It takes place as magma wells up beneath mid-ocean rifts and is pushed upward onto the seafloor. As it cools, the magma gets pushed away and is replaced by new magma. The cooling material becomes oceanic crust as it spreads across the ocean floor, pushing continental plates and changing the seafloor as it moves.

Seafloor spreading has all kinds of effects. It can cause sea level rise, pushes tectonic plates around and can affect the amount of carbon dioxide in the atmosphere by powering volcanoes.

Fifteen million years ago, the researchers found, the seafloor spread about 200 millimeters per year. But since then, it has slowed significantly. When the researchers analyzed magnetic field data, which tracks seafloor change, from 18 ridges, they found that some were moving up to 100 millimeters more slowly now, with the fastest moving only 140 millimeters per year.

The change could affect the atmosphere; because volcanoes are partially driven by seafloor spread, a slowdown could reduce volcanic activity. In turn, that could spew fewer greenhouse gases into the atmosphere.

Crust production fell by 37 percent over the past 19 million years, the researchers conclude — and they’re not sure why. The change appears to be global in nature, but it will take more research to figure out what’s driving it.

In a release, Colleen Dalton, a geophysicist at Brown University and the study’s lead author, says she thinks both convection — currents that take heat from the Earth’s interior and push them to the planet’s surface, causing tectonic plates to move — and friction at some spots where tectonic plates rub against one another could be to blame.

Next, Dalton will look at other ways to calculate the rate of speed in the hopes of pinpointing the factor or factors that are making the seafloor spread more slowly.

Geophysical Research Letters

agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL097937