Earth's Atmosphere

Earth's Atmosphere

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The Earth's atmosphere is more than just the air we breathe. It's also a buffer that keeps us from being peppered by meteorites, a screen against deadly radiation, and the reason radio waves can be bounced for long distances around the planet.

The air that accomplishes all of this is composed of five major layers.

The lowest is the troposphere, which is the layer that provides most of our weather. It contains about four-fifths of the Earth's air, but extends only to a height of about 11 miles (17 kilometers) at the Equator and somewhat less at the Poles.

The name comes from a Greek word that refers to mixing. And mixing is exactly what happens within the troposphere, as warm air rises to form clouds, rain falls, and winds stir the lands below. Typically, the higher you go in the troposphere, the colder it gets.

Above the troposphere is the stratosphere. It extends to a height of about 30 miles (50 kilometers) and includes the ozone layer, which blocks much of the sun's harmful ultraviolet rays.

The stratosphere is warmer than the troposphere because of the energy from the ultraviolet light absorbed by the ozone. At its base, the stratosphere is extremely cold, about -110 degrees Fahrenheit (-80 degrees Celsius). At its top, the temperature has risen back nearly to freezing.

Next comes the mesosphere. In this layer, the air temperature drops again, down to nearly -180 degrees Fahrenheit (-120 degrees Celsius) at the top. Meteors generally burn up in the mesosphere, which extends to a height of about 52 miles (85 kilometers). This is why the Earth's surface isn't pocked with meteor craters, like the moon's.

Entering Outer Space:-
Above the mesosphere is the ionosphere. It extends to about 430 miles (690 kilometers) and is so thin it's generally considered part of outer space. The International Space Station and many satellites orbit within the ionosphere.

The ionosphere is named for the ions created within this layer by energetic particles from sunlight and outer space. These ions create an electrical layer that reflects radio waves, allowing radio messages to be sent across oceans in the days before communication satellites. Electrical displays in the ionosphere also create the auroras called the Northern and Southern Lights.

Beyond the ionosphere lies the exosphere. This tenuous portion of the Earth's atmosphere extends outward until it interacts with the solar wind. Solar storms compress the exosphere. When the sun is tranquil, this layer extends further outward. Its top ranges from 620 miles (1,000 kilometers) to 6,214 miles (10,000 kilometers) above the surface, where it merges with interplanetary space. 

 Climate
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Climate isn't the same thing as weather. Weather is the condition of the atmosphere over a short period of time; climate is the average course of weather conditions for a particular location over a period of many years.

One of the factors that influences climate is the angle of the sun's rays. In the tropics, between 23.5° N and 23.5° S, there is at least one time of year when the noontime sun is directly overhead and its rays hit at a direct angle. This produces a hot climate with relatively small temperature differences between summer and winter.

In the Arctic and Antarctic (north or south of 66.5° latitude), there are times of year when the sun is above the horizon 24 hours a day (a phenomenon known as midnight sun) and times when it never rises. Even in the summer, the sun is low enough for temperatures to be lower than in the tropics, but the seasonal changes are much greater than in equatorial regions. Interior Alaska has seen temperatures as high as 100 degrees Fahrenheit (38 degrees Celsius).

Farther from the Equator lie the temperate regions. These include the United States, Europe, China, and parts of Australia, South America, and southern Africa. They have the typical four seasons: winter, spring, summer, and fall.

Outside Influences:-
Climate is also controlled by wind, oceans, and mountains.
Winds bring moisture to land. North and south of the Equator the trade winds blow from the northeast and southeast, respectively. These winds converge in the tropics, forcing air to rise. This produces thunderstorms, humidity, and monsoons.

North and south of the trade winds, about 30° from the Equator, there is relatively little wind, and therefore little moisture blowing inland from the oceans. Also, dry air is sinking back to the surface, warming in the process. This is why many of the world's great desert regions—the Sahara, Arabia, Iran, Iraq, and chunks of Mexico—lie at the same latitude. A similar band of deserts lies to the south in Australia, South America, and southern Africa.

Mountains force wind to rise as it crosses over them. This cools the air, causing moisture to condense in clouds and rain. This produces a wet climate on the upwind side of the mountains and an arid "rain shadow" on the downwind side.

Oceans provide moisture that fuels rainstorms. They also buffer the temperature of coastal regions, regardless of latitude.

Climate Groups:-
In the early 1900s, climatologist Wladimir Köppen divided the world into five major climate groups.
Moist, tropical climates are hot and humid. Steppes and deserts are dry, with large temperature variations. Plentiful lakes, rivers, or nearby oceans give humid, midlatitude climates cool, damp winters, but they have hot, dry summers. Some of these climates are also called Mediterranean. Continental climates occur in the centers of large continents. Mountain ranges (or sheer distance) block off sources of moisture, creating dry regions with large seasonal variations in temperature. Much of southern Canada, Russia, and parts of central Asia would fall into this category. Cold, or polar, climates round out Köppen's list. A sixth region, high elevations, was later added to the classification system.
 

