Beginning about 15 million years ago, ocean crust production declined by one-third over 10 million years to a slow pace that pretty much continues to today.
In seafloor trenches around the world, slabs of old ocean crust fall in slow motion into the mantle, while fresh slabs are built at midocean ridges, where magma emerges at the seams between separating tectonic plates.
Seafloor spreading is captured in magnetic zones on the ocean floor. Every million years or so, Earth's magnetic field flips, and this reversal is frozen in the rocks forged at the midocean ridges. Ship-based observations of the alternating magnetic"stripes" that result as slabs of ocean crust unfurl from the seafloor spreading centers helped give credence to the theory of plate tectonics in the 1960s.
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