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Land Lines: Mt Elden

Look at this–a crinoid stem. How can this be? A sea-floor fossil perched at 9000 feet in a volcanic field that stretches for miles in every direction?

Just north of Flagstaff, Mount Humphreys stands 12,633 feet above sea level, the highest summit in Arizona. Humphreys and the rest of the San Francisco Peaks are old volcanoes. Surrounding them is a necklace of dome-shaped mountains—Sugarloaf, O’Leary, Kendrick, the Dry Lake Hills, and Elden Mountain. They’re volcanic too, but they formed in a different way.

Elden, about three thousand feet lower than Humphreys, and its kin are made of dacite and rhyolite—stickier magmas less inclined to smoothly ooze out on the surface as lava flows. Instead, these domes started as blisters that swelled as magma was injected into underground chambers. As those chambers expanded, they lifted the overlying rock layers.

Drill a hole a few thousand feet beneath Flagstaff and you’ll go through pretty much the same sequence as the walls in Grand Canyon. At Elden, magma was injected into these deep sedimentary strata. The Paleozoic layers were lifted and tilted as Elden inflated half a million years ago.

If you hike up the Sandy Seep Trail on Elden’s east flank, you’ll be surrounded by dark volcanic dacite.  But along the way, you’ll also encounter red and white sedimentary beds –  first the Kaibab, and then, because the layers are tilted so steeply, progressively older rocks as you go higher. Near the top, in the Redwall Limestone, you’ll find those crinoid stems crinoids, creatures that lived on an ocean floor 330 million years ago, long before the San Francisco Peaks were even a glimmer in Earth’s eye.

To imagine how Elden formed, think Jiffy Pop. Remember how you shook the container over a campfire, and how the popcorn started to sputter and the aluminum bag began to swell? Elden Mountain is nothing but an oversized bag of Jiffy Pop.

Geologists call this a laccolith—literally "stone cistern." There are quite a few laccoliths scattered across the Colorado Plateau.  Navajo Mountain, up on the Arizona/Utah border, is a good example of one that inflated and stopped before it burst. But at Elden, the Jiffy Pop bag broke. The magma ruptured near the top of the dome and poured down in bulbous lobes that barely reached the new mountain’s feet. That dacite was fiery hot. As it fanned down the mountainside, it cooled, thickened, and cracked under the strain of gravity. Crevasses formed on the lobes, much like the surface of a glacier.

Usually sticky dacite flows so slowly that you can out-walk its sluggish advance. But at Elden, the bottom of some lobes broke away, and pyroclastic surges were unleashed that would have burned and buried neighborhoods had Flagstaff existed then. The specter of such an event reminds us what philosopher Will Durant once said, "civilization exists by geologic consent, subject to change without notice".

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