Carved in Stone


July 5, 2011

S is for sunset crater

A scenic trek always includes an interesting landmark — a volcanic cinder cone.

By Bill Langer

My wife, Pammy, and I regularly make a trek from Denver, Colo., to Phoenix, Ariz., to visit our daughter Kimberly, son-in-law Rob, and grandkids Donovan and Delaney. Part of the trip takes us along Interstate 40, from Albuquerque, N.M., to Flagstaff, Ariz. About 5 miles east of Flagstaff, I point out one of my favorite landmarks — a quarry in the side of a cinder cone. As we head south out of Flag (as the locals call it), our heartbeats quicken: mine because of the quarry; Grammy’s because it’s only about an hour and a half to the kids and grandkids.

Prior to a volcanic eruption in the Flagstaff area that took place 900 years ago, Native Americans lived in the region known as the San Francisco Volcanic Field in pit houses dug partially into the ground. After the eruption, the land became unlivable and is now mined for light-weight aggregates.

The rocks from the quarry, called cinders, are used to make cinder block and can also be used as a high-quality, lightweight aggregate. Cinders tend to be a marginal conventional aggregate, but are used effectively in some applications such as road base and as a surface for low-volume roads. Some cinders are hauled by truck to the Phoenix area, with trucks returning to Flagstaff loaded with normal-weight aggregate mined in Phoenix. Backhauling is cost effective even though the cost to transport aggregate in mountainous areas can be three to four times as expensive as for open-highway transportation.

But let’s get back to our trip. When we spotted our first cinder cone, we were driving through a region referred to as the San Francisco Volcanic Field. Almost all the mountains and hills in the volcanic field are extinct volcanoes, most relatively small basalt cinder cones like the one being quarried. They form in a matter of months or years when gas-charged frothy blobs of magma erupt into the air, cool nearly instantly, and fall to the ground as solid rock containing cavities created by trapped gas bubbles. When the fragments accumulate around the vent, they form a cone-shaped hill. After sufficient gas pressure has been released from the magma chamber, the bombardment stops and lava flows quietly from the cinder cone.

Eruptions began in the Flagstaff area about 6 million years ago. The last eruption, which took place about 900 years ago, resulted in the creation of Sunset Crater, located about 12 miles north of Flagstaff. At that time, the area was occupied by Native American people; farmers living in pit houses dug partially into the ground adjacent to their corn fields.

Tremors and earthquakes preceding the eruptions warned the people of the impending disaster. A fountain of fire shot hundreds of feet into the air. Blazing hot rocks exploded from the ground and rained down on pit houses and farmland. The horizon glowed fiery red. Billowing ash and smoke from forest fires blackened the sky. Perhaps some stayed and watched as their homes and farmland were buried under slow-moving lava flows. But following the eruption, the Sunset Crater area was no longer farmable, and people relocated elsewhere.

Ranching, logging, mining, and the railroad arrived in the late 1800s, and tourism followed. In 1928, a movie company wishing to film a landslide proposed blowing up Sunset Crater. The public, fearing irreversible damage to the volcano, pushed for its protection, and on May 26, 1930, President Herbert Hoover established the Sunset Crater National Monument.

Someday soon, Pam and I are going to retire, move to Phoenix, and live right across the street from our family. One of the first RV adventures we will take with Donovan and Delaney will be to Sunset Crater. And as we head north out of Flag, our heartbeats will quicken.

Bill Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey and can be reached at 303-236-1249 or via e-mail at

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