The Science Moab Team
Volcanologist Dr. Michael Ort studies explosive events, and is particularly interested in interactions between volcanoes and humans. Science Moab talks to Dr. Ort about his work as part of a team helping to reconstruct the timeline and human consequences of a cataclysmic eruption in northern Arizona about 1000 years ago.
Science Moab
Let’s set the scene for the San Francisco Volcanic Field—the present day San Francisco Peaks in the area of Flagstaff, Arizona. Can you talk a bit about the geologic setting, and what has captured your interest there?
Michael Ort
There are eight or nine larger vents and about 600 scoria cones (smaller vents), the most recent being Sunset Crater, which erupted just under 1000 years ago. Scoria cones are formed from basalt (a dark, low silica rock) that is thrown out of the vent into the air. As it falls, this ejected rock (known as tephra) piles up around the vent at what’s called the angle of repose. Here’s how to think about this: imagine taking a bunch of rocks and piling them up. If you add one more rock to the pile and it starts to slide; that means that you’ve got that pile as steep as it can be—the angle of repose—usually around 30 degrees. And it piles up to make a volcano around the vent where it’s all popping out. So that’s the basic idea of a scoria cone.
Science Moab
What made you interested in this most recent scoria cone at Sunset Crater?
Ort
So I got started on it, actually, because an archaeologist asked me to come out and look at it. He wanted to know, at a particular site, whether a black ash that he was finding was primary, (came from the eruption) or if humans had [collected and put] it there. I was able to show him that the ash seems to be primary, which means that the [ancient] structure he was studying was built before Sunset Crater erupted. And then, it appears that a little clay floor was installed on top of that black ash [and the structure remained in use by humans] afterwards. Working with archaeologists, soil scientists, and dendrochronologists (scientists who study tree rings), we worked to figure out the timing of the eruption and what humans were doing at the site at the time of the eruption.
Science Moab
What key pieces of evidence have you found to understand how the Sunset Crater eruption affected humans living nearby?
Ort
[Our team’s understanding is that] elevations between about 6200 feet and about 7500 feet above sea level are ideal for growing corn. Below 6200 feet it’s too dry and hot and there’s just not enough water and the plants die. Above 7500 feet the growing seasons are too short to get a harvest. Sunset Crater erupted at just about 7000 feet—right in this sweet spot for growing corn. This area was devastated with a meter or more of scoria tephra (ejected basalt fragments) over the area from the eruption, and farms are wiped out.
But something very interesting happened further downhill in the area that these scoria and tephra deposits were blown to. If you get a deposit of tephra between three and six inches deep, it acts like a mulch. The rain that falls on it moves through it very easily and doesn’t wick back up, so it prevents evaporation.
All of a sudden, a large area below that 6200 feet threshold (down at 5000 feet) with this tephra “mulch” on it became ideal to grow corn because it needed less water input and the frost-to-frost period was much longer at that elevation. This was a more assured crop. With this more reliable agricultural zone, the ancestral Puebloan population around [modern] Wupatki National Monument increased dramatically. People started building small pueblos, allowing larger groups to live together with more sustained farming ability.
This went on for about 150 years or so, and then the area appears to have been vacated. Part of the reason for that may have been a decreased supply of this volcanic mulch as the cinders blew away. We see evidence that people were trying to control this by making little windbreaks around their farms to try to keep the cinders from moving.
Science Moab
Do we have any idea how far-reaching this eruption might have been? How many communities might have felt or seen Sunset Crater explode?
Ort
Say you’re a person living in northern Arizona 1000 years ago; when an eruption happens, that’s really loud. [Louder than lightning, thunder, flooding rivers, or wind in the trees.] So, if you’re close to the volcano, just the noise of it would have been something that made quite an impression.
But also, it’s putting up this cloud pretty darn high into the air—some of our calculations suggest the cloud may have gone up over 20 kilometers (over 12 miles) into the air. That’s really high. You might have seen it from as far away as the mountains near Las Vegas, Nevada; down near Tucson, Arizona; off towards Albuquerque, New Mexico. This was a regional phenomenon. Everybody in the area knew about this event.