 Clouds
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Clouds form when humid air cools enough for water vapor to condense into droplets or ice crystals. The altitude at which this happens depends on the humidity and the rate at which temperature drops with elevation.

Normally, water vapor can only condense onto condensation nuclei—tiny particles that serve as kernels around which drops can form.

Condensation nuclei are often nothing but natural dust. But soot particles from automobile exhaust or other types of pollution can also serve the purpose. One study has found that changing levels of air pollution cause different rates of cloud formation (and rain) on weekends and weekdays, at least in humid climates with lots of cities.

Cloud Types:-
Clouds are classified into four basic categories, depending largely on the height of their bases above the ground.

High-level clouds, called cirrus clouds, can reach heights of 20,000 feet (6,000 meters) and are typically thin. They do not produce rain and often indicate fair weather. They are usually made up of ice.

Midlevel clouds form between 6,500 feet (2,000 meters) and cirrus level. They are referred to as "alto-" clouds and bear such names as altostratus or altocumulus, depending on their shape. (Altostratus clouds are flat; altocumulus clouds are puffy.) They frequently indicate an approaching storm. They themselves sometimes produce virga, which is rain or snow that does not reach the ground.

Low-level clouds lie below 6,500 feet (2,000 meters). Meteorologists refer to them as stratus clouds. They're often dense, dark, and rainy (or snowy) though they can also be cottony white clumps interspersed with blue sky.

Storm Clouds:-
The most dramatic types of clouds are cumulus and cumulonimbus, or thunderheads. Rather than spreading out in bands at a fairly narrow range of elevations, like other clouds, they rise to dramatic heights, sometimes well above the level of transcontinental jetliner flights.

Cumulus clouds are fair-weather clouds. When they get big enough to produce thunderstorms, they are called cumulonimbus. These clouds are formed by upwelling plumes of hot air, which produce visible turbulence on their upper surfaces, making them look as though they are boiling.

Just as it takes heat to evaporate water from the surface of the Earth, heat is released when water condenses to form clouds. In thunderheads, this energy can produce hail, damaging winds, lightning, torrential rain, and sometimes tornadoes.

As thunderheads reach high elevations, their tops encounter high winds that cause them to spread out sideways, earning them the nickname "anvil tops." They can reach elevations of 50,000 feet (15,000 meters).

Weather
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Weather is the state of the atmosphere at a specific time and place, with respect to temperature, precipitation, and other factors such as cloudiness. Weather is generated by many forces, some obvious, some not. Warm, humid air masses blowing in from oceans, for example, fuel rains. Sunlight heats the land, generating thermals that help produce summer thunderstorms.

Mountains and cities also affect the weather. In mountains this occurs because the wind must rise as it crosses over the ridge. This lifts the air, causing it to cool. That produces clouds, rain, or snow.

Cities, on the other hand, produce urban "heat islands" where roads, parking lots, and rooftops warm in the sun. This not only raises the city's temperature, but it can affect the weather, producing thunderstorms in some cities or altering storm tracks in others.

Predicting the Weather:-
Weather forecasting is the art of predicting what will happen in the future. In its simplest form, it's merely a matter of looking out the window to see what types of clouds are around and which way they are moving. Knowledge of local weather patterns can then allow fairly good predictions for the next 12 to 24 hours.

Professional forecasters have a wide variety of other tools. Weather stations scattered around the globe allow them to make detailed weather maps, as do satellites, which allow forecasters to see what is happening far out to sea, where there are no weather stations. Weather balloons and radar also contribute.

Nevertheless, long-run weather forecasting is notoriously difficult. That's because weather prediction involves a mathematical concept called chaos theory, in which extremely small errors in measuring today's weather conditions can snowball into large, seemingly random, errors in long-range forecasts.

It has been said, for example, that a butterfly flapping its wings today in China could produce (or prevent) tornadoes two weeks from now in Kansas. While this so-called butterfly effect is undoubtedly overstated, the basic concept is simple: Even the most minor factors can alter long-term weather forecasts.

Most weather forecasters believe that accurate forecasting more than two weeks into the future will always be impossible. Today, anything beyond five to seven days involves substantial guesswork and is often wrong.

Wind
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The energy that drives wind originates with the sun, which heats the Earth unevenly, creating warm spots and cool spots. Two simple examples of this are sea breezes and land breezes.

Sea breezes occur when inland areas heat up on sunny afternoons. That warms the air, causing it to rise. Cooler air rushes in from the ocean to take its place and presto, a wind is born. By late afternoon, a strong breeze can be blowing dozens of miles inland. A similar effect can occur near big lakes, where the wind is referred to as a lake breeze.

Land breezes come at night, when inland temperatures drop enough that the ocean is now warmer than the land, reversing the effect.

