When Plates Pull Apart

• When the plates pull apart, two types of phenomena occur depending on whether the movement takes place in the oceans or on land. When plates pull apart on land, deep valleys known as rift valleys form.
• An example of a rift valley is the Great Rift Valley that extends from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the oceans, long, sinuous chains of volcanic mountains called mid-ocean ridges form, and new seafloor is created at the site of these ridges.
• Rift valleys are also present along the crests of the mid-ocean ridges.
• Most scientists believe that gravity and heat from the interior of the Earth cause the plates to move apart and to create new seafloor.

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Plate Tectonics

• Opposing this tendency toward leveling is a force responsible for raising mountains and plateaus and for creating new landmasses. These changes to Earth’s surface occur in the outermost solid portion of Earth, known as the lithosphere.
• The lithosphere consists of the crust and another region known as the upper mantle and is approximately 65 to 100 km (40 to 60 mi) thick. Compared with the interior of the Earth, however, this region is relatively thin. The lithosphere is thinner in proportion to the whole Earth than the skin of an apple is to the whole apple.
• According to the theory, the lithosphere is divided into large and small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, the Indo-Australian plate, and the African plate.
• Smaller plates include the Cocos plate, the Nazca plate, the Philippine plate, and the Caribbean plate. Plate sizes vary a great deal. The Cocos plate is 2,000 km (1,000 mi) wide, while the Pacific plate is nearly 14,000 km (nearly 9,000 mi) wide.
• These plates move in three different ways in relation to each other. They pull apart or move away from each other, they collide or move against each other, or they slide past each other as they move sideways.
• The movement of these plates helps explain many geological events, such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.

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Erosion

• Erosion is the process that removes loose and weathered rock and carries it to a new site. Water, wind, and glacial ice combined with the force of gravity can cause erosion.

• Erosion by running water is by far the most common process of erosion. It takes place over a longer period of time than other forms of erosion. When water from rain or melted snow moves downhill, it can carry loose rock or soil with it. Erosion by running water forms the familiar gullies and V-shaped valleys that cut into most landscapes.
• The force of the running water removes loose particles formed by weathering. In the process, gullies and valleys are lengthened, widened, and deepened. Often, water overflows the banks of the gullies or river channels, resulting in floods.
• Each new flood carries more material away to increase the size of the valley. Meanwhile, weathering loosens more and more material so the process continues.
• Erosion by glacial ice is less common, but it can cause the greatest landscape changes in the shortest amount of time. Glacial ice forms in a region where snow fails to melt in the spring and summer and instead builds up as ice.
• For major glaciers to form, this lack of snowmelt has to occur for a number of years in areas with high precipitation. As ice accumulates and thickens, it flows as a solid mass.
• As it flows, it has a tremendous capacity to erode soil and even solid rock. Ice is a major factor in shaping some landscapes, especially mountainous regions. Glacial ice provides much of the spectacular scenery in these regions. Features such as horns (sharp mountain peaks), arêtes (sharp ridges), glacially formed lakes, and U-shaped valleys are all the result of glacial erosion.
• Many factors determine the rate and kind of erosion that occurs in a given area. The climate of an area determines the distribution, amount, and kind of precipitation that the area receives and thus the type and rate of weathering.
• An area with an arid climate erodes differently than an area with a humid climate. The elevation of an area also plays a role by determining the potential energy of running water.
• The higher the elevation the more energetically water will flow due to the force of gravity. The type of bedrock in an area (sandstone, granite, or shale) can determine the shapes of valleys and slopes, and the depth of streams.
• A landscape’s geologic age—that is, how long current conditions of weathering and erosion have affected the area—determines its overall appearance. Relatively young landscapes tend to be more rugged and angular in appearance. Older landscapes tend to have more rounded slopes and hills.
• The oldest landscapes tend to be low-lying with broad, open river valleys and low, rounded hills. The overall effect of the wearing down of an area is to level the land; the tendency is toward the reduction of all land surfaces to sea level.

