An improved age framework for late Quaternary silicic eruptions in northern Central America

Five new stepwise-heating Ar-40/Ar-39 ages and one new high-sensitivity C-1 4 date of ash-fall and ash-now deposits from late Quaternary silicic volcan oes in northern Central America,document the eruption rates and frequencies of five major rhyodacite and rhyolite calderas (Atitlan, Amatitlan, Ayarza , Coatepeque, and Ilopango) located north of the basalt, andesite, and daci te stratovolcanoes of the Central American volcanic front. These deposits f orm extensive time-stratigraphic horizons that intercalate regionally, and knowledge of dates and stratigraphy provides a valuable framework for age d eterminations of more localized volcanic and nonvolcanic events. The new da ta; especially when integrated with previous stratigraphic and dating work, show that all five calderas erupted several times in the past 200 ka and, despite a lack of historic activity, should be considered as active centers that could produce highly explosive eruptions again. Because of their loca tions near the highly vulnerable economic hearts of Guatemala and El Salvad or, the risks of eruptions from these calderas should be carefully consider ed along with risks of major earthquakes and volcanic front volcanoes, whic h are much more frequent but inflict less severe and extensive damage. This investigation also includes some examples of dating efforts that failed to produce reasonable results.