sea level is rising presently at a rate of one foot (0.3 m) per century, although this rate seems to be accelerating. This rising sea level will obviously change the coastline dramatically—a one-foot (0.3-m) rise in sea level along a gentle coastal plain can be equated with a 1,000-foot (300-m) landward migration of the shoreline. The world will look significantly different when sea levels rise significantly. Many of the world's low-lying cities like New York, New orleans, London, Cairo, Tokyo, and most other cities in the world may look like Venice in a hundred or several hundred years. The world's rich farmlands on coastal plains, like the East Coast of the united states, northern Europe, Bangladesh and much of China will be covered by shallow seas. if sea levels rise more significantly, as they have in the past, then vast parts of the interior plains of North America will be covered by inland seas, and much of the world's climate and vegetation zones will be shifted to different latitudes.

Governments must begin to plan for how to deal with rising sea levels, yet very little has been done so far. It is time that groups of scientists and government planners begin to meet to first understand the magnitude of the problem, then to study and recommend which tactics to initiate to mitigate the effects of rising sea levels.

See also El Niño and the Southern Oscillation (ENSO); glacier, glacial systems; plate tectonics.


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

ken, N.J.: John Wiley & Sons, 2003. Burkett, Virginia R., D. B. Zikoski, and D. A. Hart. "Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana." In U.S. Geological Survey Subsidence Interest Group Conference, Proceedings for the Technical Meeting, 63-70. Reston, Va.: U.S. Geological Survey, 2003. Davis, R., and D. Fitzgerald. Beaches and Coasts. Malden,

Mass.: Blackwell Publishing, 2004. Douglas, Bruce C., Michael S. Kearney, and Stephen P. Leatherman. Sea Level Rise: History and Consequences. San Diego, Calif.: Academic Press, 2000. Intergovernmental Panel on Climate Change. Available online. URL: 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. Cambridge: Cambridge University Press, 2007.

Kusky, T. M. Climate Change: Shifting Deserts, Glaciers, and Climate Belts. New York: Facts On File, 2008.

-. The Coast: Hazardous Interactions within the

Coastal Zone. New York: Facts On File, 2008. Schneider, D. "The Rising Seas." Scientific American (March 1997): 112-117.

seawater The oceans cover more than 70 percent or the Earth's surface and extend to an average depth of a couple of miles (several kilometers). As part of the hydrologic cycle, each year approximately 1.27 x 1016 cubic feet (3.6 x 1014 m3) of water evaporate from the oceans with about 90 percent of this returning to the oceans as rainfall. The remaining 10 percent falls as precipitation on the continents, where it forms freshwater lakes and streams and seeps into the groundwater system for temporary storage before eventually returning to the sea. During its passage over and in the land, the water erodes huge quantities of rock, soil, and sediment, and dissolves chemical elements such as salts from the continents, carrying these and other sediments as dissolved, suspended, and bed load to the oceans. Water transports more than 50 million tons of continental material into the oceans each year. Most of the suspended and bed load material is deposited as sedimentary layers near passive margins, but the dissolved salts and ions derived from the continents play a major role in determining seawater chemistry, as listed in the table "Composition of Typical Seawater." The most abundant dissolved salts are chloride and sodium, which together with sulfate, magnesium, calcium, potassium, bicarbonate, bromide, borate, strontium, and fluoride form more than 99.99 percent of the total material dissolved in seawater.

In addition to the elements listed in the table "Composition of Typical Seawater," a number of additional minor and trace elements dissolved in seawater are important for the life cycle of many organisms. For instance, nitrogen, phosphorus, silicon, zinc, iron, and copper play important roles in the growth of tests and other parts of some marine organisms. Seawater also contains dissolved gases, including nitrogen, oxygen, and carbon dioxide. The amount of oxygen dissolved in the surface layers of




Concentration in Parts per Thousand

Percentage of Dissolved Material
















































seawater is about 34 percent of the total dissolved gases, significantly higher than the 21 percent of the total atmospheric gases. Marine organisms generate this oxygen through photosynthesis. Some of it exchanges with the atmosphere across the air-water interface, and some sinks and is used by deep aerobic organisms. The amount of carbon dioxide dissolved in seawater is about 50 times greater than its concentration in the atmosphere. C02 plays an important role in buffering the acidity and alkalinity of seawater where, through a series of chemical reactions, it keeps the pH of seawater between 7.5 and 8.5. Marine organisms make carbonate shells out of the dissolved Co2, and some is incorporated into marine sediments where it is effectively isolated from the atmosphere. The total amount of Co2 stored in the ocean is very large, and as a greenhouse gas, if it were to be released to the atmosphere, it would have a profound effect on global climate.

