National Geographic reports on a study that I heard about at the SAA meetings in Sacramento. The study suggests volcanic ash from eruptions in the neighboring highlands now and then enriched the poor soils of the Maya lowlands (that are formed from weathered limestone). Beige clay discovered at Tikal turns out to be smectite which is a mineral that derives from breakdown of volcanic ash. Ash was deposited at Tikal from about 340 BC to AD 990 and thus the eruptions were frequent phenomena. It is suggested that the ash makes the soil more fertile because its permeability and porosity increases and therefore it can retain more water. The ash also contains iron and magnesium. The ashfall would have been thin in order to enrich the soil.
I am not entirely convinced by the study and some of the comments of the article explain the problems. Inyohualli suggests that “a “thin dusting” of volcanic ash has no discernible agricultural benefits and in any case would be completely washed off cultivable areas after a single rainy season, which in this region are veritable deluges. The cited archaeologist’s [Payson Sheets] declaration about the benefits of volcanic ash reiterates the common misconception that it acts like commercial fertilizer. Volcanic ash usually takes many, many years to weather sufficiently in order to improve soils for farming. If crops really were to improve with a “dusting” of volcanic ash, then there would now be truckloads of it being shipped from the highlands to farms in the Peten (Tikal’s general region), but of course this is not happening, despite well-developed roads, cheap hauling fees and, sadly, intensive farming in the former jungle.”
The reason why I am skeptical has more to do with another side effect of a volcanic eruption. Richardson Gill has for long argued that volcanic eruptions above a certain magnitude cause climate change. The Terminal Classic collapse(s) is/are believed to have followed famines caused by droughts that in themselves were caused by volcanic eruptions changing the pattern of rain distribution during a couple of years with 50 years intervals (AD 760, 810, 860 and 910).
Hence, in one study volcanoes fertilizes the Maya sites and in another study they destroy the sites. Now, this may of course be the case since the mechanisms are different between soil fertilizing and global climate systems. What troubles me is the use of reductionist approaches, the search for an ultimate cause that triggers whatever you want to explain. These are complex systems and even though volcanoes can bifurcate various systems in different and even contradicting directions, there are other parts of these systems that tend to be downplayed in studies like this. This is yet another Tikal-centered approach and I doubt that volcanic ash can be used as explanation for thousands of sites. I am sure volcanic eruptions continued in the Postclassic period as well. Why did not people continue to inhabit the Peten to the same extent as before? I believe the answers must be sought beyond the volcanoes.