Deep Sea Rocks Ignite During Escanaba Trough Study

USGS researchers found that some metal sulfide rocks collected from the Escanaba Trough seafloor off the Oregon-California border can spontaneously heat and combust after being brought to the surface and processed, revealing a potential hazard for future deep-sea mining.

The samples were collected in 2022 during a USGS investigation with the Bureau of Ocean Energy Management and NOAA. Researchers used a remotely operated vehicle to collect 57 hydrothermal rock samples from four regions of the trough.

According to USGS, two samples ignited during laboratory processing after freeze-drying and crushing. The reactions produced temperatures above 100 °C and completely oxidized the original material.

The USGS identified the finding as a safety concern for possible seafloor massive sulfide mining operations.

"The potential hazard that could merit special consideration if deep-sea mining of seafloor massive sulfide deposits moves forward" was highlighted by the USGS in describing the discovery.

The study found the combustible rocks were mainly composed of nanocrystalline marcasite, an iron sulfide form that can become unstable when exposed to oxygen. Researchers compared the mineral structure, chemistry and thermal behavior of samples that combusted with those that did not.

"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" was stated in the recent research article.

Seafloor massive sulfide deposits form around hydrothermal vents and can contain metals including copper, zinc and iron. The USGS said these deposits may react differently from similar mineral deposits mined on land.

"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 explained.

The agency described how environmental changes after removal from the deep ocean could influence mineral reactions and heat production.

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

The research does not show that all seafloor sulfide deposits are combustible, but it indicates that individual mineral characteristics must be assessed before any future extraction activities.