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EARTH
Terre-Erde-Chikyu-Tierra-Terra-Gaia
The sun is a star -- one of billions of stars that make up a galaxy called the Milky Way. The Milky Way and as many as 100 billion other galaxies make up the universe.
NASA Earth Observatory image by Jesse Allen, using data provided courtesy of the Ocean Color Web team.
Blue Marble 2012 - Arctic View: Fifteen orbits of the recently launched Suomi NPP satellite provided the VIIRS instrument enough time (and longitude) to gather the pixels for this synthesized view of Earth showing the Arctic, Europe, and Asia. Credit: NASA/GSFC/Suomi NPP
The planet Earth is only a tiny part of the universe, but it is the home of human beings and, in fact, all known life in the universe. Animals, plants, and other organisms live almost everywhere on Earth's surface. They can live on Earth because it is just the right distance from the sun. Most living things need the sun's warmth and light for life. If Earth were too close to the sun, it would be too hot for living things. If Earth were too far from the sun, it would be too cold for anything to live. Living things also must have water to live. Earth has plenty. Water covers most of Earth's surface.
The study of Earth is called geology, and scientists who study Earth are geologists. Geologists study different physical features of Earth to understand how they were formed and how they may have changed over time. Much of Earth, such as the deep interior, cannot be studied directly. Geologists must often study samples of rock and use indirect methods to learn about the planet. Today, geologists can also view and study the entire Earth from space.
Earth ranks fifth in size among the nine planets. Jupiter, the largest planet, is about 11 times larger in diameter than Earth. Pluto, the smallest planet, has a diameter less than one-fifth that of Earth. Earth, like all the planets in our solar system, travels around the sun in a path called an orbit. Earth is about 93 million miles (150 million kilometers) from the sun. It takes one year for Earth to complete one orbit around the sun. The innermost planet, Mercury, is only about one-third as far from the sun as Earth and circles the sun in only 88 days. Pluto, the outermost planet, is 40 times as far from the sun as Earth and takes 248 Earth years to circle the sun.
Earth has three motions. It (1) spins like a top around an imaginary line called an axis that runs from the North Pole to the South Pole, (2) it travels around the sun, and (3) it moves through the Milky Way along with the sun and the rest of the solar system. Earth takes 24 hours to spin completely around on its axis so that the sun is in the same place in the sky. This period is called a solar day. During a solar day, Earth moves a little around its orbit so that it faces the stars a little differently each night. Thus, it only takes 23 hours 56 minutes 4.09 seconds for Earth to spin once so that the stars appear to be in the same place in the sky. This period is called a sidereal day. A sidereal day is shorter than a solar day, so the stars appear to rise about 4 minutes earlier each day. Earth takes 365 days 6 hours 9 minutes 9.54 seconds to circle the sun. This length of time is called a sidereal year. Because Earth does not spin a whole number of times as it goes around the sun, the calendar gets out of step with the seasons by about 6 hours each year. Every four years, a day is added to bring the calendar back into line with the seasons. These years, called leap years, have 366 days. The extra day is added to the end of February and occurs as February 29. The distance around Earth's orbit is 584 million miles (940 million kilometers). Earth travels in its orbit at 66,700 miles (107,000 kilometers) an hour, or 18.5 miles (30 kilometers) a second. Earth's orbit lies on an imaginary flat surface around the sun called the orbital plane.
Earth's axis is not straight up and down, but is tilted by about 23 1/2 degrees compared to the orbital plane. This tilt and Earth's motion around the sun causes the change of the seasons. In January, the northern half of Earth tilts away from the sun. Sunlight is spread thinly over the northern half of Earth, and the north experiences winter. At the same time, the sunlight falls intensely on the southern half of Earth, which has summer. By July, Earth has moved to the opposite side of the sun. Now the northern half of Earth tilts toward the sun. Sunlight falls intensely over the northern half of Earth, and the north experiences summer. At the same time, the sunlight falls less intensely on the southern half of Earth, which has winter.
