Ultramafic rocks generally form in earth’s mantle, starting some 12 miles under the surface and extending down hundreds of miles. Bits of these rocks—peridotite, dunite, lherzholite and others—may be squeezed to the surface when continental plates collide with oceanic plates, or, less often, when the interiors of continents thin and develop rifts. Because of their chemical makeup, when the rocks are exposed to carbon dioxide, they react to form common limestone and chalk in a process called mineral carbonation.

A map accompanying the report shows that most such rocks are found in and around coastal mountain ranges, with the greatest concentrations in California, Oregon and Washington, and along the Appalachians from New England to Alabama. Some also occur in the interior, including Montana. Worldwide, other formations are scattered across Eurasia and Australia.

Earth Institute scientists are experimenting with ways to speed the natural process of mineral carbonation for use in carbon sequestration. Report lead author Sam Krevor, a graduate student working through the Earth Institute’s Lenfest Center for Sustainable Energy, said “We’re trying to show that anyone within a reasonable distance of these rock formations could use this process to sequester as much carbon dioxide as possible.”

Klaus Lackner, who directs the Lenfest Center, helped originate the idea of mineral sequestration in the 1990s. The US survey is the first of what Lackner hopes will become a global mapping effort.

Another rock, common volcanic basalt, also reacts with CO₂, and efforts are underway to map this in detail as well. The US Department of Energy has been working on a basalt atlas for the northwestern United States as part of its Big Sky Carbon Sequestration Partnership; extensive mapping in Washington, Oregon and Idaho has already been done through Idaho State University.

The major drawback to natural mineral carbonation is its slow pace: normally, it takes thousands of years for rocks to react with sizable quantities of CO₂. But scientists are experimenting with ways to speed the reaction up by dissolving carbon dioxide in water and injecting it into the rock, as well as capturing heat generated by the reaction to accelerate the process.

Juerg Matter, a scientist at Columbia’s Lamont-Doherty Earth Observatory, and his colleague Peter Kelemen are currently researching peridotite formations in Oman, which they say could be used to mineralize as much as 4 billion tons of CO₂ a year, or about 12% of the world’s annual output. And in Iceland, Matter is about to participate in the first major pilot study on CO₂ sequestration in a basalt formation.

In May, he and three other Lamont-Doherty scientists will join Reykjavik Energy and others to inject CO₂-saturated water into basalt formations there. Over nine months, the rock is expected to absorb 1,600 tons of CO₂ generated by a nearby geothermal power plant. Matter and another Lamont-Doherty scientist, David Goldberg, are also involved in a study Pacific Northwest National Laboratory, which will eventually inject 1,000 tons of CO₂ into formations beneath land owned by a paper mill near Wallula, Wash.

Combining rocks and carbon dioxide could provide an added benefit, as Krevor points out. For decades, some large US peridotite formations were mined for asbestos, used for insulation and other purposes. After a link between asbestos and cancer was proven, the substance was banned for most uses, and the mines were closed. Mine tailings left behind, at Belvidere Mountain in Vermont and various sites in California, provide a ready supply of crushed rocks. These potentially hazardous tailings would be rendered harmless during the mineralization process.

Resources

• Mapping the Mineral Resource Base for Mineral Carbon-Dioxide Sequestration in the Conterminous United States