Why Some Volcanoes Don’t Explode

Explore

SScientists who study volcanoes have long believed that rising magma rich in gas bubbles should quickly reach the top of a volcano and erupt.

But famous eruptions, like Quizapu in Chile and Mount St. Helens in Washington state, don’t fit that mold. Between 1846 and 1847, Quizapu gently discharged one of South America’s largest lava flows ever documented, leaving behind piles of rock spanning approximately 20 square miles.

And a few months before Mount St. Helens erupted for nine hours in 1980, something strange happened: Lava full of gas and seemingly ready to explode quickly flowed quietly inside the volcano’s cone. A massive explosion was only triggered when an earthquake and resulting avalanche struck, causing the north face of the mountain to collapse. (This released pressurized gases inside the volcano, prompting magma beneath the surface to rise and spread into the air.)

Now, researchers say they have identified a kind of pressure relief valve that allows thick, gas-rich lava to slowly escape, according to a paper published in the journal. Science.

Gas bubbles are thought to appear as magma rises in volcanoes and ambient pressure decreases – like opening a bottle of champagne, the bubbles quickly drive the sizzling magma stew towards explosion.

But, in some cases, bubbles can form in a tube-shaped channel called a conduit, in which magma surges from an underground reservoir before being released as lava. When emerging deep into the conduit, these bubbles can coalesce and form channels allowing gas to escape before pressure builds.

“We can therefore explain why some viscous magmas flow slowly instead of exploding, despite their high gas content – ​​a puzzle that has puzzled us for a long time,” study co-author Olivier Bachmann, a volcanologist at ETH Zürich in Switzerland, said in a statement.

Read more: »The sound is so loud that it circles the Earth four times»

This early bubbling occurs thanks to shear forces. The process can be compared to brewing honey. At the edge of the pot, more friction causes the honey to move more slowly than the sweet liquid in the center. Similarly, friction in a volcano’s conduit is higher along the edge than in the center. “This essentially ‘kneads’ the molten rock, producing gas bubbles” deep within the conduit, according to the release.

The team of scientists identified this phenomenon when modeling volcanic activity in the laboratory by saturating a thick liquid that mimics molten rock with carbon dioxide and then applying shear forces to this substance.

Bachmann now hopes that volcano models will better incorporate shear forces, helping to paint a clearer picture of the potential dangers of volcanoes.

Enjoy Nautilus? Subscribe for free to our newsletter.

Main image: Aaron Rutten / Shutterstock

• Molly Glick

  Published on November 26, 2025

  Molly Glick is the newsletter editor of Nautilus.