"Thar She Blows !!!" Or not: This time, volcanic...

[Nik_SpeakerToCats]

It's been one of Volcanology's nastier little mysteries:
Why do some volcanoes switch to-and-fro between effusive lava and FWOOOSH !!

Okay, caldera-forming, ring-faulted lid collapses aside --Preferably 'Be Not There' aside !!-- why do some gassy lavas 'effuse' yet some less-gassy lavas 'explode' ??

This report seems to have nailed it...

Why some volcanoes don't explode
https://phys.org/news/2025-11-volcanoes-dont.html
Which references...
Shear-induced bubble nucleation in magmas { Paywalled }
https://www.science.org/doi/10.1126/science.adw8543

Quote:
The explosiveness of a volcanic eruption depends on how many gas bubbles form in the magma—and when. Until now, it was thought that gas bubbles were formed primarily when the ambient pressure dropped while the magma was rising.
...
However, this explanation is incomplete—because the lava from some volcanoes, such as Mount St. Helens in the state of Washington, U.S., or the Chilean volcano Quizapu, has sometimes flowed out gently despite the presence of highly explosive magma with a high gas content.
...
In an article in the journal Science, researchers show that gas bubbles can form in the rising magma not only due to a drop in pressure but also due to shear forces. If these gas bubbles grow deep in the volcanic conduit, they can combine with one another and therefore form degassing channels. Gas can then escape at an early stage, and the magma flows out calmly.
...
The researchers' experiments show that bubbles are formed primarily near the edges of the conduit, where the shear forces are strongest. Existing bubbles further strengthen this effect.

"The more gas the magma contains, the less shear is needed for bubble formation and bubble growth," says Bachmann.
/
Nik-Note: Didn't see this one coming !!!

[Rocket J Squrriel]

Short answer there is a subterranean Taco Bell feeding some of these volcanoes.