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Weathering

• Weathering is the breakdown of rock at and near the surface of Earth. Most rocks originally formed in a hot, high-pressure environment below the surface where there was little exposure to water.
• Once the rocks reached Earth’s surface, however, they were subjected to temperature changes and exposed to water. When rocks are subjected to these kinds of surface conditions, the minerals they contain tend to change.
• These changes constitute the process of weathering. There are two types of weathering: physical weathering and chemical weathering.
• Physical weathering involves a decrease in the size of rock material. Freezing and thawing of water in rock cavities, for example, splits rock into small pieces because water expands when it freezes.
• Chemical weathering involves a chemical change in the composition of rock. For example, feldspar, a common mineral in granite and other rocks, reacts with water to form clay minerals, resulting in a new substance with totally different properties than the parent feldspar.
• Chemical weathering is of significance to humans because it creates the clay minerals that are important components of soil, the basis of agriculture.
• Chemical weathering also causes the release of dissolved forms of sodium, calcium, potassium, magnesium, and other chemical elements into surface water and groundwater.
• These elements are carried by surface water and groundwater to the sea and are the sources of dissolved salts in the sea.

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Changes to Earth’s Surface

• Earth’s surface has been constantly changing ever since the planet formed. Most of these changes have been gradual, taking place over millions of years.
• Nevertheless, these gradual changes have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of huge supercontinents, and the breakup of supercontinents into smaller continents.
• The weathering and erosion that result from the water cycle are among the principal factors responsible for changes to Earth’s surface.
• Another principal factor is the movement of Earth’s continents and seafloors and the buildup of mountain ranges due to a phenomenon known as plate tectonics. Heat is the basis for all of these changes.
• Heat in Earth’s interior is believed to be responsible for continental movement, mountain building, and the creation of new seafloor in ocean basins.
• Heat from the Sun is responsible for the evaporation of ocean water and the resulting precipitation that causes weathering and erosion. In effect, heat in Earth’s interior helps build up Earth’s surface while heat from the Sun helps wear down the surface.

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EARTH’S SURFACE

• Earth’s surface is the outermost layer of the planet. It includes the hydrosphere, the crust, and the biosphere.

Hydrosphere

• The hydrosphere consists of the bodies of water that cover 71 percent of Earth’s surface. The largest of these are the oceans, which contain over 97 percent of all water on Earth.
• Glaciers and the polar ice caps contain just over 2 percent of Earth’s water in the form of solid ice. Only about 0.6 percent is under the surface as groundwater. Nevertheless, groundwater is 36 times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere as water vapor.
• Only 0.017 percent of all the water on Earth is found in lakes and rivers. And a mere 0.001 percent is found in the atmosphere as water vapor. Most of the water in glaciers, lakes, inland seas, rivers, and groundwater is fresh and can be used for drinking and agriculture.
• Dissolved salts compose about 3.5 percent of the water in the oceans, however, making it unsuitable for drinking or agriculture unless it is treated to remove the salts.

Crust

• The crust consists of the continents, other land areas, and the basins, or floors, of the oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km (9 to 47 mi) thick.
• The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km (3 to 6 mi). The crust has a definite boundary called the Mohorovičić discontinuity, or simply the Moho.
• The boundary separates the crust from the underlying mantle, which is much thicker and is part of Earth’s interior.
• Oceanic crust and continental crust differ in the type of rocks they contain. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock, called magma, cools and solidifies.
• Sedimentary rocks are usually created by the breakdown of igneous rocks. They tend to form in layers as small particles of other rocks or as the mineralized remains of dead animals and plants that have fused together over time.
• The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks are altered by heat and pressure deep underground.
• Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continental crust consists of lighter-colored, less dense igneous rocks, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks.

Biosphere

• The biosphere includes all the areas of Earth capable of supporting life. The biosphere ranges from about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor.
• For a long time, scientists believed that all life depended on energy from the Sun and consequently could only exist where sunlight penetrated. In the 1970s, however, scientists discovered various forms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlight penetrated.
• They learned that primitive bacteria formed the basis of this living community and that the bacteria derived their energy from a process called chemosynthesis that did not depend on sunlight.
• Some scientists believe that the biosphere may extend relatively deep into Earth’s crust. They have recovered what they believe are primitive bacteria from deeply drilled holes below the surface.

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EARTH’S ATMOSPHERE

• The atmosphere is a layer of different gases that extends from Earth’s surface to the exosphere, the outer limit of the atmosphere, about 9,600 km (6,000 mi) above the surface.

• Near Earth’s surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent).
• The remaining 1 percent of atmospheric gases consists of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapor; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.

