Eruptions Happen
Sunset Crater, the most recent cinder cone in the San Francisco Volcanic Field, is estimated to have erupted between A.D. 1085 and 1090. In areas where the resulting lava flows were greater than 30cm thick, people may have been forced to migrate, leaving behind long-established homesteads and agricultural fields. On the positive side, lower elevation areas with cinder fall in the range of 3-10 cm would have become more suitable for farming, due to the beneficial effects of the cinder mulch. We talk with Volcanologist Michael Ort, who, together with a team of scientists, has unraveled the timing of these events and the implications of volcanic activity for humans at the time.
Meet the Scientist: Michael Ort
Michael Ort is a volcanologist who specializes in processes of explosive eruptions. He is interested in the processes that break magma into fragments (pyroclasts) and the ways that the pyroclasts then move in currents across the landscape. Michael also works on how eruptions change the environment.
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Interview Excerpt - Living with Volcanoes
Today we are talking with volcanologist Michael Ort about a volcanic eruption that happened about 1000 years ago, and how that affected humans living at the time.
Science Moab: I wanted to have you set the scene for the San Francisco Volcanic Field, the present day San Francisco Peaks in the area of Flagstaff, AZ . Can you talk a bit about the geologic setting?
Ort: The earliest magmas that we see are about 6 million years ago. there’s 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, which is a low silica (less than 50% SiO2) basalt that is thrown out of the vent into the air. it follows basic ballistic trajectories: it just goes up in an arc and falls back down. As it falls, it piles up around the vent (known as “tephra”) at what’s called the angle of repose. That’s a really easy thing to think about. It’s if you take a bunch of rocks and you pile them up. If you add one more to it, and it starts to slide, it means that you’ve got it piled up at the angle of repose, it is as steep as it can be. that’s usually in the 30 ish degree range. And it piles up to make a volcano around the vent where it’s all popping out. So that’s the basic idea of scoria cone.
Science Moab: What drew you to be interested in this most recent scoria cone at Sunset Crater and how it affected humans at the time.
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 this black ash that he was finding was primary, meaning that it came from the eruption, or if humans dumped it there, which it could certainly be. Basically, I was able to show him that it seems to be primary, which means that that structure was built before the eruption. And then they put a little clay floor on top of that black ash, which means then they continue to live there afterwards. So it had a human implication. Working with archaeologists, soil scientists, and dendrochronologists, we were trying to figure out the timing and what was happening with humans at the same time of the eruption.
Science Moab: What key pieces of evidence have you found to understand how this was affecting those living around it?
Ort: The lava flows could have happened in the space of a few weeks. That’s not saying it did happen in a few weeks, it could have gone on for considerably longer. There’s actually a maximum amount of time you can have for these eruptions, on the order of about a year, because if it took longer than that, you’d have to restart with a new eruption and a new batch of magma because it’s too slow of a magma supply rate. There were lots of pit houses around that area at the time of the eruption and the area between about about 6200 feet and about 7500 feet above sea level elevation was the best place for growing corn. That’s because 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 for you to get a harvest. Sunset Crater erupted at just about 7000 feet. So right in this sweet spot for growing corn. Then the area is devastated with a meter or more of scoria tephra over the area from the eruption and all the farms are wiped out. But something really interesting happened further downhill in the area that these scoria and tephra deposits were blown to. If you get a deposit of tephra, anywhere between three and six inches deep, it acts like a mulch. The rain that falls on it moves through it easily, as it is a very open framework, but it doesn’t wick back up. So it prevents evaporation. So all of a sudden a bunch of area below 6200 feet, down at 5000 feet, with this tephra on it was ideal to grow corn because it needed less water input and the frost to frost period was much longer at that elevation. So now you had a more assured crop because you had more time to plant it in and it was going to grow. Because there was a more reliable agricultural system, the population in the area around Wupatki increased dramatically. They started building actual small pueblos, which was a change in style of living from the previous pit houses where they would just be probably small family groups living together to these larger groups living together and with a more sustained farming ability. This went on for about 150 years or so. Then they left. Part of the reason for that may have been decreased mulch as the cinders had blown away. We see evidence that they were trying to control this by making little windbreaks around their farms to try to keep the cinders from moving.
Science Moab: Is there any understanding of how far reaching this eruption might have been felt? or seen?
Ort: Say you’re a person living in northern Arizona 1000 years ago, your life is mostly fairly quiet. Yeah, there’s lightning and thunder. That’s loud, you know, you’re getting close to a river at flood and it’ll be somewhat loud, the wind in the trees. But when an eruption happens, that’s really loud. So if you’re close to the volcano, just the noise of it would have been something that really would have made quite an impression besides all this other. But also, it’s putting up this cloud pretty darn high into the air, some of our calculations suggest it may have gone up over 20 kilometers into the air that’s really high, the area where you might have seen a stable, not, you know, not moving Dark Cloud, which is just not normal. You might have seen it from as far away as you know, like, the mountains near Las Vegas, down near Tucson, off towards Albuquerque. This was a regional phenomenon. Everybody knew about this.
