas other types of telescopes work in the infrared and ultraviolet wavelengths. These telescopes increase the angular size and apparent brightness of distant objects by using a series of curved optical elements (lenses or mirrors) to gather the light and focus it at a focal point where it is enhanced from the original strength. The different types of optical telescopes include the following:

• refracting telescopes, which use lenses to enhance the light and form an image

• reflecting telescopes, which use mirrors to form the image

• catadioptric telescopes, which use a combination of lenses and mirrors to form the image

Radio telescopes collect electromagnetic radiation from distant objects using directional radio antennas with a parabolic shape, and these are often arranged in groups. They are designed using a conductive wire mesh with openings that are smaller than the wavelength being observed. When these large antennae are arranged in groups they can collect data with a wavelength that is similar in size to the separation between the antenna dishes. one such array of radio telescopes is the Very Large Array located in Socorro, New Mexico. The individual telescopes in this array can be moved so that they have different separations; in this way they can be used to collect data from a wide variety of wavelengths. This process is known as aperture synthesis. Distant radio telescopes can be linked in this process to study very long wavelengths, a process known as Very Long Baseline Interfer-ometry (VLBI). The largest array size exceeds the diameter of the Earth. The VLBI Space Observation Program satellite uses a space-based system established by Japan in 2005. Radio telescopes can also be used to collect and study microwave radiation, such as signals from distant and faint quasars.

X-ray and gamma-ray telescopes collect radiation of these wavelengths that can pass through most metal and glass. Since the Earth's atmosphere is opaque to X-rays and gamma-rays, these telescopes must be based in space or from high-flying balloons. Most of these telescopes use a system of ring-shaped glancing mirrors that reflect the rays only a few degrees and do not completely focus the radiation. Instead the signal is interpreted using a system called coded aperture masks, where the patterns of shadows on the altered images can be interpreted to form an image.

See also astronomy; Brahe, Tycho; cosmic microwave background radiation; electromagnetic spectrum; Galilei, Galileo; Kepler, Johannes; quasar; radio galaxies; remote sensing.

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. Hewitt, Adelaide, ed. Optical and Infrared Telescopes for the 1990s. Proceedings. Tucson: Kitt Peak National Observatory, 1980. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West Publishing Company, 1991.

Tertiary The Tertiary is first period of the Ceno-zoic era, extending from the end of the Cretaceous of the Mesozoic at 66 million years ago until the beginning of the Quaternary 1.6 million years ago. The Tertiary is divided into two periods, the older Paleogene (66-23.8 Ma) and the younger Neogene (23.8-1.8 Ma), and further divided into five epochs, the Paleocene (66-54.8 Ma), Eocene (54.8-33.7 Ma), Oligocene (38.7-23.8 Ma), Miocene (23.8-5.3 Ma), and Pliocene (5.3-1.6 Ma). The term Tertiary was first coined by the Italian geologist Giovanni Arduino in 1758 and later adopted by Charles Lyell in 1833 for his post-Mesozoic sequences in western Europe. The term Tertiary is being gradually replaced by the terms Paleogene and Neogene periods.

The Tertiary is informally known as the "age of mammals" for its remarkably diverse group of mammals, including marsupial and placental forms that appeared abruptly after the extinction of the dinosaurs. The mammals radiated rapidly in the Tertiary while climates and seawater became cooler. The continents moved close to their present positions by the end of the Tertiary, with major events including the uplift of the Himalayan-Alpine mountain chain.

Pangaea continued to break apart through the early Tertiary, while the African and Indian plates began colliding with Eurasia, forming the Alpine-

Schematic plate reconstruction of western North America and the NE Pacific Ocean for the Tertiary, showing the Kula-Farallon spreading center interacting with the convergent margin of North America and meeting the Kula-Pacific and Farallon-Pacific spreading centers far offshore in the paleo-Pacific Ocean

Himalayan mountain chain. Parts of the Cordilleran mountain chain experienced considerable amounts of strike-slip translation of accreted terranes, with some models suggesting thousands of kilometers of displacement of individual terranes. The Cordillera of western North America experienced an unusual geologic event with the subduction of at least one oceanic spreading ridge beneath the convergent margin. The boundaries between three plates moved rapidly along the convergent margin from about 60 million years ago in the north to about 35 million years ago in the south, initiating a series of geological consequences including anomalous magmatism, metamorphism, and deformation. New subduction zones were initiated in the southwest Pacific (South east Asia) and in the Scotia arc in the south Atlantic. The Hawaiian-Emperor sea mount chain formed as a hot-spot track, with the oldest preserved record starting about 70 million years ago and a major change in the direction of motion of the Pacific plate recorded by a bend in the track near Midway Island formed 43 million years ago.

