6.2 The impact of volcanic ash on human settlements

That volcanic eruptions are capable of causing major social and environmental catastrophes, effecting land abandonment and population displacement is indisputable and readily observed from historical accounts. For example, the eruption of Mount Vesuvius in the Bay of Naples in A.D. 79 destroyed the cities of Pompeii and Herculaneum, and being witnessed and described by Pliny the Younger, resulted in his name being given to these so-called 'plinian eruptions' of explosive ash and gas-ejecting volcanoes. In more recent times the eruptions of Mount St. Helens in North America and Mount Pinotuba in the Philippines bear witness to the widespread havoc such eruptions can cause. There have been several recent examples world wide, from Japan and Indonesia to Iceland , and in Ecuador itself of both Guagua Pichincha and Tunguragua, and other numerous impressive examples. The 'plinian' eruption of Quilotoa is known to have been an extremely powerful event which left an ash layer approximately 10 cms thick over an area of about 37,000 km² of the interandean valley (Mothes and Hall 1998b:111). Explosive eruptions of this kind which generate an enormous quantity of ash are typical of the 'dacitic' volcanism common to the Cordillera Occidental, wherein large explosive eruptions are separated by relatively long inactive periods (Mothes and Hall 1998b:112).

Natural catastrophes of this order may produce profound social consequences, albeit in ways which are difficult to measure, particularly in prehistoric contexts. Large populations, dependent upon an intensive method of food production which suddenly collapses, would undoubtedly have been plunged into crisis and it is not difficult to imagine that this may have generated a general abandonment of extensive areas most affected by the ash-falls . There is no evidence that the camellones in and around the Quito basin were ever recovered following this event. Finer volcanic tephras are normally subject to weathering processes such as rain, which may wash them from slope surfaces into depressions and rivers, transporting them away from the original fall-out area. Generally, however, tephra does not rapidly disappear on its own, but is gradually subsumed within the overall soil-making processes through natural biological and weathering action across many years, after which the land is once more available for cultivation, and indeed highly fertile as a consequence (Isaacson 1987:32). To accelerate this process it presumably must be physically removed, possibly heaped up in what one may imagine to have been a considerable corporate work effort for peoples of the affected areas. Except in regions of very thick accumulations of ash, such as in the Quito basin, those affected by falls of a few centimetres, such as in the northern sierra, may have been more likely to have responded by organising land-clearance operations. As responses of prehistoric populations to such events are still largely speculative, it is illuminating to be able to observe at Hacienda Zuleta an actual land-reclamation programme.

The discovery of a post-eruption phase of camellones in sector 4 (Phase 4 of Figure 11) demonstrates that the land continued to be occupied and cultivated productively. This is also witnessed by the presence of microstructural features in the post-eruption soils indicative of anthropogenic activity. In addition to this, several of the larger tolas, where it is possible to observe their construction through erosion or through cuts made by farm machinery, have the same ash redeposited in them. This can be clearly observed in the tola profile excavated in 1997 which was constructed of earthen blocks, some of which contained this ash. Whilst tolas of small to medium size occur from at least AD 700 onwards in the sierra, the larger quadrilateral 'pyramid' mounds of very different form and construction are of much later date and appear to post-date the Quilotoa eruption. They perhaps represent a direct response to that event (Knapp and Mothes 1998, 148-55).


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