Some of the world’s most cataclysmic volcanic eruptions are associated with the collapse of a caldera, a depression in the top of the volcano that forms and deepens as a reservoir of magma below it empties out.

The two largest of the 20th century — Pinatubo in 1991 in the Philippines and Novarupta in Alaska that formed the spectacular Valley of Ten Thousand Smokes in 1912 — both blew their tops this way.

Caldera collapses are rare, and when they do occur they usually happen quickly, in hours or a few days. But when Bardarbunga, a large volcano under an ice cap in central Iceland, erupted in August 2014, the caldera sank and collapsed gradually over the course of the six-month eruption. That gave scientists a unique opportunity to study it.

In a report published Thursday in the journal Science, volcanologists reveal that the sinking caldera actually helped drive the eruption, by keeping pressure on the magma chamber as the hot rock flowed out.