Earth's orbit is not a perfect circle. Earth is slightly closer to the sun in early January (winter in the Northern Hemisphere) and farther away in July. In January, Earth is 91.4 million miles (147.1 million kilometers) from the sun, and in July it is 94.5 million miles (152.1 million kilometers) from the sun. This variation has a far smaller effect than the heating and cooling caused by the tilt of Earth's axis.
Earth and the solar system are part of a vast disk of stars called the Milky Way Galaxy. Just as the moon orbits Earth and planets orbit the sun, the sun and other stars orbit the tightly packed center of the Milky Way. The solar system is about two-fifths of the way from the center of the Milky Way and revolves around the center at about 155 miles (249 kilometers) per second. The solar system makes one complete revolution around the center of the galaxy in about 220 million years.
Russian weather satellite Elektro-L No.1 image
Most people picture Earth as a ball with the North Pole at the top and the South Pole at the bottom. Earth, other planets, large moons, and stars -- in fact, most objects in space bigger than about 200 miles (320 kilometers) in diameter -- are round because of their gravity. Gravity pulls matter in toward the center of objects. Tiny moons, such as the two moons of Mars, have so little gravity that they do not become round, but remain lumpy instead. To our bodies, "down" is always the direction gravity is pulling. People everywhere on Earth feel "down" is toward the center of Earth and "up" is toward the sky. People in Spain and in New Zealand are on exactly opposite sides of Earth from each other, but both sense their surroundings as "right side up." Gravity works the same way on other planets and moons. Earth,
however, is not perfectly round. Earth's spin causes it to bulge slightly
at its middle, the equator. The diameter of Earth from North Pole to South
Pole is 7,899.83 miles (12,713.54 kilometers), but through the equator it
is 7,926.41 miles (12,756.32 kilometers). This difference, 26.58 miles
(42.78 kilometers), is only 1/298 the diameter of Earth. The difference is
too tiny to be easily seen in pictures of Earth from space, so the planet
appears round. Earth is the third planet from the Sun and the fifth largest in the solar system.
Earth's diameter is just a few hundred kilometers larger than that of Venus. The four seasons are a result of Earth's axis of rotation being tilted more than 23 degrees.
Visible Planet
Orbits Oceans at least 4 km deep cover nearly 70 percent of Earth's surface. Fresh water exists in the liquid phase only within a narrow temperature span (0 degrees to 100 degrees Celsius). This temperature span is especially narrow when contrasted with the full range of temperatures found within the solar system. The presence and distribution of water vapor in the atmosphere is responsible for much of Earth's weather. Near the surface, an ocean of air that consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent other ingredients envelops us. This atmosphere affects Earth's long-term climate and short-term local weather; shields us from nearly all harmful radiation coming from the Sun; and protects us from meteors as well - most of which burn up before they can strike the surface. Satellites have revealed that the upper atmosphere actually swells by day and contracts by night due to solar activity. Our planet's rapid spin and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape. The solar wind is a stream of charged particles continuously ejected from the Sun. The magnetic field does not fade off into space, but has defi- nite boundaries. When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above our planet's magnetic poles. These air molecules then begin to glow and are known as the aurorae, or the Northern and Southern Lights.
Credit:ESA
Earth's land surfaces are also in motion. For example, the North American continent continues to move west over the Pacific Ocean basin. Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate. These movements are known as plate tectonics.
NASA Image Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic.
Earth's Moon NASA Image Earth has only one natural satellite, the Moon, which is 384,000 km (211,265 miles) away.Earth's moon has a diameter of 2,159 miles (3,474 kilometers) -- about one-fourth of Earth's diameter.
