Twenty million years ago there was no Atlantic Ocean. But then, thanks to plate tectonics, the South American and North American continents were separated by a rift valley that eventually turned into the Atlantic Ocean. The complementary shapes of the South American and African continents have long been noticed, but it wasn't until the 1960s that the theory of plate tectonics became accepted as the explanation.
These days, the Atlantic Ocean is growing at a rate of five centimetres per year, as new sea floor is created by volcanic activity along its mid-ocean ridge.
On the other hand, the much older Pacific Ocean is currently estimated to be shrinking by two to three centimetres each year. Again, this comes down to plate tectonics because the Pacific Ocean has subduction zones on three sides - where the Pacific plate submerges beneath other plates.
Since there are very few subduction zones in the Atlantic, Indian and Arctic oceans, these basins are expanding due to seafloor spreading. On the other hand, most subduction zones are found at the margins of the Pacific Ocean. Since subduction rates are much greater than the rates of producing new seafloor at the mid-oceanic ridges, simply because there are more spreading regions than subduction zones, the result is that the Pacific Ocean is shrinking on a geological time scale
The Pacific's "Ring of Fire" broadly maps the areas where this subduction occurs and accounts for around 90 per cent of the world's earthquakes.
Credits: Sabitha
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