Mantle Convection Quotes

If we extrapolate this rate of overturn back in geologic time, the ocean floor has apparently been rejuvenated at least two dozen times since the Earth formed. When Earth was younger and hotter, however, the pace of convection may have been faster, and the ocean floor may have been resurfaced more frequently. But this leads to a conundrum: If convection had been faster in the past, as most geoscientists think it was, ocean crust would have arrived at subduction zones at a younger average age, still too hot and buoyant to be assimilated back into the mantle. This suggests that true plate tectonics, with rigid crustal slabs, efficient recycling of ocean crust via subduction, and water-assisted production of low-temperature melts, may not have occurred on the early Earth. Instead, plate tectonics could begin only when the Earth had reached a degree of thermal maturity, probably about 2.5 billion years ago (around the close of the Archean eon and the beginning of the Proterozoic). Before this, Earth's mixer settings—and the extent to which surface water was stirred back into the interior—were probably different. We can look to rocks formed in these distant times, Earth's record of its childhood and youth, for clues.

Three dominant hypotheses explain what drives plate tectonic motion. Each one relies on the convention of the mantle — the movement of heated rock materials beneath earth’s crust — but each one focuses on a different piece of the cycle: Mantle convection hypothesis: This hypothesis proposes that heated materials inside the earth move up and down in a circular motion (like the wax in a lava lamp) and the continental plates resting on this mat-erial are moved in the direction of the circular motion. Ridge-push hypothesis: This hypothesis states that the creation of new rock materials along mid-ocean ridges continually pushes oceanic crustal plates upward and outward, so that the far edges are forced into collisions with other plates. Slab-pull hypothesis: This hypothesis is the opposite of the ridge-push model. It proposes that the heavy, dense outer edges of crustal plates sink into the mantle at plate boundaries and pull the rest of the plate along with them.