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Santiaguito volcano, active for the past 62 yr, has produced almost 1 km3 of compositionally uniform, soda-rich, calc-alkalic dacite lava that is preserved in 22 distinct extrusive units. The volcanic activity has consisted of endogenous extrusions, block-lava flows, Merapi-type block and ash flows, vertical pyroclastic eruptions, and lahars. Since 1960, block-lava flows have been volumetrically more important than endogenous domes, which were dominant in earlier periods. A 10–12-yr cyclic pattern of activity is marked by the alternation of 3–5 yr periods of high extrusion rates with longer periods of low extrusion rates. The most recent period of high extrusion rates (1972–75), was marked by several large lava flows accompanied by large block and ash flows. During the high extrusion rate period, two distinct vents erupted. However, since 1977 only the Caliente vent, the principal vent at Santiaguito, has remained active. From 1975 through 1984, vertically directed pyroclastic eruptions were especially conspicuous. These erupted bread-crust blocks and locally dispersed fine phreatomagmatic ash. Since 1979, small Merapi-type block and ash flows have been erupted repeatedly from the toe of a small, 400-m-long block-lava flow that is being extruded from the Caliente vent and is flowing down a steep southern slope. On the basis of past behavior, a new period of high extrusion rates is overdue. It may cause an increased extrusion rate for the block-lava flow, or it may result in the extrusion of a new lateral dome unit. At present, the most serious hazards are from mudflows, which occur in each monsoon season and which affect areas more than 10 km from the dome. This problem is worsened by the mid-1970’s shift of extrusion to the east side of the dome, which affects river valleys that have been resettled during the past 40 yrs. If the next expected high extrusion rate period focuses on the Caliente vent, as the trends now suggest, the hazard could increase markedly. The last extrusion rate maximum of this type, in 1929–33, resulted in very large block and ash flows that caused many deaths.

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