“No one ever talks about monitoring,” Dr. Gislason said. “This is where we score very highly.”

Mineral carbonation can occur in many kinds of rock, but often it is extremely slow. The CarbFix approach accelerates the process by injecting into basalt, a very reactive rock. And few places in the world can top Iceland for basalt; the country is made almost entirely of it. The island sits atop the Mid-Atlantic Ridge, the boundary between two of the planet’s largest tectonic plates, where basaltic magma rises from deep within the earth to form new crust.

What Iceland lacks, however, are significant CO 2 emissions. Geothermal generating stations, like the Hellisheidi plant across a road from the CarbFix site, do emit some CO 2 — it and other gases bubble up naturally along with the hot water and steam used to generate electricity — but the amounts are only about 5 percent of the emissions from an equivalent natural-gas plant.

“We can never do large-scale CO 2 injection” in Iceland, Dr. Aradottir said. But because of the geology, the country is an ideal place to demonstrate to potential users like power companies that the process works. (Since the initial test, CarbFix has scaled up its process and is now injecting 10,000 tons of gas per year from the plant at a nearby site.)

Large basalt deposits are found in other locales, including the Pacific Northwest in the United States. There, at a site in the Columbia River basin near Wallula, Wash., a similar test project — the only other one in the world — is also in an analysis phase, having completed the injection of 1,000 tons of carbon dioxide in 2013.