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.
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.
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.
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.
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.
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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.
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.
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.
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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