Turrialba (10°02′N, 83°45′W) is a 3,349-m high stratovolcano belonging to the Holocene "Cordillera Central" volcanic belt of Costa Rica. The summit consists of three EW-oriented craters (East, Central, and West).

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Turrialba volcano's high summit elevation and steep slopes, its position upwind of the Central Valley, and its record of explosive eruptions all suggest that it poses a signifi cant threat to Costa Rican population and economy. To better understand the nature and signifi cance of this threat, the geology, stratigraphy, and recent eruptive history of Turrialba were investigated. Outcrops of lava and pyroclastic units from at least 20 eruptions of basalt to dacite are recorded in Turrialba's summit area. The majority of these eruptions preceded a major erosional period that may have involved glaciation and that produced a prominent northeast-facing valley at Turrialba. This period also was apparently marked by a dearth of volcanism. The post-erosional period began with eruptions of massive andesite to dacite lava fl ows ca. 9300 yr B.P. Five of the six most recent eruptions, including the eruption of 1864-1866 A.D., were small volume (<0.03 km 3) phreatic and phreatomagmatic explosive eruptions involving basalt and basaltic andesite. The exception was a Plinian eruption of silicic andesite at ca. 1970 yr B.P. with a volume of ~0.2 km 3. Turrialba's next eruption will likely be similar to the recent eruptions of basaltic to basaltic andesitic composition, although a larger volume and more destructive eruption of silicic andesite to dacite also is possible.

Explosive activity at Arenal and associated tephra fall that has occurred over the 14-year period from 1987-2001 is described. Explosions have been notably variable in both frequency and size. A marked decrease in both frequency and quantity of tephra fallout occurred in early 1998 until the end of 2001. Grainsize distributions of cumulative tephra samples collected once a month are typically bimodal. Aggregation causing premature fallout of fine ash and possibly fallout from ash plumes produced by pyroclastic flows are considered responsible for the bimodality of fallout. Scanning electron microscopy of the glass component of tephra from single explosions show predominantly blocky and blocky/fluidal clast types, interpreted as being the product of vulcanian type explosions. Fragmentation of a mainly rigid, degassed magma body, and a minor molten component is inferred for these explosions. Pyroclastic flows were produced either associated with the larger explosions by a mechanism of column collapse (1987)(1988)(1989)(1990), or unrelated to explosions by partial collapse of the crater wall (1993, 1998, 2000, 2001). Pyroclastic flow activity has migrated from west to north during the period reported. Pyroclastic flow deposits are variable in the quantity of juvenile material and any associated surge component. Large juvenile blocks were partially molten on emplacement and many have a typical cauliform texture. Blocks with both juvenile and lithic textures indicate that at the summit magma was in intimate contact with the pre-existing edifice, rather than as a simple open crater or lava pool. Crater wall collapse may have been promoted by the reduction in explosive activity, which has increased the lava accumulation at the summit and in turn increased instability of the summit region. Thus although explosive activity has waned, if the lava output is maintained, the hazard of pyroclastic flows is likely to continue.