Sunset Crater blast worse than we thought

And that makes the way people recovered from the volcanic eruption that created Sunset Crater a thousand years ago even more remarkable.

Those conclusions emerge from a remarkable reconstruction of the most recent blast of fire, lava and sun-blotting smoke and ash in the vast lava fields around Flagstaff, sending a 18-mile-tall column of ash and smoke into the atmosphere.

The explosion rivaled the devastating explosion of Mount St. Helens in more recent times. That blast in 1980 spewed 540 tons of ash, which settled over 22,000 square miles. The drifting cloud of the finest ash circled the globe within two weeks. The blast killed 57 people and destroyed 200 homes and covered nearly 200 miles of highway. It blew more than 1000 feet of rock off the top of the mountain, leaving a mile-wide crater.

Now imagine that explosion taking place in sight of Mount Humphreys, with thousands of people living in the area, tending their crops and nourishing an inventive, peaceful 1,000-year-old civilization. Previous studies have documented a remarkable recovery from that explosion in approximately AD 1085, the most recent upheaval in a vast landscape with more than 600 volcanoes. The explosion left a 1,000-foot-tall, sunset red cinder cone, now preserved as a national monument.

But how do you reconstruct a 1,000-year-old explosion?

The latest findings emerged from an ingenious analysis of all the fissures, splatters and geochemical makeup of the molten stone and ash left behind. Researchers from the Arizona State University School of Earth and Space Exploration researchers led by Amanda Clarke published their findings in Nature Communications. The website Science Daily published a summary.

The researchers hoped to not only understand the scale of the explosion, but to gather clues on whether the volcanic field that spawned such violence will ever awaken again. The 12,000-foot-tall peak of Mount Humphreys is itself the remains of a nearly 3-million-year-old volcano, now considered dormant.

The ASU researchers showed the importance of carbon dioxide released from deep inside the earth in the timing and violence of the eruption of Sunset Crater.

The researchers analyzed the change in the chemical composition of gases trapped in stone produced before, during and after the eruption to reconstruct violent events far beneath the surface. The melt inclusions less than a thousandth of an inch across get embedded in crystals in the volcanic plumbing system.

Clark described the trapped gases as a fizzy, soda mix captured from the surrounding molten rock.

Turns out the basaltic magma feeding Sunset Crater had the consistency of maple syrup and contained a lot of water.

So what drove the violence of the final explosions?

To answer that question, the researchers focused on carbon dioxide and sulfur dioxide, which proved surprisingly common in the trapped gases. The deeper you go, the greater the concentration of this volatile mix. The researchers relied on a custom-built spectrometer at ASU to analyze the response of the gases in the melt inclusions. The spectrometer used a blue sapphire laser. Complex calculations allowed the researchers to estimate the speed with which those gases rose through the magma from sources perhaps 10 miles beneath the surface.

The researchers found parallels between the chemistry of the Sunset Crater explosions and the volcanic eruptions of Pinatubo in the Philippines in 1991 and the explosion of Tambora in Indonesia in 1815.

Pinatubo released 10 billion tons of rock and ash, produced a global haze of sulfuric acid, eroded the earth’s protective ozone layer and had a global effect on the weather. Tambora was the biggest volcanic explosion in recorded human history. The death toll was estimated at 71,000 and a cloud of smoke and ash covered the planet and made 1816 “the year without a summer.” Crops failed all across distant North America that summer, resulting in the worst famine of the century.

Both Tambora and Pinatubo were larger explosions than Sunset Crater, but the chemistry of the basaltic magma loaded with carbon dioxide was similar.

The researchers noted that Sunset Crater cannot erupt again, but the surrounding volcanic field is still considered active, despite the 1,000 years of silence.

Remarkably, other studies have shown that the people living in the area recovered with surprising speed, according to research by Archaeology Southwest prior to the widening of I-17. The explosions created an estimated 2,000 climate refugees whose homes were destroyed and cropland buried in ash.

However, in a wider area, a “mulch” of ash trapped more moisture in the soil in subsequent years, increasing the growing season in a region where people practiced drylands agriculture.

The researchers concluded that the Sinagua were culturally adapted to disaster. They were flexible and decentralized, which means individual groups made the best decisions for their particular situation. They had already adapted to tough conditions. Given the unreliable rainfall, their survival depended on risk reduction, storing food in good years and flexible settlement patterns. Finally, they could migrate freely, leaving areas where production dropped and taking advantage of new areas with improved conditions.

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