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Bubbling basalt:
Molten rock invades the Colorado Plateau

Drippy texture testifies to the once molten surface of Bonito Lava Flow. Photo by Melanie Moreno, 10/98

Bubbling basalt!

As you walk along the trail toward Sunset Crater Volcano everywhere you look you'll see black, bubbly basalt. Basalt is the most common volcanic rock you'll see on the Earth's surface. When most people think of lava or what is generally called 'lava rock', they are usually thinking of basalt.

Basalt, like all volcanic rocks, starts as hot lava that erupts at or near the Earth's surface. Once it's outside, contact with the atmosphere or water cools the hot lava very quickly. Quick cooling prevents minerals from growing to a large size. That's why basalt and other volcanic rocks are made mostly of tiny mineral grains that can only be identified with a microscope.

The first thing people most people notice about basalt is its very dark color. Basalt has lots of iron and magnesium-rich minerals that give it its distinctive, almost black color. Despite basalt's dark color, the single most abundant material (40-50%) in it is silica, the stuff that window glass is made of.


Close-up of Sunset Crater basalt with thumb for scale.  Photo by Melanie Moreno, USGS, 10/98

The second thing people most people notice about basalt along the Lava Flow trail is its bubbly texture. Geologists call these small bubbles vesicles. They can be found in any type of volcanic rock. Vesicles are evidence that the basaltic lava had plenty of gas. Given enough time, the gas will its way out of the lava and escape into the air. These gas bubbles were frozen into place before they could escape.

TAKE A SIDE TRIP: Close-up look at a monster vesicle
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