USGS Finds Deep Sea Rocks Can Ignite After Recovery

Some mineral-rich rocks recovered from the deep ocean can heat up and ignite during processing, according to a US Geological Survey study of samples collected from the Escanaba Trough off the Oregon-California border. The findings identify a potential fire hazard for future seafloor mining, where recovered materials encounter oxygen and environmental conditions different from those on the ocean floor.

The USGS-led research examined 57 hydrothermal rock samples collected in 2022 by a remotely operated vehicle across four areas of the Escanaba Trough. The deposits contain metal sulfides, including copper, zinc, and iron, according to the study.

Two samples ignited during laboratory processing after freeze-drying and crushing, reaching temperatures above 100 C and becoming fully oxidized, according to findings reported by ScienceAlert and the USGS.

The researchers identified nanocrystalline marcasite, an iron sulfide mineral, as the main component of the combustible samples. The mineral can become unstable when exposed to oxygen, while crushing and other mechanical forces may trigger oxidation and self-heating.

The USGS described the discovery as a potential safety concern if extraction of seafloor massive sulfide deposits advances.

"A potential hazard that could merit special consideration if deep-sea mining of seafloor massive sulfide deposits moves forward" was the USGS's description of the risk.

The agency also highlighted the possibility that heat-generating chemical reactions could create dangerous conditions during the handling and transport of recovered materials.

"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," the USGS said.

To investigate the differences between combustible and noncombustible material, the researchers compared mineral composition, chemical characteristics, and thermal behavior across the samples. Their findings indicated that the properties of individual deposits are important when evaluating potential hazards.

The agency emphasized that the behavior of deep-sea minerals may differ from that of comparable materials extracted on land.

"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," the USGS said.

Seafloor sulfide deposits develop around hydrothermal vents, where hot fluids release metals that accumulate on the ocean floor. International exploration of these resources has increased, although commercial mining has not begun, according to the source report.

The transition from deep-ocean conditions to the surface can expose recovered rocks to substantial environmental changes, including differences in oxygen availability, temperature, pressure, and moisture.

"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 said.

Those changes could influence the chemical reactions occurring within sulfide minerals and the amount of heat they release.

"Such changes could affect how sulfide minerals react and how much heat they generate," the USGS said.

The study does not establish that all seafloor massive sulfide deposits are combustible. The USGS said the findings point to the need for deposit-specific risk assessments and appropriate safety measures if future mining operations proceed.