Scientists investigating the possibility of a 'gold rush' on the bottom of the ocean have hit upon an unforeseen 'landmine'.
Researchers at the United States Geological Survey (USGS) are now warning that some deep-sea minerals may spontaneously combust when they are brought to the surface.
In 2022, that happened to two samples of metal-sulfide rocks collected from the deep seafloor.
The event caused temperatures above 100 °C in the lab and completely oxidized the original samples.
An article on the discovery, published in Scientific Reports, is being shared early, before full editing, to give the public faster access to new research.
According to the USGS, this is a "potential hazard that could merit special consideration if deep-sea mining of seafloor massive sulfide deposits moves forward".
"The production of heat through exothermic reactions has not been investigated in seafloor massive sulfide deposits but has the potential to cause rapid onset fires on ships," claims the recent article.
The combustible rocks were collected by the USGS in 2022, during a deep-sea investigation of the Escanaba Trough, in collaboration with the Bureau of Ocean Energy Management (BOEM) and the National Oceanic and Atmospheric Administration (NOAA).
The Escanaba Trough sits in the US Exclusive Economic Zone (EEZ) off the Oregon–California border, where hydrothermal activity has created deep-sea deposits rich in metals such as copper, zinc, and iron.
Using a remotely operated vehicle, researchers collected 57 hydrothermal rock samples from four regions in the trough.
Most of these samples contained metal sulfides as a major mineral.
When researchers freeze-dried and crushed them in the laboratory, however, two of the samples spontaneously combusted.
The discovery that some rocks from the deep may catch fire on land comes at an important time.
The International Seabed Authority (ISA) has granted exploration contracts for sulfide minerals on two deep-sea ridges, the Northern Mid-Atlantic Ridge and Southwest Indian Ridge.
Mining has not yet taken place, but when it does, there is reason to tread cautiously.
Sulfide minerals are special because they can contain precious metals, like gold, silver, copper, or zinc.
But they can also spontaneously combust.
Heavy sedimentation in the deep ocean, for instance, can promote the formation of hydrothermally derived petroleum.
This material can then be hidden within metal sulfide minerals, and on land, it has the ability to self-heat.
"The new research suggests that some seafloor massive sulfide rocks have physical and chemical properties that could make them behave very differently from comparable mineral deposits mined on land," warns a press release from the USGS.
To figure out what sets these combustible rocks apart, researchers compared the mineralogy, chemical composition, and thermal behavior of the samples they collected in 2022.
Tar and petroleum didn't seem to be the problem. Instead, researchers found that the self-combusting rocks were made mostly of nanocrystalline marcasite.
This is a form of iron sulfide that appears to be especially unstable in the presence of oxygen.
The addition of heat and mechanochemical forces, like crushing, breaking, and grinding, can supply energy to initiate oxidation.
When this sample oxidizes, it can lead to self-heating.
This doesn't mean every rock on the ocean floor containing marcasite is a hazard, but it is a potential danger that could help shape safety regulations for a new industry.
"If seafloor massive sulfide mining occurs, rocks brought to the surface would experience conditions dramatically different from those on the seafloor, including exposure to oxygen and changes in temperature, pressure, and moisture," the USGS explains.
"Such changes could affect how sulfide minerals react and how much heat they generate."
Deep-sea mining comes with many warnings, not just for the safety of humans.
Many environmentalists are concerned that this burgeoning new industry may be the last straw for precious ocean ecosystems, already at risk.
The consequences, if we are not careful, could blow up in our faces.
The research has been published in Scientific Reports.
This article was fact-checked by Peter Dockrill and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.