4. Third rock from the Sun - restless Earth
Remodeling the Earth's surface
Continents, oceans and ocean floors
There are two major types of terrain on Earth - the high, dry continents and the low, wet floor of the ocean. Between them, and partially surrounding many continents, is a narrow strip of shallow ocean called the continental shelf. Today, the oceans cover 71 percent of the Earth's surface, and the world's continents amount only to scattered and isolated masses surrounded by water.
Ongoing erosion will wear down the world's highest mountains in just a few hundred million years, which is just a fraction of the Earth's age of 4.6 billion years. If the planet was a perfectly smooth sphere, the oceans would cover the entire globe to a depth of 2.8 thousand meters. So, we can tell right away that high, dry land must be continuously recreated and pushed up out of the water.
The idea that continents have not always been fixed in their present positions was suggested more than three centuries ago, in 1596 by the Dutch map maker Abraham Ortelius (1527-1598) in his work Thesaurus Geographicus. However, the theory of moving continents was not developed into a thorough scientific hypothesis until the early 20th century, by the German meteorologist Alfred Wegener (1880-1930) in his influential and controversial book Die Entstehung der Kontinente und Ozeane, or The Origin of Continents and Oceans. Wegener noticed that the outlines of the continents themselves exhibit a number of remarkable symmetries. For example, the eastern edge of South America would fit snugly into the western edge of Africa, a remarkable fit noticed by Ortelius. In fact, much of the east and west shores of the Atlantic are as well matched as the shores of a river.
Wegener based his concept of continental drift not only on the similar shapes of the present continental edges, but also on the striking match of certain rocks and geologic formations, fossil creatures, and ancient climates along the borders of continents on opposite sides of the ocean. He concluded that all of the continents were once a part of single land mass that fragmented and drifted apart. If spacecraft had existed back then, their camera eyes would have seen one large continent and a single ocean surrounding it. This hypothetical super-continent is called Pangaea, a Greek word meaning "all lands" and pronounced pan-gee-ah.
The bottom of the ocean is not flat. It contains underwater mountains and valleys that are as grand as those on any continent. Although we cannot see these features in the inky darkness of the deep sea, we can use sound waves to reach down and touch them. Their distance is determined by recording the time it takes for electrically-generated sound signals, called pings, to travel from a ship to the floor and back.
Nowadays, the United States Navy detects enemy submarines or ships with sonar, an acronym for sound navigation and ranging, transmitting a continuous rain of pulsed sound waves and using the same echo technique to measure distance. Many modern ships, including warships and some commercial fishing boats are also equipped with sonar to aid in navigation. Navy ships and research vessels equipped with sonar can now map a two-thousand meter swath at the bottom of the ocean in a single ping of the sonar. Gravitational data, obtained from satellites that bounce radio beams off the sea surface, complement the sonar data, and they together result in highly detailed maps of the entire ocean floor.
The global mid-ocean ridge is a gigantic network of underwater mountain ranges. The submerged mountains stand higher than the greatest peaks on land, and meander for more than 75 million meters, creating the longest mountain chain on Earth. It is long enough to accommodate the total length of the Alps, Andes, Himalayas and Rockies. The mid-ocean ridge winds around the Earth, girdling the globe like the stitched seams of a baseball, not in simple lines but in offset segments, and when the undersea mountains reach the surface they can form islands, like Iceland and its new neighboring island Surtsey, named after the Icelandic god of fire, Surtur.
Even more remarkable are the deep canyons, collectively known as the Great Global Rift, that run along the mid-ocean ridges, splitting them as though they had been sliced with a giant's knife. Hot magma emerges from beneath the sea floor, and oozes into the canyons of the Great Global Rift, filling them with lava. As the lava cools in the ocean water, it expands and pushes the ocean crust away form the ridge. More lava then fills the widening crack, creating new sea floor that moves laterally away from the ridge on both sides, with bilateral symmetry.
As it migrates away from the hot rift of its beginning, the new ocean floor grows colder and denser, subsiding to greater depths as it ages. After traveling across the Earth, in conveyer belt fashion for many millions of years, the older heavier floor bends and descends back into the Earth, often at the edges of continents, creating a deep ocean trench in the underlying rock. Such trenches are found all around the edges of the Pacific Ocean, and they can sink as far below sea level as the tallest mountains rise above it. The over-all concept is known as sea-floor spreading.
