Most discussions of deep-sea mining focus on environmental disruption or regulatory uncertainty. A new study adds a different kind of risk to the list: some seafloor rocks can catch fire when brought to the surface.
That finding comes from a USGS-led research team whose work was published in Scientific Reports in late 2026. The study is narrow in scope and explicit about its limits. However, it raises a genuine safety question that the industry has not previously had to address, and it applies specifically to one of the three main categories of deep-sea mineral deposit.
Understanding which category it affects — and why — is the starting point for any serious evaluation of the finding.
Three Deposit Types, Three Different Risk Profiles
Deep-sea mining exploration currently targets three distinct types of mineral deposit. Each forms through different geological processes, occurs at different depths, and contains a different mix of economically valuable metals.
Polymetallic nodules lie scattered across abyssal plains at depths of 4,000 to 6,000 metres. They contain nickel, copper, cobalt and manganese, and have attracted the most commercial attention to date. The Clarion-Clipperton Zone in the Pacific is the most extensively explored area.
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Cobalt-rich crusts form on the flanks of seamounts at depths of roughly 800 to 2,500 metres. They contain cobalt, platinum, vanadium and manganese, and are harder to extract because they are bonded to underlying rock.
Seafloor massive sulfides (SMS) form at hydrothermal vents, where superheated fluids moving through the Earth’s crust carry dissolved metals upward and deposit them as those fluids meet colder seawater. These deposits contain copper, zinc, gold and silver, and occur at depths of approximately 1,400 to 3,700 metres along mid-ocean ridges.
The combustion risk identified by the USGS study applies exclusively to SMS deposits. It does not affect polymetallic nodules or cobalt-rich crusts, which have different mineralogies and form through entirely different processes.
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What the Researchers Found
Using a remotely operated vehicle, the research team collected 57 samples of metal sulfide-rich rocks from four locations across the Escanaba Trough, a hydrothermal area along the southern Gorda Ridge off the US West Coast. The study was conducted in collaboration with the Bureau of Ocean Energy Management and the National Oceanic and Atmospheric Administration.
When the samples were crushed and freeze-dried in the laboratory — standard processing steps — two of the 57 samples underwent spontaneous combustion upon exposure to air.
The researchers then analysed what made those two samples different. The combusting rocks were composed primarily of nanocrystalline marcasite, a form of iron sulfide that had replaced the blades of an earlier mineral called pyrrhotite, preserving their original shape in the process.
Marcasite is a metastable phase, meaning it exists in a state that is not thermodynamically stable over geological time. The study found it to be especially prone to rapid oxidation. When sulfide minerals oxidise, they release heat. Under the right conditions, that heat can trigger further self-heating — and, as these samples demonstrated, eventual ignition.
The study also used thermodynamic calculations to confirm that metal sulfide oxidation produces more heat when oxygen is the oxidant. That is directly relevant to the surface conditions that SMS rocks would encounter during mining operations: exposure to both seawater and open air.
Why This Differs from Land-Based Sulfide Mining
Sulfide deposits are not unique to the seafloor. Ancient seafloor that has been exposed on land is mined in many places as volcanogenic massive sulfide (VMS) deposits. Those operations have their own documented history of self-heating and spontaneous combustion events.
However, the researchers found that SMS rocks from the Escanaba Trough have mineralogical characteristics distinct enough from their land-based equivalents that terrestrial safety protocols may not translate directly. The pressure, temperature and chemical environment on the seafloor differ significantly from surface conditions. When SMS material is brought to the surface, it experiences a sudden shift in all three.
The USGS statement put the operational implication plainly: 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. Those changes could affect how sulfide minerals react and how much heat they generate.
What the Study Does Not Say
The researchers are precise about the boundaries of their finding, and those boundaries matter.
The study does not establish that all SMS deposits will combust. It documents the behaviour of specific samples from one location — the Escanaba Trough — and identifies the mineralogical conditions that led to ignition in two of 57 samples collected there.
