Volcanic eruptions are driven by underground combustion or chemical fires involving substances such as sulfur, coal, or other combustible materials.

To an early observer, a volcano could look like an enormous underground fire.

There was heat, glowing material, choking gas, explosions, and sulfur around volcanic vents. Sulfur itself burns, and real coal seams can catch fire underground. Without a modern picture of magma and Earth’s interior, subterranean combustion was a plausible explanation.

Different versions proposed burning sulfur, coal, pyrite, metals, or other reactive material beneath the surface.

The geology did not fit well enough.

Lava behaves as molten rock, and large volcanic formations contain enormous amounts of material that had clearly melted and then solidified. Field relationships also linked volcanic rocks to deeper bodies of once-molten rock rather than to local deposits of fuel.

Experiments in the eighteenth and nineteenth centuries showed that lava could be remelted and would solidify again into volcanic rock. As understanding of Earth’s internal heat improved, volcanism came to be explained through melting inside the planet rather than combustion.

Modern eruptions are driven by magma and gas. Magma can rise through the crust because it is less dense than surrounding rock. It also contains dissolved water, carbon dioxide, sulfur compounds, and other gases.

As magma rises, pressure falls. Just as opening a carbonated drink lets dissolved gas form bubbles, falling pressure allows gases to come out of the magma and expand. That expansion can help drive an explosive eruption.

Sulfur is genuinely part of volcanic chemistry. It is not the underground fuel powering a giant fire.