Precession Nutation

The chances of experiencing a large meteorite impact on Earth are small, but the risks associated with large impacts are extreme. Small objects hit the Earth many times every day but burn up in the atmosphere. Events that release enough energy to destroy a city happen about once every thousand years, while major impacts that can significantly alter the Earth's climate happen every 300,000 years. Truly catastrophic impacts that cause mass extinctions, death of at least 25 percent of the world's population, and could lead to the end of the human race occur about every 100 million years.

Specific hazards from impacts include atmospheric shock waves and air blasts, major earthquakes, monstrous tsunamis, and global firestorms that throw so much soot in the air the impact is followed by a global winter that could last years. Carbon dioxide can be released by impacts as well and then can act as a greenhouse gas leading to global warming.

More than 20,000 near-Earth objects are thought to have a potential to collide with Earth, and more than 150 of these are larger than half a mile (1 km) across. A variety of programs to detect and track these near Earth objects is under way, yet most meteorite impacts and near collisions in the past few years have been complete surprises. If a large asteroid is found to be on a collision course with Earth, several strategies have been devised that may be able to move the object out of its collision course with Earth. The asteroid could be attacked with nuclear weapons that could vaporize the object, removing the threat. However, this could also break up the asteroid and send thousands of smaller, now radioactive, fragments to Earth. If nuclear weapons are detonated near the asteroid, the force of the explosions may be enough to push it out of its collision course. A massive spacecraft could be crashed into the asteroid, changing its momentum and moving it from orbit. It might be possible to install rocket propulsion systems on the asteroid and have it steer itself out of Earth orbit. A variety of other techniques have been proposed to deflect asteroids, including beaming solar radiation at the body, or attaching thermal blankets or sails, to have the solar radiation pressure move the asteroid out of its collision course.

See also asteroid; comet; solar system.

FURTHER READING

Alvarez, W. T. Rex and the Crater of Doom. Princeton,

N.J.: Princeton University Press, 1997. Angelo, Joseph A. Encyclopedia of Space and Astronomy.

New York: Facts On File, 2006. Chapman, C. R., and D. Morrison. "Impacts on the Earth by Asteroids and Comets: Assessing the Hazard." Nature 367 (1994): 33-39. Cox, Donald, and James Chestek. Doomsday Asteroid: Can

We Survive? New York: Prometheus Books, 1996. Elkens-Tanton, Linda T. Asteroids, Meteorites, and Comets. New York: Chelsea House, 2006. Kusky, T. M. Asteroids and Meteorites: Catastrophic Collisions with Earth. New York: Facts On File, 2009.

Martin, P. S., and R. G. Klein, eds., Quaternary Extinctions. Tucson: University of Arizona Press, 1989. Melosh, H. Jay. Impact Cratering: A Geologic Process.

New York: Oxford University Press, 1988. National Aeronautic and Space Administration (NASA). NASA's Web site on Lunar and Planetary Science. Available online. URL: http://nssdc.gsfc.nasa.gov/ planetary/planets/asteroidpage.html. Accessed February 17, 2008.

Poag, C. Wylie. Chesapeake Invader, Discovering America's Giant Meteorite Crater. Princeton, N.J.: Princeton University Press, 1999. Sharpton, Virgil L., and P. D. Ward. "Global Catastrophes in Earth History." Special Paper 247, Geological Society of America. 1990. Stanley, S. M. Extinction. New York: Scientific American Library, 1987.

meteoric Water that has recently come from the Earth's atmosphere is called meteoric water. The term is usually used in studies of groundwater, to distinguish water that has resided in ground for extended periods of time versus water that has recently infiltrated the system from rain, snow melt, or stream infiltration. Measurements of oxygen isotopes and other elements are typically used to aid this differentiation, as water from different sources shows different isotopic compositions. See also groundwater.