The san Andreas fault system was initiated about 30 million years ago as the East Pacific rise was subducted beneath western North America, and the relative motions between the Pacific plate and the North American plate became parallel to the margin. Around 3.5 million years ago the Panama arc grew, connecting North and south America, dramatically changing the circulation patterns of the world's oceans and influencing global climate. The East African rift system began opening about 5-2 million years ago, forming the sheltered environments that hosted the first known Homo sapiens.

Climate records show a general cooling of ocean waters and the atmosphere from the earliest Tertiary through the Paleocene, with warming then cooling in the Eocene. The oceans apparently became stratified with cold bottom waters and warmer surface waters in the Eocene, with further cooling reflecting southern glaciations in the Oligocene. Late Oligocene through Early Miocene records indicate a period of warming, followed by additional cooling in the mid-Miocene with the expansion of the Antarctic ice sheet that continued through the end of the Miocene. Pliocene climates began fluctuating wildly from warm to cold, perhaps as a precursor to the Pleistocene ice ages and interglacial periods. The Late Pliocene climates and change into the Pleistocene ice ages were strongly influenced by the growth of the Panama arc and the closing of the ocean circulation routes between the Pacific and Atlantic oceans. The Panama isthmus blocked warm Caribbean waters from moving west into the Pacific Ocean, but forced these waters into the Gulf Stream that brings warm water northward into the Arctic Ocean basin. Warm waters here cause increased evaporation and precipitation, leading to rapid growth of the northern glaciers.

Nearly all of the mammals present on the Earth today appeared in the Cenozoic, and most in the Tertiary, with the exception of a primitive group known as the pantotheres, which arose in the Middle Cretaceous. The pantotheres evolved into the first marsupial, the opossum, which in turn branched into the first placental mammals that spread over much of the northern continents, India, and Africa by the Late Cretaceous. Pantotheres and earlier mammals laid eggs, whereas marsupial offspring emerge from an eggshelllike structure in the uterus early in their development but then develop further in an external pouch. In contrast, placental mammals evolve more fully inside the uterus and emerge stronger with a higher likelihood of surviving infancy. It is believed that this evolutionary advantage led to the dominance of placental mammals and the extinction of the pantotheres.

Mammalian evolution in the Tertiary was strongly influenced by continental distributions. Some continents like Africa, Madagascar, India, and Australia were largely isolated. Connections or land-bridges between some of these and other continents, such as the Bering landbridge between Alaska and Siberia, allowed communication of taxa between continents. With the land distribution patterns, certain families and orders evolved on one continent and others on other continents. Rhinoceroses, pigs, cattle, sheep, antelope, deer, cats, and related families evolved primarily in Asia, whereas horses, dogs, and camels evolved chiefly in North America, with some families reaching Europe. Horses have been used as a model of evolution with progressive changes in the size of the animals, as well as the complexity of their teeth and feet.

Marine faunas included gastropods, echinoids, and pelecypods along with bryozoans, mollusks, and sand dollars in shallow water. Coiled nautiloids floated in open waters, whereas sea mammals including whales, sea cows, seals, and sea lions inhabited coastal waters. The Eocene-Oligocene boundary is marked by minor extinctions, and the end of the Pliocene saw major marine extinctions caused by changes in oceanic circulation with massive amounts of cold waters pouring in from the Arctic and from meltwater from growing glaciers.

See also Cenozoic; historical geology; neogene; plate tectonics.

FURTHER READING

Bradley, Dwight C., Timothy M. Kusky, Peter Haeussler, David C. Rowley, Richard Goldfarb, and Steve Nelson. "Geologic Signature of Early Ridge Subduction in the Accretionary Wedge, Forearc Basin, and Mag-matic Arc of South-Central Alaska." In Geology of a Transpressional Orogen Developed During a Ridge-Trench Interaction along the North Pacific Margin, Special Paper, edited by Virginia B. Sisson, Sarah Roeske, and Terry L. Pavlis. Denver: Geological Society of America, 2003. Pomeral, C. The Cenozoic Era. New York: John Wiley & Sons, 1982.

