An excellent example is the west coast of South America. Active margins are commonly the sites of tectonic activity : earthquakes, volcanoes, mountain building, and the formation of new igneous rock. Because of the mountainous terrain, most of the rivers are fairly short, and the continental shelf is narrow to non-existent, dropping off quickly into the depths of the subduction trench.
Passive continental margins are found along the remaining coastlines. Because there is no collision or subduction taking place, tectonic activity is minimal and the earth's weathering and erosional processes are winning. This leads to lots of low-relief flat land extending both directions from the beach, long river systems, and the accumulation of thick piles of sedimentary debris on the relatively wide continental shelves.
Earthquake and volcanic activity are prominent here. Since active continental margins occur along many coasts of the Pacific Ocean, these types of margins are also known as "Pacific-type" margins. The two types of continental margins, passive and active, tell about the geologic history of Earth and the activities that are continually affecting its surface. The continents are not stationary, but move about the planet's surface. That movement varies, but in general is extremely slow, only about 2 inches 5 centimeters per year.
Over millions of years, however, the continents have made their way along on an endless journey across the planet's surface, repeatedly crashing into or breaking away from one another. Continental shelves have not always been covered by water. During Earth's history, changes in sea level have alternately exposed, then covered portions of the shelves.
Scientists estimate that during the last glacial period over 10, years ago, much of Earth's water was trapped in the polar ice sheets. The level of the oceans may have dropped as much as feet meters below the current level.
At times of low sea level, land plants and animals, including humans and their ancestors, lived on the shelves. Evidence of this lies in their remains that are often found there in the present day.
Twelve-thousand-year-old bones of mastodons, extinct relatives of the elephant, have been recovered off the coast of the northeastern United States. Because water above continental shelves is not that deep, sunlight is able to penetrate, helping plants grow. This, in turn, leads to a rich web of sea life. Most commercial fishing takes place in these waters. Extensive deposits of oil, natural gas, minerals, and other natural resources lie beneath continental shelves.
The economic benefit of the fish and natural resources is important to many nations, which claim territorial ownership of the continental shelves adjacent to their land areas. Many political disagreements have arisen because of this.
In spite of their desire to reap the benefits contained over and in the shelves, many nations illegally dump much of their waste in the ocean over these areas. In German geophysicist Alfred Wegener — published a book in which he presented geological evidence that all the continents had once been joined together in a supercontinent he called Pangaea pronounced pan-JEE-ah; from the Greek words meaning "all lands".
Wegener suggested that the Atlantic Ocean and the Indian Ocean formed when this supercontinent broke apart and the continents drifted away from each other. He called his hypothesis continental drift. A hypothesis is an educated guess, while a theory is a principle supported by extensive scientific evidence and testing.
Wegener formed his hypothesis after he had observed that the present-day continental margins along some of the continents seemed to fit together like pieces in a jigsaw puzzle: eastern South America with western Africa, eastern North America with western Europe, and India and Antarctica with eastern Africa.
When the continents are linked, some of their geological features, such as mountain ranges and mineral deposits, also match. In addition, related species of land animals are often present on both side of the present-day oceans. Wegener argued further that if the continents had drifted, they would have passed through various climate zones. This explains how evidence of past glaciers could be found in the Sahara desert region and why fossil coral reefs appear north of the Arctic Circle.
What Wegener lacked, however, was a convincing explanation as to what moved the continents along the surface. Evidence to support his hypothesis did not come until the early s when geologists developed the theory of plate tectonics from the Greek word tekton, meaning "builder".
A revolutionary idea, it helps geologists and others understand how Earth has changed over long periods of time. Changes in the positions and features of the continents and oceans have had a profound effect on everything from global climate to the evolution of life.
Simply, the theory states that the surface of the planet is broken into sections—some large, some small—called tectonic plates. As these plates drift slowly over Earth, they slide past, collide with, and move away from each other. The boundaries where the plates meet and interact are called plate margins. What moves the plates along occurs within the planet. Geologists divide Earth into three distinct layers: the crust, the mantle, and the core.