Layers of the Atmosphere

• The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi) above sea level at the equator.
• Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi) is called the mesosphere. At an altitude of about 90 km, temperatures begin to rise.
• The layer that begins at this altitude is called the thermosphere because of the high temperatures that can be reached in this layer (about 1200°C, or about 2200°F).
• The region beyond the thermosphere is called the exosphere. The thermosphere and the exosphere overlap with another region of the atmosphere known as the ionosphere, a layer or layers of ionized air extending from almost 60 km (about 50 mi) above Earth’s surface to altitudes of 1,000 km (600 mi) and more.
• Earth’s atmosphere and the way it interacts with the oceans and radiation from the Sun are responsible for the planet’s climate and weather. The atmosphere plays a key role in supporting life.
• Almost all life on Earth uses atmospheric oxygen for energy in a process known as cellular respiration, which is essential to life. The atmosphere also helps moderate Earth’s climate by trapping radiation from the Sun that is reflected from Earth’s surface.
• Water vapor, carbon dioxide, methane, and nitrous oxide in the atmosphere act as “greenhouse gases.” Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun in the lower atmosphere and thereby help warm Earth’s surface. Without this greenhouse effect, heat radiation would escape into space, and Earth would be too cold to support most forms of life.

The Atmosphere and the Water Cycle

• The water cycle simply means that Earth’s water is continually recycled between the oceans, the atmosphere, and the land. All of the water that exists on Earth today has been used and reused for billions of years. Very little water has been created or lost during this period of time.
• Water is constantly moving on Earth’s surface and changing back and forth between ice, liquid water, and water vapor. The water cycle begins when the Sun heats the water in the oceans and causes it to evaporate and enter the atmosphere as water vapor.
• Some of this water vapor falls as precipitation directly back into the oceans, completing a short cycle. Some of the water vapor, however, reaches land, where it may fall as snow or rain.
• Melted snow or rain enters rivers or lakes on the land. Due to the force of gravity, the water in the rivers eventually empties back into the oceans. Melted snow or rain also may enter the ground. Groundwater may be stored for hundreds or thousands of years, but it will eventually reach the surface as springs or small pools known as seeps.
• Even snow that forms glacial ice or becomes part of the polar caps and is kept out of the cycle for thousands of years eventually melts or is warmed by the Sun and turned into water vapor, entering the atmosphere and falling again as precipitation. All water that falls on land eventually returns to the ocean, completing the water cycle.

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EARTH, THE SOLAR SYSTEM, AND THE GALAXY

• Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation.

• The Earth travels at a velocity of about 107,000 km/h (about 67,000 mph) as it orbits the Sun. All but one of the planets orbit the Sun in the same plane—that is, if an imaginary line were extended from the center of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line.
• The exception is Pluto, which has an eccentric (unusual) orbit. Earth’s orbital path is not quite a perfect circle but instead is slightly elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km (about 95 million mi) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi) from the Sun.
• If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun. The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of stars bound together by gravity. The Milky Way has armlike discs of stars that spiral out from its center.
• The solar system is located in one of these spiral arms, known as the Orion arm, which is about two-thirds of the way from the center of the Galaxy. In most parts of the Northern Hemisphere, this disc of stars is visible on a summer night as a dense band of light known as the Milky Way.
• Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles.
• Its polar diameter, measured from the North Pole to the South Pole, is somewhat less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets—Mercury, Venus, Earth, and Mars—that make up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune.
• For example, the largest planet, Jupiter, has a diameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. A famous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earths would fit inside it. Earth has one natural satellite, the Moon.
• The Moon orbits the Earth, completing one revolution in an elliptical path in 27 days 7 hr 43 min 11.5 sec. The Moon orbits the Earth because of the force of Earth’s gravity. However, the Moon also exerts a gravitational force on the Earth.
• Evidence for the Moon’s gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than 4 billion years ago when a large meteorite or small planet struck the Earth.
• As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min 4.1 sec.
• The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr 48 min 46 sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It takes more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy’s center.

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Earth (planet)

• Earth (planet), one of nine planets in the solar system, the only planet known to harbor life, and the “home” of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans.

• About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans. Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space.
• Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form. Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it.
• It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets—Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth’s atmosphere includes just the right amount of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form.
• The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth’s atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth and is also neither too near nor too far from the Sun. But because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot—462°C (864°F)—hot enough to melt lead and too hot for life to exist.
• For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas—for example, that the Earth was a sphere and that it traveled around the Sun—were based on brilliant reasoning.
• However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system.
• By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. And with the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered. Life itself contributes to changes on Earth, especially in the way living things can alter Earth’s atmosphere.
• For example, Earth at one time had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth’s atmosphere and made it possible for animal life to evolve on land.

