Slow spread could mean reduced greenhouse gas emissions from volcanoes
Federation of Geometry of the United States of America
WASHINGTON — A new global analysis Over the past 19 million years, the rate at which they spread on the seafloor suggests they are slowing down. Geologists want to know why the seafloor is shrinking.
New oceanic crust forms continuously along thousands of mile-long cracks in the seafloor, created by plate tectonics. As subduction pulls the old crust down, the cracks open like those in an erupting volcano, pulling the hot crust to the surface. Once at the surface, the crust begins to cool and is pushed out of the crack, replaced by a hotter, younger crust.
This cycle is called seafloor spreading, and its speed shapes many global processes, including sea levels and the carbon cycle. Faster velocities tend to cause more volcanic activity, which releases greenhouse gases, so decoding the propagation speed helps identify lasting changes in the atmosphere.
Today, the rate of spread peaks around 140 mm per year, but peaked at about 200 mm per year just 15 million years ago in some places, according to new research. The study was published in the journal AGU . Geophysical research letterwhich publishes high-impact, short-format reports with immediate impact on all disciplines of Earth and space sciences.
The slowdown is a global average, the result of different rates of spread between the slopes. The study looked at 18 ridges, but looked especially closely at the eastern Pacific Ocean, home to some of the world’s fastest-spreading ridges. Because they slow down so much, some nearly 100 millimeters slower per year than 19 million years ago, they’ve dragged down the world’s average rate of spread.
It is a complex problem to solve, made more difficult by the slow and steady self-destruction of the seafloor.
“We know a lot more about the surfaces of several planets other than our own seabeds,” said Colleen Dalton, a geophysicist at Brown University who led the new study. “One of the challenges is the lack of perfect preservation. The seabed was destroyed, so we were left with an incomplete record. ”
The seafloor is destroyed in subduction zones, where oceanic crust slides under the continents and sinks back into the mantle, and is regenerated at spreading ledges on the seafloor. This cycle of creation and destruction occurs about every 180 million years, the age of the oldest seafloor. The crust’s magnetic field profile tracks this pattern, creating identifiable bands each time Earth’s magnetic field reverses.
Dalton and her co-authors studied the magnetic records of 18 of the world’s largest spreading mountain ridges, using the age of the seafloor and their area to calculate how much oceanic crust each ridge has produced. in the past 19 million years. Each ridge grows a little differently: some lengthen, some shrink; some speed up, but most slow down. The overall result of Dalton’s work is that the average seafloor spreading rate slowed by 40% during that time.
The driving force here may lie in subduction zones rather than spreading ridges: for example, as the Andes grow along the western edge of mainland South America, mountains push down into the crust.
“Think of it as an increase in friction between two colliding tectonic plates,” Dalton said. “The decrease in convergence there could eventually cause a slowdown in propagation in neighboring ridges.” A similar process may have been operating beneath the Himalayas, with the rapidly growing range slowing spreading along ridges in the Indian Ocean.
However, Dalton points out, this increased friction cannot be the sole driver of the slowdown, because she sees the pace of slowdown globally and that mountain growth is regional. Larger-scale processes, such as changes in mantle convection, may also play a role. In all likelihood, she concluded, it was a combination of the two. To learn more, Dalton hopes to gather absolute disk speeds, rather than the relative speeds used in this study, which will allow her to better determine the cause of the slowdown.
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Notes for journalists:
This study is available for free until the end of the month. Download a PDF copy of the article here. Neither the newspaper nor this press release are embargoed.
Newspaper headline:
“Evidence for widespread global seafloor degradation since 15 million”
Author information:
- Colleen Dalton (corresponding author), Timothy Herbert, Department of Earth, Environment, and Planetary Sciences, Brown University, Providence, RI, USA
- Douglas S. Wilson, Institute of Marine Science, University of California-Santa Barbara, CA, USA
JOURNEYS
Geophysical research letter
DOI
ARTICLE TITLE
Evidence for widespread global seafloor degradation since 15 million
ARTICLE PUBLICATION DATE
March 28, 2022
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