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| Oceanic crust undergoing extension. |
New oceanic crust is constantly being created at the midocean ridges as the result of plate spreading. Each year twenty cubic kilometers of new oceanic crust are created along this interconnected network of volcanoes. As the plates that make up the surface of Earth move apart along midocean ridges, magma or molten rock rises from Earth's mantle below and solidifies to rock in the cracks created by the separation. Further plate separation splits this new rock and creates more cracks into which magma again flows from the mantle below. The ocean basins, which cover approximately 70 percent of Earth's surface, are created through this ongoing process.
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| Pillow lavas |
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| Fissure |
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| Fissure |
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When the magma erupts onto the surface of the volcanoes that make up the ocean floor, it is called lava. Some of the volcanic features observed on midocean ridges include pillow lavas (rounded forms created when lava erupts under water), sheet flows (thin, flat flows similar to eruptions on Hawaii), and collapsed lava pits (created when the tops of lava tubes collapse inward). When the cracks that form due to plate separation intersect the ocean floor and don't get filled in by magma, they are called fissures.
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| This black and white map shows how ocean crust gets older as it spreads away from the mid-ocean ridge. |
On the map you may notice the smaller lines that cut across the midocean ridge like rail road tracks and cause the steplike appearance of the midocean ridge system. These "train tracks" represent valleys bounded by faults and are called fracture zones. The forces that separate the oceanic plates are not always evenly distributed. As the plates move in response to these forces, they fracture or tear to accommodate the uneven forces. This fracturing and tearing process causes earthquakes. The earthquake-prone San Andreas fault along the West Coast of the United States is very similar to these fracture zones.
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| Bathymetry map shows the depth from the sea surface to the sea floor. |
Notice that the ocean basins are much deeper (purple) farther away from the midocean ridges and shallower (blue) closer to the midocean ridges. The change in bathymetry stems from the fact that old oceanic crust is cold and dense whereas new oceanic crust is warm and buoyant (less dense). New oceanic crust is created at the midocean ridges. Plate separation then moves this crust away from the ridges. Oceanic crust is therefore younger near the midocean ridges and increasingly older farther from the midocean ridges. The new crust is formed from molten rock or magma that rises from the mantle to fill the gap left between the plates as they separate. The magma is very hot (roughly 1,200°C), and the new crust formed by the freezing of this magma is therefore still warm. As the newly formed, warm crust moves away from the ridge, it ages and cools. Warm crust is more buoyant than cold crust, and warm, buoyant crust "floats" at a higher level on the underlying mantle than does cold, dense crust. Thus, the crust ages as it moves away from the midocean ridge, cools, and sinks a bit into the mantle below. Note that along some margins with continents, the oceanic crust gets so dense that it sinks below the adjacent continent ‹causing a deep trench (see portions of the margins of the Pacific Ocean basin). The greatest ocean depths, more than 5,000 meters, are found in these trenches!
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