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Mid-Oceanic Ridges are formed when two plates move apart, and magma upwells from the upper mantle and forms a new ocean-floor layer. Once the magma begins to cool, the magnetic minerals in the ejected material take on the same sense of polarity as that of the magnetic field of the Earth at the time of crystallization. The Earth's magnetic field periodically reverses its polarity, the new basaltic lava forming at the ridge also takes in the reversed polarity magnetization.
The seafloor spreading process takes place evenly. The lavas that come out are carried away equal distances on either side of the ridge. This causes a symmetrical pattern of magnetic stripes around the ridge. This way the ocean crust creates a magnetic record of its formation. The magnetic stripes are numbered symmetrically from 1 at the ridge to about 180-2001 at the edge of the ocean boundries. These magnetic patterns tell that the major ocean floors of the world have all been created since the early Jurassic period and that the continents must have moved apart to accommodate the formation of new oceanic crust.
The world's oceans are in different stages of opening and closure. The Red Sea is in a narrow, early stage. The Atlantic Ocean is continuiung to spread on both sides. The Indian Ocean is opening on the west but subducting on it's east side. The Pacific Ocean is being subducted on both sides and will soon disappear as Asia collides with the Americas.
The rate of plate movement away from the ridge, which is half of the rate at which an ocean expands, varies from 1 cm/yr in the North Atlantic and the Red Sea to 4.4 cm/yr2 in the East Pacific.
The Mid-Atlantic Ridge is been formed due to the effect of plate tectonics. Over the due cource of time this ridge will develop into a belt of islands, like the islandic country of Iceland. With the current direction of plates, soon the Pacific will be extint and Atlantic the largest ocean on Earth. What happens as time passes by is just a mystery.
1; 2 Volcanoes Online
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