Like a modern-day version of Medusa, an Icelandic project turns enemies to stone. The modern enemy is carbon dioxide, and success relies on the basaltic rocks on which the island sits.

Carbon capture and sequestration (CCS) has been promoted as fossil fuels' savior in a warming world. The idea is to collect carbon dioxide from coal- and gas-fired power stations and pump it into the Earth rather than into the atmosphere.

Special circumstances aside, however, CCS has been an expensive failure. Among the many reasons is the fear the carbon dioxide won't stay down. Any large-scale release would be catastrophic for local populations. Even a slower leakage at rates of more than 0.1 percent per year would destroy the value of the operation.

In 2012, the Hellisheidi power plant in Hengill, southwest Iceland, pumped 230 tonnes (254 tons) of carbon dioxide – mixed with water and 18 tonnes (20 tons) of hydrogen sulphide – into wells drilled 150 to 3,000 meters (500 to 10,000 feet) through basaltic lavas. The carbon dioxide was spiked with extra carbon-14 so that it could be distinguished from naturally occurring gas. Injection has now since increased to 4,500 tonnes (5,000 tons) a year.

^{Hellisheidi is the largest geothermal plant in the world, and far cleaner than coal or gas, but it is aiming to do even better. Kevin Krajick/Lamont-Doherty Earth Observatory}

A study published in Science found that more than 95 percent of the original trial's carbon was converted to rocky carbonate within two years. This astonishingly rapid transformation, the paper notes, “contrasts with the common view that the immobilization of CO₂ as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years.”

The process was found to work even when the carbon dioxide was mixed with a high concentration of hydrogen sulphide, which was thought might interfere with the carbonization process.

The authors attribute the rapid mineralization to calcium, iron, and magnesium ions released by the basalt and the alkaline waters into which the carbon dioxide was injected.

Storing the carbon as a solid, rather than a gas, removes the danger associated with conventional reservoirs such as earthquakes, removal of the holding rock or gas leakage through small pores.

Hellisheidi is a surprising place for such a trial. It is geothermal, running on the heat generated by the volcanic conditions. The carbon dioxide and hydrogen sulphide are volcanic gasses released with the steam that drives the station's turbines. The 36,000 tonnes (40,000 tons) of CO₂ released annually are 5 percent of what an equivalently-sized coal-fired power plant would emit, but Iceland is aiming for complete carbon neutrality.

“In the future, we could think of using this for power plants in places where there's a lot of basalt – and there are many such places," said Professor Martin Stute of Columbia University in a statement. Indeed, one-tenth of continental rock is. Virtually the entirety of the ocean floor is also basalt, although pumping carbon dioxide far out to sea is expensive.

Where basalt and ample water are conveniently available to the point of capture, the authors calculate that a fully developed version of the system might produce a price for CCS that may be as low as $30 per ton, compared to $130 for scaled-up versions of other methods. Yet even this may struggle to compete with renewables in producing cheap, pollution-free electricity.