Each layer has its own unique properties and composition. As already mentioned, the crust is the thin shell of rock that covers Earth. Two types of crust exist: continental crust, which underlies the continents, and oceanic crust, which underlies the oceans. Varying in thickness, the crust is thickest below land and thinnest below the oceans. Underneath the crust is the mantle, which is separated into two layers: The uppermost part of the mantle is solid.
Along with the overlying crust, it forms what is called the lithosphere pronounced LITH-uh-sfeer. It is the brittle lithosphere that has broken into the tectonic plates. Under the lithosphere is the part of the mantle known as the asthenosphere pronounced as-THEN-uh-sfeer.
Beneath Earth's surface, temperature and pressure increase with increasing depths. Rock in the asthenosphere is hot. Cross-section of Earth's interior, with the solid inner core, molten outer core, mantle, and crust. It is puttylike in its consistency, or what geologists call "plastic. The core, lying at the center of the planet, is divided into a liquid outer layer and a solid inner layer. Made up of the metallic elements iron and nickel, the core is almost five times as dense as rock on Earth's surface.
This heat energy moves the tectonic plates across the planet's surface. It is carried to the area beneath the plates by convection currents, which act similar to the currents produced in a pot of boiling liquid on a hot stove.
When a liquid in a pot begins to boil, it turns over and over. Liquid heated at the bottom of the pot rises to the surface because heating has caused it to expand and become less dense lighter. Once at the surface, the heated material cools and becomes dense once more. It then sinks back down to the bottom to become reheated. This continuous motion of heated material rising, cooling, and sinking forms the circular convection currents. Like a gigantic furnace, the core heats the mantle rock immediately above it.
Expanding and becoming less dense, the heated rock slowly rises through cooler, denser mantle rock. When the heated rock reaches the lithosphere, it moves along its base, exerting dragging forces on the tectonic plates. This causes the plates to move. In the process, the heated rock begins to lose heat. Cooling and becoming denser, the rock then sinks back toward the core, where it will be heated once more.
It takes an estimated million years for heated mantle rock to make the circular trip from the core to the lithosphere and back again. As the plates move, they interact in several different ways. Where they diverge, or move away from each other, new lithospheric rock is created as magma rises up through the crust at the plate boundary and gradually begins to cool. Many boundaries of diverging plates are on the floors of the oceans.
They are known as mid-ocean ridges, and the process of new ocean crust forming at the ridges is known as seafloor spreading. Where plates converge, or move into one another, either they crumple up and compress or one plate slides beneath the other. When two continental land plates converge, the crust bends and breaks from the collision, forming complex mountain ranges and very high plateaus.
When a continental plate and an oceanic plate converge, the oceanic plate which is thinner, yet denser bends and plunges at an angle into the asthenosphere beneath the continental plate. As it does so, its leading edge begins to melt because of high temperature and pressure in the mantle. It is characterized by its relatively steep slope of o abyssal plain A flat region of the ocean floor, usually at the base of a continental rise, whose slope is less than It is formed by deposition of gravity-current and pelagic sediments that obscure the preexisting topography mid ocean ridge A continuous, seismic, median mountain range extending through the North and South Atlantic Oceans, the Indian Ocean, and the South Pacific Ocean.
It is a broad, fractured swell with a central rift valley and usually extremely rugged topography; it is km in height and about km in width and over 84, km in length oceanic islands Island either composed of basalt or of biogenic origin e.
Seamounts may be either discrete, arranged in a linear or random grouping, or connected at their base and aligned along a ridge or rise oceanic plateaus A broad, more or less flat-topped and ill-defined elevation of the sea floor, generally over meters in height passive margin trailing edge A continental margin formed by rifting and continental rupture; characterized by broad continental shelves and located within plates rather than at the leading edge of a plate active margin convergent plate boundary Marking the boundary between two plates that are moving toward each other; the leading edge of a tectonic plate ocean basin The area of the sea floor between the base of the continental margin, usually the foot of the continental rise, and the mid-ocean ridge trench A narrow, elongate depression of the deep-sea floor associated with a subduction zone.
Hawaiian Islands , island arcs e.
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