meteorology Meteorology is the study of the Earth's atmosphere, along with its movements, energy, interactions with other systems, and weather forecasting. The main focus of meteorology is short-term weather patterns and data within a specific area, in contrast to climatology, which is the study of the average weather on longer timescales and often on a global basis. Different aspects of meteorology include the study of the structure of the atmosphere, such as its compositional and thermal layers, and how energy is distributed within these layers. It includes analysis of the composition of the atmosphere, how the relative and absolute abundance of elements have changed with time, and how different interactions of the atmosphere, biosphere, and lithosphere contribute to the atmosphere's chemical stability. A fundamental aspect of meteorology is relating how different factors, including energy from the Sun, contribute to cloud formation, movement of air masses, and weather patterns at specific locations. Meteorologists interpret these complex energy changes and moisture changes and try to predict the weather using this knowledge. Increasingly, meteorologists are able to use data collected from orbiting satellites to aid their interpretation of these complex phenomena. Satellites have immensely improved the ability to monitor and predict the strength and paths of severe storms, such as hurricanes, as well as monitor many aspects of the atmosphere, including moisture content, pollution, and wind patterns.

The Earth's atmosphere is rich in nitrogen and oxygen and has much lower abundances of water vapor, carbon dioxide, and other gases. Some gases, such as water vapor and carbon dioxide, have a tendency to trap heat in the atmosphere. Called greenhouse gases, such gases have varied in abundance throughout Earth history, causing large temperature changes of several to tens to even hundreds of degrees in the past 4.5 billion years.

The atmosphere is divided into several layers, including the lower troposphere (where most weather events take place), the stratosphere, the mesosphere, and finally the thermosphere, which is the hottest part of the atmosphere. The topmost layer of the atmosphere is called the exosphere, where many gas molecules escape from the gravitational pull of the Earth, and which grades into the highly charged ionosphere where many free electrons and ions exist.

Weather events in the atmosphere are driven by heat and energy transfer. Latent heat, the amount of energy in the form of heat that is absorbed or released by a substance during a change in state such as from a liquid to a solid, is an important source of atmospheric energy. Heat transfer by convection is also important in the atmosphere, as moving air transfers energy from one region to another. Radiation, or the transfer of energy by electromagnetic waves, is a third important source of energy in the atmosphere. The Sun emits energy as shortwave radiation that the Earth absorbs and subsequently emits as long wavelength infrared radiation. Water vapor and carbon dioxide can absorb energy at these wavelengths, warming the atmosphere. The atmosphere warms since it allows the Sun's short wavelength radiation through, but then traps the energy absorbed from the long wavelengths emitted from the Earth. The Earth then cools by radiation, which operates most efficiently on clear nights when the clouds do not trap the outgoing radiation.

Seasons on the Earth are caused by the Earth's tilt on its axis, which results in a seasonal variation in the amount of sunlight received in different hemispheres at different times of the year. Longer hours of more intense sunlight are associated with summer, and fewer hours of less intense sunlight are associated with winters in both hemispheres.

The daily variations in temperature in any place are controlled mainly by the balance between energy input from the Sun versus energy output by convection and radiation. With radiative cooling at night, the ground surface often cools more quickly than the overlying air, resulting in an inversion with the coldest air right next to the surface.

Water is an important element in the atmosphere. Absolute humidity is the density of water vapor in a given volume of air. Relative humidity is a measure of how close the air is to being saturated with water vapor, which also depends on temperature. The dew point is a measure of how much the air would have to be cooled for saturation to occur. When the air temperature and dew point are close the air feels much more humid and the relative humidity is high. Condensation occurs when the temperature reaches the dew point, and then small droplets of water form in the atmosphere or on surfaces, forming fog. If these small droplets of water freeze it produces small frozen droplets. Condensation above the surface produces clouds, which are classified according to their height and physical appearance and are commonly divided into high, middle, and low groups plus those that cut across many atmospheric levels.

Clouds tend to form horizontal layers in stable atmospheric conditions, but in unstable conditions parcels of air that get uplifted are warmer and lighter than surrounding air, so they continue to rise, forming large vertical clouds such as the towering cumulonimbus or thunderhead clouds. on warm days simple surface heating can cause cumulus clouds to form, at heights determined by the temperature and moisture content of the surface air. As droplets of moisture in clouds coalesce by moving in the con-vecting clouds and hitting each other, they gradually get large enough to form raindrops, or ice if the temperatures are low. Precipitation can have a variety of forms when it reaches the surface depending on the form it took in the cloud and on the near surface and surface temperatures. If the surface air is cold but the air aloft is warm, raindrops may fall and freeze on impact, a phenomenon called freezing rain. snow can develop when both surface and higher level air is cold, and may fall as snowflakes, pellets, or grains. In situations where surface air is warm but cold air is aloft and there is a strong updraft (such as in a cumulonimbus cloud), hail stones may form in the cloud and hit the surface as balls of ice. Conditions in which there is warm air aloft and also on the ground cause precipitation to fall as rain.

