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USGS Geology in the Parks

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More Bubbling Basalt

Limestone xenolith preserved in basalt flow. Photo by Melanie Moreno, USGS,10/98
Limestone xenolith preserved in Bonito Lava Flow.

What's down there?

When you reach the first stop you'll see an something very unusual at Sunset Crater-a rock that's not volcanic! Entombed within the once molten basalt is a rock that provides evidence of what lies buried beneath Sunset Crater's lava flows. A close-up look reveals that this little rock is a piece of ancient limestone, probably 260 million years old, give or take a few million. Where did it come from? How did it get here?

Escape from the depths

930 years ago a simmering mass of basaltic lava lay trapped 10 kilometers (6.2 miles) below the Earth's surface. The basalt didn't remain there long before it found a weakness in the overlying layered rock. The lava was suddenly injected upwards, traveling through a dike, an almost vertical conduit, up to Sunset Crater Volcano's central vent.

Paleozoic layers of Grand Canyon National Park. Photo by Melanie Moreno, 1997
Sedimentary rock layers of Grand Canyon National Park

As the fiery liquid made its way through thick layers of sedimentary rock, pieces of limestone were plucked from the sides and carried upward. Most were probably melted by the liquid basalt, but some limestone chunks from layers nearest the surface are preserved in the lava flows. These bits of 'foreign rock' called xenoliths provide evidence of hidden rock layers now mantled by lava flows and cinders.

Geologists now know that beneath this 700 to 1000 feet thick volcanic cap they will find sedimentary rocks. If you've ever visited the Grand Canyon, you've seen sedimentary layers like those that lie beneath your feet. This bit of limestone is from the one of the top-most layers you'll see at the Grand Canyon.

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