When was seafloor spreading discovered




















Seafloor spreading occurs along mid-ocean ridge s—large mountain range s rising from the ocean floor. The East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate.

The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate. Seafloor spreading is not consistent at all mid-ocean ridges. Slowly spreading ridges are the sites of tall, narrow underwater cliff s and mountains. Rapidly spreading ridges have a much more gentle slopes. The Mid-Atlantic Ridge, for instance, is a slow spreading center.

It spreads centimeters. The East Pacific Rise, on the other hand, is a fast spreading center. It spreads about centimeters inches every year. There is not an ocean trench at the East Pacific Rise, because the seafloor spreading is too rapid for one to develop! The newest, thinnest crust on Earth is located near the center of mid-ocean ridge—the actual site of seafloor spreading.

The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge. The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century.

Basalt, the once- molten rock that makes up most new oceanic crust , is a fairly magnetic substance, and scientists began using magnetometer s to measure the magnetism of the ocean floor in the s. Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge. The continual process of seafloor spreading separated the stripes in an orderly pattern. Oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading.

As it moves, it becomes cooler, more dense, and more thick. Eventually, older oceanic crust encounters a tectonic boundary with continental crust. In some cases, oceanic crust encounters an active plate margin. An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquake s and volcano es. Oceanic crust created by seafloor spreading in the East Pacific Rise, for instance, may become part of the Ring of Fire , the horseshoe-shaped pattern of volcanoes and earthquake zones around the Pacific ocean basin.

In other cases, oceanic crust encounters a passive plate margin. Passive margins are not plate boundaries, but areas where a single tectonic plate transition s from oceanic lithosphere to continental lithosphere. Passive margins are not sites of fault s or subduction zone s.

Thick layers of sediment overlay the transitional crust of a passive margin. The oceanic crust of the Mid-Atlantic Ridge, for instance, will either become part of the passive margin on the North American plate on the east coast of North America or the Eurasian plate on the west coast of Europe. New geographic features can be created through seafloor spreading. The Red Sea, for example, was created as the African plate and the Arabian plate tore away from each other.

Eventually, geologist s predict, seafloor spreading will completely separate the two continent s—and join the Red and Mediterranean Seas. Mid-ocean ridges and seafloor spreading can also influence sea level s.

As oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes more dense. This increases the volume of the ocean basin and decreases the sea level. The Rim of Fire, which is named for its volcanoes and earthquakes, is created by a series of subduction zones along the coastlines surrounding the Pacific Ocean-from western South and Central America to the Aleutian Islands in Alaska, down the western Pacific, from Japan and the Philippines, all the way to Indonesia and New Zealand.

In , a Canadian geophysicist, J. Tuzo Wilson, combined the continental drift and seafloor spreading hypotheses to propose the theory of plate tectonics. In the asthenosphere, rocks are under such tremendous heat and pressure that they behave like a viscous liquid like very thick honey.

Tuzo Wilson predicted three types of boundaries between plates: mid-ocean ridges where ocean crust is created , trenches where the ocean plates are subducted and large fractures in the seafloor called transform faults, where the plates slip by each other.

Plate tectonics has provided a unifying theory that explains the fundamental processes that shape the face of the Earth. Alfred Wegener was born in Berlin, Germany on November 1, He spent a great deal of time in Greenland as part of several exploration and research expeditions. Wegener died on the ice in Greenland in Nov. Reconstruction of the map of the world according to drift theory for three epochs, the upper one is the upper Carboniferous, the middle one is for the Eocene, and the lower one is for the Lower Quaternary.

In , he added a geologic mechanism to account for Wegener's moving continents. It was possible, he said, that molten magma from beneath the earth's crust could ooze up between the plates in the Great Global Rift. As this hot magma cooled in the ocean water, it would expand and push the plates on either side of it -- North and South America to the west and Eurasia and Africa to the east. This way, the Atlantic Ocean would get wider but the coastlines of the landmasses would not change dramatically.

If, as Georges LeMaitre suggested for visualizing the early universe, you play the "film" of this phenomenon backwards, the continents come closer together until Brazil fits right into the Gulf of Guinea.



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