Natural CO2 sequestration.

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. . .

Another rock, common volcanic basalt, also reacts with CO2 . . .

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. . .

One model is to capture CO2 directly from power-plant smokestacks or other industrial facilities, combine it with water and pipe it into the ground, as in the upcoming Iceland project. Lackner and his colleagues are also working on a process using "artificial trees" that would remove CO2 already emitted into the atmosphere.

Combining rocks and carbon dioxide could provide an added benefit, as Krevor points out. For decades, some large U.S. 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.

There's no information given here about costs, but other work pegs the costs as no greater than current estimates for emissions reductions, and potentially much less.