The sun's gravity acts on Earth and the moon as if they were a single body with its center about 1,000 miles (1,600 kilometers) below Earth's surface. This spot is the Earth-moon barycenter. It is the point of balance between the heavy Earth and the lighter moon. The path of the barycenter around the sun is a smooth curve. Earth and the moon circle the barycenter as they orbit the sun. The motion of Earth and moon around the barycenter makes them "wobble" in their path around the sun.
Earth has a modest magnetic field produced by electric currents in the outer core. The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth.
The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.
The magnetosphere is that area of space, around the Earth, that is controlled by the Earth's magnetic field.The magnetosphere extends into the vacuum of space from approximately 80 to 60,000 kilometers (50 to 37,280 miles) on the side toward the Sun, and trails out more than 300,000 kilometers (186,500 miles) away from the Sun.
Credit UCAR
Temperature Records
Wettest Location
The Earth is 4.5 billion years old It is the home of 6 Billion human beings and millions of species. Its is at least 4 1/2 billion years old. It weighs 6.6 sextillion tons-6,000,000,000,000,000,000,000,000 (6E+24) kilograms.
The hydrosphere About 71 percent of Earth's surface is covered by water, most of it in the oceans. Ocean water is too salty to drink. Only about 3 percent of Earth's water is fresh water, suitable for drinking. Much of Earth's fresh water is not readily available to people because it is frozen in the polar ice caps or beneath Earth's surface. Polar regions and high mountains stay cold enough for water to remain permanently frozen. The region of permanent ice on Earth is sometimes called the cryosphere. The lithosphere The crust and upper mantle of Earth from the surface to about 60 miles (100 kilometers) down make up the lithosphere. The thin crust is made up of natural chemicals called minerals composed of different combinations of elements. Oxygen is the most abundant chemical element in rocks in Earth's crust, making up about 47 percent of the weight of all rock. The second most abundant element is silicon, 27 percent, followed by aluminum (8 percent), iron (5 percent), calcium (4 percent), and sodium, potassium, and magnesium (about 2 percent each). These eight elements make up 99 percent of the weight of rocks on Earth's surface. Two elements, silicon and oxygen, make up almost three-fourths of the crust. This combination of elements is so important that geologists have a special term for it: silica. Minerals that contain silica are called silicate minerals. The most abundant mineral on Earth's surface is quartz, made up of pure silica. Another plentiful group of silicates are the feldspars, which consist of silica, aluminum, calcium, sodium, and potassium. Other common silicate minerals on Earth's surface are pyroxene (PY rahk seen) and amphibole (AM fuh bohl), which consist of combinations of silica, iron, and magnesium. Another important group of minerals are the carbonates, which contain carbon and oxygen along with small amounts of other elements. The most important carbonate mineral is calcite, made up of calcium, carbon, and oxygen. Limestone, a common rock used for building, is mostly calcite. Another important carbonate is dolomite, composed of carbon, oxygen, calcium, and magnesium. Earth has two kinds of crust. The dry land of the continents is made up mostly of granite and other light silicate minerals, while the ocean floors are composed mostly of a dark, dense volcanic rock called basalt. Continental crust averages about 25 miles (40 kilometers) thick, but it is thicker in some areas and thinner in others. Most oceanic crust is only about 5 miles (8 kilometers) thick. Water fills in the low areas over the thin basalt crust to form the world's oceans. There is more than enough water on Earth to completely fill the oceanic basins, and some of it spreads onto the edges of the continents. This portion of the continents surrounded by a band of shallow ocean is called the continental shelf. The biosphere Earth is the only planet in the universe known to have life. The region containing life extends from the bottom of the deepest ocean to a few miles or kilometers into the atmosphere. There are several million known kinds, called species, of living things, and scientists believe that there are far many more species not yet discovered. Life affects Earth in many ways. Life has actually made the atmosphere around us. Plants take in water and carbon dioxide, both of which contain oxygen. They use the carbon in carbon dioxide and the hydrogen in water to make chemicals of many kinds and give off oxygen as a waste product. Animals eat plants to get energy and return water and carbon dioxide back into the environment. Living things affect the surface of Earth in other ways as well. Plants create chemicals that speed the breakdown of rock. Grasslands and forests slow the erosion of soil. Earth's rocks