Horizontal changes in temperature in the atmosphere produce areas with high and lower pressure. Plots of the height of equal air pressure show that low areas correspond to low pressure, and high areas to high pressure. The difference in the air pressure creates a force called the pressure gradient force that sets the air in motion in winds. This moving air is then acted on by the Coriolis force, which tends to move air to the right of its intended course in the

Northern Hemisphere and to the left in the southern Hemisphere. Winds in the Northern Hemisphere bend clockwise around high pressure and counterclockwise around low pressure centers. The Coriolis force causes the opposite pattern in the southern Hemisphere.

There are many variations of microscale and mesoscale winds near the surface of the planet. The surface layer of air, extending to about half a mile (1 km) above the surface, is affected by surface friction, causing different types of winds to develop around different obstructions. Wind produces sand dunes and ripples in deserts and in snow fields and may deform vegetation near mountaintops where the winds are consistently strong. Mountains can produce strong rotations of the air downwind of the range, and frictional effects of fast-moving air aloft in jet streams can produce strong eddies in the surface layer, associated with strong turbulence. Local winds that blow uphill through mountain valleys during the day are called valley breezes, and those that flow downhill at night are called mountain breezes. strong downslope winds are called katabatic winds. Larger scale mesocale wind systems often form near boundaries between the ocean and land, where differential heating of the land and water creates pressure differences that generate winds. Where winds blow across a large body of water, differential heating of the land and sea in different seasons may cause the winds to shift direction with the seasons, producing wind systems called monsoons.

There are many large-scale patterns of wind and pressure that persist around the world. Trade winds are those that blow toward the equator from the semipermanent high pressure zones located at 30° latitude. The trade winds from the Northern and southern Hemispheres converge along the intertropical convergence zone. Poleward of the high pressure belts is a zone where the winds blow predominantly to the west (the westerlies). These meet a more poleward belt of east-flowing winds known as the easterlies along the polar front. Annual shifts in the positions of these belts produce the annual changes in patterns of precipitation that characterize many regions.

Jet streams form where strong winds aloft get concentrated into narrow bands, such as the polar jet stream that forms in response to temperature differences along the polar front, while subtropical jets form at high elevations above the subtropics along an upper level boundary called the subtropical front.

Interactions between the atmosphere and ocean are complex. surface winds form ocean currents, and yet the oceans release energy that helps maintain atmospheric circulation. Atmospheric circulation patterns may change on seasonal or other timescales.

When warm air and water from the Austral-Indonesia region flows eastward toward South America it can form an El Niño, choking off the nutrient-rich upwelling and wreaking havoc on the environment and economics of South and Central America. The opposite effects, called La Niña, often dominate, and the alternating cycle of winds and currents is called the Southern Oscillation.

Air moves as coherent masses along boundaries called fronts. Stationary fronts have no movement, with cold air on one side and warm on the other. Winds usually blow parallel to fronts and in opposite directions on either side of the front. Fronts more typically move across continents and oceans, being driven by global atmospheric circulation. Leading edges of cold fronts are usually associated with showers as the cold air forces the warm air upward and replaces it, but in warm fronts the warm air rises over the colder surface air, producing cloudiness and widespread precipitation.

Mid-latitude cyclones form when an upper-level low-pressure trough forms west of a surface low-pressure area and when a shortwave disturbs this system, setting up surface and upper-level winds that enhance the development of the surface storm. The air converges at the surface level and rises in the center of the storm, forming precipitation. As the warm air rises and cool air sinks, energy is released, and the storm grows in strength as the potential energy is converted into kinetic energy. The storm may be steered by mid- or upper-level winds in the atmosphere.

