Reports of the wildfire in Indonesia’s Bromo Tengger Semeru National Park mention hotspots “near the caldera”. The word appears often in volcano coverage and is rarely explained.

Not the same as a crater

A crater is the opening at the top of a volcano, formed by material being blasted outwards. It is typically a few hundred metres across.

A caldera is formed the opposite way — by collapse rather than ejection. When a very large eruption empties the magma chamber beneath a volcano, the ground above has nothing left to support it and falls inward.

The result is a broad basin, often many kilometres wide, ringed by steep walls that are the edges of the collapse. The word comes from the Spanish for cauldron, which describes the shape well.

What they look like afterwards

Calderas frequently fill with water, producing near-circular lakes with unusually steep sides. Others hold new volcanic cones that grow inside the old basin as activity resumes — which is exactly the arrangement at Bromo, where a younger cone sits within a much larger ancient caldera floored with volcanic sand.

Some are so large that they are difficult to recognise from the ground. People live inside them, farm them, and are entirely unaware of the shape of the landform they inhabit, because the rim is beyond the horizon.

Why the distinction is useful

A caldera is evidence that a very large eruption happened. That is not a prediction — most calderas are quiet for tens of thousands of years, and many will never erupt on that scale again.

But it tells volcanologists what a given system has been capable of, which is the starting point for monitoring it. And it explains why a national park built around one contains both fragile specialised habitat and enough open, wind-exposed terrain for fires to spread quickly.

Prompted by reporting on the Bromo Tengger Semeru National Park fire, August 2026.