The solid part of Earth consists of rocks, which are sometimes made up of a single mineral, but more often consist of mixtures of minerals. Geologists classify rocks according to their origin. Igneous rocks form when molten rock cools and solidifies. Sedimentary rocks form when grains of rock or dissolved chemicals are deposited in layers by wind, water, or glaciers. Over time, the layers harden into solid rock. Metamorphic rocks develop deep in Earth's crust when heat or pressure transform other types of rock. Igneous rocks form from molten material called magma. Most of Earth's interior is solid, not molten, but it is extremely hot. At the base of Earth's crust, the temperature is about 1800 degrees F (1000 degrees C). In some portions of the crust, conditions are right for rocks to melt. Rocks can melt more easily near the crust if they contain water, which lowers their melting point. Where conditions are right, small pockets of magma form beneath and within the crust. Some of this magma reaches the surface, where it erupts from volcanoes as lava. Igneous rocks formed this way are called volcanic or extrusive. Vast quantities of magma, however, never reach the surface. They cool slowly within the crust and may only be exposed long afterward by erosion. Such igneous rocks are called plutonic or intrusive. Plutonic rocks cool slowly. During this slow cooling, their minerals form large crystals. Plutonic rocks tend to be much coarser than volcanic rocks. Igneous rocks that are rich in silica tend to be poor in iron and magnesium, and the opposite is also true. Volcanic rocks that are iron-rich and silica-poor are basalt. Plutonic rocks of the same makeup are called gabbro. Silica-rich volcanic rocks are called rhyolite (RY uh lyt), and plutonic rocks of the same composition are granite. Granite lies under most of the continents, while basalt lies under most of the ocean floors. Sedimentary rocks Rocks on Earth's surface are under constant attack by chemicals and mechanical forces. The processes that break down rocks are called weathering. Water is effective at dissolving minerals. When water freezes, it expands, so expanding ice helps pry apart mineral grains in rocks. In addition, living things produce chemicals that help dissolve rocks. Once rocks break apart, the loose material is often carried away by erosion. Running water erodes rocks. Wind and glaciers also contribute to erosion. Erosion is usually a relatively slow process, but over millions of years, erosion can uncover even rocks many miles or kilometers below the surface. Materials derived from weathering and erosion of rocks are eventually deposited to form sedimentary rocks. Rocks that are made up of small pieces of other rocks are called clastic rocks. Rocks containing larger pebbles are called conglomerate. The particles in these rocks are cemented together when minerals dissolved in the water crystallize between the grains. The most abundant sedimentary rocks, called mudrocks, consist of tiny particles. Some of these rocks, called shale, split into thin sheets when broken. Sandstone is a sedimentary rock made up of sand cemented together. Other sedimentary rocks form when dissolved materials undergo chemical reactions and settle out as tiny solid particles. These rocks are called chemical sedimentary rocks. Common chemical sedimentary rocks include some types of limestone and dolomite. Some chemical sedimentary rocks form when water evaporates, leaving dissolved materials behind. Rock salt and a mineral called gypsum form this way. Some sedimentary rocks, called biogenic, are formed by the action of living things. Coal is the remains of woody plants that have been transformed into rock by heat and pressure over time. Most limestone is formed by microscopic marine organisms that secrete protective shells of calcium carbonate. When the animals die, the shells remain and solidify into limestone. Metamorphic rocks When rocks are buried deeply, they become hot. Earth's crust grows hotter by about 70 degrees F per mile (25 degrees C per kilometer) of depth. Pressure also increases with depth. At a depth of 1 mile (1.6 kilometers) beneath the surface, the pressure is about 6,000 pounds