Thunderstorms commonly develop when there is a humid layer of surface air, sunlight to heat the ground, and unstable air aloft. In these conditions the heated air may quickly rise, forming large cumulonimbus clouds that may drop locally heavy rains. When a strong vertical wind shear exists, severe thunderstorms may form. Supercells are large rotat

Thunderhead cloud (cumulonimbus) rising (Greg F. Riegler, Shutterstock, Inc.)

ing thunderstorm systems that may persist for hours. Many thunderstorms form along frontal boundaries where cold air forces the warm air to rise, forming lines and clusters of storms called mesoscale convective complexes. Tornados, rapidly circulating columns of air that reach the ground, are often associated with supercells, and can have winds that reach a couple hundred miles per hour (few hundred km/hr) in the tornado core, typically less than several hundred yards (meters) wide.

Hurricanes are tropical cyclones with winds exceeding 74 miles per hour (119 km/hr) and include a well-organized mass of thunderstorms rotating about a central low pressure region in the storm's eye. Hurricanes form over warm tropical waters where surface winds converge along a tropical wave, initiating central airs to rise, forming a tropical depression. As the air continues to move into the storm system and rise in its center, much latent heat is released, causing more air to rise, and central pressures to reduce further, leading the storm to grow further. Energy is released by the storm in the cloud tops by radiational cooling so the strengthening of the storm depends on the balance between the energy gained by converting sensible and latent heat into kinetic energy in the storm eye and the energy lost in the cloud tops by radiation. Most hurricanes are steered to the west by the easterly winds in the tropics but may move westward when they move into mid-latitudes. Since the storms gain energy (and keep their energy balance) from the warm water, hurricanes rapidly lose strength when they move over cool water or land masses. Most damage from hurricanes is associated with the large storm surges that some generate, as well as the high winds and flooding rains.

See also atmosphere; climate; climate change; clouds; El Niño and the Southern Oscillation (ENSO); energy in the Earth system; greenhouse effect; hurricanes; monsoons, trade winds; precipitation; Sun; thermodynamics.

FURTHER READING

Ahrens, C. D. Meteorology Today: An Introduction to Weather, Climate, and the Environment. 8th ed. Pacific Grove, Calif.: Brooks/Cole, 2007. 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. Cambridge: Cambridge University Press, 2007. Also available online. URL: http://www.ipcc. ch/index.htm. Accessed October 10, 2008.

National Aeronautic and Space Administration (NASA). Earth Observatory. Available online. URL: http:// earthobservatory.nasa.gov/. Accessed October 9, 2008; updated daily. U.S. Environmental Protection Agency. Climate Change home page. Available online. URL: http://www.epa. gov/climatechange/. Updated September 9, 2008.

Milankovitch, Milutin M. (1879-1958) Serbian Mathematician, Physicist Milutin Milankovitch was born and educated in Serbia, and was appointed to a chair in the University of Belgrade in 1909, where he taught courses in mathematics, physics, mechanics, and celestial mechanics. He is well known for his research on the relationship between celestial mechanics and climate on the Earth, and he is responsible for developing the idea that rotational wobbles and orbital deviations combine in cyclic ways to produce the climatic changes on the Earth. He determined how the amount of incoming solar radiation changes in response to several astronomical effects such as orbital tilt, eccentricity, and wobble. These changes in the amount of incoming solar radiation in response to changes in orbital variations occur with different frequencies, and produce cyclical variations known as Milankov-itch cycles. Milankovitch's main scientific work was published by the Royal Academy of Serbia in 1941, during World War II in Europe. He calculated that the effects of orbital eccentricity, wobble, and tilt combine every 40,000 years to change the amount of incoming solar radiation, lowering temperatures and causing increased snowfall at high latitudes. His results have been widely used to interpret climatic variations, especially in the Pleistocene record of ice ages, and also in the older rock record.

See also climate change; Milankovitch cycles.

Milankovitch cycles Systematic changes in the amount of incoming solar radiation, caused by variations in Earth's orbital parameters around the Sun, are known as Milankovitch cycles. These changes can affect many Earth systems, causing glaciations, global warming, and changes in the patterns of climate and sedimentation.