Savage, R. J. G., and M. R. Long. Mammal Evolution: An Illustrated Guide. New York: Facts On File, 1986.

thermodynamics Thermodynamics is the study of the transformation of heat into and from other forms of energy, particularly mechanical, chemical, and electrical energy. The science is concerned with energy conversions into heat and the relations of this conversion to variables including pressure, temperature, and volume. The name comes from the Greek therme, meaning heat, and dynamis, meaning power. Thermodynamics forms the basis of many principles of chemistry, physics, and earth sciences. The core of the science is based on statistical predictions of the collective motion of atoms and molecules based on their microscopic behavior. In this sense heat means energy in transit, and dynamics refers to movement, so thermodynamics can also be thought of as the study of the movement of energy. To study the movement of heat and energy between different objects, it is important to define systems and surroundings. For thermodynamics a system is defined as a group of particles whose average motion defines its properties, which are related to each other by equations of state (thermodynamic equations that describe the state of matter under a given set of physical conditions such as temperature, pressure, volume, or internal energy). Thermodynamics uses these equations to describe how systems respond to changes in their surroundings.

The study of thermodynamics rose from the study of steam engines and efforts to find ways to make them more efficient. The first law of thermodynamics states that energy can be neither created nor destroyed and that heat and mechanical work are mutually convertible. This is why moving engines get hot: The mechanical energy is transformed into heat energy. The second law of thermodynamics states that it is impossible for an unaided self-acting machine to transfer heat from a low-temperature body to a higher-temperature body. As an example, an ice cube can not make a cup of coffee warmer. Fundamental to the second law of thermodynamics is the quantity entropy (abbreviated as S), which is a measure of the unavailability of a system's energy to do work and is basically a measure of the randomness of the molecules in the system. The third law of thermodynamics states that it is impossible to reduce any system to absolute zero temperature (0°K, -273°C, or -459°F).

Energy is the capacity to do work, and it can exist in many different forms. Potential energy is energy of position, such as when an elevated body exhibits gravitational potential in that it can move to a lower elevation under the influence of gravity. Kinetic energy is the energy of motion and can be measured as the mean speed of the constituent molecules of a body. Einstein's theory of relativity showed that mass too can be converted to energy, as

E=mc2

where E = energy, m = mass, and c = the speed of light. This remarkable relationship forms the basis of atomic power, and many mysteries of the universe.

Heat is a form of kinetic energy that manifests itself as motion of the constituent atoms of a substance. According to the laws of thermodynamics, heat may be transferred only from high-temperature bodies to lower-temperature bodies, and it does so by convection, conduction, or radiation. The specific heat of a substance is the ratio of the quantity of heat required to raise the temperature of a unit mass of the substance through a given range of temperature to the heat required to raise the temperature of an equal mass of water through the same range.

Conduction is the flow of heat through a material without the movement of any part of the material. The heat is transferred as kinetic energy of the vibrating molecules, which is passed from one molecule or atom to another. Convection is the transfer of heat through a fluid (liquid, gas, or slow-moving solid such as the Earth's mantle) by moving currents. Radiation is a heat transfer by infrared rays. All materials radiate heat, but hotter objects emit more heat energy than cold objects. Infrared radiation can pass through a vacuum and operates at the speed of light. Radiative heat can be reflected and refracted across boundaries, but it does not affect the medium through which it passes.

See also atmosphere; black holes; clouds; convection and the Earth's mantle; energy in the Earth system; geochemistry; geophysics; granite, granite batholith; hot spot; mantle plumes; radioactive decay; thunderstorms, tornadoes.

FURTHER READING

Cengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. New York: McGraw-Hill, 2005.

Dunning-Davies, Jeremy. Concise Thermodynamics: Principles and Applications. Chichester, U.K.: Horwood Publishing, 1997. Van Ness, H. C. Understanding Thermodynamics. New York: Dover Publications, 1969.