The salinity and temperature of seawater are important in controlling mixing between surface and deep water and in determining ocean currents. Temperature is controlled largely by latitude, whereas river input, evaporation from restricted basins, and other factors determine the total dissolved salt concentration. Density differences caused by temperature and salinity variations induce ocean currents and thermohaline circulation, distributing heat and nutrients around the globe.

See also geochemical cycles; hydrosphere; ocean basin; ocean currents; oceanography; thermohaline circulation.


Allaby, Alisa, and Michael Allaby. A Dictionary of Earth Sciences. 2nd ed. Oxford: Oxford University Press, 1999.

Erickson, Jon. Marine Geology: Exploring the New Frontiers of the Ocean. Rev. ed. New York: Facts On File, 2003.

Sedgwick, Adam (1785-1873) British Geologist One of the founders of geology as a science, Adam Sedgwick was born on March 22, 1785, in Yorkshire, England. He was the third of seven children of the Anglican vicar of the town of Dent, and he spent a happy childhood with many hours exploring the countryside collecting fossils and rocks. Sedgwick attended the Sedberg School in Yorkshire, then he was admitted to Trinity College at Cambridge University on a special scholarship, obtaining his bachelor of arts in natural sciences in 1808. The college made him a fellow in 1810, when he was charged with supervising six students, which he noted seriously held him back in his own studies.

In 1818 Sedgwick was appointed the Wood-wardian professor of geology at Cambridge, which had been endowed by natural historian John Woodward in the early 1700s. Until this time, Sedgwick had not had any formal studies in geology, and is credited with saying, "Hitherto I have never turned a stone; henceforth I will leave no stone unturned," upon his appointment to the post. While in this post Sedgwick taught himself geology and paleontology and paid great attention to expanding the geological collections of Cambridge university, while gaining experience by doing field work throughout the British Isles. Sedgwick became a very popular lecturer and went against tradition of the times by allowing women to attend his courses. In 1829 he was elected president of the Geological Society of London, and in 1845 he became a vice-master of Trinity College. Sedgwick's health began faltering in the 1850s, and he stopped giving lectures due to his health in 1871.

Adam Sedgwick was exploring the field of geology in England at a time when the science was in its infancy. He met and worked with gentleman geologist Roderick Murchison, and the two jointly presented their research on some fossiliferous rocks of Devonshire, England. The distinctive fossil assemblage in these rocks led Sedgwick and Murchison to propose a new division of geologic time for these rocks—the Devonian period. In the early 1830s the two began working on the folded and faulted rocks of Wales. Murchison worked on the fossil assemblages and determined that they appeared more primitive (containing fewer fish) than the rocks of Devonshire, so he assigned these rocks to an older period, naming it the Silurian after the Silures, a Celtic tribe who lived in the Welsh borderlands in Roman times. Sedgwick then suggested that even older rocks existed in central Wales, and he named these the Cambrian, after Cambria, the Latin name for Wales. Sedgwick and Murchison then presented their descriptions of the rocks and stratigraphic divisions of England and Wales in a famous paper called "On the Silurian and Cambrian Systems, Exhibiting the order in Which the older Sedimentary Strata Succeed Each other in England and Wales." The paper became famous as it offered the first division of lower Paleozoic time. During these studies Sedgwick became the first geologist to clearly distinguish between the structures of jointing, slaty cleavage, and stratification.

There was some overlap between the upper part of the Cambrian as proposed by Sedgwick and the lower part of the Silurian as proposed by Murchison. This led to a major dispute between Sedgwick and Murchison, with both claiming they were correct. At stake was the honor of being the first person to describe the rocks that seemingly contained the earliest record of life on Earth since, at that time, sedimentary rock, sedimentation

the oldest fossils known were Cambrian. Murchison claimed that Sedgwick's Cambrian rocks were not sufficiently different from his Silurian rocks to warrant a further division of geologic time. The debate was resolved in 1879 when British geologist Charles Lapworth proposed a new division of geologic time between the Cambrian and Silurian, which he called the Ordovician after a Celtic tribe in Wales. The Ordovician included both the disputed Upper Cambrian and Lower Silurian strata.

