Volcanic rocks can quickly turn pollution into stone

Transforming ‘greenhouse’ gas into solid rock may be better than other methods at fighting climate change

Greenhouse gases are a major contributor to climate change. But a new technique can turn one of these gases into solid rock before it floats up into the atmosphere. If widely adopted, such a move might just help slow global warming.

As part of a test program, researchers in Iceland injected gaseous carbon dioxide (CO2) into basalt (Bah-SALT). It’s the type of rock that lava turns into when it cools quickly. About two years later, the scientists checked back. More than 95 percent of the gas had turned to stone, they found.

Solidifying the gas effectively locked it away. Now the gas is no longer a threat to Earth’s climate. Such a relatively fast process could help counteract the world’s greenhouse-gas emissions, the researchers conclude.

“It’s working,” says Jeurg Matter. “It’s feasible and it’s fast enough to be a permanent solution for storing CO2 emissions,” he adds. As a geochemist, Matter studies the chemical composition of solid material on Earth — and how it changes. He works at the University of Southampton in England.

Matter’s group published its new results in the June 10 Science.

What makes basalt special

People have tried different approaches for storing CO2. Many rely on pumping the gas into rocky vaults underground. But sometimes the gas can leak back to the surface. That’s unlikely to happen with basalt, however. As much as one-fourth of basalt is made up of elements that react with CO2.. This allows them to form solid minerals, such as limestone.

This process is called mineralization. It occurs naturally as basalt is exposed to harsh weather. Researchers had thought this process took hundreds to thousands of years. That would make it far too slow to be useful for combating the immediate threat of climate change.

For their new experiment, Matter and his colleagues mixed groundwater with 230 tons of CO2 . The gas had been emitted by a geothermal-energy power plant. Combining that gas with the water created a mixture that was a lot like seltzer water. The researchers injected this bubbly liquid into basaltic rock some 400 to 800 meters (about 1,300 to 2,600 feet) below ground. When the team sampled that rock about two years later, almost all of the CO2 had mineralized.

Turning CO2 emissions into minerals is not cheap. It costs $17 per ton. That's roughly twice as much as existing storage methods. But it doesn’t require long-term monitoring to prevent leaks, Matter notes. A second benefit: This technique requires only water and basalt. And in theory, he adds, “We have enough basalt globally to take care of all anthropogenic CO2 emissions.”

A second group’s work seems to back up the new findings. Peter McGrail is a geochemist at the Energy Department’s Pacific Northwest National Laboratory in Richland, Wash. He and his colleagues did similar tests using pure CO2, no groundwater added. They haven’t yet published their results. But McGrail says their findings resemble those that Matter’s group has just reported.

Power Words

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anthropogenic    An adjective that describes a human influence on something. It was coined by putting together the prefix “anthro,” meaning human, and suffix “genic,” meaning caused by.

atmosphere   The envelope of gases surrounding Earth or another planet.

basalt   A fine-grained rock that forms when lava cools quickly.

carbon dioxide (or CO2)   A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

climate   The weather conditions prevailing in an area in general or over a long period.

climate change   Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

geochemistry   A science that deals with the chemical composition of and chemical changes in the solid material of Earth or of another celestial body (such as the moon or Mars). Scientists who study geochemistry are known as geochemists.

geothermal energy   The internal heat of the Earth or other planet. It can be tapped as an energy source to heat buildings at the surface.

greenhouse gas    A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

groundwater   Water that is held underground in the soil or in pores and crevices in rock.

lava    Molten rock that comes up from the mantle, through Earth’s crust, and out of a volcano.

limestone     A natural rock formed by the accumulation of calcium carbonate over time, then compressed under great pressure. Most of the starting calcium carbonate came from the shells of sea animals after they died. However, that chemical also can settle out of water, especially after carbon dioxide is removed (by plants, for instance).

mineral     The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns).

mineralization      The process by which gas and other organic matter turns into minerals.

seltzer water      Another name for the soft drink known as carbonated water or club soda.