Global Patterns:-
Similar forces produce global wind patterns that affect climate. The tropics, for example, are always hot. Air rises here and spreads north and south, high above the land. Lower down, air is pulled in from the north and south. The coriolis effect, an offshoot of the Earth's rotation, makes moving air masses curve, so that the winds converging on the Equator come from the northeast in the Northern Hemisphere and the southeast in the Southern Hemisphere. These winds are called the trade winds.

Farther from the Equator, the surface winds try to blow toward the Poles, but the coriolis effect bends them the opposite direction, creating westerlies. This is why so many weather events in the United States come from the west.

At latitudes higher than about 60°, cold surface winds try to blow toward the Equator, but, like the trade winds, they are bent by the coriolis effect, producing polar easterlies.

Highs and Lows:-
Within the mid-latitudes, weather effects create high- and low-pressure zones, called highs and lows, respectively. Air moves from areas of high pressure to low pressure. As it moves, however, it spirals due to the coriolis effect, producing the shifting winds we experience from day to day, as highs and lows drift under the influence of the prevailing westerlies.

Winds reaching the center of a low-pressure area have nowhere to go but up. This causes moisture to condense into clouds, producing storms. At the center of high-pressure areas, dry air descends from above, producing fair weather.

On a smaller scale, colliding wind patterns can produce convergence, in which air also has nowhere to go but up. If one of the winds is a humid flow from a warm ocean such as the Gulf of Mexico, the result can be powerful thunderstorms and tornadoes.

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The Dynamic Earth

The Dynamic Earth

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We tend to think of the ground beneath our feet as solid, steady, and unchanging. But forces all around us are constantly at work shaping Earth's surface—usually at a pace too slow to be noticed but occasionally in cataclysmic fits that leave no doubt about their power. Explore the forces that move and shake our dynamic Earth.

Erosion and Weathering
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Weathering and erosion slowly chisel, polish, and buff Earth's rock into ever evolving works of art—and then wash the remains into the sea.

The processes are definitively independent, but not exclusive. Weathering is the mechanical and chemical hammer that breaks down and sculpts the rocks. Erosion transports the fragments away.

Working together they create and reveal marvels of nature from tumbling boulders high in the mountains to sandstone arches in the parched desert to polished cliffs braced against violent seas.

Water is nature's most versatile tool. For example, take rain on a frigid day. The water pools in cracks and crevices. Then, at night, the temperature drops and the water expands as it turns to ice, splitting the rock like a sledgehammer to a wedge. The next day, under the beating sun, the ice melts and trickles the cracked fragments away.

Repeated swings in temperature can also weaken and eventually fragment rock, which expands when hot and shrinks when cold. Such pulsing slowly turns stones in the arid desert to sand. Likewise, constant cycles from wet to dry will crumble clay.

Bits of sand are picked up and carried off by the wind, which can then blast the sides of nearby rocks, buffing and polishing them smooth. On the seashore, the action of waves chips away at cliffs and rakes the fragments back and forth into fine sand.

Plants and animals also take a heavy toll on Earth's hardened minerals. Lichens and mosses can squeeze into cracks and crevices, where they take root. As they grow, so do the cracks, eventually splitting into bits and pieces. Critters big and small trample, crush, and plow rocks as they scurry across the surface and burrow underground. Plants and animals also produce acids that mix with rainwater, a combination that eats away at rocks.

Rainwater also mixes with chemicals as it falls from the sky, forming an acidic concoction that dissolves rock. For example, acid rain dissolves limestone to form karst, a type of terrain filled with fissures, underground streams, and caves like the cenotes of Mexico's Yucatán Peninsula.

Back up on the mountains, snow and ice build up into glaciers that weigh on the rocks beneath and slowly push them downhill under the force of gravity. Together with advancing ice, the rocks carve out a path as the glacier slumps down the mountain. When the glacier begins to melt, it deposits its cargo of soil and rock, transporting the rocky debris toward the sea. Every year, rivers deposit millions of tons of sediment into the oceans.

Without the erosive forces of water, wind, and ice, rock debris would simply pile up where it forms and obscure from view nature's weathered sculptures. Although erosion is a natural process, abusive land-use practices such as deforestation and overgrazing can expedite erosion and strip the land of soils needed for food to grow.

Plate Tectonics
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There are a few handfuls of major plates and dozens of smaller, or minor, plates. Six of the majors are named for the continents embedded within them, such as the North American, African, and Antarctic plates. Though smaller in size, the minors are no less important when it comes to shaping the Earth. The tiny Juan de Fuca plate is largely responsible for the volcanoes that dot the Pacific Northwest of the United States.

The plates make up Earth's outer shell, called the lithosphere. (This includes the crust and uppermost part of the mantle.) Churning currents in the molten rocks below propel them along like a jumble of conveyor belts in disrepair. Most geologic activity stems from the interplay where the plates meet or divide.

The movement of the plates creates three types of tectonic boundaries: convergent, where plates move into one another; divergent, where plates move apart; and transform, where plates move sideways in relation to each other.

