Researchers have turned carbon dioxide (CO2) into solid rock by injecting volcanic basalt rock with pressurised liquid CO2, and letting natural chemical reactions trigger the transformation.

The technique, which takes two years to achieve, gives scientists another option for capturing and storing the excess CO2 humans are pumping into the atmosphere – and could one day be scaled up to take significant levels of carbon out of circulation.

The research was conducted by a team from the the US Department of Energy's Pacific Northwest National Laboratory (PNNL), and builds on a similar experiment in Iceland earlier this year, which dissolved CO2 in water and injected it into a basalt formation.

In the latest study, undiluted CO2 was used, and much more of it was stored at once: 1,000 tonnes of fluid carbon dioxide.

The PNNL team had already shown that the chemical reactions could happen in lab conditions, but until now, they didn't know how long the reactions would take in a real-world setting.

"Now we know that this mineral trapping process can occur very quickly, it makes it safe to store CO2 in these formations," says researcher Pete McGrail. "We know now that in a short period of time the CO2 will be permanently trapped."

In their field study, the researchers injected the fluid carbon dioxide into hardened lava flows some 900 metres (2,952 feet) underground, near the town of Wallula in Washington State.

At that depth, minerals including calcium, iron, and magnesium make up part of the basalt formations. These minerals become unstable, and then dissolve in the acidic conditions created by the CO2.

The dissolving minerals react with the carbon dioxide to form the carbonate material ankerite, which is similar to limestone, and binds with the basalt.

You can see the end results marked by the white areas in the sample shown in the image above.

While turning CO2 into rock isn't a new idea, scientists are working to make the process quicker and more efficient – as original estimates predicted the reactions could take thousands of years. 

Basalts are found all around the world, including North America and Iceland, which is one of the reasons the technique could be an effective way of dealing with excess CO2.

But before we get too excited about sending all of our excess carbon underground, there are still some issues to resolve.

Capturing carbon remains relatively expensive, and scientists aren't sure how well these experiments will ultimately scale up, particularly as more and more existing basalt formations turn into carbonate.

Then there's the question of accurately gauging how much storage capacity the basalt actually offers.

Scientists have recently found that our calculations of another carbon storage method – soil's natural capacity to absorb and store CO2 – had been overestimated by as much as 40 percent.

So we need to wait for further tests to be carried out before declaring all our carbon worries over, but it's a promising area of research, and it gives us an end result that's difficult to beat: CO2 in a safe and solid form deep below the ground, where it can't do any harm to the atmosphere or oceans.

"[The CO2] can't leak, there's no place for it to go, it's back to solid rock," explains McGrail. "There isn't a more safer or permanent storage mechanism."

The findings are published in Environmental Science & Technology Letters.