Plate Tectonics By madikreitz Mar 1, 1000 Continental Drift In the early 20th century, German scientist Alfred Wegener published a book explaining his theory that the continental landmasses, far from being immovable, were drifting across the Earth. He called this movement continental drift. Wegener noticed that the coasts of western Africa and eastern South America looked like the edges of interlocking pieces of a jigsaw puzzle. He was not the first to notice this, but he was the first to formally present evidence suggesting that the two continents had once been connected. Wegener was convinced that the two continents were once part of an enormous, single landmass that had split apart. He knew that the two areas had many geological and biological similarities. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. The presence of mesosaurus suggests a single habitat with many lakes and rivers.Wegener believed that all the continents—not just Africa and South America—had once been joined in a single supercontinent. This huge ancient landmass is known as Pangaea, which means “all lands” in Greek. Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their present positions as the continents.Tectonic ActivityAt first, other scientists did not accept Wegener’s theory of continental drift. But scientists now know that the continents rest on massive slabs of rock called tectonic plates. The plates are always moving and interacting in a process called plate tectonics. Over time, tectonic activity changes the Earth’s surface, rearranging and reshaping its landmasses.Today, scientists believe that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These include Pannotia, which formed about 600 million years ago, and Rodinia, which existed more than a billion years ago.The continents are still moving today. Underwater exploration has revealed seafloor spreading. Seafloor spreading is the process of new crust forming between two plates that are moving apart. Along a network of mountain ranges in the ocean, molten rock rises from within the Earth and adds new seafloor to the edges of the old. As the seafloor grows wider, the continents on opposite sides of the ridges move away from each other.North America and Europe, for example, are moving away from each other at the rate of about 2.5 centimeters (1 inch) per year. If you could visit the planet in the future, you might find part of California separated from North America, becoming an island in the Pacific Ocean. Africa will eventually split in two along the Great Rift Valley. It is even possible that another supercontinent may form someday. May 12, 1200 Sea Floor Spreading Seafloor spreading is a process of plate tectonics. New oceanic crust is created as large slabs of the Earth's crust split apart from each other and magma wells up to fill the gap.The large slabs of rock that make up the Earth’s crust are called tectonic plates. As they slowly move away from each other beneath the ocean floor, hot magma from the Earth’s mantle bubbles to the surface. This magma is then cooled by seawater. The new rock forms a new part of the Earth’s crust. Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor.The newest oceanic crust is located near the center of the ridge, the actual site of seafloor spreading. The Mid-Atlantic Ridge, which separates the North American plate from the Eurasian plate, and the South American plate from the African plate, is the site of new oceanic crust in the middle of the Atlantic Ocean. Over time, new oceanic crust pushes older crust farther away. New bodies of water and even continents can be created through seafloor spreading. The Red Sea, for example, was created through seafloor spreading, as the African plate and the Arabian plate tear away from each other. Today, the northern Sinai Peninsula connects the Middle East (Asia) with North Africa. Eventually, geologists predict, seafloor spreading will expand the Red Sea so that it will completely separate the two continents. Rift valleys, which exist on continental crust as well as oceanic crust, can be created by seafloor spreading. Two of the largest rift valleys in the world, the Mid-Atlantic Ridge and the East Pacific Rise, are products of seafloor spreading.Seafloor spreading disproves an early part of the theory of continental drift. Continental drift was one of the first theories that the Earth's crust was dynamic and always in motion. Supporters of continental drift originally theorized that the continents moved (drifted) through unmoving oceans. Seafloor spreading proves that the ocean floor itself is the site of tectonic activity.Subduction is the opposite of seafloor spreading. Subduction happens where tectonic plates crash into each other instead of spreading apart. In subduction zones, the edge of the heavier plate subducts, or slides, beneath the lighter one. It then melts back into the Earth's mantle.Seafloor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remains constant. Oct 13, 1500 Subduction Zone The older, heavier