Convergent Boundaries:-
Where plates serving landmasses collide, the crust crumples and buckles into mountain ranges. India and Asia crashed about 55 million years ago, slowly giving rise to the Himalaya, the highest mountain system on Earth. As the mash-up continues, the mountains get higher. Mount Everest, the highest point on Earth, may be a tiny bit taller tomorrow than it is today.

These convergent boundaries also occur where a plate of ocean dives, in a process called subduction, under a landmass. As the overlying plate lifts up, it also forms mountain ranges. In addition, the diving plate melts and is often spewed out in volcanic eruptions such as those that formed some of the mountains in the Andes of South America.

At ocean-ocean convergences, one plate usually dives beneath the other, forming deep trenches like the Mariana Trench in the North Pacific Ocean, the deepest point on Earth. These types of collisions can also lead to underwater volcanoes that eventually build up into island arcs like Japan.

Divergent Boundaries:-
At divergent boundaries in the oceans, magma from deep in the Earth's mantle rises toward the surface and pushes apart two or more plates. Mountains and volcanoes rise along the seam. The process renews the ocean floor and widens the giant basins. A single mid-ocean ridge system connects the world's oceans, making the ridge the longest mountain range in the world.

On land, giant troughs such as the Great Rift Valley in Africa form where plates are tugged apart. If the plates there continue to diverge, millions of years from now eastern Africa will split from the continent to form a new landmass. A mid-ocean ridge would then mark the boundary between the plates.

Transform Boundaries:-
The San Andreas Fault in California is an example of a transform boundary, where two plates grind past each other along what are called strike-slip faults. These boundaries don't produce spectacular features like mountains or oceans, but the halting motion often triggers large earthquakes, such as the 1906 one that devastated San Francisco.

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Surface of the Earth

 Surface of the Earth

Learn more about the Earth's dramatic landforms, from the tops of majestic mountains to the depths of mysterious caves and everything in between.

Canyons 

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Bound by cliffs and cut by erosion, canyons are deep, narrow valleys in the Earth's crust that evoke superlatives and a sense of wonder. Layers of rock outline stories of regional geology like the table of contents to a scientific text.

The landforms commonly break parched terrain where rivers are the major force to sculpt the land. They are also found on ocean floors where the torrents of currents dig underwater graves.

"Grand" is the word used to describe one of the most famous canyons of all. Cut by the Colorado River over the last few million years, the Grand Canyon is 277 miles (446 kilometers) long, more than 5,000 feet (1,500 meters) deep, but only 18 miles (29 kilometers) across at its widest yawn.

Layers of rock in the Grand Canyon tell much about the Colorado Plateau's formative years: a mountain range built with two-billion-year-old rock and then eroded away; sediments deposited from an ancient sea; more mountains; more erosion; another sea; a burst of volcanic activity; and the birth of a river that has since carved the chasm by washing the layers away.

Each layer erodes differently. Some crumble into slopes, others sheer cliffs. They stack together like a drunken staircase that leads to the river's edge. A mixture of minerals gives each layer a distinctive hue of yellow, green, or red.

Canyon Types
Other canyons start where a spring sprouts from the base of a cliff as if out of nowhere. Such cliffs are composed of permeable, or porous, rock. Instead of flowing off the cliff, water seeps down into the rock until it hits an impermeable layer beneath and is forced to leak sideways. Where the water emerges, the cliff wall is weakened and eventually collapses. A box canyon forms as sections of wall collapse further and further back into the land. The heads of these canyons are marked by cliffs on at least three sides.

Slot canyons are narrow corridors sliced into eroding plateaus by periodic bursts of rushing water. Some measure less than a few feet across but drop several hundred feet to the floor.

Submarine canyons are similar to those on land in shape and form, but are cut by currents on the ocean floor. Many are the mere extension of a river canyon as it dumps into the ocean and flows across the continental shelf. Others are gouged from turbid currents that occasionally plunge to the ocean floor.

Caves
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A veil of darkness cloaks the natural beauty of caves. Some are found in cliffs at the edge of the coastline, chipped away by the relentless pounding of waves. Others form where a lava tube's outer surface cools and hardens and the inside of the molten rock drains away. Caves even form in glaciers where meltwater carves tunnels at the beginning of its journey to the sea.

But most caves form in karst, a type of landscape made of limestone, dolomite, and gypsum rocks that slowly dissolve in the presence of water with a slightly acidic tinge. Rain mixes with carbon dioxide in the atmosphere as it falls to the ground and then picks up more of the gas as it seeps into the soil. The combination is a weak acidic solution that dissolves calcite, the main mineral of karst rocks.

The acidic water percolates down into the Earth through cracks and fractures and creates a network of passages like an underground plumbing system. The passages widen as more water seeps down, allowing even more water to flow through them. Eventually, some of the passages become large enough to earn the distinction of cave. Most of these solutional caves require more than 100,000 years to widen large enough to hold a human.

