‘Rock flour’: The unassuming material that could soak up carbon emissions

Minik Rosing grew up around the fine mud flowing from Greenland’s glaciers. It wasn’t until much later, when his own daughter had grown up and was in her mid-20s, that he realized how special it is.

During a family vacation in rural Greenland, where there was no electricity, she was fishing ice out of a milky-blue fjord for a gin and tonic when that mud gripped her feet so tightly that she had to abandon one of her boots.

As temperatures rise, meltwater is flushing out millions of tons of this stuff: ultrafine powder ground down by the island’s melting glaciers. Geologists have a culinary-sounding name for the microscopic particles: “rock flour.”

The loss of his daughter’s boot got Rosing thinking. Maybe those tiny grains of rock could be used to trap something much bigger: the carbon emissions that are altering the frozen landscape and way of life on the island.

“Greenland has been seen as the example and the horror story of climate change, and never been portrayed as a part of the solution,” said Rosing, a geology professor at the University of Copenhagen in Denmark who was born in Greenland.

As global emissions continue to rocket, he is part of a growing group of scientists looking for ways to suck carbon right out of the sky, an example of a sometime contentious suite of technologies called geoengineering.

For Rosing, the massive Arctic island’s exceptional mud represents not only a way of dialing back global warming but also an opportunity to change “the rest of the world’s impression of the Arctic” from ground zero for climate change to a solution for it.


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Petrifying the air

Give it enough time and most of the carbon dioxide that humanity is pumping into the air will be taken back by the planet. CO2 dissolves in rainwater and reacts with rocks to form carbon-containing compounds that lock the gas out of the atmosphere. That naturally occurring process, called “chemical weathering,” literally petrifies the air.

The problem - at least for us humans - is that chemical weathering takes millennia to work its carbon-absorbing magic. Humanity doesn’t have that kind of time: The U.N. Intergovernmental Panel on Climate Change says society needs to drastically reduce CO2 emissions by the end of the decade. The situation has gotten so bad that the panel of scientists says we need to develop ways of pulling carbon from the air to avert catastrophe.

So what if we could speed things up? What if, Minik Rosing and other scientists wonder, we exposed more carbon-absorbing rocks to the carbon-laden air? They call that technique “enhanced weathering.”

Most enhanced-weathering proposals involve pulverizing tons of basalt or other rocks and spreading them across the land. But all that crushing would consume an enormous amount of energy that might result in more greenhouse-gas emissions.

That’s where rock flour comes in.

Glaciers flow over the bedrock like a slow-moving river. Over centuries, the tremendous weight of the ice grinds the rock underneath into a fine powder only a few ten-thousandths of a centimeter, or microns, in diameter - finer than most sand found on a beach.

During a Zoom interview, Rosing poured a bit of cream-colored rock flour from Greenland onto his index finger and held it up to the camera.

“If you see here on my finger now,” he said, “there’s absolutely no grains you can see.” Greenland’s ice sheets have already done the hard, dirty work of milling the rocks. Rubbing his fingers, he said it felt as soft as talc, an ingredient in many baby powders.

The fineness of the grains is the flour’s advantage. It gives the substance an enormous surface area to expose to the air, making it an attractive candidate for enhanced weathering. That high surface area is also what gripped the boots of Minik Rosing’s daughter, Johanne Aviaja Rosing, so thoroughly. “Every time I tried to pull one foot up,” she recalled, “the other foot just went down.”

“Other groups or companies are looking at using other types of rocks for enhanced rock weathering, but they have to crush the material,” said Christiana Dietzen, a soil scientist working with Rosing. “Even if they’re doing that, they’re kind of lucky to get to a hundred microns.”

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The power of ‘rock flour’

To test how well rock flour stashes carbon, Rosing and Dietzen hauled about 200 tons of the stuff from Greenland for experiments.

The material packed a one-two punch, according to a pair of papers the researchers published last year: Not only did it suck up carbon when spread over farm fields in southern Denmark, but it also enriched the soil with nutrients and increased the yield of corn and potatoes in the first year of application.

The researchers estimate that, given enough time, spreading rock flour on all agricultural land in Denmark would suck up a quantity of carbon approximately equal to the annual emissions of that country (or of Hong Kong or Syria). Preliminary results show longer-lasting crop yields in nutrient-poor soil in Ghana.

“There’s a novelty to the idea in using pre-ground material,” said Bob Hilton, an Oxford geochemist not involved in the research. “There’s interest in the idea because glacier processes produce huge amounts of this material.”

But there is still a lot of work to do before any farmer begins dusting their fields with rock flour.


So far, the studies have only measured the crop yield and carbon-absorbing effects over short periods of time. The rock flour works best in certain soil - slightly but not too acidic. Rosing and other enhanced-weathering researchers need to measure more precisely how much CO2 their techniques are taking up, so farmers and others can eventually make money by selling carbon credits.

“It is not easy to figure out how much CO2 has been sequestered in these field operations,” said Susan Brantley, a geochemist at Pennsylvania State University.

Even though no extra energy needs to go into crushing rock flour, it may come with other environmental costs, such as the greenhouse-gas pollution from shipping it across the ocean and impacts on local ecosystems. And even though Greenland’s frigid waters slow the weathering process, rock flour would have already reacted with some CO2.

“Just because these areas look like sediment factories, if you like, doesn’t mean that the grains haven’t already reacted in some way,” Hilton said.

Last year, Rosing helped found the Rock Flour Company. It has raised $2 million and plans to seek approval from the government of Greenland, an autonomous territory of Denmark, to mine and export rock flour. The process of assessing the environmental and social impacts will take years.

“We have quite high environmental standards regarding our mining sector,” said Naaja Nathanielsen, a minster overseeing minerals in Greenland. “It’s a long process, and it’s not a one size fits all.” But she added that she expects scooping rock flour from the shore would have a lower impact than hard-rock mining for nickel and other minerals that other firms are eyeing in Greenland.

“It’s a tension I think about a lot in that I would love to accelerate the process from a personal perspective in order to have climate impact,” said Clive Eley, a Rock Flour Company board member. “But at the same time, you don’t want to accelerate to the detriment of its full potential.”

Rosing hopes rock flour not only alters the trajectory of climate change, but changes the perception of his birthplace, too. He was born in a small settlement on Nuuk Fjord in West Greenland, not far from where he extracted his magic mud.

“Greenland is already seen as the parking lot of problems,” he said. “The Arctic is seen as a victim with no agency, and it would be really nice for Greenland to be relevant to the world in a positive way.”