Why Some Deep-Sea Sulfide Rocks Heat Up in Air

A small number of iron sulfide samples collected from the Escanaba Trough heated and combusted during laboratory handling after contact with air.

Nature & AnimalsOctober 11, 20263 min readHow Sureno writes this

The short version

Some rocks rich in metal sulfides, collected from the seafloor, became hot and combusted during laboratory work after meeting air. ¹³
The reported samples rose above 100°C and were entirely oxidized. ¹
The combusted rocks consisted mainly of nanocrystalline marcasite, an iron sulfide mineral. ³
This suggests a possible handling risk, but does not mean every deep-sea sulfide rock will heat or burn. ¹³

Why can deep-sea minerals heat up in air?

The rocks in this report contained abundant metal sulfides, minerals made from sulfur and metals. When these particular samples were exposed to air during laboratory work, reactions involving oxygen released heat. A reaction that gives off heat is called exothermic. ¹³

The samples that combusted consisted mainly of nanocrystalline marcasite, a fine-grained form of iron sulfide. The research summaries identify this composition as a notable feature, but do not explain in detail why it made the samples react so intensely. ³

What happened to the samples?

Two metal-sulfide rocks collected in 2022 heated during laboratory processing. Their temperatures exceeded 100°C, and the original material was entirely oxidized, meaning it reacted with oxygen and changed chemically. ¹

Researchers also compared them with rocks from the same area that did not combust. The reports describe an event involving particular samples during processing, not a general reaction in every rock taken from the seafloor. ¹³

Where did the rocks come from?

The samples came from the Escanaba Trough, a hydrothermal area on the southern Gorda Ridge off the US West Coast. Hydrothermal fluids carry metals through Earth’s crust. When these fluids encounter colder seawater, minerals can accumulate in deposits called seafloor massive sulfides. ³

The 2022 collection was part of an investigation involving the US Geological Survey, the Bureau of Ocean Energy Management and the National Oceanic and Atmospheric Administration. ¹

Could this matter for deep-sea mining?

Seafloor massive sulfide deposits can contain metals such as copper, zinc and iron, and researchers are considering them as possible sources of minerals in the future. ³ If some rocks have the same reactive properties, heat or combustion during handling could become a safety issue.

The USGS called the finding a possible hazard to consider if seafloor massive sulfide deposits are mined. The reports flag a concern; they do not establish how frequently such a reaction would occur in an industrial operation. ¹

Does every deep-sea sulfide rock catch fire?

No. The reports describe two samples that combusted, and researchers also examined rocks from the same area that did not. The evidence shows that some samples can react this way, not that all sulfide-rich rocks will heat up or burn. ¹³

The available summaries do not establish how widespread this mineral composition is, what precise conditions set off the reaction, or what controls would be needed for handling larger quantities.

What we don't know yet

How widespread is the mineral composition found in the samples that combusted?
What specific exposure and handling conditions start the reaction, and how quickly does it develop?
How likely is this reaction during large-scale recovery, transport or processing?
Which procedures would reliably prevent dangerous heating or combustion?

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Questions people ask

Can deep-sea rocks really catch fire when brought to the surface?

Some sulfide-rich samples from the Escanaba Trough combusted during laboratory work after exposure to air. The finding concerns those samples and does not establish that all deep-sea rocks behave this way. ¹³

What is marcasite, and why is it mentioned?

Marcasite is a form of iron sulfide. The rocks that combusted were made mainly of nanocrystalline marcasite, according to the research summary. ³

How hot did the deep-sea samples get?

During laboratory processing, temperatures rose above 100°C. The samples were entirely oxidized. ¹

Where were the rocks collected?

They were collected in 2022 at the Escanaba Trough, a hydrothermal area on the southern Gorda Ridge off the US West Coast. The investigation involved the USGS, BOEM and NOAA. ¹³

Does this mean deep-sea mining is unsafe?

The finding raises a possible concern about handling certain seafloor sulfide rocks. It does not show how often the reaction might happen in mining operations or resolve the broader question of mining safety. ¹³

Sources

Scientists Collected Deep-Sea Rocks. They Spontaneously Caught Fire in The Lab. — ScienceAlert, 2026-10-11
Escanaba Trough Samples Collected By USGS Reveal Combustion Hazard In SMS Rock Handling — Ocean News & Technology, 2026-10-08