per square inch (41,360 kilopascals). As rocks are heated and subjected to pressure, minerals react and the rocks become metamorphic. Shale is transformed to slate, limestone, and eventually into marble under pressure. Many metamorphic rocks contain recognizable features that tell of their origin, but others change so much that only the chemical makeup provides evidence of what they originally were. Cycles on and in Earth Earth can be thought of as a huge system of interacting cycles. In each cycle, matter and energy move from place to place and may change form. Eventually, matter and energy return to their original condition and the cycle begins again. The cycles affect everything on the planet, from the weather to the shape of the landscape. There are many cycles on and within Earth. A few of the most important are (1) atmospheric circulation, (2) ocean currents, (3) the global heat conveyor, (4) the hydrologic cycle, and (5) the rock cycle. Atmospheric circulation Air warmed by the sun near the equator rises and flows toward Earth's poles, returning to the surface and flowing back to the equator. This motion, combined with the rotation of Earth, moves heat and moisture around the planet creating winds and weather patterns. In some areas, the winds change directions with the seasons. These patterns are often called monsoons. In summer, air over Asia is heated by the sun, rises, and draws moist air from the Indian Ocean, causing daily rains over most of southern Asia. In winter, the air over Asia cools, sinks, and flows out, pushing the moist air away and creating dry weather. A similar pattern occurs in the Pacific Ocean near Mexico and brings moist air and afternoon thunderstorms to the southwestern United States in the summer. Ocean currents are driven by the winds and follow the same general pattern. The continents block the flow of water around the globe, so ocean currents flow west near the equator, then turn toward the poles when they strike a continent, turn east, then flow back to the equator on the other side. In all the oceans, the ocean currents form great loops called gyres. The gyres flow clockwise north of the equator and counterclockwise south of it. The global heat conveyor is an enormous cycle of ocean water that distributes the oceans' heat around Earth. Water in the polar regions is very cold, salty, and dense. It sinks and flows along the sea floor toward the equator. Eventually, the water rises along the margins of the continents and merges with the surface water flow. When it reaches the polar regions, it sinks again. This three-dimensional movement of water mixes heat throughout the oceans, warming polar waters. It also brings nutrients up from the deep ocean to the surface, where they are available for marine plants and animals. The hydrologic cycle Water from the oceans evaporates and is carried by the atmosphere, eventually falling as rain or snow. Water that falls on the land helps break rocks down chemically, nourishes plants, and wears down the landscape. Eventually, the water returns to the sea to start the cycle over again. The rock cycle
Earth has many more kinds of rocks compared to other planets because there are so many processes acting to form and break down rocks. Geologists sometimes speak of the rock cycle to explain how different rock types are related. The cycle may begin with a flow of lava from a volcano cooling to form new igneous rocks on Earth's surface. As the rock is exposed to water, it breaks down and the resulting materials may be carried away to be deposited as sedimentary rocks. These rocks may eventually be so deeply buried that they change in form to become metamorphic rocks. They may even melt, creating the raw material for the next generation of igneous rocks. Rocks rarely go through the entire rock cycle. Instead, some steps may be skipped or repeated. For example, igneous rocks can be subjected to heat and pressure and transformed directly to metamorphic rocks. Sedimentary rocks can be broken down by weathering and then reassembled into a new generation of sedimentary rocks. Metamorphic rocks can also be weathered to form the raw material for a new generation of sedimentary rocks. Any rock type, igneous, metamorphic, or sedimentary, can be transformed into any other type. Earth's Layers
The earth is divided into four main layers: the inner core, outer core, mantle, and crust
Geologists
cannot study the interior of Earth directly. The deepest wells drilled
reach less than 8 miles (13 kilometers) below the surface. Geologists know
that the whole Earth differs in composition from its thin outer crust.