Medium-term climate changes include those that alternate between warm and cold on time scales of 100,000 years or less. These medium-term climate changes include the semi-regular advance and retreat of the glaciers during the many individual ice ages in the past few million years. The last 2.8 million years have been marked by large global climate oscillations that have been recurring at approximately a 100,000-

year periodicity at least for the past 800,000 years. The warm periods, called interglacial periods, appear to last approximately 15,000 to 20,000 years before regressing to a cold ice age climate. The last of these major glacial intervals began ending about 18,000 years ago, as the large continental ice sheets covering North America, Europe, and Asia began retreating. The main climate events related to the retreat of the glaciers, can be summarized as follows:

• 18,000 years ago: The climate begins to warm

• 15,000 years ago: Advance of glaciers halts and sea levels begin to rise

• 10,000 years ago: Ice Age megafauna goes extinct

• 8,000 years ago: Bering Strait land bridge becomes drowned, cutting off migration of people and animals.

• 6,000 years ago: The Holocene maximum warm period

• So far in the past 18,000 years, the Earth's temperature has risen approximately 16°F (10°C) and the sea level has risen 300 feet (91 m).

This past glacial retreat is but one of many in the past several million years, with an alternation of warm and cold periods apparently related to a 100,000 year periodicity in the amount of incoming solar radiation, causing the alternating warm and cold intervals. These systematic changes are known as Milankovitch cycles, after Milutin Milankovitch (1879-1958), a Serbian scientist who first clearly elucidated the relationships between the astronomical variations of the Earth orbiting the Sun and the climate cycles on Earth. Milankovitch's main scientific work was published by the Royal Academy of Serbia in 1941, during World War II in Europe. He was able to calculate that the effects of orbital eccentricity, wobble, and tilt combine every 40,000 years to change the amount of incoming solar radiation, lowering temperatures and causing increased snowfall at high latitudes. His results have been widely used to interpret the climatic variations, especially in the Pleistocene record of ice ages, and also in the older rock record.

Astronomical effects influence the amount of incoming solar radiation; minor variations in the path of the Earth in its orbit around the Sun and the inclination or tilt of its axis cause variations in the amount of solar energy reaching the top of the atmosphere. These variations are thought to be responsible for the advance and retreat of the Northern and Southern Hemisphere ice sheets in the past few million years. In the past two million years alone, the Earth

Orbital variations of the Earth cause changes in the amount of incoming solar radiation, known as Milankovitch cycles. Shown here are changes in the eccentricity of the orbit, the tilt of the spin axis (nutation), and precession of the equinoxes.

Precession Nutation

has seen the ice sheets advance and retreat approximately 20 times. The climate record as deduced from ice-core records from Greenland and isotopic tracer studies from deep ocean, lake, and cave sediments suggest that the ice builds up gradually over periods of about 100,000 years, then retreats rapidly over a period of decades to a few thousand years. These patterns result from the cumulative effects of different astronomical phenomena.

Several movements are involved in changing the amount of incoming solar radiation. The Earth rotates around the Sun following an elliptical orbit, and the shape of this elliptical orbit is known as its eccentricity. The eccentricity changes cyclically with time with a period of 100,000 years, alternately bringing the Earth closer to and farther from the Sun in summer and winter. This 100,000-year cycle is about the same as the general pattern of glaciers advancing and retreating every 100,000 years in the past two million years, suggesting that this is the main cause of variations within the present-day ice age. Presently, the Earth is in a period of low eccentricity (~3 percent) and this yields a seasonal change in solar energy of ~7 percent. When the eccentricity is at its peak (~9 percent), "seasonality" reaches ~20 percent. In addition a more eccentric orbit changes the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes.

The Earth's axis is presently tilting by 23.5°N/ S away from the orbital plane, and the tilt varies between 21.5°N/S and 24.5°N/S. The tilt, also known as obliquity, changes by plus or minus 1.5°N/ S from a tilt of 23°N/S every 41,000 years. When the tilt is greater, there is greater seasonal variation in temperature. For small tilts, the winters would tend to be milder and the summers cooler. This would lead to more glaciation.

Wobble of the rotation axis describes a motion much like a top rapidly spinning and rotating with a wobbling motion, such that the direction of tilt toward or away from the Sun changes, even though the tilt amount stays the same. This wobbling phe nomenon is known as precession of the equinoxes, and it has the effect of placing different hemispheres closest to the Sun in different seasons. This precession changes with a double cycle, with periodicities of 23,000 years and 19,000 years. Presently the precession of the equinoxes is such that the Earth is closest to the Sun during the Northern Hemisphere winter. Due to precession, the reverse will be true in ~11,000 years. This will give the Northern Hemisphere more severe winters.

