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Contents of 12.3 What is the evidence that plates move apart at divergent plate boundaries?

• Plates move apart at divergent boundaries by seafloor spreading, which creates new, mafic ocean crust and mantle.
• Abundant earthquakes, basaltic volcanism, high heat flow, high seafloor elevation, and a seismically slow upper mantle confirm plate tectonic predictions for mid-ocean ridges.
• Magnetic polarity of seafloor crust, along with isotopic dating, demonstrates that oceanic crust is progressively older away from mid-ocean ridges. Seafloor spreading predicts this.
• Divergent plate boundaries can originate within continents, forming rift valleys. This may result in separation into two smaller continental fragments. Not all rift valleys will extend sufficiently to accomplish this.

12.3 What is the evidence that plates move apart at divergent plate boundaries?