Deep in Earth, pressures are so great that minerals can be compressed into
dense forms not found on the surface. Unlike chondrites, Earth is made up of layers that contain different amounts of various chemical elements. Geologists learn about Earth's interior by studying vibrations generated by earthquakes, using instruments called seismographs. The speed and motion of vibrations traveling through Earth depends on the composition and density of the material they travel through. Geologists can determine many properties of Earth's interior by analyzing such vibrations. The mantle Beneath the crust, extending down about 1,800 miles (2,900 kilometers), is a thick layer called the mantle. The mantle is not perfectly stiff but can flow slowly. Earth's crust floats on the mantle much as a board floats in water. Just as a thick board would rise above the water higher than a thin one, the thick continental crust rises higher than the thin oceanic crust. The slow motion of rock in the mantle moves the continents around and causes earthquakes, volcanoes, and the formation of mountain ranges. The core At the center of Earth is the core. The core is made mostly of iron and nickel and possibly smaller amounts of lighter elements, including sulfur and oxygen. The core is about 4,400 miles (7,100 kilometers) in diameter, slightly larger than half the diameter of Earth and about the size of Mars. The outermost 1,400 miles (2,250 kilometers) of the core are liquid. Currents flowing in the core are thought to generate Earth's magnetic field. Geologists believe the innermost part of the core, about 1,600 miles (2,600 kilometers) in diameter, is made of a similar material as the outer core, but it is solid. The inner core is about four-fifths as big as Earth's moon. Earth gets hotter toward the center. At the bottom of the continental crust, the temperature is about 1800 degrees F (1000 degrees C). The temperature increases about 3 degrees F per mile (1 degrees C per kilometer) below the crust. Geologists believe the temperature of Earth's outer core is about 6700 to 7800 degrees F (3700 to 4300 degrees C). The inner core may be as hot as 12,600 degrees F (7000 degrees C) -- hotter than the surface of the sun. But, because it is under great pressures, the rock in the center of Earth remains solid. Earth's crust The hot rock deep in Earth's mantle flows upward slowly, while cooler rock near the surface sinks because hot materials are lighter than cool materials. The rising and sinking of materials due to differences in temperature is called convection. As Earth's mantle flows, it breaks the crust into a number of large slabs called tectonic plates, much as slabs of ice break apart on a pond. The slow flow of Earth's mantle drags the crust along, causing the continents to move, mountains to form, and volcanoes and earthquakes to occur. This constant motion of Earth's crust is called plate tectonics. In some places, usually under the oceans, Earth's plates are spreading apart. New magma from the mantle rises to fill the cracks between the plates. Places where plates spread apart are called spreading centers. Many volcanoes occur where plates pull apart and magma wells up from within the mantle to fill the gap. The material from the mantle is made of iron and magnesium-rich silicate rocks. It hardens to form rocks and creates oceanic crust made of basalt. Subduction Earth's crust cannot spread apart everywhere. Somewhere, an equal amount of crust must be removed. When two plates push together, one of the plates sinks back into Earth's mantle, a process called subduction. The sinking plate eventually melts into magma in Earth's interior. Much of the magma created in subduction zones does not reach the surface and cools within the crust, forming plutonic rocks. The heat from the magma also helps create metamorphic rocks. Because continental crust is too thick and light to sink into Earth's interior, only plates made of dense oceanic crust are subducted. The boundary where the two plates meet is marked by a deep trench on the ocean floor. The trenches are the deepest places in the oceans, up to 36,000 feet (11,000 meters) deep. The upper plate that remains on the surface may be continental crust or oceanic crust. This plate is also changed by subduction. As the two plates move together, the edge of the upper plate is compressed. The crust becomes thicker and higher, creating a mountain range. When the rocks of the sinking plate reach a depth of about 60 miles (100 kilometers), they begin to melt and form magma. Some of the magma reaches the surface to form volcanoes. Regions with many volcanoes, such as Peru, Japan, and the northwestern United States, lie near areas where subduction is happening. Mountain building Occasionally, as a plate sinks into Earth's mantle, it drags along a continent or a smaller land mass. Continental crust is too thick and light to sink. Instead, it collides with the opposing plate. If the opposing plate is also a continent, neither plate