The Mid Ocean Ridge Is Approximately 75000 Miles Long

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Berzin, R., O. Oncken, J. H. Knapp, A. Perez-Estaun, T. Hismatulin, N. Yunusov, and A. Lipilin. "Orogenic Evolution of the Ural Mountains: Results from an Integrated Seismic Experiment." Science 274 (1996): 220-221.

Bogdanova, Svetlana V., R. Gorbatschev, and R. G. Garetsky. "The East European Craton." In Encyclopedia of Geology, vol. 5, edited by R. C. Selley, L. R. Cocks, and I. R. Plimer, 34-49. Amsterdam; London: Elsevier Academic, 2005. Condie, Kent C., and Robert Sloan. Origin and Evolution of Earth: Principles of Historical Geology. Upper Saddle River, N.J.: Prentice Hall, 1997. Goodwin, Alan M. Precambrian Geology. London: Academic Press, 1991. Kusky, Timothy M. Precambrian Ophiolites and Related

Rocks. Amsterdam: Elsevier, 2004. Sengor, A. M. C., and B. A. Natal'in. "Tectonics of the Altaids: An Example of a Turkic Type Orogen." In Earth Structure, 2nd ed., edited by B. A. van der Pluijm and S. Marshak, 535-546. New York: W. W. Norton, 2004.

Windley, Brian F. The Evolving Continents. 3rd ed. Chichester, England: John Wiley & Sons, 1995.

Saturn Saturn is the sixth planet, residing between Jupiter and Uranus, orbiting at 9.54 astronomical units (888 million miles, or 1,430 million kilometers) from the Sun, twice the distance from the center of the solar system as Jupiter, and having an orbital period of 29.5 Earth years. The mass of Saturn is 95 times that of Earth, yet it rotates at more than twice the rate of Earth. The average density of this giant gaseous planet is only 0.7 grams/cm3, less than water. The planet has a molecular hydrogen interior with a radius of 37,282 miles (60,000 km), a metallic hydrogen core with a radius of 18,641 miles (30,000 km), and a rocky/icy inner core with a radius of 9,320 miles (15,000 km).

The most striking features of Saturn are its many rings and moons, with the rings circling the planet along its equatorial plane and their appearance from Earth changing with the seasons because of the different tilt of the planet as it orbits the Sun. The rings are more than 124,275 miles (200,000 km) in diameter but are less than 30 feet (10 m) thick. They are composed of numerous small particles, most of which are ice between less than an inch (a few millimeters) and about 50 feet (a few tens of meters) in diameter. The breaks in the rings result from gravitational dynamics between the planet and its many moons.

Saturn has a yellowish-tan color produced largely by gaseous methane and ammonia, but the atmosphere consists of 92.4 percent molecular hydrogen, 7.4 percent helium, 0.2 percent methane, and 0.02 percent ammonia. These gases are stratified into three main layers, including a 62-124-mile- (100-200-km-) thick outer layer of ammonia, a 31-62-mile- (50-100-km-) thick layer of ammonium hydrosulfide ice, and a deeper 31-62-mile- (50-100-km-) thick layer of water ice. The atmosphere of Saturn is somewhat colder and thicker than that of Jupiter. Atmospheric winds on Saturn reach a maximum eastward-flowing velocity of 930 miles per hour (1,500 km/hr) at the equator and diminish with a few belts of high velocity toward the poles. Like Jupiter, Saturn has atmospheric bands related to these velocity variations, as well as turbulent storms that show as spots, and a few westward-flowing bands.

Many moons circle Saturn, including the large rocky Titan, possessing a thick nitrogen-argon-rich atmosphere that contains hydrocarbons including methane, similar to the basic building blocks of life on Earth. Other large to midsize moons include, in increasing distance from the planet, Mimas, Ence-ladus, Tethys, Dione, Rhea, and Iapetus. About a dozen other moons of significant size also circle the planet.

See also Earth; Jupiter; Mars; Mercury; Neptune; solar system; Uranus; Venus.

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. National Aeronautic and Space Administration. "Solar System Exploration page. Saturn." Available online. URL: http://solarsystem.nasa.gov/planets/profile. cfm?Object=Saturn. Last updated June 25, 2008. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West Publishing Company, 1991.

Mosaic of Saturn taken by Cassini, October 2004: The mosaic is made from 126 images processed to show the planet in natural color. Saturn's equator is tilted relative to its orbit by 27 degrees, very similar to the 23-degree tilt of the Earth. As Saturn moves along its orbit, first one hemisphere, then the other is tilted toward the Sun. This cyclical change causes seasons on Saturn, just as the changing orientation of Earth's tilt causes seasons on our planet. Saturn's rings are incredibly thin, with a thickness of only about 30 feet (10 m). The rings are made of dusty water ice, in the form of boulder-sized and smaller chunks that gently collide with each other as they orbit around Saturn. Saturn's gravitational field constantly disrupts these ice chunks, keeping them spread out and preventing them from combining to form a moon. The rings have a slight pale reddish color due to the presence of organic material mixed with the water ice. Saturn is about 75,000 miles (120,000 km) across and is flattened at the poles because of its very rapid rotation. A day is only 10 hours long on Saturn. Strong winds account for the horizontal bands in the atmosphere of this giant gas planet. The delicate color variations in the clouds are due to smog in the upper atmosphere, produced when ultraviolet radiation from the Sun shines on methane gas. Deeper in the atmosphere, the visible clouds and gases merge gradually into hotter and denser gases. (NASA Jet Propulsion Laboratory)