During some of his field work in Wales, Sedgwick took a student from Cambridge along as a field assistant—the young Charles Darwin. Darwin was in Sedgwick's geology lecture course and wanted more experience so took the employment from Sedgwick. This experience proved invaluable, as during Darwin's famous voyage on the HMS Beagle (1831-36) Darwin sent many rock samples and descriptions of South America back to Sedgwick, who read and helped interpret the work. Sedgwick also highly recommended Darwin's work to the Geological Society of London, improving the career and reputation of his former student and then colleague. However, later Sedgwick did not approve of Darwin's theory of evolution, writing in a letter to Darwin after reading his On the Origin of Species that "other parts I read with absolute sorrow; because I think them utterly false and grievously mischievous—You have deserted—after a start in that tram-road of all solid physical truth—the true method of induction."

Sedgwick was a geologic catastrophist, believing most Earth history events could be explained by a series of major catastrophes, much as described by the French geologist Georges Cuvier (1769-1832), and opposed to the gradualistic models of Sir Charles Lyell. Sedgwick's main opposition to Darwin's model for evolution was its apparent lack of any involvement of a divine being or creation. Although Sedg-wick believed in the great lengths of geological time, he thought that there was a god in the evolution of life and the Earth, arguing with Darwin that "there is a moral or metaphysical part of nature as well as a physical."

See also Cambrian; Carboniferous; Darwin, Charles; life's origins and early evolution; Lyell, Sir Charles; Ordovician; Sorby, Henry Clifton.


Clark, J. W., and T. M. Hughes. The Life and Letters of the Reverend Adam Sedgwick. 2 vols. Cambridge:

Cambridge University Press, 1890.

sedimentary rock, sedimentation Sedimentary rocks are rocks that have consolidated from accumulations of loose sediment produced by physical, chemical, or biological processes. Common physical processes involved in the formation of sediments include the breaking, transportation of fragments, and accumulation of older rocks; chemical processes include the precipitation of minerals by chemical processes or evaporation of water; common biological processes include the accumulation of organic remains.

Soils and other products of weathering of rocks are continuously being removed from their sources and deposited elsewhere as sediments. This process can be observed as gravel in streambeds, on alluvial fans, and in wind-blown deposits. When these sediments are cemented together, commonly by minerals deposited from water percolating through the ground, they become sedimentary rocks. Other types of sedimentary rocks are purely chemical in origin, and were formed by the precipitation of minerals from an aqueous solution.

Clastic sediments (also detritus) are the accumulated particles of broken rocks, some with the remains of dead organisms. The word clastic is from the Greek word klastos, meaning broken. Most clastic particles have undergone various amounts of chemical change, and some have a continuous gradation in size from huge boulders to submicroscopic particles. Size is the main basis for classifying clastic sediments and sedimentary rocks. The textures of the sedimentary rocks or individual sedimentary particles act as additional criteria for the classification of sedimentary rocks.

Clastic sediments can be transported by wind, water, ice, or gravity, and each method of transport leaves specific clues as to how it was transported and deposited. For instance, deposits from sediments transported by gravity in a landslide will consist of a poorly sorted mixture including everything that was in the path, whereas sediment transported by wind will have a very uniform grain size and typically forms large dunes. Clastic sediments are deposited when the transporting agent can no longer carry them. For instance, if the wind stops, the dust and sand will fall out, whereas sediments transported by streams are deposited when the river velocity slows down. This happens either where the stream enters a lake or the ocean or when a flood stage lowers and the stream returns to a normal velocity and clears up. Geologists can look at old rocks and tell how fast the water was flowing during deposition and can also use clues such as the types of fossils or the arrangement of the individual particles to decipher the ancient environment.

Chemical sediment is sediment formed when minerals precipitate from solution. They may result from biochemical activities of plants and animals

sedimentary rock, sedimentation that live in the water, or they may form from inorganic reactions in the water, induced by things such as hot springs or simply the evaporation of seawater. This produces a variety of salts, including ordinary table salt. Chemical sedimentary rocks are classified according to their main chemical component, with common types including limestone (made of predominantly calcite), dolostone (consisting of more than 50 percent dolomite), rock salt (composed of NaCl), and chert (whose major component is sio2).

Evaporite sediments include salts precipitated from aqueous solutions, typically associated with the evaporation of desert lake basins known as playas, or the evaporation of ocean waters trapped in restricted marine basins associated with tectonic movements and sea level changes. They are also associated with sabkha environments along some coastlines such as along the southern side of the Persian (Arabian) Gulf, where seawater is drawn inland by capillary action and evaporates, leaving salt deposits on the surface.