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OCEANOGRAPHIC SUBMERSIBLES

• The American explorer Charles William Beebe was the first to observe marine species at depths that could not be reached by a diver. He and the engineer Otis Barton designed a spherical steel vessel called a bathysphere that could be lowered from a ship, suspended from a cable.

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• In 1930 Beebe and Barton reached a depth of 435 m (about 1,425 ft), and in 1934 a depth of 923 m (3,028 ft). The danger of this submersible was that if the cable broke, the occupants could not return to the surface.
• With this in mind, Swiss physicist Auguste Piccard designed his first bathyscaphe, a navigable deep-sea vessel consisting of a pressure sphere that is kept buoyant by a float (a large container filled with gasoline).
• With this bathyscaphe, Piccard reached (1954) a depth of 4,000 m (13,125 ft). In 1960 his son Jacques Piccard reached the record depth of 10,915 m (about 35,810 ft) in the Mariana Trench off the island of Guam, with Trieste (the second bathyscaphe built by Piccard in 1953).

• A large number of occupied submersibles for deep-sea exploration are now employed by different countries around the world. These craft can dive as deep as 6,000 m (about 20,000 ft), and are equipped with underwater lights, cameras, television systems, and mechanical manipulators to collect bottom samples.
• Unmanned, robot submersibles are also being used for underwater exploration and are capable of descending to even greater depths.
• One such craft, called Argo, was used in 1985 to locate the wreck of the Titanic, and a smaller robot, called Jason, was used to explore the wreck.

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OCEANOGRAPHIC INSTRUMENTATION

• The first instruments used for the investigation of the sea bottom was the sounding weight, with which British explorer Sir James Clark Ross reached a depth of 3,700 m (12,140 ft) in 1840.

• The sounding weights used on the Challenger, called Baillie sounding machines, were provided with a tube into which a sample of the seabed was forced when the weight hit the bottom of the ocean.
• Also used on the Challenger were dredges and scoops, suspended on ropes, with which samples of the sediment and biological specimens of the seabed could be obtained.
• A modern version of the Baillie sounding machine is the gravity corer. The corer consists of an open-ended tube with a lead weight and a trigger mechanism that releases the corer from its suspension cable when the corer is lowered over the seabed and a small weight touches the ground.
• The corer falls into the seabed and penetrates it to a depth of up to 10 m (33 ft). By lifting the corer, a long, cylindrical sample is extracted in which the structure of the seabed's layers of sediment is preserved. Samples of deeper layers can be obtained with a corer mounted in a drill.
• The drilling vessel JOIDES Resolution (see Ocean Drilling Program) is equipped to extract cores from depths of as much as 1,500 m (4,900 ft) below the ocean bottom. Other instruments for deep-sea exploration are high-resolution television cameras, movie cameras, thermometers, pressure meters, flow meters, and seismographs.
• These instruments are either lowered to the sea bottom on long cables or attached to submersible buoys; they sometimes are provided with a sound source, making depth determination possible.
• Deep-sea currents can be determined by floats carrying an ultrasonic sound source so that their movements can be followed aboard the research vessel.
• Such vessels themselves require precise navigational instrumentation, such as satellite navigation devices, and special positioning systems that keep the vessel in a fixed position relative to a sonar beacon on the bottom of the ocean.

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Deep-Sea Exploration

• Deep-Sea Exploration, investigation of physical, chemical, and biological conditions at the bottom of the ocean, for scientific and commercial purposes.
• The depths of the sea have been investigated with precision only during comparatively recent years; compared to the other areas of geological research, they still form a relatively unexplored domain.
• Modern scientific study of the deep sea can be said to have begun when the French scientist Pierre Simon de Laplace calculated the average depth of the Atlantic Ocean from tidal motions registered on the Brazilian and African coasts.
• He determined this depth to be 3,962 m (13,000 ft), a value proven quite accurate by later soundings. The first investigations of the sea bottom were undertaken because of the need for accurate soundings when laying submarine cables.
• That life exists at great depths was discovered in 1864, when Norwegian researchers sampled a stalked crinoid at a depth of 3,109 m (10,200 ft).
• The most important knowledge of deep-sea conditions has been gained since 1870, beginning with the Challenger expedition sent out by the British government in 1872.
• It engaged in global oceanographic investigations for nearly four years, during which 715 new genera and 4,417 new species of marine organisms were discovered.

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