thermohaline circulation Thermohaline circulation refers to the vertical mixing of seawater driven by density differences caused by variations in temperature and salinity. Variations in formation and circulation of ocean water driven by thermohaline circulation may cause some of the thousands-of-years to decadal scale variations in climate. Cold water forms in the Arctic and Weddell Seas. This cold salty water is denser than other water in the ocean, so it sinks to the bottom and gets ponded behind seafloor topographic ridges, periodically spilling over into other parts of the oceans. The formation and redistribution of North Atlantic cold bottom water accounts for about 30 percent of the solar energy budget input to the Arctic Ocean every year. Eventually, this cold bottom water works its way to the Indian and Pacific Oceans where it upwells, gets heated, and returns to the North Atlantic. Variations in temperature and salinity that drive thermohaline circulation are found in waters that occupy different ocean basins and in those found at different levels in the water column. When the density of water at one level is greater than or equal to that below that level, the water column becomes unstable and the denser water sinks, displacing the deeper, less-dense waters below. When the dense water reaches the level at which it is stable it spreads out laterally and forms a thin sheet, forming intricately stratified ocean waters. Thermohaline circulation is the main mechanism responsible for the movement of water out of cold polar regions, and it exerts a strong influence on global climate. The upward movement of water in other regions balances the sinking of dense cold water, and these upwelling regions typically bring deep water, rich in nutrients, to the surface. Thus, regions of intense biological activity are often associated with upwelling regions.

The coldest water on the planet is formed in the polar regions, with large quantities of cold water originating off the coast of Greenland, and in the Weddell Sea of Antarctica. The planet's saltiest ocean water is found in the Atlantic Ocean, and this is moved northward by the Gulf stream. As this water moves near Greenland it is cooled and then sinks to flow as a deep cold current along the bottom of the western North Atlantic. The cold water of the Weddell sea is the densest on the planet, where surface waters are cooled to -35.4°F (-1.9°C), then sink to form a cold current that moves around Antarctica. Some of this deep cold water moves northward into all three major ocean basins, mixing with other waters and warming slightly. Most of these deep ocean currents move at a few to 10 centimeters per second.

Presently, the age of bottom water in the equatorial Pacific is 1,600 years, and in the Atlantic it is 350 years. Glacial stages in the North Atlantic have been correlated with the presence of older cold bottom waters, approximately twice the age of the water today. This suggests that the thermohaline

Warm less saline shallow currents

Cold saline deep currents

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Heat releases to atmosphere

Heat releases to atmosphere

Map of the world's oceans showing main warm and cold currents driven by thermohaline circulation circulation system was only half as effective at recycling water during recent glacial stages, with less cold bottom water being produced during the glacial periods. These changes in production of cold bottom water may in turn be driven by changes in the North American ice sheet, perhaps itself driven by 23,000-year orbital (Milankovitch) cycles. Scientists suggest that a growth in the ice sheet would cause the polar front to shift southward, decreasing the inflow of cold saline surface water into the system required for efficient thermohaline circulation. Several periods of glaciation in the past 14,500 years (known as the Dryas) are thought to have been caused by sudden, even catastrophic injections of glacial meltwater into the North Atlantic, which would decrease the salinity and hence density of the surface water. This in turn would prohibit the surface water from sinking to the deep ocean, inducing another glacial interval.

Shorter-term decadal variations in climate in the past million years are indicated by so-called Heinrich events, defined as specific intervals in the sedimentary record showing ice-rafted debris in the North Atlantic. These periods of exceptionally large iceberg discharges reflect decadal-scale sea-surface and atmospheric cooling and are related to thickening of the North American ice sheet followed by ice stream surges associated with the discharge of the icebergs. These events flood the surface waters with low-salinity freshwater, leading to a decrease in flux to the cold bottom waters, and hence a short-period global cooling.

Changes in the thermohaline circulation rigor have also been related to other global climate changes. Droughts in the Sahel and elsewhere are correlated with periods of ineffective or reduced thermohaline circulation, because this reduces the amount of water drawn into the North Atlantic, in turn cooling surface waters and reducing the amount of evaporation. Reduced thermohaline circulation also reduces the amount of water that upwells in the equatorial regions, in turn decreasing the amount of moisture transferred to the atmosphere, reducing precipitation at high latitudes.

Atmospheric levels of greenhouse gases such as carbon dioxide (C02) and atmospheric temperatures show a correlation to variations in the thermohaline circulation patterns and production of cold bottom waters. Co2 is dissolved in warm surface water and transported to cold surface water, which acts as a sink for the Co2. During times of decreased flow from cold, high-latitude surface water to the deep ocean reservoir, Co2 can build up in the cold polar waters, removing it from the atmosphere and decreasing global temperatures. In contrast, when the thermohaline circulation is vigorous, cold oxygen-rich surface waters downwell, and dissolve bur ied CO2 and even carbonates, releasing this C02 to the atmosphere and increasing global temperatures.

