Abyssal hills form at mid-ocean ridges, such as the Mid-Atlantic Ridge (shown). Some researchers had suggested that these hills also record the influence of climate cycles on sea floor eruptions—but a new study finds this tectonically unlikely.

Abyssal hills form at mid-ocean ridges, such as the Mid-Atlantic Ridge (shown). Some researchers had suggested that these hills also record the influence of climate cycles on sea floor eruptions—but a new study finds this tectonically unlikely.

NCEI/NOAA

Climate cycles didn’t shape ocean’s abyssal hills

Can Earth’s ice ages be seen in the undulating fabric of the sea floor? Earlier this year, a pair of papers suggested that long-term cycles of glaciation and melting trigger pulses of lava that harden into sea floor hills. But now, a new study throws cold water on that hypothesis, finding that these climate-driven pulses did not significantly shape the sea floor. Instead, they say, the underwater hills likely come from faulting action and steady—rather than climate-driven—magma eruptions.

“The main point is that the crustal bathymetry is complex,” says David Lund, a paleoceanographer at the University of Connecticut, Avery Point, who was not involved with the study. With so many processes shaping the sea floor, he says, climate-related signals are extremely difficult to detect.

A map of Earth’s ocean floor reveals a jagged landscape of tall mid-ocean ridges flanked by a rippling series of smaller hills and grooves. These so-called abyssal hills, some a few hundred meters high, are the most abundant topographic feature on Earth, covering about a third of the ocean floor. Given their ubiquity, geologists have long tried to understand their origin. They have, of course, long known that mid-ocean ridges are the birthplace of new sea floor. As two tectonic plates pull apart, magma wells into the gap, cooling into new rock that is then dragged away from the ridge. Then, as the crust stretches and cools, it cracks, forming faults that allow up-and-down movement. Some scientists have argued that this process alone is sufficient to produce the rugged terrain at most ridges. But others say that periodic fluctuations in magma volume can also help make the abyssal hills.

Earlier this year, two papers—one published in Science and the other in Geophysical Review Letters—added a new wrinkle to the debate. They suggested that long-term climate cycles could be modulating the amount of magma erupting on the sea floor. As glaciers grew and retreated, sea levels rose and fell. Those massive fluctuations in pressure would drive periodic pulses of magma to erupt. The Science paper further suggested the distribution of hills at one mid-ocean ridge could be matched with three well-known climate cycles—the Milankovitch cycles—that take place every 23,000, 41,000, and 100,000 years. These Milankovitch cycles are tied to Earth’s wobbly orbital axis, its oscillating axial tilt, and its orbital eccentricity.

Jean-Arthur Olive

But Jean-Arthur Olive, a geodynamicist at Columbia University and lead author of the new paper, doubted that seafloor topography would be sensitive enough to record such relatively rapid changes in magma supply. Even long-term climate cycles are short relative to geologic time, where “we’re talking about hundreds of thousands to millions of years for plate tectonics,” Olive says.

To find out whether the climate-driven hypothesis was possible, Olive and his colleagues modeled three different ways in which such rapid pulses of magma might change the face of the sea floor. First, they examined the strength of the tectonic plate itself. The bulk of the plate is made of oceanic crust, a relatively light layer of rock that floats on the denser mantle like a boat floating on water. Newly formed sea floor becomes part of that crust.

But most new crust is not erupted and added to the sea floor, but is but is tacked on to the base of the crust. So the researchers wanted to determine whether pulses of magma would produce a crust thick and heavy enough to warp its surface. The models suggested it wouldn’t: The magma inputs didn’t significantly alter the surface of the sea floor. “The plates at the ridge axis simply have too much strength to deform enough to create topography that way,” Olive says.  

A second test looked at how much sea floor a mass of magma could make. The team found that magma pulses that accumulated over 100,000 years—the longest of the three Milankovitch cycles—could produce tens of meters of crust. But the abyssal hills at some ridges can reach 200 meters high, so high that a few extra tens of meters wouldn’t make much of a difference.

Finally, Olive and his team looked at how climate-driven magma pulses might interact with active faults to shape the sea floor. Different mid-ocean ridges spread at different rates; if magma pulses helped shape the abyssal hills on the ridges’ flanks, then the fastest-spreading ridges would have hills spaced farther apart, while slower-spreading ridges would have hills clustered closer together. But the reverse has long been known to be true.  So Olive and his team devised a new explanation: At fast-spreading ridges with abundant magma eruptions, the cooling, stretching crust forms new faults in rapid succession as it continues to spread. Under normal conditions, “You end up with a lot of closely spaced faults, and [few hills],” says Olive.

Richard Katz, a geodynamicist at the University of Oxford in the United Kingdom who was an author on the previous Science paper, says he welcomes the new ideas. “The paper we published—we always knew it would be controversial.” But Katz contends the new study’s models are too simplistic. “Those models weren’t developed to consider processes at shorter times and spatial scales. While I think it’s reasonable to do what they’ve done, it’s also reasonable to say that these models aren’t detailed enough to capture these observations,” Katz says. The best way to resolve this, he says, is to gather and analyze more data from additional ridges.

Gathering data from additional ridges is one strategy, Lund agrees. Another, he says, is to find a different proxy for climate-driven pulses of magma altogether—one that is independent of tectonics. To that end, he says, he and others have been examining hydrothermal vents at a mid-ocean ridge known as the East Pacific Rise. Such vents can serve as records of heat activity—which may include pulses of magma—at the ridge through time. 

Olive acknowledges that they’ve used a simplified model, but he doesn’t think a more detailed one will produce different results.  Rather than focusing on the surface expression of the abyssal hills, he and his colleagues suggest that researchers hunting for signals from climate cycles use seismic imaging at the base of the oceanic crust, where much of the new sea floor accretes and rapid pulses of magma might be more observable. “We’re not contradicting the idea that the modulations exist,” he emphasizes. “We’re contradicting the idea that it leads to the sea floor landscapes.”