The water courses down through the Earth until it reaches the zone where the rocks are completely saturated with water. Here, masses of water continually slosh to and fro, explaining why many caverns lay nearly horizontal.

Fanciful Features
Hidden in the darkness of caves, rock formations called speleothems droop from the ceilings like icicles, emerge from the floor like mushrooms, and cover the sides like sheets of a waterfall. Speleothems form as the carbon dioxide in the acidic water escapes in the airiness of the cave and the dissolved calcite hardens once again.

The icicle-shaped formations are called stalactites and form as water drips from the cave roof. Stalagmites grow up from the floor, usually from the water that drips off the end of stalactites. Columns form where stalactites and stalagmites join. Sheets of calcite growths on cave walls and floor are called flowstones. Other stalactites take the form of draperies and soda straws. Twisty shapes called helictites warp in all directions from the ceiling, walls, and floor.

 

Coastlines
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Like change? Then head down to the coastline, that narrow strip of land that borders the sea along a continent or an island. The nonstop wave action there means nothing ever stays the same. Breakers gnaw away at cliffs, shift sand to and fro, breach barriers, build walls, and sculpt bays. Even the gentlest of ripples constantly reshape coastlines in teeny, tiny ways—a few grains of sand at a time.

Glaciers, rivers, and streams deliver a steady supply of building material for nature's unending job. And not to be outdone, the tectonic forces that move giant pieces of Earth's crust will periodically bump the bedrock here and squeeze fresh lava out there, adding their own flourish to the coastal redesign.

Waves are the busiest sculptors on the coastline. Built up by winds far out at sea, they unleash their energy and go to work when they break on the shore. The upward rush of water, called swash, delivers sand and gravel to the beach. On the return, backwash carries sand and gravel out to sea. Since waves usually hit the beach from one side or the other but always return at a right angle to the beach, the motion drifts sand and gravel along the shore.

The ebb and flow of the tides is an added partner in the dance of breaking waves and shifting sands, helping to sculpt an array of landforms for temporary display, such as narrow spits, barrier islands, and lofty dunes. The delivery of sediment from muddy rivers and streams keeps the coastal construction on the go.

Along much of the coastline, pounding waves slowly chip away the base of cliffs, forcing chunks of rock to crumble and slide into the sea. Where a band of solid rock gives way, waves claw at weaker clays behind to sculpt a cove or a bay. Headlands form where the coastline gives on either side, leaving a lone rocky mass to get hammered by the sea.

Mountains
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Mountains are the wrinkles of age and pimples of youth on Earth's crusty outer skin. They rise up as the crust collides, cracks, crumbles, folds, and spews. By definition, they dominate their surroundings with towering height.

The mighty chunks rise all over the world, including the oceans. They usually have steep, sloping sides and sharp or rounded ridges. The highest point is called the peak or summit. Most geologists classify a mountain as a landform that rises at least 1,000 feet (300 meters) or more above its surrounding area. A mountain range is a series or chain of mountains that are close together.

The world's tallest mountain ranges form when pieces of Earth's crust—called plates—smash against each other, in a process called plate tectonics, and buckle up like the hood of a car in a head-on collision. The Himalaya in Asia formed from one such massive wreck that started about 55 million years ago. Thirty of the world’s highest mountains are in the Himalaya. The summit of Mount Everest, at 29,035 feet (8,850 meters), is the highest point on Earth.

The tallest mountain measured from top to bottom is Mauna Kea, an inactive volcano on the island of Hawaii in the Pacific Ocean. Measured from the base, Mauna Kea stands 33,474 feet (10,203 meters) tall, though it only rises 13,796 feet (4,205 meters) above the sea.

Types of Mountains:-
Volcanic mountains form when molten rock from deep inside the Earth erupts through the crust and piles up on itself. The island chain of Hawaii is actually the tops of volcanoes. Well-known volcanoes on land include Mount St. Helens in Washington State and Mount Fuji in Japan. Sometimes volcanic eruptions break down mountains instead of building them up, like the 1980 eruption that blew the top off Mount St. Helens.

When magma pushes the crust up but hardens before erupting onto the surface, it forms so-called dome mountains. Wind and rain pummel the domes, sculpting peaks and valleys. Examples include the Black Hills of South Dakota and the Adirondack Mountains of New York. Plateau mountains are similar to dome mountains, but form as colliding tectonic plates push up the land without folding or faulting. They are then shaped by weathering and erosion.

Other types of mountains form when stresses within and between the tectonic plates lead to cracking and faulting of the Earth's surface, which forces blocks of rock up and down. Examples of fault-block mountains include the Sierra Nevada in California and Nevada, the Tetons in Wyoming, and the Harz Mountains in Germany.

Mountains often serve as geographic features that define natural borders of countries. Their height can influence weather patterns, stalling storms that roll off the oceans and squeezing water from the clouds. The other side is often much drier. The rugged landscapes even provide refuge—and protection—for fleeing and invading armies.