Evaporites are typically associated with continental breakup and the initial stages of the formation of ocean basins. For instance, the opening of the south Atlantic ocean about 110 million years ago is associated with the formation of up to 3,280 feet (1,000 m) of salts north of the Walvus-Rio Grande Ridge. This ridge probably acted as a barrier that episodically (during short sea-level rises) let seawater spill into the opening Atlantic ocean, where it evaporated in the narrow rift basin. A column of ocean water about 18.5 miles (30 km) thick would be necessary to form the salt deposits in the south Atlantic, suggesting that water spilled over the ridge many times during the opening of the basin. The evaporate-forming stage in the opening of the Atlantic probably lasted about 3 million years, perhaps involving as many as 350 individual spills of seawater into the restricted basin. salts that form during the opening of ocean basins are economically important because when they get buried under thick piles of passive continental margin sediments, the salts typically become mobilized and intrude overlying sediments as salt diapirs, forming salt domes and other oil traps exploited by the petroleum industry.

salts can also form during ocean closure, with examples known from the Messinian (Late Miocene) of the mediterranean region. In this case thick deposits of salt with concentric compositional zones reflect progressive evaporation of shrinking basins, when water spilled out of the Black sea and Atlantic into a restricted Mediterranean basin. so-called closing salts are also known from the Hercenian orogen north of the Caspian sea and in the European Permian Zechstein basin in the foreland of the collision.

As seawater evaporates, a progressive sequence of different salts forms from the concentrated brines.

Typically, anhydrite (Caso4) is followed by halite (NaCl), which forms the bulk of the salt deposits. A variety of other salts can form depending on the environment, composition of the water being evaporated, when new water is added to the brine solution, and whether or not it partly dissolves existing salts.

Most sedimentary rocks display a variety of internal and surface markings known as sedimentary structures that can be used to interpret the conditions of formation. stratification results from a layered arrangement of particles in a sediment or sedimentary rock that accumulated at the surface of the Earth. The layers are visible and distinct from adjacent layers because of differences (such as size, shape, or composition) in the particles between successive layers and because of differences in the way the particles are arranged between different layers. Bedding is the layered arrangement of strata in a body of rock. Parallel strata are sedimentary layers in which individual layers lie parallel to one another. The presence of parallel strata usually means that the sediments were deposited underwater, such as in lakes or in the deep sea. some sediments with parallel layers have a regular alternation between two or more types of layers, indicating a cycle in the depositional environment. These can be daily, yearly, or some other rhythm influenced by solar cycles. one unusual type of layered rock is a varve, which is a lake sediment that forms a repeating cycle of coarse-grained sediments with spring tides, and fine clay with winter conditions, when the suspended sediments gradually settle out of the water column. Cross strata are layers that are inclined with respect to larger layers in which they occur. Most cross-laminated deposits are sandy or coarser, and they form as ripples that move along the surface. The direction of inclination of the cross strata is the direction that the water formerly flowed.

sorting is a sedimentary characteristic that refers to the distribution of grain sizes within a sediment or sedimentary rock. sediments deposited by wind are typically well sorted, but those deposited by water may show a range of sorting. A bed is called uniform if its layers contain grains with the same size throughout. A gradual transition from coarse- to fine-grained, or fine- to coarse-grained, is known as a graded bed. Graded beds typically reflect a change in current velocity during deposition. Nonsorted layers represent a mixture of different grain sizes, without any apparent order. These are common in rock falls, avalanche deposits, landslides, and from some glaciers. Rounding is a textural term that describes the relative shape or roundness of grains. When sediments first break off from their source area, they tend to be angular and reflect the shape of joints or internal

Cross-bedded Navajo sandstone from the Jurassic period in Zion National Park, Utah (François Gohier/ Photo Researchers, Inc.)

mineral forms. With progressive transportation by wind or water, abrasion tends to smooth the grains and make them rounded. The greater the transport distance, in general, the greater the rounding.

Surface features on sedimentary layers also yield clues about the depositional environment. Like ripple marks or footprints on the beach, many features preserved on the surface of strata offer clues about the origin of sedimentary rocks and the environments in which they formed. Ripple marks show the direction of ancient currents, whereas tool marks record places where an object was dragged by a current across a surface. Turbulent eddies in a current produce grooves in the underlying sediment called flute marks, by scouring out small pockets on the paleo-surface. Mud cracks reveal that the surface was wet, then desiccated by subaerial exposure. Other types of surface marks include footprints and animal tracks in shallow water environments, and raindrop impressions in subaerial settings.

