Seafloor spreading
The formation of new areas of oceanic crust, which occurs through the up welling of magma at mid ocean ridges.
Harry Hess was a geologist who studied chains of islands with active volcanoes. He was in the Navy in WWll and commanded an attack transport ship. His ship was equipped for sounding the ocean floor which was useful to him also as a geologist. In 1945, Hess measured the oceans to the deepest points to date which is about 7 miles deep. He discovered hundreds of flat top mountains on the pacific floor. He named them guyots after the first geology professor at Princeton. The tops of the guyots looked like they were eroded but they were 2 kilometers under water. Years after the war, a volcanic valley running along the mid ocean ridges was discovered and Hess thought of his data he collected during the war. He hypothesized that the sea floor was spreading from vents in the rift where hot magma oozed up. As the magma cooled it forced the existing sea floor away from the rift on either side. The guyots fit in to the seafloor spreading picture. He believed they were once active volcanoes which rose above the water surface before being eroded by the atmosphere down to sea level. As the sea floor moved, it carried the eroded volcanoes along with it and the guyots were moved to deeper and deeper water.
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Sea floor spreading occurs along mid ocean ranges - large mountain ranges rising from the ocean floor. The farther away you travel from a ridge the older the crust is and the older the sediments on top of the crust. 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 cooled by sea water and the new rock forms a new part of the Earth's crust. This map shows the ages of the crustal rocks that make up the floor of the Atlantic Ocean. Red represents the youngest rocks; the deepest red marks the mid oceanic ridge, where continental plats are pulling apart and new crust is being formed. Older rocks are yellow, green and blue: The deepest blue rocks along the coastal line of Africa, Europe and the Americas are Jurassic in age, showing the time of formation of the Atlantic Ocean. The red is where the magma is oozing up from the mantle spreading the ocean floor. New continents and even new bodies of water can be created through seafloor spreading.
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Subduction is the opposite of seafloor spreading. Subduction happens where tectonic plates crash in to 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. Where the plates collide, great mountain ranges may be pushed up or if one plate sinks below another, deep oceanic trenches and chains of volcanoes are formed.
Sea floor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other so the shape and diameter of the Earth remain constant.
Sea floor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other so the shape and diameter of the Earth remain constant.
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The Earth is like a giant magnet with a North and South pole. Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes.” These stripes hold a record of reversals in Earth’s magnetic field. The rock of the ocean floor contains iron. The rock began as molten material that cooled and hardened. As the rock cooled, the iron bits inside lined up in the direction of Earth’s magnetic poles. This locked the iron bits in place, giving the rocks a permanent “magnetic memory.” Using sensitive instruments, scientists recorded the magnetic memory of rocks on both sides of a mid-ocean ridge. They found that stripes of rock that formed when Earth’s magnetic field pointed north alternate with stripes of rock that formed when the magnetic field pointed south.
The crust surrounding the midocean ridges showed the alternating bands by drilling in the crust of the Earth -- each band magnetized with a polarity opposite the surrounding bands. They suggested that as new sea-floor crust was formed around the rift in the mid ocean ridge, it magnetized differently, depending upon the polarity of the planet at that time. This supported the theory that Harry Hess had put forth, that the ocean progressivley widens as new sea floor is created along a crack that follows the crest of midocean ridges. This confirmed the theory that Earth's magnetic field had flip-flopped through the planet's life, and it made Matthews and Vine's 1963 finding quite clear. They realized that the pattern of reversals matched perfectly the magnetic profile they had compiled of the sea floor. This discovery, together with data from a 1964 research vessel, transformed the field of geology. It confirmed sea-floor spreading as hypothesized by Hess, and thus "continental drift," originally proposed by Alfred Wegener back in 1912. It convinced many that plate tectonics was the best theory to unify nearly all the previously accumulated, but disjoint geological data.
The crust surrounding the midocean ridges showed the alternating bands by drilling in the crust of the Earth -- each band magnetized with a polarity opposite the surrounding bands. They suggested that as new sea-floor crust was formed around the rift in the mid ocean ridge, it magnetized differently, depending upon the polarity of the planet at that time. This supported the theory that Harry Hess had put forth, that the ocean progressivley widens as new sea floor is created along a crack that follows the crest of midocean ridges. This confirmed the theory that Earth's magnetic field had flip-flopped through the planet's life, and it made Matthews and Vine's 1963 finding quite clear. They realized that the pattern of reversals matched perfectly the magnetic profile they had compiled of the sea floor. This discovery, together with data from a 1964 research vessel, transformed the field of geology. It confirmed sea-floor spreading as hypothesized by Hess, and thus "continental drift," originally proposed by Alfred Wegener back in 1912. It convinced many that plate tectonics was the best theory to unify nearly all the previously accumulated, but disjoint geological data.