Pangaea: All Lands
All of the evidence suggests that 225 million years ago all of the land masses of the earth were locked together as a great supercontinent called Pangaea, a Greek word meaning 'all lands.' Panthalessa, Greek for "all seas," was the name given to the resulting world ocean. Eventually, tectonic forces caused teh break-up of Pangaea, leading to the current, albeit temporary, arrangement of oceans, continents, and other land masses. |
Scientists believe that the Earth's crust has been undergoing these transformations for most of its 4.6 billion year history. Two hundred million years from now a new Pangaea will be formed when all lands again converge. Then, inevitably, another break-up will ensue as our restless planet continues to reform and reshape itself.
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Mid-Ocean Ridges
Did you know that the Earth's longest mountain range is under water? The Mid-Ocean Ridge system, shown to the right snaking its way between the continents, is more than 35,000 miles long! This series of mountains and valleys marks where the tectonic plates are moving apart. |
At Mid-Ocean Ridges, two plates are pulling apart from each other as hot magma (liquid rock) emerges from the mantle and oozes forth as lava to fill the crack continuously created by the plate separation. The lava cools and attaches itself to the trailing edge of each plate, forming new ocean floor crust in a process commonly known as sea-floor spreading.
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Birth of an Island:
Sometimes the Mid-Ocean Ridge shows itself above the surface of the sea. On November 14, 1963 something amazing occurred. Early that morning, several miles off the southern coast of Iceland, fisherman noticed black smoke bubbling from the sea. Some thought it was a boat on fire. Others believed it might be an undersea volcano erupting far below on the ocean floor, but by evening, a ridge of hardening lava was noticed just below the waves. And by the following morning, a tiny island had emerged above the surface! The island was given the name Surtsey - after the Icelandic god of fire, Surtur. The island grew for three and a half years and eventually grew to an area of approximately one square mile. Iceland lies on the Mid-Atlantic Ridge, and continued spreading and eruptions along this section of the ridge widens the island by about one inch every year. |
The highest mountain range in the world, the snow-capped Himalayas, is an example of a continent-to-continent collision. This huge mountain range began to form when two large landmasses, India and Asia, driven by tectonic plate movement collided. Since both land masses have about the same rock density, one plate could not be subducted under the other. The pressure fo the colliding plates could only be relieved by thrusting skyward. The folding, bending, and twisting of the collision zone formed the jagged Himalayan peaks. This string of towering peaks is still being thrust up as India continues to crunch relentlessly into Tibet.
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Mountain Ranges
While new ocean crust is constantly being created at mid-ocean ridges, old crust must either be destroyed or reduced at the same rate (or else the planet would be continually expanding and increasing in volume). The plates, therefore, are almost all headed on a collision course! When two continents carried on converging plates ram into each other, they crumple and fold under the enormous pressure, creating great mountain ranges. |
Trench Flipping
If the descending oceanic plate is carrying a continent, the less dense continental material cannot sink, so it dives into the trench behind the leading oceanic crust until it gets stuck. This crumples its edge into folded mountains and causes some of the oceanic crust of the overlying plate to be deposited on top of the continent. Pressure steadily builds up until the trench 'flips,' and the previously overriding oceanic plate dives under the continental crust. This could explain why most ocean trenches are found along the edges of continents. |
Subduction Zones
When two oceanic plates collide, the younger of the two plates will ride over the edge of the older plate. The older, heavier plate bends and plunges steeply and descends into the earth and forms a trench that can be as much as 70 miles wide, more than a thousand miles long, and several miles deep. The Marianas Trench is the deepest sea floor in the world. It curves northward from near the island of Guam and its bottom lies close to 36,000 feet below the surface of the Pacific Ocean. |
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The Ring of Fire
In the not too distant past, cartographers charted the deep ocean trenches, seismologists plotted earthquakes beneath the trenches, and volcanologists studied the overlying volcanoes. But these researchers worked alone, and were not aware that the things they studied were all part of a single process. Today, the ideas of sea-floor spreading and subduction explain clearly why so many of the world's volcanoes are situated on the Pacific island arcs, the Ring of Fire, where the tectonic plates are being subducted beneath deep ocean trenches. After about 10 million years, the final stage of subduction begins. At depths of as much as 450 miles, the plate becomes so hot that it softens and stops generating earthquakes. But the descent and melting continue until, at some unknown depth, the plate blends with the surrounding mantle material. Eventually this material will emerge along Mid-Ocean Ridges as new sea-floor crust or escape as volcanic lava as the process of subduction comes full circle and our tectnoc planet continues to evolve. |
Faults
Aside from Mic-Ocean Ridges, subduction zones, and collision zones, there are areas where the stretching or compression of plates causes slight fractures in the earth's crust. These areas of crustal stress are called faults and there are essentially two types. Normal faults occur where tension within the Earth stretches the crust to form a basin, or range, with fault-blocked mountains flanking the basin. The southern Rockies (mountains in western U.S.) include a basin and range area formed as a result of a normal fault. Reverse faults occur where compression squeezes the crust together as one block of land slides over another, forming overthrust mountains. Impressive examples of overthrust formations can be seen in Montana's Glacier National Park. |
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Geologists believe that a huge column of upwelling lava, known as 'plume,' a lies at a fixed position under the Pacific Plate. As the ocean floor moves over this 'hot spot' at about 5 inches per year, the upwelling lava creates a steady succession of new volcanoes that migrate along with the plate - a veritable conveyor belt of volcanic islands.
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Hot Spots
The Hawaiian Islands provide striking evidence of yet another tectonic phenomenon. Stretching to the west and to the north of the big island of Hawaii is a string of smaller islands and submerged volcanoes 3,700 miles long. There is convincing evidence that every one of these islands has been formed in the exact place where Hawaii now stands. What is the nature of forces at work here? |