Changing species dynamics and locations aren’t the only things happening in our ocean, but not every story is about climate change messing up our world. Sometimes, our world changes for reasons that have nothing to do with humanity but everything to do with the planet itself.

Take plate tectonics for example. We have zero control over this process, and it’s probably one of the biggest factors in shaping the world. Weirdly, it’s one of the more recent theories to be developed and accepted, with textbooks changing while I was in grade school. But we don’t understand everything about the process of plate tectonics, and now we even have something new to grasp – seafloor spreading is slowing down.

Seafloor spreading is just what it sounds like: two plates are moving apart in the ocean, causing the crustal rocks to thin and allowing the warmer, molten mantle to rise up. At places like the Mid-Atlantic Ridge, this process creates new rocks, pushing the European and North American plates apart. Meanwhile, on the other side of the globe, the pushing from that process is causing subduction around the Pacific Rim, and that is where existing rocks are being destroyed as they are driven down into the mantle.

It’s a plate-eat-plate world, apparently.

Overall, plate tectonics keeps our planet at the same size, destroying older rocks to make new ones and leaving us with an incomplete record of our planet’s history. And all that destruction and creation also involves volcanoes, which then shape other aspects of Earth such as sea level, the carbon cycle, and atmospheric greenhouse gases.

Now, new research published in Geophysical Research Letters looks at how the rate of seafloor spreading has changed in the past 19 million years and possibly why it has changed. It turns out that the rate peaked for some ridges about 15 million years ago at 200 millimeters per year, and now it has slowed down, dropping by almost 100 millimeters per year in that entire 19 million year time. The study measured the spreading rates at eighteen ridges around the world, but most of the slowdown is occurring in the eastern Pacific. Overall, the drop in the average global rate was about 40%.

One of the possible reasons for this slowdown may actually be those subduction zones. Take the Andes mountains, for example, which are created due to subduction off the coast of South America. As those mountains continue to grow and rise up through volcanic activity, they also create pressure, pushing down on the crust as it gets heavier and heavier. That means that the subduction gets more and more difficult. Lead author Colleen Dalton explains: Think of it as increased friction between the two colliding tectonic plates. A slowdown in convergence there could ultimately cause a slowdown in spreading at nearby ridges.

Dalton notes that the increased friction cannot be the only cause of the slowdown since mountain growth is regional, and the slowdown is seen globally. Something might be going on with the mantle convection processes as well, and she hopes to collect more data and find the absolute rather than relative plate speeds to determine the complete cause.

More Information

AGU press release

“Evidence for a Global Slowdown in Seafloor Spreading Since 15 Ma,” Colleen A. Dalton, Douglas S. Wilson, and Timothy D. Herbert, 2022 March 25, Geophysical Research Letters