What's the Difference Between Magma and Lava?

And why do some volcanoes explode and others only ooze? It all comes down to their gooey centers.

By Michael Greshko
Published 8 May 2018, 22:22 BST
Lava from Hawaii's volcanoes tends to be comparatively runny due to its lower silica content.
Lava from Hawaii's volcanoes tends to be comparatively runny due to its lower silica content.
Photograph by U.S. Geological Survey

Hawaii's Kilauea volcano keeps erupting with syrupy lava flows, serving as a fiery reminder of nature's destructive power. But as the ongoing eruption captures headlines, a question might occur to you: what's the difference between magma and lava?

The distinction between magma and lava is all about location. When geologists refer to magma, they're talking about molten rock that's still trapped underground. If this molten rock makes it to the surface and keeps flowing like a liquid, it's called lava.

Magmas vary in their chemical composition, which gives them—and the volcanoes that contain them—different properties.

Related: Stunning Photos Of Volcanoes From Around The World

Mafic magmas like those in Hawaii tend to form when the heavier crust that forms the ocean floor melts. They contain between 47 to 63 percent silica, the mineral that makes up glass and quartz. As far as molten rocks go, mafic magmas are fairly runny, with viscosities ranging from molasses to peanut butter. They're also the hottest variety of magma, reaching temperatures of between 1000 and 1200oC.

Silicic magmas, on the other hand, tend to form when the lighter continental crust melts. These magmas are more than 63 percent silica, which makes them more viscous. At their runniest, silicic magmas flow about as well as lard or caulk—which is to say not well at all. They're also cooler than mafic magmas. Rhyolite, an especially silica-rich type of lava, hits temperatures of between 650 and 800oC.

Since silicic magmas are colder and gloopier than mafic magmas, dissolved gases have a harder time venting out of them. This makes silicic magmas more dangerous. As more gas builds up in the magma, it becomes more explosive, like adding more and more carbonation to a fizzy drink.

Kilauea is a shield volcano, with gentle slopes formed by oozing lava.
Photograph by U.S. Geological Survey

When silicic magmas are no longer confined under sufficiently high pressure, the gases dissolved within them come out of solution and form bubbles. And just like opening a shaken-up can of cola, the resulting rush of vapour triggers an explosive eruption. Iconic cone-shaped volcanoes called stratovolcanoes, such as Mount Pinatubo, are loaded with silicic magmas.

Hawaii's volcanoes, on the other hand, contain especially low-silica magmas made of basalt, which means they have much less explosive oomph. Instead, they ooze and spatter, creating shield volcanoes—gently sloped formations that have become the islands' signature geological silhouette.


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