Oceans
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Vast in scope and size, there is only one true ocean on Earth. This connected body of water surrounds the continents and is divided into five major regions: the Pacific, Atlantic, Indian, Arctic, and Southern oceans. (The Black and Caspian Seas are enclosed by the landmass that makes up Europe and Asia.) Taken together, the oceans cover more than 70 percent of the Earth's surface and give the planet the appearance, from space, of a blue marble.

The deep waters of these oceans hide from view rugged mountains, vast plateaus, active volcanoes, and seemingly bottomless trenches. These underwater landscapes are in an endless cycle of construction and destruction as new crust is born along mid-ocean ridges, pushing old crust into the depths of the fiery mantle.

The oceans appeared hundreds of millions of years ago as planet Earth cooled from its wild and hot adolescence. Layers of rock piled together and the vast amounts of steam that volcanoes spewed into the atmosphere turned to water vapor, condensed, and fell as rain. And rain it did. For thousands of years, the rains fell hard and filled giant depressions—forming the world's first seas.

Continental crust is less dense and thicker than the surface of the deep ocean. The transition from land to sea begins at the continental shelf, a gently sloping, submerged extension of the continent. The shelf ends at a break, where the increased steepness is defined as the continental slope. The slope leads down to the ocean abyss and its plains, plateaus, mountains, ridges, and trenches—hidden from view except for the tops of certain features that rise above the water's surface as islands.

Ever New Crust
The oldest rocks in the ocean date back only 200 million years, quite young for a planet thought to be about 4.5 billion years old. New crust constantly rises to the ocean surface along the mid-ocean ridge system, a giant underwater mountain range that snakes through the oceans like the stitching on a baseball. The birth of new crust pushes apart pieces of Earth's crust, called plates.

The pushing forces the old oceanic crust on the plate margins to bump into the edges of other plates. Where it collides with continental crust, the denser ocean crust dives beneath in a process called subduction. Once deep in the mantle, the crust melts into magma only to be spewed back to the surface along a mid-ocean ridge or an isolated volcano.

Subduction zones are marked by deep trenches, and just beyond them island arcs like Japan and mountains like the Andes in South America often rise. Where pieces of ocean crust collide, especially deep trenches form. The Mariana Trench in the North Pacific drops to a depth of 35,827 feet (10,920 meters) below the ocean surface at a point called Challenger Deep—the deepest spot on Earth.

Plateaus
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Plateaus are sculpted by geologic forces that lift them up and the wind and rain that wear them down into mesas, buttes, and canyons. Monument Valley and the Grand Canyon, both icons of the American Southwest, were chiseled from the Colorado Plateau.

Plateaus are built over millions of years as pieces of Earth's crust smash into each other, melt, and gurgle back toward the surface. Some owe their creation to a single process; others have been subjected to more than one during different epochs of Earth's history.

The highest and biggest plateau on Earth, the Tibetan Plateau in East Asia, resulted from a collision between two tectonic plates about 55 million years ago. The land buckled up along the seam of the collision and formed the Himalaya mountain range. Farther away, the crust uplifted but didn't crumple and wrinkle, creating instead a raised, flat, and wide open expanse known as the "roof of the world."

Many plateaus form as magma deep inside the Earth pushes toward the surface but fails to break through the crust. Instead, the magma lifts up the large, flat, impenetrable rock above it. Geologists believe a cushion of magma may have given the Colorado Plateau its final lift beginning about ten million years ago.

Repeated lava flows that spill out from cracks in the ground and spread out over hundreds of square miles can also slowly build up massive plateaus. The Columbia Plateau in the U.S. Pacific Northwest and the Deccan Plateau of west-central India were formed by these runny lava flows.

Plateaus also form in the ocean, such as the Mascarene Plateau in the Indian Ocean, one of the few underwater features clearly visible from space. It extends approximately 770 square miles (2,000 square kilometers) between the Seychelles and Mauritius Islands.

The Power of Wind and Water
Other plateaus are created over time as wind and rain wear away the side of an uplifted region, giving it geographic distinction from the surrounding terrain. Wind and rain eventually wear plateaus down to mesas and buttes and sculpt odd landforms like the arches and hoodoos found in southern Utah's famed national parks.

Water is the greatest erosive force on plateaus. As they course along, rivers carve valleys into the rock, washing the sediment toward the sea. Over time, these valleys become giant, majestic chasms like the Grand Canyon, which is continually carved by the Colorado River.

Plains
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Broad and flat, plains are well named. Some appear when glaciers and streams erode away elevated terrain; others spread where rising magma pushes, erupts, and spews. Some plains spill into the oceans, and others are bound by mountains on several sides. They all hide a tumultuous geologic history beneath their level disguise.

The base of the vast Great Plains in North America formed when several small pieces of continental crust collided and welded together more than a billion years ago. As time marched forward, the base was filled with marine sediments as periodic shallow seas covered the region and glaciers, rivers, and streams eroded the Rocky and Appalachian Mountains. Today, mountain erosion continues to carry debris out onto the plains.

