The Seafloor Spreading Hypothesis - Scientists brought all of these observations together in the early 1960s to create the seafloor spreading hypothesis. They suggested that hot mantle material rises up toward the surface at mid-ocean ridges. This hot material is buoyant and causes the ridge to rise, which is one reason that mid-ocean ridges are higher than the rest of the seafloor.
The hot magma at the ridge erupts as lava that forms new seafloor. When the lava cools, its magnetite crystals take on the current magnetic polarity. The polarity is locked in when the lava solidifies and the magnetite crystals are trapped in position. Reversals show up as magnetic stripes on opposite sides of the ridge axis. As more lava erupts, it pushes the seafloor that is at the ridge horizontally away from ridge axis.
This continues as the formation of new seafloor forces older seafloor to move horizontally away from the ridge axis. The magnetic stripes continue across the seafloor. If the oceanic crust butts up against a continent, it pushes that continent away from the ridge axis as well. If the oceanic crust reaches a deep sea trench, it will sink into it and be lost into the mantle. In either case, the oldest crust is coldest and lies deepest in the ocean.
It is the creation and destruction of oceanic crust, then, that is the mechanism for Wegener’s drifting continents. Rather than drifting across the oceans, the continents ride on a conveyor belt of oceanic crust that takes them around the planet’s surface. One of the fundamental lines of evidence for continental drift is the way the coastlines of continents on both sides of the Atlantic Ocean fit together.
New oceanic crust is forming at the mid-ocean ridge that runs through the center of the Atlantic Ocean basins, which is called the Mid-Atlantic Ridge. Stripes of different magnetic polarity are found on opposite sides of the Mid-Atlantic Ridge. These stripes go all the way to the continents, which lie on opposite sides of the Atlantic. So new seafloor forming at the Mid-Atlantic Ridge is causing the Americas and Eurasia to move in opposite directions.
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