plate bends and plunges steeply through the athenosphere, and descending into the earth, it 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, where the enormous Pacific Plate is descending under the leading edge of the Eurasian Plate, 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. Jan 13, 1700 Earthquake Patterns Earthquake belts and distribution. Earthquakes occur in well-defined belts that correspond to active plate tectonic zones. The circum-Pacific belt (also called the Rim of Fire) follows the rim of the Pacific Ocean and hosts over 80 percent of the world's shallow and medium-depth earthquakes and 100 percent of the deep earthquakes. Other earthquake zones are the Mediterranean-Himalayan belt and the midoceanic ridges that split the crust at the bottom of the world's oceans. Plate boundaries and associated earthquakes. Distribution plots reveal that many earthquakes are associated with andesitic volcanic action and oceanic trenches that occur over subduction zones in the circum-Pacific belt. Oceanic trenches are narrow, deep troughs that mark where two plates converge, usually along the edge of a continent or island are where andesitic volcanoes typically occur. Earthquakes originate in Benioff zones, zones that slope downward from the trenches and under the overlying rocks at 30 to 60 degrees. Benioff zones are closely associated with the subduction of a crustal plate below an adjacent plate. Almost all earthquakes occur at the edges of the crustal plates. The constant bumping, grinding, and lateral movement along crustal boundaries can create sudden movements that result in earthquakes. Each of the three types of plate boundaries—convergent, divergent, and transform—has a distinctive pattern of earthquakes.There are two kinds of convergent boundaries: subduction and collision. A subduction boundary is marked by the oceanic crust of one plate that is being pushed downward beneath the continental or oceanic crust of another plate. A collision boundary separates two continental plates that are pushed into contact; the suture zone is the line of collision. Both types of boundaries have distinctive earthquake patterns. Earthquakes associated with a collision boundary define shallow, broad zones of seismic activity that form in complex fault systems along the suture zone. Earthquake patterns in subduction zones are more complex. As the oceanic crust begins to descend, it begins to break into blocks because of tension stress. Shallow earthquakes in the upper part of the subduction zone are a result of shallow-angle thrust faults, in which slices of plates slide like cards in a deck that is being shuffled. Earthquakes also periodically occur as the plate continues to subduct up to a depth of about 670 kilometers (400 miles). First-motion studies of these earthquakes suggest they result from both compressional and tensional forces on the subducting plate.Earthquakes are relatively abundant in the first 300 kilometers (180 miles) of a subduction zone, are scarce from 300 to 450 kilometers (180 to 270 miles), and then increase slightly again from 450 to 670 kilometers (270 to 400 miles). It is possible that these deepest quakes are related to sudden mineral transformations and resultant energy releases or volume changes. It has been theorized that earthquakes do not occur at depths greater than 670 kilometers because the subducting plate is not brittle anymore and has become hot enough to flow plastically.The distribution of earthquake foci along a subduction zone gives an accurate profile of the angle of the descending plate. Most often, plates start subducting at a shallow angle, which becomes steeper with depth. The angle of subduction is proportional to the density of the plate material, the amount of faulting and thrusting, and the tearing or crumpling of the descending plate.Divergent boundaries are those at which crustal plates move away from each other, such as at midoceanic ridges. These huge underwater mountains often have a central graben feature, or rift valley, that forms at the crest of the ridge. The formation of new ocean crust that is pushed away from both sides of the ridge fault creates a tensional setting that results in the formation of the graben. Earthquakes are located along the normal faults that form the sides of the rift or beneath the floor of the rift. Divergent faults and rift valleys within a continental mass also host shallow-focus earthquakes. Shallow-focus earthquakes occur along transform boundaries where two plates move past each other. The earthquakes originate in the transform fault, or in parallel strike-slip faults, probably when a frictional resistance in the fault system is overcome and the plates suddenly move. Aug 22, 1960 Plate Techtonics 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 BoundariesWhere 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 BoundariesAt 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 BoundariesThe 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.
