The emerging industry of deep-sea mining could end up in flames after researchers discovered that some rocks collected from the bottom of the ocean can heat up and even spontaneously combust once they’re brought to the surface and exposed to air.

In a recent paper published in Scientific Reports, researchers from the United States Geological Survey (USGS) warn that some rocks containing deep-sea minerals may catch fire once at the surface. The findings highlight a potential hazard that needs to be better understood before mining massive sulfide deposits from the seafloor.

Using a remotely operated vehicle, the researchers collected 57 samples of metal sulfide-rich rocks from four different regions in the Escanaba Trough. The hydrothermal area along the southern Gorda Ridge off the U.S. West Coast created deep-sea deposits rich in metals like zinc, copper, and iron.

The rocks were crushed and freeze-dried in the lab, with two of the samples experiencing spontaneous combustion when exposed to air. To figure out how and why the rocks performed this pyrotechnic trick, the researchers tested how the samples responded to increasing temperatures. They then compared the chemical composition and mineralogy between the combustable rocks and other samples.

The combusting rocks were composed primarily of nanocrystalline marcasite, a form of iron sulfide that appears to be unstable when exposed to oxygen. The use of heat and other forces, like crushing, breaking, and grinding the rocks, can supply enough energy to initiate oxidation. That can then lead to the rock samples self-heating.

The discovery offers a dangerous precaution to the industry of deep-sea mining.

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