Because each of these astronomical factors acts on a different time scale, they interact in a complicated way, known as Milankovitch cycles. Using the power of understanding these cycles, we can make predictions of where the Earth's climate is heading, whether we are heading into a warming or cooling period and whether we need to plan for sea level rise, desertification, glaciation, sea level drops, floods, or droughts. When all the Milankovitch cycles (alone) are taken into account, the present trend should be toward a cooler climate in the Northern Hemisphere, with extensive glaciation. The Milankovitch cycles may help explain the advance and retreat of ice over periods of 10,000 to 100,000 years. They do not explain what caused the Ice Age in the first place.

Western aspect of the Pelmo massif in the Italian Dolomite Mountains. The cyclical layering recorded by the beds (horizontal) is interpreted as records of Milankovitch climate cycles. (Gillian Price/Alamy)

The pattern of climate cycles predicted by Milan-kovitch cycles is made more complex by other factors that change the climate of the Earth. These include changes in thermohaline circulation, changes in the amount of dust in the atmosphere, changes caused by reflectivity of ice sheets, changes in concentration of greenhouse gases, changing characteristics of clouds, and even the glacial rebound of land that was depressed below sea level by the weight of glaciers.

Milankovitch cycles have been invoked to explain the rhythmic repetitions of layers in some sedimentary rock sequences. The cyclical orbital variations cause cyclical climate variations, which in turn are reflected in the cyclical deposition of specific types of sedimentary layers in sensitive environments. There are numerous examples of sedimentary sequences where stratigraphic and age control are sufficient to be able to detect cyclical variation on the time scales of Milankovitch cycles, and studies of these layers have proven consistent with a control of sedimentation by the planet's orbital variations. Some examples of Milankovitch-forced sedimentation have been documented from the Dolomite Mountains of Italy, the Proterozoic Rocknest Formation of northern Canada, and from numerous coral reef environments.

Predicting the future climate on Earth involves very complex calculations, including inputs from the long- and medium-term effects described in this entry, and some short-term effects such as sudden changes caused by human inputs of greenhouse gases to the atmosphere, and effects such as unpredicted volcanic eruptions. Nonetheless, most climate experts expect that the planet will continue to warm on the hundreds-of-years time scale. However, based on the recent geological past, it seems reasonable that the planet could be suddenly plunged into another ice age, perhaps initiated by sudden changes in ocean circulation, following a period of warming. Climate is one of the major drivers of mass extinction, so the question remains if the human race will be able to cope with rapidly fluctuating temperatures, dramatic changes in sea level, and enormous shifts in climate and agriculture belts.

See also climate change; Milankovitch, Milutin M.; sequence stratigraphy; stratigraphy, stratification, cyclothem.

FURTHER READING

Ahrens, C. D. Meteorology Today, An Introduction to Weather, Climate, and the Environment. 6th ed. Pacific Grove, Calif.: Brooks/Cole, 2000. Allen, John R. Sedimentary Structures: Their Character and Physical Basis. Amsterdam: Elsevier, 1982. Allen, P. A., and J. R. Allen. Basin Analysis, Principles and Applications. Oxford: Blackwell Scientific Publications, 1990.

Dawson, A. G. Ice Age Earth. London: Routledge, 1992. Erickson, J. Glacial Geology: How Ice Shapes the Land.

New York: Facts On File, 1996. Goldhammer, Robert K., Paul A. Dunn, and Lawrence A. Hardie. "High-Frequency Glacial-Eustatic Sea Level Oscillations with Milankovitch Characteristics Recorded in Middle Triassic Platform Carbonates in Northern Italy." American Journal of Science 287 (1987): 853-892. Grotzinger, John P. "Upward Shallowing Platform Cycles: A Response to 2.2 Billion Years of Low-Amplitude, High-Frequency (Milankovitch Band) Sea Level Oscillations." Paleoceanography 1 (1986): 403-416. Hayes, James D., John Imbrie, and Nicholas J. Shakelton. "Variations in the Earth's Orbit: Pacemaker of the Ice Ages." Science 194 (1976): 2,212-2,232. Imbrie, John. "Astronomical Theory of the Pleistocene Ice Ages: A Brief Historical Review." Icarus 50 (1982): 408-422.

