More Volcano Stuff

Sunset Crater Volcano

The effect of the eruption of Sunset Crater Volcano on the prehistoric population of northern Arizona has long been a topic of interest to archaeologists.  As I’ve mentioned recently, in the 1930s and 1940s Harold Colton of the Museum of Northern Arizona in Flagstaff came up with a theory to explain the settlement dynamics of the Wupatki area northeast of Sunset Crater.  In Colton’s view, the eruption resulted in a level of volcanic ash falling on Wupatki that acted as a mulch to retain water and make that very arid area suitable for farming for the first time, resulting in a “land rush” in which people from all over the region converged on Wupatki to farm the newly available land.  Over time, however, the ash began to blow away and the land became less productive, so people aggregated into the large pueblos for which Wupatki is best known, then left entirely when the area could no longer support its population.  Dendrochronological evidence from timbers at Wupatki Pueblo later provided a basis for dating the eruption to around AD 1064, which would put the “land rush” shortly after that.  Other evidence has shown that the abandonment of the area probably occurred some time in the thirteenth century, a time when many parts of the Southwest were being abandoned as well.

As I’ve mentioned, recent archaeological survey at Wupatki has cast doubt on some aspects of this model.  The main influx of population seems to have come after AD 1130, a few decades after the eruption, and the scale of that influx was probably quite a bit lower than Colton estimated, since many of the sites he counted to compute his population estimates were probably season field houses or other temporary structures rather than permanent habitations.  This implies that there wasn’t really a “land rush” the way Colton described it, but rather a substantial increase in population at some point after the eruption, perhaps in response to drought or other problematic conditions in other parts of the Southwest.

A few parts of Colton’s model do seem to hold up, however.  Experiments have shown that the levels of ash found at Wupatki do indeed work well as a mulch.  Without this mulch, dry farming in the area with any reasonable measure of reliability is basically impossible, since there just isn’t enough rain, and irrigation or floodwater farming isn’t possible on any substantial scale either due to the geological conditions and the lack of permanent surface water sources.  Furthermore, the Wupatki survey showed that this lack of agricultural suitability made the area essentially uninhabited before the eruption.  Of nearly a thousand datable sites recorded by the survey, only two dated to before the eruption.  The biggest influx of population came after about 1130, but there was already a fairly significant movement of people into the area in the immediate post-eruptive period.  Perhaps these people first experimented with agriculture using the ash as a mulch, and were so successful that when conditions deteriorated elsewhere others joined them.  The ash was liable to blow away in the strong winds, however, and over time the advantages it offered as a mulch would have diminished as a result of this and other factors, so it’s quite possible that it was declining agricultural productivity, perhaps exacerbated by warfare to defend land claims, that led the area to be abandoned in the thirteenth century.

Volcanic Rock in Masonry at the Citadel, Wupatki National Monument

That’s all well and good, but where did the people who moved to Wupatki after the eruption come from?  Colton saw them as coming from all over, but at least in the immediate post-eruptive period a more specific answer is tempting: perhaps they came from the area right around the volcano, which would have been rendered uninhabitable (and certainly unfarmable) by lava flows and massive ash fall.  A relatively recent paper takes a close look at the circumstances of the Sunset Crater eruption and its likely effects on local people, and basically comes to this conclusion.

From a detailed analysis of the details of the eruption, the authors of this paper found that the area of the heaviest ash fall and the largest lava flows was probably densely populated and heavily farmed before the eruption.  They cast some doubt on the tree-ring evidence pointing to an AD 1064 date for the eruption itself, but they argue on other grounds that the eruption likely took place between AD 1050 and 1100 and that it was relatively quick, lasting from a few weeks to a few years at the most.  Because the high-elevation area where the eruption took place gets more precipitation than lower-elevation Wupatki, it would have been the most favorable area for farming at the time, and a large number of homes and farms were likely buried by the lava and ash.  The amounts of ash falling right around the volcano would have been much too thick to serve as a mulch.  The ash itself is sterile, so it could only function effectively as a mulch if plants could reach their roots down through it to the soil underneath.  The few inches of ash cover at Wupatki would have allowed this, but the uplands immediately around the volcano got over a foot of ash, which would have effectively killed any agricultural potential.

Lava at Sunset Crater

Thus, the effects of Sunset Crater on local agriculturalists were two-fold: they were forced to leave a rather large and previously quite productive agricultural area around the volcano, but they were able to go to a previously unproductive area nearby that was made newly fertile by the ash.  Cinder-cone eruptions like the one that created Sunset Crater rarely cause much direct loss of life, and that would have been particularly the case in this context, since the pre-eruption populations lived in dispersed farmsteads and were probably not organized sociopolitically at any level above the household or extended family.  This would have allowed rapid reactions to the eruption, which would primarily have taken the form of migration away from the immediate area.  Since the population was so dispersed, people fleeing the ash-fall zone would likely have had relatives or friends in less affected areas to whom they could go for shelter and assistance in the immediate aftermath of the eruption.  The population movements spurred by the eruption, however, could well have resulted in groups infringing on territory claimed by others and resulting violence and loss of life.  Within this context, the relatively empty Wupatki area may have seemed particularly attractive even before its enhanced potential for farming was discovered.

Another reaction of people in the local area to the eruption, which was documented in an earlier paper by some of the same authors, was the apparent practice of placing corncobs in the path of lava and carrying the resulting “corn rocks,” with visible imprints of the cobs (which were vaporized by the heat) to rather distant settlements.  Given the amount of effort this would have required, it probably had some ritual significance, perhaps to appease the spirits of the volcano or something similar.

Mt. Trumbull from Pipe Spring National Monument

In addition to the Sunset Crater eruption, the authors of this paper also discussed a smaller and less studied eruption that likely took place about the same time at Little Springs, next to Mount Trumbull on the Arizona Strip just north of the Grand Canyon.  Here there was relatively little ash fall, so the loss of productive land and enhanced productivity of other land seen in the Sunset Crater case did not occur.  Instead, the main effect was a lava flow, with the land immediately surrounding it continuing to be largely ash-free and fertile.  The people who had lived and farmed in the immediate area covered by the lava flow would have had to leave, but people clearly continued to live and farm right around the lava, and they also built sites on top of the flow itself.  These sites have few artifacts and likely served defensive purposes, a theory that is supported by the presence of an elaborate system of trails on the lava flow that would have made it an effective refuge in times of war.  The use of defensive refuges or strongholds separate from ordinary living quarters is well-attested in the prehistoric and historic record of the Southwest.  Similar to the corn rocks at Sunset Crater, in this area there were some rocks with potsherds embedded in them, a sign of similar ritual behaviors with respect to the volcano.

These two eruptions and the different reactions to them by local populations show the effects that sudden, catastrophic events can have on human societies.  The eruption of the much larger White River Volcano a bit earlier and its effect on local Athapaskan populations in Alaska and the Yukon is another example.  Unlike many other catastrophes, volcanic eruptions are generally pretty visible in the archaeological record, which makes them a useful source of information on how societies adapt to sudden shocks.

ORT, M., ELSON, M., ANDERSON, K., DUFFIELD, W., & SAMPLES, T. (2008). Variable effects of cinder-cone eruptions on prehistoric agrarian human populations in the American southwest Journal of Volcanology and Geothermal Research, 176 (3), 363-376 DOI: 10.1016/j.jvolgeores.2008.01.031