Explosive Past of Arizona’s Sunset Crater Eruption May Inform Threats of Similar Volcanoes Today

Around A.D. 1085, along the southern rim of Northern Arizona’s elevated Colorado Plateau, a volcano erupted, forever changing ancient Puebloan fortunes and all nearby life. Among the 600 or so volcanoes that dot the landscape of the San Francisco volcanic fields, this one blew. It was the very first (and last) eruption for what came to be known as Sunset Crater, aptly named for its multihued, 1,000-foot-tall cinder cone.

Today, Arizona State University School of Earth and Space Exploration scientist Amanda Clarke and her team have been working to solve the mysterious root cause of the Sunset Crater eruption and any lessons learned to better understand the threats similar volcanoes may pose around the world today.

“This is a common thing in volcanology, to reconstruct past eruptions to try to understand what the volcano or region might do in the future,” Clarke said. “We did the field work and we combined data from a previous study and used some modern techniques to put the story together.”

Working alongside several collaborators, they have painstakingly mapped every fissure, eruption deposit and ancient lava flow of Sunset Crater to reconstruct the complete splatter patterns and geochemical compositions of all ejected materials, or tephra, from the eruption.

An explosive past

“When you visit the site, there are these lava flows that are obvious, but also this big tephra blanket that extends far beyond the volcanic edifice itself, way beyond the vent,” Clarke said. “My interest was first piqued when I learned on a field trip many years ago with former ASU Professor Stephen Self, that Sunset Crater had an explosive past.”

Melt Inclusions

To reconstruct the past, ASU scientists have taken advantage of extensive microanalyses from the tiniest blobs and bubbles that are the best representation of the composition of magma from Sunset Crater before the eruption, known as melt inclusions. How tiny? Melt inclusions are less than a thousandth of an inch across. They become embedded in time within growing crystals of the magma plumbing system that forms before a volcano erupts. Credit: Amanda Clarke, Arizona State University

In a previous study, Clarke’s group first showed that the volcanic activity developed in seven or eight distinct phases: initial fissure phases, followed by highly explosive phases, and finally, low-explosivity, waning phases.

“It’s not clear how this happens, but eventually, the eruption settled on this single pipeline to the surface, and that’s where a lot of our work picks up the story,” Clarke said.

At several points during the explosive phase, the sky was filled with basaltic, cindery ash up to 20 to 30 kilometers high, making it one of the most explosive volcanic eruptions of its kind ever documented in the world.

“People in Winslow (100 km away) would have been able to see it,” Clarke said.

To give one an idea of the eruption size, they measured the total volume of eruption material, or 0.52 km3 dense rock equivalent (DRE) — which, by comparison, turned out to be similar to the volume of the infamous 1980 Mount St. Helens eruption.

“It was very similar to Mount St. Helens in terms of height and volume,” Clarke said. “You think these things that are cinder cones are going to be something like Stromboli in Italy — a fire fountain of a couple of hundred meters and people might be able to watch it from their terrace — but this peak phase was St. Helens scale.”

Mysterious magma

But as to why it erupted, that has remained a mystery, until now.

“The science question is how these more liquidy magmas behave like viscous magmas,” Clarke said.

The study, published in the journal Nature Communications was the result of a collaboration between School of Earth and Space Exploration PhD alumna Chelsea Allison (now at Cornell University) and research scientist Kurt Roggensack.

“Chelsea was a graduate student who did some innovative analysis and Kurt has this expertise in petrology and more small-scale analysis while I am more of a physical volcanologist; so that’s where we came together,” Clarke said.

Measuring the factors that led to the Sunset Crater eruption 1,000 years later is an extremely difficult task because the gases that make up the magma usually escape into the sky during the eruption, forever lost in time. But to better reconstruct the past, the group have taken advantage of extensive microanalyses from the tiniest blobs and bubbles that are the best representation of the composition of magma from Sunset Crater before the eruption, known as melt inclusions. Roggensack is recognized as a world expert in innovative melt inclusion analysis, especially in basaltic magmas.

How tiny? Melt inclusions are less than a thousandth of an inch across. They become embedded in time within growing crystals of the magma plumbing system that forms before a volcano erupts.

“They’ve been liberated from the magma in the explosion,” Clarke said.

They are like a fizzy soda concoction of trapped gas, frozen in time from the surrounding magma as they crystalize, yet able to reveal the gas composition and secret history of an eruption so long ago.

Think of the basaltic Sunset Crater having more of a maple syrup consistency versus the peanut butter variety of the rhyolite magma of Mount St. Helens.

“Those are viscous magmas that can have a lot of water stuffed in them,” Clarke said.

What were the conditions and ingredients that could lead to the Sunset Crater eruption?

“That leads to the big questions of what is the volatile content of the magma because that is going to control the explosivity,” Clarke said. “To answer the questions, you have to dig down deep into the plumbing system, and that’s what we did.”

Clarke’s group is among the first to show the importance of carbon dioxide in volcanic eruptions, partly because it wasn’t an easy task to measure in the first place. “We think this eruption could have pumped a fair amount of carbon dioxide and also sulfur dioxide into the atmosphere,” Clarke said.

“Water is usually the main component (as in Mount St. Helens) but what we are finding at Sunset is that carbon dioxide is very abundant and that tends to be more critical in the deeper part of the system to get the magma moving toward the surface. We think that played a big role in this. And the carbon dioxide is probably coming from deep in the mantle within the source area.”

The melt inclusions (MIs) were specifically chosen to provide a representative sample of textural features observed in the Sunset Crater eruption (e.g., varying bubble volumes, sizes and shapes). Some of the tools of the trade used were microscopes to bring the details of crystallization and bubble formation for each tiny melt inclusion to life, as well as sensitive instruments to measure the amount of volatiles trapped in the quenched glass.

“That can tell us some of the details of the last moments of the magma before it was quenched.”

Tiny bubbles