Perhaps the most decisive evidence for sea-floor spreading was the discovery of regular magnetic-field patterns in the ocean floor. Magnetic detectors towed behind ships and carried in aircraft could measure very small differences in the Earth's magnetic field from place to place, known as magnetic anomalies. Positive magnetic anomalies are places where the magnetic field is stronger than expected, and negative ones are weaker than anticipated. The pattern of magnetic anomalies was symmetrically placed, or mirrored, on each side of the mid-ocean ridge. The symmetric magnetic anomalies on the sea floor exactly match these polar reversals recorded on land.
By radioactive dating of volcanic rocks on land, it is possible to tell when they solidified and to build up a chronology of the magnetic changes. This chronology can then put dates on the reversals found in the sea floor, and from the distances traveled it is possible to compute the rate of sea-floor spreading, assuming that the floor has moved at a constant rate. The ocean floor moves away from the ridge at rates of 0.02 to 0.20 meters per year depending on the location, or just a little faster than your fingernails grow. When sustained for 200 million years, the spreading sea floor can push continents apart by between 4 million and 40 million meters - entirely adequate to explain the widths of the great oceans. At the measured rate, it took just 150 million years for a slight fracture in an ancient former continent to widen into today's Atlantic Ocean.
The rind of the Earth, its outer shell known as the lithosphere, is subdivided into a mosaic of large plates, million of meters across. They vaguely resemble the cracked pieces of an egg shell.
Six of the nine major plates are named for continents embedded in them: the North American, South American, Eurasian, African, Indo-Australian, and Antarctic Plates. The other three are almost entirely oceanic: the Pacific, Nazca, and Cocos Plates. Accompanying them are a host of smaller plates.
Driven by heat from below, the plates move with respect to one another, accounting for most of our world's familiar surface features and phenomena, such as mountains, earthquakes and ocean basins. The continents are implanted within the moving plates, and continental drift is a consequence of the motion of plates carried along by the sea-floor spreading. So the moving plates carry the continents with them, on an endless journey with nowhere special to go.
The rigid plates are in continual, relentless movement, and they deform at their boundaries. Like drops of olive oil gliding across a warm frying pan, the continents sometimes collide and coalesce, sometimes slide and rub against each other, and at other times break up and scatter. The transformations produced by these interactive motions are known as plate tectonics, from the Greek word tectonic for "carpenter or building". They are forever reconstructing the face of the Earth.
Radio interferometer measurements indicate that the Pacific Plate is migrating with a northwestward velocity of 0.048 meters per year, carrying Los Angeles northward and producing earthquakes along the edge of the plate. At this rate, Los Angeles will be a suburb of San Francisco in 10 million years. The interferometer observations also indicate that the Atlantic Ocean is widening by 0.017 meters per year, so it was 8.7 meters narrower when Columbus crossed it in 1492.
An earthquake is a trembling or shaking of the ground caused by a sudden release of energy stored in the rocks below the Earth's surface. The devastating tremors and after shocks can ravage large sections of the land, flattening entire cities, awakening dormant volcanoes and creating new ones, draining lakes and causing floods, avalanches and fires.
In addition to diverging plates, that are moving apart at a mid-ocean ridge, and convergent plates, that are heading toward a collision, there is a third type of plate boundary known as a transform fault. It is a place where plates move past one another, neither toward or away, and this is where earthquakes can occur. When the two plates meet along a transform fault, they "transform" their encounter into a slipping, sliding horizontal motion, and a sudden lurch in this motion can produce an earthquake.
The two plates on each side of a transform fault bump, crush, grind, rub and slide against each other, without creating or destroying crust, like two high-speed cars sideswiping each other, but in slow motion. A famous and visible example is the San Andreas Fault in California that marks the meeting of the Pacific Plate with the North American Plate.
