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USGS: Deep-Sea Sulfide Rocks Can Spontaneously Catch Fire in the Lab
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In brief
A USGS-led team reports that iron-sulfide-rich rocks collected from the seafloor off the Oregon–California coast spontaneously combusted after being brought into a laboratory. The study, published in Scientific Reports, says the reaction poses a possible fire hazard for future deep-sea mining operations that has not been fully accounted for in current safety rules.
Scientists led by the U.S. Geological Survey (USGS) have found that certain metal-sulfide rocks pulled from the deep ocean floor can heat up and catch fire on their own once exposed to air in a laboratory setting. The finding comes from samples collected in the Escanaba Trough, a hydrothermal area along the southern Gorda Ridge in the U.S. Exclusive Economic Zone off the Oregon–California border.
What happened
According to the USGS, researchers gathered 57 hydrothermal rock samples from four regions of the Escanaba Trough using a remotely operated vehicle during a 2022 expedition. The work involved collaboration with the Bureau of Ocean Energy Management (BOEM) and the National Oceanic and Atmospheric Administration (NOAA), according to ScienceAlert's reporting on the study.
Most samples contained metal sulfides rich in copper, zinc and iron, but two of them spontaneously combusted after being freeze-dried and crushed in the lab, ScienceAlert reported. The USGS said the combusting rocks were primarily made of nanocrystalline marcasite, an iron-sulfide mineral, which had replaced an earlier mineral called pyrrhotite. Ocean News also listed isocubanite and chalcopyrite among minerals present in the deposits.
Why the rocks ignited
The USGS explains that marcasite underwent exothermic oxidation when exposed to oxygen, releasing heat. ScienceAlert reported that this reaction pushed sample temperatures above 100°C, with the original rock material becoming fully oxidized. Mechanical actions such as grinding during lab processing appear to have supplied additional energy that helped trigger the self-heating, according to ScienceAlert.
The research, published in the journal Scientific Reports under the title "Spontaneous combustion of metal sulfide minerals and implications for seafloor massive sulfide mining," concludes that the nanocrystalline, altered form of the mineral is less stable than its primary precursor, making it more prone to this kind of reaction.
Why it matters for deep-sea mining
The study's authors frame the discovery as a hazard relevant to any future mining of seafloor massive sulfide (SMS) deposits — mineral-rich formations around hydrothermal vents that have drawn interest as potential sources of copper, zinc and other metals. Bringing such rocks to the surface exposes them to sharply different oxygen levels, temperature, pressure and moisture compared with the seafloor, conditions the USGS says could trigger self-heating.
The finding identifies a potential hazard that could be important to understand if seafloor massive sulfide deposits are ever mined.
Ocean News reported that the study stresses the mineral characteristics of individual deposits matter when assessing risk, and that existing safety protocols developed for land-based sulfide mining may not directly apply to material recovered from the deep sea. ScienceAlert noted the researchers specifically warned the reaction could cause rapid-onset fires aboard ships handling such cargo.
What is not yet known
The reported findings are based on a limited number of combustion events — two out of 57 samples — from one site, the Escanaba Trough. The sources reviewed do not state how frequently this reaction would occur at other hydrothermal sites with different mineral compositions, nor do they outline specific new safety protocols that mining operators would need to adopt. The study instead calls for dedicated risk assessments and engineering controls tailored to SMS extraction, according to Ocean News.
Key facts at a glance
- Samples collected in 2022 from Escanaba Trough, off the Oregon–California coast, via remotely operated vehicle.
- 57 hydrothermal rock samples gathered; 2 spontaneously combusted during lab processing.
- Combustible rocks were composed mainly of nanocrystalline marcasite, which replaced earlier pyrrhotite.
- Sample temperatures exceeded 100°C during the reaction, per ScienceAlert's reporting.
- Study published in Scientific Reports; led by USGS with involvement of BOEM and NOAA.
Questions
- What minerals caused the rocks to catch fire?
- The combustion was linked mainly to nanocrystalline marcasite, an iron-sulfide mineral that had replaced an earlier mineral called pyrrhotite, according to the USGS.
- Where were the rocks collected?
- They came from the Escanaba Trough, a hydrothermal area along the southern Gorda Ridge in the U.S. Exclusive Economic Zone off the Oregon–California border, collected via remotely operated vehicle in 2022.
- How hot did the rocks get?
- ScienceAlert reported that affected samples reached temperatures exceeding 100°C and were fully oxidized during the reaction.
- Does this mean deep-sea mining is unsafe?
- The study identifies a potential fire hazard that researchers say merits further risk assessment; it does not state that all seafloor sulfide mining is unsafe, and notes existing terrestrial mining safety rules may not directly apply.
- Who conducted the research?
- A USGS-led team carried out the study, with collaboration from the Bureau of Ocean Energy Management and NOAA, published in the journal Scientific Reports.
Sources
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