Colorado’s got a ton of volcanoes. One of ’em blasted out the biggest eruption in all of Earth history. Others produced some of our most important diamond, gold and fossil deposits.
If you imagine the Earth as a giant avocado, Earth’s “pit” is a radioactive core of ultra-hot metal. It’s surrounded by a hot gooey layer called the mantle (the guacamole ingredient). We live on the skin of the avocado—perched on tectonic plates that cruise around atop the mantle like bumper-cars. Where plates collide, one of them often gets shoved down toward the mantle, where it sweats out water and many of its gases. These cause nearby rock to melt into magma, which then burbles upward toward the surface. This process has happened many times beneath Colorado.
Depending on its chemistry, magma erupts in two main forms—oozing or explosive. Lava that oozes out gently forms a rock called basalt, thick flows of which cool slowly and develop dramatic vertical columns (think: Devil’s Tower). Colorado’s Flat Tops are a great example—they’re frosted with so much basalt that they don’t have jagged peaks like the rest of the Rockies. When the blend of volatile gases and water is just right, magma blasts upward and outward as ash and vapor, forming slippery soils and muds like those near Silverton, Creede and Del Norte.
Perhaps Colorado’s coolest volcano is northwest of Fort Collins, where there’s a 10-mile-wide ring representing a remnant volcanic neck. This so-called kimberlite pipe is so prominent you can see it from outer space. What’s unique about it is that it carried diamonds from the mantle up to the surface—making Colorado one of only two states that are rich in diamonds (the other is Arkansas).
The easiest belch of a volcano for Front Rangers to ogle is the one that produced North and South Table Mountain. These flat-topped mesas formed as lava spurted from the Ralston Dike about 2 miles northwest of N. Table Mountain, filling in local valleys and streams with a lake of lava. This volcanic vent erupted at least four different times, about 60 million years ago. Fast forward to today, and the softer strata that surrounded these valley-filling flows have eroded away—leaving the resistant caps of fossil lava behind. To see these up close, drive Highway 58 between Wheat Ridge and Golden. Or better yet, hike to the top of its volcano-scape, starting at the Table Mountain Trailhead off Highway 93.
Colorado’s most famous volcanoes erupted about 35 million years ago, destroying nearly all life in the region and producing two gigantic Pompeii-like deposits. At Florissant, about 60 miles west of Colorado Springs, ash eruptions and ash avalanches dammed a nearby river. The water backed up, creating a huge lake in which animals and plants were frequently buried. The fine ash helped to preserve delicate fossils like leaves and insects; it even entombed a grove of giant sequoia trees. Check them out in Florissant National Monument, or if you’d like to collect, stop by the “pay-to-dig” in town.
About 100 miles southwest of the Springs is the big kahuna of volcanoes—the San Juan Volcanic Field. It is so immense it’s hard to fathom. Among its many eruptions was one that expelled nearly 5,000 times the amount of material produced by Mount St. Helens. Enough debris was ejected to cover the state with a six-inch-thick blanket of hot ash. This killed many plants and animals, and contaminated the air so much that sunlight would have been blocked enough to reduce temperatures by one to three degrees for quite a while. Glad we missed that one.
Although the last eruption in the San Juans was 30 million years ago, the earth is still hot below the surface. Water percolating down and back up again produces many of the area’s hot springs—Mount Princeton and Conundrum among them.
Our cutest volcano erupted a mere 4,100 years ago. Look for it next time you’re driving west on I-70, about a mile before you hit Dotsero. On your right is the reddish-brown edge of the half-mile-wide crater. On your left is the lava flow that flowed southward from the crater and dammed the Eagle River. The resulting basalt and its frothy cousin, scoria, is currently being mined for landscape stone.
The Rockies are still volcanically active—witness the supervolcano that burbles beneath Yellowstone. But not to worry—seismic (earthquake-monitoring) networks will give us plenty of time to leave the blast zone beforehand.
So if you dig volcanoes and a trip to Hawaii isn’t in the budget, perhaps take a road trip this summer. Visit some of our very own, and very awesome, volcanoes.
James Hagadorn, Ph.D., is a scientist at the Denver Museum of Nature & Science. Suggestions and comments are welcome at email@example.com.