Fossils are remains of animals and plants preserved in the rock that can also reveal clues about past environments. For instance, deep marine fossils are not found in lake environments, and dinosaur footprints are not found in deep marine environments.

See also Arabian geology; basin, sedimentary basin; continental margin; historical geology; ocean basin; petroleum geology; Pet-tijohn, Francis John; sequence stratigraphy; stratigraphy, stratification, cyclothem.


Allen, P. A., and J. R. Allen. Basin Analysis, Principles and Applications. Oxford: Blackwell Scientific Publications, 1990.

Bouma, Arnold H. Sedimentology of Some Flysch Deposits: A Graphic Approach to Facies Interpretation. Amsterdam, Elsevier, 1962. Cefrey, Holly. Sedimentary Rocks. New York: Rosen Publishing Group, 2003. Pettijohn, Francis J. Sedimentary Rocks. London: Harper, 1957.

Prothero, Donald, and Robert Dott. Evolution of the Earth.

6th ed. New York: McGraw-Hill, 2002. Stanley, Steven M. Earth and Life through Time. New York: Freeman, 1986.

seismology The study of the propagation of seismic waves through the Earth, including analysis of earthquake sources, mechanisms, and the determination of the structure of the Earth through variations in the properties of seismic waves is called seismology.

Determination of the structure of the deep parts of the Earth can be achieved only by remote geophysical methods such as seismology. Seismographs are stationed all over the world, and studying the propagation of seismic waves from natural and artificial source earthquake and seismic events allows for the calculation of changes in the properties of the Earth in different places. If the Earth had a uniform composition, seismic wave velocity would increase smoothly with depth, because increased density is equated with higher seismic velocities. However, by plotting the observed arrival time of seismic waves, seismologists have found that the velocity does not increase steadily with depth but that several dramatic changes occur at discrete boundaries and in transition zones deep within the Earth.

One can calculate the positions and changes across these zones by noting several different properties of seismic waves. Some are reflected off interfaces, just like light is reflected off surfaces, and other waves are refracted, changing the velocity and path of the rays. These reflection and refraction events happen at specific sites in the Earth, and the positions of the boundaries are calculated using wave velocities. The core-mantle boundary at 1,802 miles (2,900 km) depth in the Earth strongly influences both P and S waves. It refracts P-waves, causing a P-wave shadow and, because liquids cannot transmit S waves, none get through, causing a huge S-wave shadow. These

Earthquake focus 0°

Earthquake focus 0°

Many P waves received

- S wave -Shadow zone

- S wave -Shadow zone

© Infobase Publishing

Cross sections of the Earth showing shadow zones that around the planet due to refraction of P and S waves at contrasting properties of P and S waves can be used to accurately map the position of the core-mantle boundary.

Variations in the propagation of seismic waves illustrate several other main properties of the deep Earth. Velocity gradually increases with depth to about 62 miles (100 km), where the velocity drops slightly between 62 and 124 miles (100-200 km) depth, in a region known as the low velocity zone. The reason for this drop in velocity is thought to be small amounts of partial melt in the rock, corresponding to the asthenosphere, the weak sphere on which the plates move, lubricated by partial melts.

Another seismic discontinuity exists at 248.5 miles (400 km) depth, where velocity increases sharply due to a rearrangement of the atoms within olivine in a polymorphic transition into spinel structure, corresponding to an approximate 10 percent increase in density.

A major seismic discontinuity at 416 miles (670 km) could be either another polymorphic transition or a compositional change, the topic of many current investigations. some models suggest that this boundary separates two fundamentally different types of mantle, circulating in different convection cells, whereas other models suggest that there is more interaction between rocks above and below this discontinuity.

The core-mantle boundary is one of the most fundamental on the planet, with a huge density contrast from 5.5 g/cm3 above, to 10-11 g/cm3 below, a contrast greater than that between rocks and air on the surface of the Earth. The outer core is made dominantly of molten iron. An additional discontinuity occurs inside the core at the boundary between the liquid outer core and the solid, iron-nickel inner core.

The properties of seismic waves can also be used to understand the structure of the Earth's crust. Andrija Mohorovicic (a Yugoslavian seismologist) noticed slow and fast arrivals from nearby earthquake source events. He proposed that some seismic waves traveled through the crust, some along the surface, and that some were reflected off a deep seismic discontinuity between seismically slow and fast material at about 18.6

develop in bands internal boundaries

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