The present-day ice sheet in Antarctica grew in the Middle Miocene, related to active thermohaline circulation that caused prolific upwelling of warm water that put more moisture in the atmosphere, falling as snow on the cold southern continent. The growth of the southern ice sheet increased the global atmospheric temperature gradients, which in turn increased the desertification of mid-latitude continental regions. The increased temperature gradient also induced stronger oceanic circulation, including upwelling and removal of Co2 from the atmosphere, lowering global temperatures, and bringing on late Neogene glaciations.

Ocean bottom topography exerts a strong influence on dense bottom currents. Ridges deflect currents from one part of a basin to another and may restrict access to other regions, whereas trenches and deeps may focus flow from one region to another.

See also climate; climate change; ocean basin; ocean currents.

FURTHER READING

Ashworth, William, and Charles E. Little. Encyclopedia of Environmental Studies. New ed. New York: Facts On File, 2001.

Botkin, D., and E. Keller. Environmental Science. Hobo-

ken, N.J.: John Wiley & Sons, 2003. Intergovernmental Panel on Climate Change. Available online. URL: http://www.ipcc.ch/index.htm. Accessed January 30, 2008. Intergovernmental Panel on Climate Change. Climate Change 2007: The Physical Science Basis. Contributions of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller, 996. Cambridge, U.K.: Cambridge University Press, 2007. Windows to the Universe, Thermohaline Ocean Circulation home page. University Corporation for Atmospheric Research. Available online. URL: http://www. windows.ucar.edu/tour/link=/earth/Water/thermoha-line_ocean_circulation.html. Accessed October 10, 2008.

thunderstorms, tornadoes Any storm that contains lightning and thunder may be called a thunderstorm. However, the term normally implies a gusty heavy rainfall event with numerous lightning strikes and thunder, emanating from a cumulonimbus cloud or cluster or line of cumulonimbus clouds. There is a large range in the severity of thunderstorms from minor to severe, with some causing extreme damage through high winds, lightning, tornadoes, and flooding rains.

Thunderstorms are convective systems that form in unstable rising warm and humid air currents. The air may start rising as part of a converging air system, along a frontal system, as a result of surface topography, or from unequal surface heating. The warmer the rising air is than the surrounding air, the greater the buoyancy forces acting on the rising air. scattered thunderstorms that typically form in summer months are referred to as ordinary thunderstorms, and these typically are short-lived, produce only minor to moderate rainfall, and do not have severe winds. However, severe thunderstorms associated with fronts or combinations of unstable conditions may have heavy rain, hail, strong winds or tornadoes, and drenching or flooding rains.

ordinary thunderstorms are most likely to form in regions where surface winds converge, causing parcels of air to rise, and where there is not significant wind shear or change in the wind speed and direction with height. These storms evolve through several stages, beginning with the cumulus or growth stage, where the warm air rises and condenses into cumulus clouds. As the water vapor condenses it releases a large amount of latent heat that keeps the cloud warmer than the air surrounding it and causes it to continue to rise and build as long as it is fed from air below. simple cumulus clouds may quickly grow into towering cumulus congestus clouds in this way. As the cloud builds above the freezing level in the atmosphere, the particles in the cloud get larger and heavier and eventually are too large to be kept entrained in the air currents, and they fall as precipitation. As this precipitation is falling, drier air from around the storm is drawn into the cloud, but as the rain falls through this dry air it may evaporate, cooling the air. This cool air is then denser than the surrounding air and it may fall as a sudden downdraft, in some cases enhanced by air pulled downward by the falling rain.

The development of downdrafts marks the passage of the thunderstorm into the mature stage, in which the upward and downward movement of air constitutes a convective cell. In this stage the top of the storm typically bulges outward in stable levels of the stratosphere, often around 40,000 feet (12,192 m), forming the anvil shape characteristic of mature thunderstorms. Heavy rain, hail, lightning, and strong, turbulent winds may come out of the base of the storms, which can be several miles in diameter. Cold downwelling air often expands out of the cloud base, forming a gust front along its leading edge, forcing warm air up into the storm. Most mature storm cells begin to dissipate after half an hour or so, as the gust front expands away from the storm and can no longer enhance the updrafts that feed the storm. These storms may quickly turn into gentle rains, and then evaporate, but the moisture may be quickly incorporated into new, actively forming thunderstorm cells.

