Two rocks hauled up from the seafloor off northern California heated themselves past the boiling point of water and burned up inside a U.S. Geological Survey laboratory. Scientists now say the reaction behind that event is a hazard that will need special attention if mining of similar deep-sea deposits ever moves forward.

The findings, published in Scientific Reports by USGS research oceanographer Amy Gartman and colleagues, trace the fires to an unstable form of iron sulfide that reacts vigorously with oxygen. According to the USGS publication summary, the paper was published Sept. 28, 2026.

The concern reaches beyond one lab. Seafloor sulfide deposits hold copper, zinc, gold, silver and critical minerals that the United States considers vulnerable to supply disruptions, and the rocks in this study came from inside the U.S. Exclusive Economic Zone.

A Spreading Ridge 200 Miles Offshore

The samples came from the Escanaba Trough, a seafloor spreading center on the southern Gorda Ridge, almost 200 miles off northern California. According to a USGS expedition summary, the trough lies about 3,300 meters (roughly 10,800 feet) deep and is the only mid-ocean spreading center within the U.S. Exclusive Economic Zone.

In 2022, a USGS-led team working with the Bureau of Ocean Energy Management and NOAA spent three weeks there aboard the research vessel Thomas G. Thompson. The remotely operated vehicle Jason collected 57 hydrothermal rock samples from four parts of the trough, most of them rich in metal sulfides.

"Any inventory of global mineral resources is incomplete without considering marine minerals," Gartman, who served as the expedition's chief scientist, said at the time.

Marcasite, Oxygen and Heat

The trouble started back on land. Two of the samples spontaneously combusted after being freeze-dried and crushed for analysis, pushing temperatures above 100 degrees Celsius (212 degrees Fahrenheit) and completely oxidizing the original rock, ScienceAlert reported.

The team ruled out tar or petroleum as the fuel. Instead, the combusting rocks turned out to be made mostly of nanocrystalline marcasite, an iron sulfide that had replaced an earlier mineral called pyrrhotite. The authors describe this altered material as less stable than the rock it replaced, an in-between phase that is especially prone to reacting with oxygen.

Sulfide oxidation releases heat. When the mineral is broken, ground, or crushed, fresh surfaces meet air, the reaction speeds up, and the rock can warm itself until it ignites. The researchers' thermodynamic calculations show the reaction releases more heat when oxygen, from air or seawater, is the oxidizing agent.

The researchers also compared the burned samples with rocks from the same site that did not ignite, measuring how each lost mass and released energy as it was heated. That side-by-side comparison is what tied the fires to marcasite rather than to sulfides in general — most of the other samples were also sulfide-rich but stayed stable.

The handling steps matter for anyone who might one day work with this material. Freeze-drying strips out water, and crushing multiplies the surface area exposed to air. Both are routine in a laboratory, and similar steps would occur at far larger scale if seafloor ore were ever dried, ground and stockpiled.

The USGS said the finding points to a "potential hazard that could merit special consideration if deep-sea mining of seafloor massive sulfide deposits moves forward."

Land-Mining Rules May Not Transfer

Self-heating of sulfide ore is a known safety problem in mines on land. The new study suggests deep-sea deposits could behave differently. The authors conclude that modern seafloor sulfide deposits, because of their distinct mineralogy, texture and setting, may have a greater potential for self-heating than the ancient deposits now mined on land.

That matters because any future operation would dredge, crush, store and ship large volumes of this material. According to Ocean News & Technology, the study calls for dedicated risk assessments for seafloor sulfide deposits and engineering controls designed to limit self-heating, noting that safety approaches developed for deposits on land may not apply directly.

The evidence has clear limits. This is a laboratory study of samples from one site, and only two of 57 samples ignited. The study does not show that all seafloor sulfide deposits will spontaneously combust, and it does not estimate how often fires might occur during real mining, shipping or storage — those questions remain open.

Where Deep-Sea Mining Stands

No commercial mining of seafloor sulfide deposits has taken place yet. The International Seabed Authority has issued exploration contracts for sulfide minerals on the Northern Mid-Atlantic Ridge and the Southwest Indian Ridge, ScienceAlert noted, and environmental groups have raised concerns about the effects of mining on deep-sea ecosystems that are already under pressure.

Interest in U.S. waters centers on critical minerals. The 2022 expedition summary notes that sulfide deposits can contain zinc, nickel, and antimony, which the government considers essential to the economy or national security and vulnerable to supply-chain disruption.

For most households, the finding carries no direct risk. The rocks pose no hazard on the seafloor, where there is little oxygen to fuel the reaction, and the event happened under specific lab conditions. Its practical weight falls on the agencies that would permit seafloor mining, the companies that might pursue it, and the port, shipping and processing workers who would handle the ore.

The next steps rest with regulators and researchers. Any federal leasing process for seafloor minerals would involve environmental and safety review, and the authors' recommendations for risk assessment and engineering controls give that review a specific question to answer. The bottom line is that a rare lab fire has exposed a real material-handling hazard, and the time to plan for it is before mining starts.

What Readers Want to Know

What happened to the deep-sea rocks?

Two of 57 rock samples collected from the Escanaba Trough in 2022 spontaneously combusted after being freeze-dried and crushed in a USGS lab, reaching temperatures above 212°F.

Where is the Escanaba Trough?

It is a seafloor spreading center on the southern Gorda Ridge, almost 200 miles off northern California and about 10,800 feet deep, inside the U.S. Exclusive Economic Zone.

Why did the rocks catch fire?

They were made mostly of nanocrystalline marcasite, an unstable iron sulfide that releases heat as it reacts with oxygen. Crushing exposed fresh surfaces to air and sped up the reaction.

Is deep-sea mining happening now?

No commercial mining of seafloor sulfide deposits has occurred. The International Seabed Authority has issued exploration contracts in the Atlantic and Indian oceans.

Does this pose a risk to the public?

Not directly. The hazard applies to handling crushed sulfide material in air, such as during mining, shipping, storage, and processing.

What do the researchers recommend?

Dedicated risk assessments for seafloor sulfide deposits and engineering controls to limit self-heating, since safety rules built for land mines may not apply directly.