Continental drift
Continental driftContinental Drift
Continental drift is the theory that the positions of the earth's continents have changed a lot through geologic time. The German meteorologist Alfred Wegener proposed the first comprehensive theory of continental drift in 1912, on the basis of the way the continents fit together on the opposing Atlantic coasts and paleontology correlation on both sides of the Atlantic. He proposed the theory that about 200 million years ago there was one great big continent, or supercontinent, called Pangaea. Pangaea split into two big landmasses, Laurasia and Gondwanaland.
The present continents separated in the next geologic era, the Mesozoic. In this process, the Earth's rotation caused horizontal alterations in the granite continents floating on the sea of the basaltic ocean floors. The frictional drag along the leading edges of the drifting continents created mountains. Wegener's theory met controversy until 1954, when British geophysicists seeking to explain the phenomenon of polar wandering revived it. Since then, the modern theory of Plate Tectonics has evolved from and replaced Wegener's original thesis.
Plate Tectonics is the theory that deals with the structure, history, and dynamics of the earth's crust. The earth's crust is formed of 13 continental plates. The boundaries of the zones are where tectonic activities occur, and where earthquakes and volcanic eruptions tend to happen. The plate tectonics theory was thought up in the 1960s and 1970s, with ideas taken from Wegener�s earlier thoughts. Earlier, Wegener and other scientists had noted that continental crust grows by additions and that a ridge in the middle of the ocean runs almost all the way around the world. It was also noted that continental plates tear apart, drift apart, and collide with each other.
Seafloor Spreading Geophysicists, mapping the ocean floor with an instrument that measures magnetic fields, showed that the rocks on one side of the midocean ridge produced an opposing image geomagnetic pattern of the rocks on the other side. When they dated these rocks, it showed that those nearest the ridge is obviously younger than those farther away are. They came to the conclusion that the midocean ridge is where new ocean crust is created, flowing out as hot lava and rapidly cooling when it comes into contact with the cold, deep-ocean water.
To make room for this continual addition of new crust, the plates on either side of the ridge must constantly move apart. This movement, about two inches per year in the Pacific, causes continents to drift. This movement is driven by heat waves of magma originating deep in the earth's mantle.
Volcanic Arcs and Subduction Seismology showed that the earthquakes associated with some seacoasts occur at shallow depths near the ocean side of volcanic island arcs, a chain of islands like Japan. The deepness increases to a maximum of about 430 miles at a distance of about 400 miles landward of the arc. Seismologists concluded that this represented a fault plane extending toward the land, down through the crust, into the upper mantle at about a 45� angle.
Scientists use the term subduction for the process that produces this earthquake pattern. They proposed that earthquake type subduction zones form when two plates moving in directions toward each other come together. Then one plate slips beneath the other at a shallow angle and then rub against the bottom of the other plate. Quieter zones form when two plates moving in the same direction come together, and the lower plate slides under at a steep angle, lessening contact between the plates.
When subducted ocean crust melts, magma rises from deep within the Earth and erupts on the earth's surface. This creates volcanic island arcs. Volcanic island arcs are where an oceanic plate subduct beneath continental crust, the magma erupts from volcanoes among long, linear, inland mountain chains, such as the Andes. The subduction zone is located along a submarine trench offshore of the continent.
Integrated Plate-Tectonics Theory Geophysicists combined the thoughts of seafloor spreading and subduction zones into a system of geodynamics. They demonstrated the global continuity of the subduction zones and showed that when the ocean floor spreads in one part of the globe, only one thing could possible happen. The expansion has to be accommodated by subduction in another part, or the earth would grow larger. The theory of plate tectonics has been debated, tested, and expanded and now forms a general idea for the geological sciences.