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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.