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Comment: 11:02 - 11:53 (00:51)
Source: Annenberg/CPB Resources - Earth Revealed - 4. The Sea Floor
Keywords: "mid-oceanic ridge", fissure, "lava flow", crust, subduction
Our transcription: The fissures atop a mid-ocean ridge are conduits through which fresh lava emerges.
The lava hardens, but soon the crust is pulled open forming new fissures and allowing more lava to emerge.
Repeated continuously over millions of years, this process forms new sea floor.
As the sea floor is pulled away from the ridge, it becomes cooler and denser.
Eventually, it becomes so cold and dense, it can sink back into the mantle against the edge of a nearby continent or smaller ocean basin.
This sinking of oceanic crust, called "Subduction", explains the absence of very ancient sea floor.
Quite simply, it disappears back into the Earth.
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