The Earth's internal heat engine
What pushes the tectonic plates across the globe? Heat, bottled up deep inside the Earth, produces internal currents that move the plates and propel the drifting continents. This heat is left over from the time of the Earth's formation, within the liquid outer core, and augmented by the continued radioactive decay of elements such as uranium and thorium. As the internal heat tries to escape, it maintains a ceaseless, wheeling, churning and roiling motion, called convection, that turns and rolls over very slowly. Convection occurs when molten rock becomes swollen by heat and rises through the cooler overlying material of lower pressure, like the currents in a pot of thick soup or oatmeal about to boil.
An impermanent face
Moving plates provide the tools for sculpting the Earth's surface and altering its landscape. They have profoundly changed the way we view the world. It's entire surface is continuously shifting about and changing in shape and form. High-standing belts of mountains and volcanoes are continuously being created when two plates converge along their borders, helping to hold the land above the sea.
As an ocean plate disappears into the trenches, great chains of towering volcanoes are created along the margins of continents. The descending slab of lithosphere causes underground rock to melt, and the magma generated rises buoyantly to widen the continents at their edges. The Andes are still growing higher in this way, as the floor of the Pacific Ocean plunges beneath the west coast of South America.
Eventually, a moving continent reaches an open trench and jams it shut, like trying to shove an eggplant down a garbage disposal. Continents are too light and thick to be subducted, and when they arrive at a trench the suture is closed up.
The violent collisions between continents have created the world's tallest mountains. When the continents meet, they buckle upward to form a range of mountains. Both land and oceanic sediment, built up over many millions of years, are tossed into the sky. The magnificent Himalayan range was formed this way, when the Indo-Australian plate, with India firmly embedded, ran into the Eurasian plate, like a head-on collision of two cars. Slowly, the Himalayas shot up as India rammed into Asia, carrying the fossilized remains of ancient sea-creatures with them. Today the plate that carries India continues to slide beneath the Eurasian plate, widening the Indian Ocean and pushing the mighty Himalayas upward.
The edges of plates are not the only place that land rises above the sea, for some oceanic islands are located millions of meters from the nearest plate boundaries. Chains or strings of such isolated islands are attributed to hot spots. The hot spots are rising plumes of magma, liquid or molten rock, anchored far beneath the ocean and deep within the mantle, even as far down as the core-mantle boundary. The relatively small, long-lasting and exceptionally hot regions provide a persistent source of magma capable of penetrating the mantle and piercing an overriding lithosphere plate, like a fixed blowtorch might melt holes in a steel plate moving by. As the plate glides slowly overhead at the rate of a few meters every century, it can leave a trail of islands that have risen out of the sea.
The Hawaiian islands were formed in such a way, as the Pacific Plate moved over a deep, stationary hot spot at the slow rate of 0.13 meters per year. Stretching to the north and west of the big island of Hawaii, they form a string of smaller islands, including Oahu and Midway, and submerged volcanoes, or seamounts, about 6 million meters long. Every one of these islands and seamounts was formed in the exact place where Hawaii now stands. The plume pushed the first Hawaiian island up above the ocean surface in this location about 70 million years ago.
Kiluea, the world's largest active volcano, is still rumbling because Hawaii has yet to move completely off the hot spot. At the same time, the underwater volcano, Loihi, is being formed as the Pacific Plate moves steadily on, continuing its relentless journey over the hot spot. In about fifty thousand years Loihi should grow high enough to form the next Hawaiian island.
If a plate carrying a continent comes to rest over a hot spot, the heat and pressure from the upwelling magma will weaken and stretch the overlying continental crust. And when the crust is stretched beyond its limits, cracks or rifts will form in it. The magma rises and squeezes through the widening cracks, forming volcanoes. If the upwelling is short-lived, the result is merely a rift scar, such as the Rhine Valley. If it persists, the rift widens and a continent can literally be split in two. In time, the gap reaches a coastline, permitting sea water to flow in and a new ocean is created.
Hot spots are now tearing Africa apart at its seams. A Great Rift Valley stretches from Ethiopia to Tanzania; as it widens the continent will break apart and the sea will eventually enter. The African and Arabian Plates have already pulled apart in another location, forming the Red Sea and the Gulf of Aden. They are developing into an ocean that may eventually rival the Atlantic Ocean in size. At the same time, the Mediterranean Sea is narrowing as Africa moves toward Europe.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University