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d5kenn
ANSWERS: 2
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Roughly 200-300MY. Basalt, which is denser than continental granite, sinks into the mantle at oceanic abysses and is formed anew at mid-oceanic ridges. Granite floats atop the basalt and is not reabsorbed into the earth -- and as a result, is much older on average.
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Oceanic Crust is mostly of Cretaceous, Tertiary and Quaternary age, with only small areas of Jurassic age in the margins of the North Atlantic Abyssal plains, and in the NW Pacific region. The oldest measured age is about 165 Million years, and it is reckoned there is none older than 200Ma. The oceanic crust is constantly recycled by convection systems in the Earth's mantle creating new rock at the spreading mid-oceanic ridges, and destroying older ocean floor by subduction at continental margins.
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