Volcanic mountains

How volcanic mountains cool the climate

Through subtle changes and tugs across the planet, activity on Earth perpetually works to balance the global thermostat, from leaves sucking carbon dioxide out of the atmosphere to polar ice reflecting the sun’s rays back into it. space. Today, scientists have delved into another source of long-term climatic equilibrium: the formation of volcanic mountain ranges.

The team, based at Rice University in Houston, focused on plutons on the edges of continents. These continental volcanic arcs rise where the oceanic crust sinks and melts beneath the continental crust, such as in the Andes and North American waterfalls.

These mountains initially warm the climate by spewing greenhouse gases from the molten seabed into the atmosphere as they form. Researchers have long focused on this effect and have generally viewed these continental arcs as net sources of global heat. But now, new findings suggest that these mountain ranges are actually counteracting their warming over geologic time by exposing cool rock that easily binds to carbon dioxide in rainwater, ultimately pushing that carbon out of the water. atmosphere towards the seabed. The team published their findings in Letters of Earth and Planetary Sciences.

“If you have a global push for these arcs, you could help create greenhouse conditions,” said Cin-Ty Lee, a Rice University geologist and author of the article. “But as soon as [the arcs] die, you should be immediately followed by more improved cooling.

The combination of freshly exposed reactive rocks and steep relief accelerates mineral degradation and erosion which leads to this cooling. The minerals in these particular types of mountains, which tend to be quite high in the elements magnesium and calcium, also make them ripe for weathering as these particular elements readily react with carbon dioxide in rainwater to form crystals. stable carbonate minerals like limestone.

In addition to weathering, the erosion of these high-relief coastal mountains also buries organic material on the seabed, blocking even more carbon from the atmosphere.

These concepts are not new. But, until now, researchers did not know to what extent the continental arcs could cancel their warming by weathering and erosion.

Measure the sink

Sedimentary rock formed from eroded volcanic arc mountains in Southern California. Credit: Cin-Ty Lee

To uncover these dynamics, Lee and his graduate student Hehe Jiang studied the chemistry of an ancient arc that was active 170 to 85 million years ago along the southern California coast in what is today the peninsular chains. They measured the amount of calcium in the original plutonic rock and in the sediments that eroded from these rocks towards the coast. A loss of calcium in the eroded sediment should mean a breakdown due to weathering, so they calculated weathering rates by measuring this loss of calcium.

They found that weathering had indeed dissolved a large mass of eroded material, up to about 50% in some places. From these calculations, they estimated that the weathering rates would have canceled out any warming due to volcanic activity over about 40 to 50 million years.

“I think it was a pretty important discovery,” said Clément Bataille, a geochemist at the University of Ottawa who studies weathering processes but was not involved in this study.

Over time, the relief of the mountains has diminished, as have the rates of erosion and weathering. But these mountains still draw carbon dioxide from the atmosphere today, in part thanks to the subsequent tectonic activity that lifted them up.

Dig deep in the continental arcs

In fact, all mountains have the potential to condition and cool the planet in this way. But some do it more vigorously than others depending on their composition, shape and location.

Volcanic islands, for example, tend to weather more easily than continental arcs because they tend to contain more reactive minerals.

“When we think of volcanic weathering, people always think of island arcs,” said Benjamin Mills, a biogeochemist at the University of Leeds in the UK who studies climate change past and present. This is one of the reasons why continental arcs have not received as much attention as potential carbon sinks as island arcs, he explained.

But continental arcs tend to cover much larger areas than island arcs, so they can contain more weather-resistant and erodible material. “It’s something we haven’t thought about as much as we should,” Mills said.

Weathering also requires rainwater or other forms of precipitation. So the features of arid lands, like the Tibetan Plateau, are not as effective in cooling the climate as wetter mountain ranges like the Andes, Lee explained.

A recent Nature The document suggests that the Earth has more reactive and weather-resistant materials today than in the recent geological past. If so, the planet’s mountains could help the climate recover from anthropogenic warming over geological time, Bataille said. “Maybe the Earth will recover a lot faster than in the past.”

—Laura Poppick (@laurapoppick), Freelance science journalist

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