Deep beneath the Mid-Atlantic Ridge, scientists have recorded earthquakes at roughly 6 to 12 miles below the seafloor, far deeper than where rock should normally crack. The finding points to a new driver of seismicity far from familiar fault zones and volcanoes.
The culprit is not extra cold rock or a hidden megafault. It looks like carbon dioxide in hot mantle melt changing volume as pressure drops, a physical nudge that can make rock fail at depth.
Deep seafloor earthquakes
A research team deployed ocean-bottom seismometers, autonomous instruments that sit on the seafloor and record tiny vibrations, during the SMARTIES campaign in 2019.
Those sensors captured clusters of microearthquakes in the warm mantle directly under the ridge axis, well below the usual brittle layer.
The work was led by Satish C. Singh of the Institut de Physique du Globe de Paris (IPGB), a group with decades of experience imaging the oceanic crust and mantle.
In this study, the team focused on a slow-spreading equatorial ridge segment where seismic activity is usually modest.
Geochemical analyses of nearby basalts showed unusually high carbon dioxide in the primary melts, about 0.4 to 3.0 percent by weight. That enrichment is consistent with melts derived from a mantle source that carries extra volatiles.
The earthquakes sit at depths where temperatures are expected to be too high for rock to break in a brittle way. That mismatch pushed the team to look beyond temperature or fault geometry and consider the physical effects of gases in magma.
How CO2 triggers seafloor earthquakes
Degassing changes the volume of a fluid, and in a confined rock, even small pressure shifts can matter.
The solubility of CO2 in basaltic melt depends strongly on pressure, so as melt rises and pressure falls, bubbles form and the melt expands.
At depth, CO2 can stay dissolved at high concentrations, then separate as a gas as pressure eases, a process that stiffens and relaxes stresses along cracks in quick bursts.
Volatile solubility studies show this pressure control is a first-order effect in mafic magmas.
Seismologists often use mantle temperatures around 700 to 900 degrees Celsius as a practical ceiling for where brittle failure can occur, which puts these Atlantic events in notably hot conditions.
That is why a stress source tied to expanding gas, not a cold, thick lid, better fits the observations.
Why it matters for ocean crust
Volatiles do more than spark eruptions at volcanoes on land. They lower melting temperatures, steer where melts collect, and change how the new ocean crust forms beneath spreading ridges.
The Atlantic results suggest volatiles can also shape where and how earthquakes happen in the mantle beneath ridges.
If CO2 rich melts stall in the mantle before feeding the crust, they can evolve chemically and mechanically at depth.
That pause can increase heterogeneity in the lithosphere, the rigid outer shell, and near the asthenosphere, the weaker layer below it.
The study also helps explain seismic reflections and partial melt hints right at the lithosphere, asthenosphere boundary in the equatorial Atlantic.
Independent work indicates that a small percentage of melt, aided by volatiles, can persist there at temperatures below the dry peridotite solidus.
What makes this different from volcano swarms
Deep earthquakes linked to magma motion show up in active volcanic zones like Iceland’s Reykjanes Peninsula.
There, researchers documented deep long period events at about 6 to 7 miles depth around the 2021 Fagradalsfjall eruption, a very different tectonic setting.
Off Mayotte in the Indian Ocean, a seismic crisis revealed drainage of a huge reservoir from roughly 15 to 22 miles down, accompanied by deformation and very long period signals.
That sequence pointed to a developing submarine volcano rather than steady ridge accretion.
In Iceland’s Northern Volcanic Zone, several nests of unusually deep quakes, sometimes deeper than 20 miles, are tied to magma movement below the usual brittle layer.
Those cases involve thickened crust and a volcanic plumbing system, unlike the Atlantic ridge segment studied here.
Fresh look at volatile-rich ridges
The Atlantic ridge segment examined spreads slowly, a regime where melt pathways are complex and tectonics can expose mantle rocks on the seafloor.
In such settings, volatile rich melts may focus and linger, priming the conditions for gas driven stress changes at depth.
Global ridge studies show that mid ocean ridge basalt glasses are commonly depleted in CO2 by the time they erupt, which complicates efforts to reconstruct their original volatile content.
Trace element ratios such as CO2 to Ba and CO2 to Rb provide workarounds to estimate the pre-eruptive load.
More deep seafloor earthquakes studies
Better constraints on volatile content and pressure in the mantle will come from longer deployments and denser arrays of seafloor instruments.
Targeted sampling of fresh basalts and melt inclusions will tighten the links between chemistry, pressure, and the mechanics of these deep events.
Models that couple gas solubility, fracture mechanics, and ridge thermal structure can test how much CO2 is needed to tip rock into failure in hot mantle.
Those models can also explain why only some ridge segments host deep quakes while others stay quiet.
The study is published in Nature Communications.
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