ROCKS THAT SET THEMSELVES ON FIRE: Why Deep-Sea Minerals From Off Oregon Burst Into Flames in a USGS Lab
Quick answer: In a U.S. Geological Survey (USGS) lab, two rocks collected from hydrothermal vents on the seafloor off the Oregon–California border heated past 100 °C and burned up on their own after being freeze-dried and crushed. A study published in Scientific Reports traced the cause to an unstable, nanocrystalline form of the iron sulfide mineral marcasite, which releases heat as it reacts with oxygen. The researchers say this is a hazard worth assessing if deep-sea mining of these deposits ever goes ahead.
Rocks are not supposed to catch fire by themselves. Yet that is exactly what happened to two samples brought up from the deep Pacific by a USGS-led team, and the story has been spreading fast online this week, with plenty of people joking that “flaming rocks” were not on their 2026 bingo card. Behind the jokes is a serious piece of science about the minerals that mining companies are eyeing on the ocean floor.
Where the rocks came from
The samples were collected during a 2022 expedition to Escanaba Trough, a seafloor spreading center on the southern Gorda Ridge, off the U.S. West Coast near the Oregon–California border. According to the USGS announcement, it is a hydrothermal area where hot fluids circulating through Earth’s crust carry dissolved metals that settle out when they meet cold seawater.
Over time, that process builds up seafloor massive sulfide (SMS) deposits. They can contain copper, zinc, iron and other economically important elements, which is why they are increasingly studied as possible future sources of minerals.
The team used a remotely operated vehicle to gather 57 hydrothermal rock samples from four parts of the trough. Forty-one of them contained metal sulfides as a major mineral, according to the peer-reviewed paper.
What happened in the lab
Back on land, the rocks were processed in the usual way: freeze-dried, then crushed. Two of the sulfide-rich samples did something unexpected. When they were crushed under normal atmospheric conditions, they began to heat up on their own, reaching temperatures above 100 °C and oxidizing completely. The study reports that after combusting, the original minerals had been transformed into maghemite, an iron oxide.
Two out of 57 may sound like a small number. But in a laboratory, an unplanned fire from a sample sitting on the bench is a safety event, and it was enough for the team to ask a bigger question: why did these rocks burn when others from the same place did not?
The culprit: an unstable form of marcasite
To find out, the researchers compared the combusting samples with rocks from the same area that stayed calm. They looked at mineralogy, chemical composition and how each sample behaved as it was heated, using thermogravimetric analysis, which tracks changes in a sample’s mass as temperature rises.
The difference was clear. The rocks that burned were made mostly of nanocrystalline marcasite, a form of iron sulfide, that had replaced the blade-shaped crystals of an earlier mineral called pyrrhotite while keeping their original shape. Geologists call this kind of replacement a pseudomorph.
Why tiny crystals matter
Minerals usually become more stable over time as they recrystallize. The USGS paper argues that this altered marcasite is the opposite: a “metastable” in-between phase that is less stable than the mineral it replaced. Its extremely fine crystals also give it a lot of surface area for oxygen to attack.
As the USGS summary puts it, earlier work had already shown that oxidizing sulfide minerals can release heat. What this study adds is that marcasite, which is not often singled out as a hazard, may be especially unstable. Under the right conditions, the heat from oxidation feeds further heating, and the reaction runs away.
Oxygen makes it worse
The team also ran thermodynamic calculations on different sulfide oxidation reactions. They found that the reactions release more heat when oxygen is the oxidant, as it is in seawater or air. That matters because a rock that is stable in the deep, cold, high-pressure environment of a vent field faces very different conditions once it is pulled to the surface.
Why this matters for deep-sea mining
Similar sulfide deposits are already mined on land, where ancient seafloor has been pushed up and exposed. These are known as volcanogenic massive sulfide (VMS) deposits. But the researchers say the minerals and setting of modern seafloor deposits are different enough that safety rules written for land mines may not transfer directly to material dredged from the ocean floor.
If SMS mining happens, rocks brought up from the seabed would suddenly meet air, warmer temperatures, lower pressure and different moisture. The study abstract concludes that the mineralogy, texture and setting of modern deposits suggest “a greater potential for self-heating” than their ancient land-based counterparts.
The authors’ recommendation is practical rather than alarmist: carry out dedicated risk assessments for SMS deposits and design engineering controls that reduce the chance of self-heating and spontaneous combustion, for example during storage and transport.
What the study does not say
The researchers are careful to stress that the work does not show that all seafloor sulfide deposits will spontaneously combust. It documents what happened to specific rocks from Escanaba Trough and explains why the mineral make-up of each deposit matters when judging its risks.
Who did the research
The paper, “Spontaneous combustion of metal sulfide minerals and implications for seafloor massive sulfide mining,” was published online on 28 September 2026 in Scientific Reports. It is led by USGS research oceanographer Amy Gartman of the Pacific Coastal and Marine Science Center in Santa Cruz, California, with co-authors including Jaycee Favela, Katlin Bowman Adamczyk and Kha Cam Phan, according to the USGS publication record.
It is a reminder that the deep ocean still surprises scientists, whether it is a Bronze Age shipwreck found with its cargo still inside or bleached coral that recovers just by touching a healthy twin. And as with the medieval supernova seen in new detail or the coldest alien world ever photographed, a closer look often turns a curiosity into a real discovery.
Frequently asked questions
Can rocks really catch fire on their own?
Some can. Certain sulfide minerals release heat when they react with oxygen. If that heat builds up faster than it escapes, the material keeps warming until it combusts. In the USGS study, two seafloor rocks heated beyond 100 °C after being freeze-dried and crushed in air.
Which mineral caused the deep-sea rocks to burn?
Nanocrystalline marcasite, a form of iron sulfide that had replaced earlier crystals of pyrrhotite. The researchers describe it as a metastable phase that is less stable than the mineral it replaced.
Where is Escanaba Trough?
It is a seafloor spreading center on the southern Gorda Ridge in the Pacific Ocean, offshore of the border between Oregon and California.
Does this mean deep-sea mining is dangerous?
The study identifies a specific hazard, not a verdict on mining as a whole. The authors say seafloor sulfide deposits should get their own risk assessments and engineering controls, because land-based safety rules may not fully apply.
Will all seafloor sulfide deposits combust?
No. The researchers stress that the findings apply to particular rocks and that the mineral characteristics of each deposit determine its risk.
Sources: U.S. Geological Survey; Gartman et al., Scientific Reports (2026); USGS Publications Warehouse.
Featured image: A hot vent near the top of a 38-m sulfide chimney, Fonualei rift, NOAA Vents Program / MARUM, University of Bremen, public domain, via Wikimedia Commons.