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. Cambridge: Cambridge university Press, 2007.

Intergovernmental Panel on Climate Change home page. Available online. uRL: http://www.ipcc.ch/index.htm. Accessed January 29, 2009.

mineral, mineralogy The branch of geology that deals with the classification and properties of minerals is closely related to petrology, the branch of geology that deals with the occurrence, origin, and history of rocks. Minerals are the basic building blocks of rocks, soil, and sand. Most beaches are made of the mineral quartz, which is very resistant to weathering and erosion by the waves. Most minerals, like quartz or mica, are abundant and common, although some minerals like diamonds, rubies, sapphires, gold, and silver are rare and very valuable. Minerals contain information about the chemical and physical conditions in the regions of the Earth that they formed in. They can often help discriminate which tectonic environment a given rock formed in, and they can tell us information about the inaccessible portions of Earth. For example, mineral equilibrium studies on small inclusions in diamonds show that they must form below a depth of 90 miles (145 km). Economies of whole nations are based on exploitation of mineral wealth; for instance, South Africa is such a rich nation because of its abundant gold and diamond mineral resources.

The two most important characteristics of minerals are their composition and structure. The composition of minerals describes the kinds of chemical elements present and their proportions, whereas the structure of minerals describes the way in which the atoms of the chemical elements are packed together.

Mineralogists have identified nearly 4,000 minerals, most made out of the eight most common mineral-forming elements. These eight elements, listed in the table "The Eight Most Common Mineral-Forming Elements," make up greater than 98 percent of the mass of the continental crust. Most of the other 133 scarce elements do not occur by themselves, but occur with other elements in compounds by ionic substitution. For example, olivine may contain trace amounts of copper (Cu), nickel (Ni), cobalt (Co), manganese (Mn), and other elements.

The two elements oxygen and silicon make up more than 75 percent of the crust, with oxygen alone forming nearly half of the mass of the continental crust. Oxygen forms a simple anion (O2-), and silicon forms a simple cation (Si4+). Silicon and oxygen combine together to form a very stable complex anion that is the most important building block for minerals—the silicate anion (SiO4)4-. Minerals that contain this anion are known as the silicate minerals, and they are the most common naturally occurring inorganic compounds in the solar system. The other, less common building blocks of minerals (anions) are oxides (O2-), sulfides (S2-), chlorides (Cl-), carbonates (CO3)2-, sulfates (SO4)2-, and phosphates (PO4)3-.

Minerals are classified into eight major groups based on the main type of cation present in the min-

THE EIGHT MOST COMMON MINERAL-FORMING ELEMENTS

Element

Abbreviation

Percentage of Continental Crust Mass

Oxygen

O

46.6

Silicon

Si

27.7

Aluminum

Al

8.1

Iron

Fe

5.0

Calcium

Ca

3.6

Sodium

Na

2.8

Potassium

K

2.6

Magnesium

Mg

2.1

Eight types of minerals: sulfur, sapphire, orpiment/ realgar, cinnabar, malachite, olivine (peridot), copper, and beryl (Charles D. Winters/Photo Researchers, Inc.)

eral structure, a classification scheme championed by James Dana in the early 1800s. His classification recognized (1) native elements; (2) sulfides; (3) oxides and hydroxides; (4) halides; (5) carbonates, nitrates, borates, and iodates; (6) sulfates, chromates, molyb-dates, and tungstates; (7) phosphates, arsenates, and vanadates; and (8) silicates.

Approximately 20 minerals are so common that they account for greater than 95 percent of all the minerals in the continental and oceanic crust; these are called the rock-forming minerals. Most rock-forming minerals are silicates and they have some common features in the way their atoms are arranged.

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  • milena awet
    What is Aquatic precession?
    10 months ago
  • Susanne
    How does earth's precessional nutation effect weather/climate?
    6 months ago
  • ashleigh ross
    How does nutation change the climate?
    3 months ago

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