Seafloor spreading, paleomagnetism, and the ages of crust

Plate tectonics became widely accepted in the 1960s, when new technology revealed the seafloor. Sonar mapping, including Marie Tharp's maps of the Mid-Atlantic Ridge, showed a global chain of mid-ocean ridges with rift valleys down their centers, and Harry Hess proposed seafloor spreading: new oceanic crust forms as magma rises at ridges and moves away on both sides. Magnetometers towed behind ships found evidence in the rocks themselves. As lava cools, iron-bearing minerals record the direction of Earth's magnetic field, which has reversed many times over geologic history, so the seafloor shows stripes of normal and reversed magnetism that are mirror images on either side of a ridge, just as spreading predicts. Drilling showed that seafloor sediment gets thicker and the crust gets older with distance from ridges, and almost all ocean floor is less than about 200 million years old, because older seafloor has been subducted back into the mantle. Continental crust, too thick and buoyant to subduct, preserves rocks up to about 4 billion years old. Today GPS measures plate motion directly, typically about 2 to 10 centimeters per year, and the relationship rate equals distance divided by time lets scientists reconstruct past plate positions and predict future ones: at 5 centimeters per year, a plate moves 50 kilometers in a million years.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report