Two important variables for comparison of these cases of volcanic impacts (Fig. 1) are the depth of burial of sites and the subsistence sustaining areas of those sites. Burial by explosive volcanism is essentially instantaneous, and the depth of burial is proportional to the deleterious impact. Ideally, we would wish to compare cases of volcanic impacts on regional scales, but many cases are weak in this regard because they focus on a single site. In conducting regional volcanological-ecological-archaeological research in El Salvador, Panama, and Costa Rica, my project members and I have been more successful in searching for, finding, and investigating sites buried by \ to 2 m of tephra. Sites buried by less than \ m of tephra are generally poorly preserved because of plowing, bioturbation, erosion, and other postimpact factors. The more deeply buried sites are increasingly difficult to find and excavate. Thus, an aspect of practicality inadvertently assists this study for purposes of comparability, as many of the sites under consideration in this chapter had approximately similar depths of burial with approximately similar tephra impacts on the environment and roughly comparable effects upon those sites and their sustaining areas. Some more deeply buried sites, such as Cuicuilco (México) and Ceren (El Salvador), are also considered here, but in less detail.

5.2.1. Arenal, Costa Rica

People occupied the Arenal area of northwestern Costa Rica for at least the last 10,000 years, most of which was prior to the earliest eruption of the Arenal Volcano in ca. 2000 B.C. (Sheets, 1994). The Arenal Vol

Mao Volcanoes Mesoamerica
FIGURE 1 Map of Middle America. Volcanoes and the pre-Columbian societies affected by them that are explored in this chapter are indicated. The dashed line in Honduras separates Mesoamerica to the west from the Intermediate Area (lower Central America).

cano erupted 10 times in four millennia with major explosive eruptions (Melson, 1994), including the 1968 eruption, an average of one per 400 years. These eruptions provide cases of sudden stresses on egalitarian social groups with diversified subsistence economies and relatively low population densities. The eruptions that are relatively well dated and understood are presented in Table 1 within the context of the regional cultural history as discovered by the Arenal Research Project (Sheets, 1994). All of Arenal's pre-Columbian eruptions affected egalitarian societies (Sheets, 1994). Thus, the Arenal area provides a contrast to most of the other cases in Middle America where explosive volcanism affected more complex societies with quite different adaptations and political and economic systems and greater population densities. Therefore, the Arenal eruptions and affected societies are examined more closely than most other cases in this chapter.

The hilly research area spans the Continental Divide, with elevations on both Atlantic and Pacific drainages ranging from 400-1000 m. Mean precipitation varies considerably, with averages as low as 1300 mm at low-elevation western stations to averages over 6000 mm at stations in the east close to the Arenal volcano. That

TABLE 1 Principal Explosive Eruptions of the Arenal Volcano, Costa Ricafl


Date (est.)

Cultural phase

Immediate effects

Long-term effects detected

Unit 10

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