Different SMS deposits will have different mineral compositions. Whether marcasite is present, in what form, and in what concentration will vary from site to site. Those variables determine whether a given deposit carries the same risk.
Consequently, the finding is best understood as a category-level alert, not a site-specific verdict. It identifies a mechanism that the industry should assess for each potential SMS development, not a universal characteristic of every hydrothermal deposit on the ocean floor.
The Regulatory and Exploration Context
The study arrives at a moment when SMS exploration is expanding. The International Seabed Authority has issued seven exploration contracts for polymetallic sulphides in areas including the Northern Mid-Atlantic Ridge, the Southwest Indian Ridge and the Central Indian Ridge. India holds two ISA contracts for sulphide exploration and currently has the largest sulphide exploration area allocated by the ISA.
None of those contracts authorise extraction. The ISA Mining Code, which would govern commercial operations, remains under negotiation. However, the exploration work underway involves collecting exactly the kind of material the USGS study examined — SMS rocks that will be brought to the surface, transported to vessels and processed in laboratories.
The study’s recommendation is therefore applicable right now, not only at some future point when commercial mining begins. The researchers call for dedicated risk assessments for SMS deposits and engineering controls specifically designed to reduce the possibility of self-heating and spontaneous combustion during handling.
Neither TMC nor Deep Sea Minerals responded to comment requests on the study, according to earlier coverage.
What This Means for the Industry
The broader deep-sea mining debate has centred on environmental disruption, jurisdictional disputes and the pace of regulatory development. The combustion finding adds a distinct dimension: operational safety during extraction and transit.
A fire aboard a mining vessel or processing ship would carry consequences extending well beyond the immediate incident. It would affect permitting timelines, insurance requirements and investor confidence. It would also invite closer regulatory scrutiny of every aspect of an already closely watched sector.
The USGS finding does not make SMS mining unviable. The same risk exists, in some form, in land-based VMS mining, and the industry has developed techniques to manage it. What it does mean is that those techniques cannot simply be carried over unchanged. SMS material from the deep seafloor requires its own safety evaluation.
What the Industry Should Watch
Mineralogical characterisation: Exploration programmes targeting SMS deposits should include analysis of marcasite content and mineral stability, not only metal concentrations and deposit size.
Risk assessment protocols: The USGS recommendation for dedicated SMS-specific risk assessments should inform how exploration contractors design sample handling and processing procedures now, not after a mining code is adopted.
Regulatory uptake: Whether the ISA and national maritime authorities incorporate the study’s findings into guidance for exploration contractors is an early indicator of how seriously the safety question will be treated institutionally.
Site-by-site differentiation: The study’s limits are important. An SMS deposit with different mineralogy may carry lower risk. Blanket assumptions in either direction — that all SMS rocks will combust, or that none will — are not supported by the evidence.
Land-based comparison: VMS operations have experience with sulfide self-heating. The gap between those established practices and what SMS conditions require is the practical engineering question the industry now needs to investigate.
A Narrow Finding With Broad Implications
Two samples out of 57 caught fire in a laboratory. That is a specific result, not a generalised catastrophe. The researchers present it as what it is: evidence of a mechanism that exists, that has not previously been systematically investigated for SMS deposits, and that carries direct implications for any operation that brings this material to the surface.
The finding adds something that most coverage of deep-sea mining has not yet addressed: that the chemical behaviour of SMS rocks under surface conditions is not fully understood, and that understanding it is a prerequisite for safe operations, not an afterthought.
That gap between what the seafloor holds and what happens when those materials reach the surface is, ultimately, the same gap that makes every aspect of deep-sea mining harder than it looks from a mineral inventory alone.
Sources:
- Spontaneous Combustion of Metal Sulfide Minerals — USGS Publication Page
- Study in Scientific Reports (Nature.com)
- Escanaba Trough Rocks Reveal Hidden Fire Risk — ECO Magazine
- Deep-Sea Mining Faces Surprise Risk of Rocks Catching Fire — Mining.com
- USGS News Release
- ISA Contract List — DSM Observer
- India Signs Second ISA Sulphide Contract — Skillings.net
