Why do (some) silicic bodies behave effusively instead behaving explosively as in common?

Why Some Silicic Magmas Just Chill and Flow (Instead of Exploding)

Volcanoes. They’re forces of nature that can either gently ooze lava or blow their tops in spectacular, terrifying displays. We tend to picture the latter – those ash plumes reaching for the sky, the ground shaking, the sheer power of an explosive eruption. But what about the times when volcanoes don’t explode? What about the slow, steady flow of lava? This is especially puzzling when we’re talking about silicic magmas.

See, silicic magmas – think rhyolites and dacites – are usually the culprits behind those massive explosions. They’re packed with silica, which makes them thick and sticky, like cold honey. They also tend to be full of dissolved gases, like water vapor and carbon dioxide. So, logically, you’d expect them to always go boom. But sometimes, they just… flow. What gives?

Well, it’s not a simple answer, but it boils down to a few key factors all working together. Think of it like baking a cake: you need the right ingredients, the right temperature, and the right timing to get it just right. With volcanoes, it’s the same principle.

One of the biggest factors is how the gases behave. Sure, a lot of gas can mean a big explosion. But it’s more about how those gases escape. Imagine slowly letting the air out of a balloon versus popping it with a pin. If the magma rises slowly, and the gases have time to seep out through cracks in the rock, you avoid that pressure cooker effect. This slow release is called “open-system degassing,” and it’s a major player in keeping things calm.

Temperature also matters. Hotter magma is runnier magma, which makes it easier for gases to escape. Silicic magmas aren’t exactly known for being scorching hot, but even a little extra heat can make a difference.

And then there’s the magma itself. Even tiny differences in the chemical makeup can change how it behaves. A little less silica, or a bit more iron, and suddenly the magma is less sticky, more willing to flow. It’s like adding a touch more water to your pancake batter – suddenly, it pours much easier.

Finally, don’t forget the surrounding rocks. If there’s a good network of cracks and faults, those gases have an easy escape route. The strength and permeability of the rocks also play a role in how the magma makes its way to the surface.

Think about the Mono-Inyo Craters in California. Those rhyolitic lava flows weren’t exactly gentle, but they were a far cry from a Mount St. Helens-style eruption. The magma rose slowly, the gases had time to escape, and the result was a relatively peaceful (though still hazardous!) flow of lava.

So, next time you see a volcano, remember that there’s more to the story than just explosions. Sometimes, these silicic beasts just want to chill out and flow. And understanding why is key to predicting their behavior and keeping people safe.