severe thunderstorms are more intense than ordinary storms, producing large hail, wind gusts of greater than 50 knots (57.5 m/hr, or 92.5 km/hr), more lightning, and heavy rain. Like ordinary thunderstorms, severe storms form in areas of upwelling unstable moist warm air, but severe storms tend to develop in regions where there is also strong wind shear. The high level winds have the effect of causing the rain that falls out of the storm to fall away from the region of upwelling air so that it does not have the effect of weakening the upwelling. In this way the cell becomes much longer lived and grows stronger and taller than ordinary thunderstorms, often reaching heights of 60,000 feet (18,288 m). hail may be entrained for long times in the strong air currents and even thrown out of the cloud system at height, falling several kilometers from the base of the cloud. Down-drafts from severe storms are marked by bulbous mammatus clouds.

supercell thunderstorms form where strong wind shear aloft is such that the cold downwelling air does not cut off the upwelling air, and a giant rotating storm with balanced updrafts and downdrafts may be maintained for hours. These storms may produce severe tornadoes, strong downbursts of wind, large (grapefruit-sized) hail, very heavy rains, and strong winds exceeding 90 knots (103.5 m/hr, or 167 km/hr).

unusual winds are associated with some thunderstorms, especially severe storms. Gust fronts may be quite strong with winds exceeding 60 miles per hour (97 km/hr), followed by cold gusty and shifty winds. Gust fronts may be marked by lines of dust kicked up by the strong winds, or ominous-looking shelf clouds formed by warm moist air rising above the cold descending air of the gust front. in severe cases, gust fronts may force so much air upward that they generate new multi-celled thunderstorms with their own gust fronts that merge, forming an intense gust front called an outflow boundary. intense down-drafts beneath some thunderstorms spread laterally outward at speeds sometimes exceeding 90 miles per hour (145 km/hr) when they hit the ground and are termed downbursts, microbursts, or macrobursts depending on their size. some clusters of thunderstorms produce another type of unusual wind called a straight line wind, or derecho. These winds may exceed 90 miles per hour (145 km/hr), and extend for tens or even hundreds of miles.

Thunderstorms often form either in groups called mesoscale convective systems or as lines of storms called squall lines. squall lines typically form along

Direction of storm motion

Debris Funnel or Wall Tail cloud tornado cloud cloud

Debris Funnel or Wall Tail cloud tornado cloud cloud

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Direction of storm motion

Scud

Scud

Gust front outflow

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Cross section of typical thunderstorm or within a zone up to a couple of hundred miles in front of the cold front where warm air is compressed and forced upward. squall lines may form lines of thunderstorms hundreds or even a thousand miles long, and many of the storms along the line may be severe with associated heavy rain, winds, hail, and tornadoes. Mesoscale convective complexes form when many individual thunderstorm cells across a region start to act together, forming an exceedingly large convective system that may cover more than 50,000 square miles (130,000 km2). These systems move slowly and may be associated with many hours of flooding rains, hail, tornadoes, and wind.

Cumulonimbus clouds typically become electrically charged during the development of thunderstorms, although the processes that lead to the unequal charge distribution are not well known. About 20 percent of the lightning generated in thunderstorms strikes the ground, with most passing from cloud to cloud. Lightning is an electrical discharge that heats the surrounding air to 54,000°F (30,000°C), causing the air to expand explosively, creating the sound waves heard as thunder. As the air expands along different parts of the lightning stroke, the sound is generated from several different places, causing the thunder to have a rolling or echoing sound, enhanced by the sound waves bouncing off hills, buildings, and the ground. Cloud-to-ground lightning forms when negative electrical charges build up in the base of the cloud, causing positive charges to build in the ground. When the electrical potential gradient reaches 3 million volts per meter along several tens of meters, electrons rush to the cloud base and form a series of stepped leaders that reach toward the ground. At this stage, a strong current of positive charge moves up, typically along an elevated object, from the ground to the descending leader. As the two columns meet huge numbers of electrons rush to the ground, and a several-centimeter wide column of positively charged ions shoots up along the lightning stroke, all within one ten-thousandth of a second. The process then may be repeated several or even dozens of times along the same path, all within a fraction of a second.

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    What are the heavy elements that were formed during the formation of plate?
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