When melting snows and heavy rains fill rivers beyond their banks, they flood. The waters spread out over the surrounding landscape and drop the load of mud, sand, and silt they normally channel downstream. Over thousands of years, the sediments build up floodplains.

Alluvial plains often form where steep mountain valley rivers gush onto more level lands, forcing the rush of heavy waters to spill over their banks and drape their sediments out like a fan.

Out on the wide-open valley floors, rivers twist and turn on a constant search for passage to the sea. The meandering path continues to widen the valley floor as falling sediment forever alters the course and builds up a river plain.

The Snake River Plain stretches from Oregon across northern Nevada and southern Idaho into Wyoming. Its geologic history is a complicated tale of normal fractures in the Earth's crust on its western edge to a more complex plot of basalt lava flows perhaps stemming from a hot plume of magma now beneath Yellowstone National Park.

Coastal plains are stretches of lowland next to oceans that are separated from the interior by highland features such as mountains and plateaus. Often the plains are portions of the ocean floor built up from the sediments rivers carry towards the sea. Geologists call the submerged part of coastal plains the continental shelves.

Valleys
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Down in the valley, the land is depressed—scoured and washed out by the conspiring forces of gravity, water, and ice. The scars left behind are known by their shapes and where they lie. Some hang; others are hollow. They all take the form of a "U" or "V."

Rivers and streams make most primary valley cuts, carving steep-walled sides and a narrow floor that from afar looks like the letter "V." The gradient of the river—how quickly it drops—helps define the steepness of the sides and the width of the floor. Mountain valleys, for example, tend to have near-vertical walls and a narrow channel, but out on the plains the slopes are shallow and the channel wide.

As the waters wind toward the sea, they accentuate natural curves in the land by stripping sediment from the outsides of bends and dumping it on the insides. The bulk of the rock and dirt is dredged from the bottom of the channel, a process called down cutting that can ultimately lead to deep, slender chasms like Black Canyon in Colorado's Gunnison National Park.

Glacier-Made Valleys
Some river and stream valleys, especially those in the mountains or located near the North and South Poles, are transformed by glaciers.

The massive blocks of snow and ice slowly creep downhill along the path of least resistance—valleys already cut by rivers and streams. As the glaciers ooze, they pick up rocks and grind away at the valley floor and sides, pressing the "V" into a "U." When the glacier melts, a U-shaped valley marks the spot where the snow and ice once flowed.

Side valleys are formed by tributaries to streams and rivers and feed the main stem. Where the main channel is carved deeper than the tributary, as commonly occurs during glaciations, the side valleys are left hanging. Waterfalls often cascade from the outlet of the upper valley into the drainage below.

Hollows, like those in Appalachia, are small valleys nestled between mountains or hills. Giant valleys, called rifts, are found where two pieces of Earth's crust are separated or split apart. One such example is the Great Rift Valley, a rift system stretching from the Middle East to southern Africa.

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Planet Earth

Earth

Earth, our home planet, is the only planet in our solar system known to harbor life. All of the things we need to survive are provided under a thin layer of atmosphere that separates us from the uninhabitable void of space. Earth is made up of complex, interactive systems that are often unpredictable. Air, water, land, and life—including humans—combine forces to create a constantly changing world that we are striving to understand.

Viewing Earth from the unique perspective of space provides the opportunity to see Earth as a whole. Scientists around the world have discovered many things about our planet by working together and sharing their findings.

Some facts are well known. For instance, 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.

Oceans at least 2.5 miles (4 kilometers) deep cover nearly 70 percent of Earth's surface. Fresh water exists in the liquid phase only within a narrow temperature span (32 to 212 degrees Fahrenheit/ 0 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.

Protective Atmosphere
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. 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 definite 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.

 Inside the Earth
The Earth's interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun.

The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannot melt.

The iron isn't pure—scientists believe it contains sulfur and nickel, plus smaller amounts of other elements. Estimates of its temperature vary, but it is probably somewhere between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius).

Above the inner core is the outer core, a shell of liquid iron. This layer is cooler but still very hot, perhaps 7,200 to 9,000 degrees Fahrenheit (4,000 to 5,000 degrees Celsius). It too is composed mostly of iron, plus substantial amounts of sulfur and nickel. It creates the Earth's magnetic field and is about 1,400 miles (2,300 kilometers) thick.

River of Rock
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The next layer is the mantle. Many people think of this as lava, but it's actually rock. The rock is so hot, however, that it flows under pressure, like road tar. This creates very slow-moving currents as hot rock rises from the depths and cooler rock descends.

The mantle is about 1,800 miles (2,900 kilometers) thick and appears to be divided into two layers: the upper mantle and the lower mantle. The boundary between the two lies about 465 miles (750 kilometers) beneath the Earth's surface.