will sink. This type of collision often forms a vast mountain chain in the middle of a continent. The Himalaya were formed in such a way from the collision of two plates of continental crust. The series of events that happen during formation of a mountain range is called orogeny. Orogeny includes the elevation of mountains, folding and crumpling of the rocks, volcanic activity, and formation of plutonic and metamorphic rocks that occur when plates collide. Long after mountains have vanished from erosion, geologists can still see the changes orogeny produces in the rocks. Terrane collisions Smaller pieces of continental crust that collide with another plate are often added to the edge of the larger plate. These small added pieces of crust are called terranes. Most of the land in the United States west of Salt Lake City has been added to North America by terrane collisions in the last 500 million years. Earthquakes Earthquakes
occur when rocks on opposite sides of a break in the crust, called a
fault, slide past each other. The boundaries between plates are faults,
but there are faults within plates as well. Occasionally, forces within
the plates cause rocks to fracture and slip even though the rocks are not
at a plate boundary. The boundaries between two plates sliding past each
other are called transform faults. The San Andreas Fault in California is
a transform fault, where a portion of crust called the Pacific Plate is
carrying a small piece of California northwest past the rest of North
America. Several times
in Earth's history, collisions between continents have created a huge
supercontinent. Although the crust of the continents is thick, it breaks
more easily than oceanic crust, and supercontinents broke quickly into
smaller pieces. Material from Earth's mantle filled the gaps, creating new
oceanic crust. As the continents moved apart, new ocean basins formed
between them. About one-third of Earth's surface is covered by continental
crust, so the pieces cannot move far before colliding. As two continents
collide, an old ocean basin is destroyed. The process of continents
breaking apart and rejoining is called the Wilson cycle, after the
Canadian geologist John Tuzo Wilson, who first described it. Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root "to build." Putting these two words together, we get the term plate tectonics, which refers to how the Earth's surface is built of plates. The theory of plate tectonics states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia).
There are eight major plates:
Tallest Mountains
History Of Earth Concealed within the rocks that make up the Earth's crust lies evidence of over 4.5 billion years of time. The written record of human history, measured in decades and centuries, is but a blink of an eye when compared with this vast span of time. In fact, until the eighteenth century, it was commonly believed that the Earth was no older than a few thousand, or at most, million, years old. Scientific detective work and modern radiometric technology have only recently unlocked the clues that reveal the ancient age of our planet. Long before scientists had developed the technology necessary to assign ages in terms of number of years before the present, they were able to develop a 'relative' geologic time scale. They had no way of knowing the ages of individual rock layers in years (radiometric dates), but they could often tell the correct sequence of their formation by using relative dating principles and fossils. Geologists studied the rates of processes they could observe first hand, such as filling of lakes and ponds by sediment, to estimate the time it took to deposit sedimentary rock layers. They quickly realized that millions of years were necessary to accumulate the rock layers we see today. As the amount of evidence grew, scientists were able to push the age of the Earth farther and farther back in time. Piece by piece, geologists constructed a geologic time scale, using increasingly more sophisticated methods for dating rock formations.
Early geologists used the relative positions of rock layers as clues to begin to unravel the complex history of our planet. However, it was not until this century that nuclear age technology was developed that uses measurements of radioactivity in certain types of rocks to give us ages in numbers of years. These ages, usually called radiometric ages, are used in conjunction with relative dating principles to determine at least an approximate age for most of the world 's major rock formations. 
The 4.55 billion-year geologic time scale is subdivided into different time periods of varying lengths. All of Earth history is divided into two great expanses of time. The Precambrian began when Earth first formed 4.55 billion years ago and ended about 570 million years ago. The Phanerozoic Eon began 570 million years ago and continues today.
Credit: NASA, EPA, USGS, University Corporation for Atmospheric Research,University of Wisconsin-Stevens Point Karen A. Lemke |