sea-level rise Global sea levels are currently rising as a result of the melting of the Greenland and Antarctica ice sheets and thermal expansion of the world's ocean waters due to global warming. Earth is presently in an interglacial stage of an ice age. sea levels have risen nearly 400 feet (122 m) since the last glacial maximum 20,000 years ago and about 6 inches (15 cm) in the past 100 years. The rate of sea-level rise seems to be accelerating, and may presently be as much as an inch (2.5 cm) every eight to 10 years. If all of the ice on both the Antarctic and Greenland ice sheets were to melt, global sea levels would rise by 230 feet (70 m), inundating most of the world's major cities and submerging large parts of the continents under shallow seas. The coastal regions of the world are densely populated and are experiencing rapid population growth. Approximately 100 million people presently live within three feet (1 m) above the present-day sea level. If sea level were to rise rapidly and significantly the world would experience an economic and social disaster of a magnitude not yet experienced by civilization. Many areas would become permanently flooded or subject to inundation by storms, beach erosion would be accelerated, and water tables would rise.

sea level has risen and fallen by hundreds of feet many times in the billions of years represented by Earth history, and it is presently slowly rising at about one foot (0.3 m) per century, a rate that may be accelerating from the effects of global warming. The causes of sea-level rise and fall are complex, operate on vastly different time scales, and may affect local regions or the entire planet. These include growth and melting of glaciers, changes in the volume of the mid-ocean ridges, thermal expansion of water from global warming, and other complex interactions of the distribution of the continental landmass in mountains and plains during periods of faulting, mountainbuilding, and basin-forming activity. Deciphering the amount of sea-level rise depends critically on correct identification and separation of the local effects of the rising and sinking of the land, known as relative sea-level changes, from global changes in sea level, that are referred to as eustatic events.

Sometimes individual large earthquakes may displace the land surface vertically, resulting in subsidence or uplift of the land relative to the sea. One of the largest and best-documented cases of earthquake-induced subsidence resulted from the March 27, 1964, magnitude 9.2 earthquake in southern Alaska. This earthquake tilted a huge approximately 125,000-square-mile (200,000 square km) area of the Earth's crust. Significant changes in ground level were recorded along the coast for more than 600 miles (1,000 km), including uplifts of up to 36 feet (11 m), subsidence of up to 6.5 feet (2 m), and lateral shifts of 30 or more feet (several to tens of meters). Much of the area that subsided was along Cook Inlet, north to Anchorage and Valdez, and south to Kodiak Island. Towns that were built around docks prior to the earthquake were suddenly located below the high tide mark, and entire towns had to move to higher ground. Forests that subsided found their root systems suddenly inundated by salt water, leading to the death of the forests. Populated areas located at previously safe distances from the high-tide (and storm) line became prone to flooding and storm surges and had to be relocated.

The fastest changes in sea level are caused by instantaneous geologic catastrophes such as meteorite impacts into the ocean, but luckily these types of events do not happen often. Seasonal changes can blow or move water to greater heights on one side of a basin, move it to lower heights on another side, and cause water to expand and contract with changes in temperature, causing small changes in sea level. Climate changes can cause glaciers and ice caps to melt and reform, causing sea levels to rise and fall on time scales of hundreds to thousands of years. Longer-term climate variations related to variations in the orbit of the Earth around the Sun can lengthen the time scale of sea-level changes related to climate change to hundreds of thousands of years for individual cycles. Plate tectonics also influences sea-level changes, but on much longer time scales than climate variations. If the processes of seafloor spreading and submarine volcanism become accelerated in any geologic time period, the volume of material that makes up the mid-ocean ridge system will be larger, and this extra volume of elevated seafloor will displace an equal amount of seawater and raise sea levels. This process typically operates with time variations on the order of tens of millions of years. An even longer-term variation in sea levels is caused by the motions and collisions of continents in supercontinent cycles. When continents collide, large amounts of continen tal material are uplifted above sea level, effectively taking this material out of the oceans, making the ocean basins bigger, and lowering sea levels. When continents rift apart, the opposite happens—more material is added to the ocean basins, and sea levels rise on the continents. These slow tectonic variations can change sea levels on time scales of tens to hundreds of millions of years.

Rising sea levels cause the shoreline to move landward, whereas a fall in sea level causes the shoreline to move oceanward. With the present sea-level rise, coastal cliffs are eroding, barrier islands are migrating (or being submerged if they were heavily protected from erosion), beaches are moving landward, and the sea is flooding estuaries. At some point in the not too distant future, low-lying coastal cities will be flooded under several feet of water, and eventually the water could be hundreds of feet deep. Cities including New Orleans, New York, Washington, Houston, London, Shanghai, Tokyo, and Cairo will be inundated; the world's nations need to begin planning how to handle this inevitable geologic hazard and encroachment of the sea.

About 70 percent of the world's sandy beaches are being eroded. The reasons for this erosion include rising sea levels, increased storminess, a decrease in sediment transport to beaches from the damming of rivers, and perhaps shifts in global climate belts. Construction of sea walls to reduce erosion of coastal cliffs also causes a decreased supply of sand to replenish the beach, so also increases beach retreat. Pumping of groundwater from coastal aquifers also results in coastal erosion, because pumping causes the surface to subside, leading to a relative sea-level rise.

When sea level rises, beaches try to maintain their equilibrium profile and move each beach element landward. A sea-level rise of 1 inch (2.5 cm) is generally equated with a landward shift of beach elements of more than four feet (>1 m). Most sandy beaches worldwide are retreating landward at rates of 20 inches to 3 feet per year (0.5-1 m), consistent with sea-level rise of an inch (2.5 cm) every 10 years.

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