The crust is the outermost layer of the Earth. It is the familiar landscape on which we live: rocks, soil, and seabed. It ranges from about five miles (eight kilometers) thick beneath the oceans to an average of 25 miles (40 kilometers) thick beneath the continents.

Currents within the mantle have broken the crust into blocks, called plates, which slowly move around, colliding to build mountains or rifting apart to form new seafloor.

Continents are composed of relatively light blocks that float high on the mantle, like gigantic, slow-moving icebergs. Seafloor is made of a denser rock called basalt, which presses deeper into the mantle, producing basins that can fill with water.

Except in the crust, the interior of the Earth cannot be studied by drilling holes to take samples. Instead, scientists map the interior by watching how seismic waves from earthquakes are bent, reflected, sped up, or delayed by the various layers.

Minerals and Gems
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More than 4,000 naturally occurring minerals—inorganic solids that have a characteristic chemical composition and specific crystal structure—have been found on Earth. They are formed of simple molecules or individual elements arranged in repeating chains, sheets, or three-dimensional arrays.

Minerals are typically formed when molten rock, or magma, cools, or by separating out of mineral-rich water, such as that in underground caverns. In general, mineral particles are small, having formed within confined areas such as lava flows or between grains of sediments. Large crystals found in geodes and other rocks are relatively rare.

Rocks themselves are made of clusters or mixtures of minerals, and minerals and rocks affect landform development and form natural resources such as gold, tin, iron, marble, and granite.

Silicates—including quartz, mica, olivine, and precious minerals such as emeralds—are the most common class of minerals, as well as the major components of most rocks. Oxides, sulfides, sulfates, carbonates, and halides are other major mineral classes.

Gemstones
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Many minerals form beautiful crystals, but the most prized of all are gemstones. Uncut gems are often fairly ordinary looking. It's only when they are cut and polished that they obtain the brilliance and luster that makes them so valued.

Historically gems have been divided into precious and semiprecious classes. There are a number of semiprecious gems, many quite beautiful, but diamonds, rubies, sapphires, and emeralds continue to qualify as "precious." (At one time, amethyst was also considered a precious gem, but large reserves were later found in Brazil, reducing its value.)

Diamonds, made of carbon atoms, are the hardest natural substance found on Earth. Formed under extremely high pressure hundreds of miles underground, they are found in very few locations around the world. Graphite is also made of carbon atoms, but with a different arrangement—explaining why diamond is the hardest mineral and graphite (used in pencil lead) is one of the softest.

Rubies are formed of a mineral called corundum, comprised of aluminum oxide. The red color is caused by traces of chromium. Corundum also forms sapphire in many colors, which generally come from trace mixtures of iron, titanium, and chromium.

Emeralds are formed of a mineral called beryl whose chemical formula is a complex mix of beryllium, aluminum, silicon, and oxygen. The color comes from additional traces of chromium and vanadium. Different trace elements can produce other colors, allowing beryl to form semiprecious stones such as aquamarine.

Minerals and gems are classified by their physical properties, including hardness, luster, color, density, and magnetism. They're also identified by the ways in which they break, or the type of mark, or streak, that they leave when rubbed on a laboratory tool called a streak plate.

Rocks
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Rocks are so common that most of us take them for granted—cursing when we hit them with the garden hoe or taking advantage of them to drive in tent pegs on summer camping trips.

But what exactly is a rock?

To geologists, a rock is a natural substance composed of solid crystals of different minerals that have been fused together into a solid lump. The minerals may or may not have been formed at the same time. What matters is that natural processes glued them all together.

There are three basic types of rock: igneous, sedimentary, and metamorphic.

Extremely common in the Earth's crust, igneous rocks are volcanic and form from molten material. They include not only lava spewed from volcanoes, but also rocks like granite, which are formed by magma that solidifies far underground.

Typically, granite makes up large parts of all the continents. The seafloor is formed of a dark lava called basalt, the most common volcanic rock. Basalt is also found in volcanic lava flows, such as those in Hawaii, Iceland, and large parts of the U.S. Northwest.

Granite rocks can be very old. Some granite, in Australia, is believed to be more than four billion years old, although when rocks get that old, they've been altered enough by geological forces that it's hard to classify them.

Sedimentary rocks are formed from eroded fragments of other rocks or even from the remains of plants or animals. The fragments accumulate in low-lying areas—lakes, oceans, and deserts—and then are compressed back into rock by the weight of overlying materials. Sandstone is formed from sand, mudstone from mud, and limestone from seashells, diatoms, or bonelike minerals precipitating out of calcium-rich water.

Fossils are most frequently found in sedimentary rock, which comes in layers, called strata.

Metamorphic rocks are sedimentary or igneous rocks that have been transformed by pressure, heat, or the intrusion of fluids. The heat may come from nearby magma or hot water intruding via hot springs. It can also come from subduction, when tectonic forces draw rocks deep beneath the Earth's surface.

Marble is metamorphosed limestone, quartzite is metamorphosed sandstone, and gneiss, another common metamorphic rock, sometimes begins as granite.

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