The A.D. 1835 eruption of Volcan Cosiguina, Nicaragua: A guide for assessing local volcanic hazards

The January 1835 eruption of Volcán Cosigüina in northwestern Nicaragua was one of the largest and most explosive in Central America since Spanish colonization. We report on the results of reconnaissance stratigraphic studies and laboratory work aimed at better defining the distribution and character of deposits emplaced by the eruption as a means of developing a preliminary hazards assessment for future eruptions. On the lower flanks of the volcano, a basal tephra-fall deposit comprises either ash and fine lithic lapilli or, locally, dacitic pumice. An overlying tephra-fall deposit forms an extensive blanket of brown to gray andesitic scoria that is 35–60 cm thick at 5–10 km from the summit-caldera rim, except southwest of the volcano, where it is considerably thinner. The scoria fall produced the most voluminous deposit of the eruption and underlies pyroclastic-surge and -flow deposits that chiefly comprise gray andesitic scoria. In northern and southeastern sectors of the volcano, these flowage deposits form broad fans and valley fills that locally reach the Gulf of Fonseca. An arcuate ridge 2 km west of the caldera rim and a low ridge east of the caldera deflected pyroclastic flows northward and southeastward. Pyroclastic flows did not reach the lower west and southwest flanks, which instead received thick, fine-grained, accretionary-lapilli–rich ashfall deposits that probably derived chiefly from ash clouds elutriated from pyroclastic flows. We estimate the total bulk volume of erupted deposits to be ∼6 km3. Following the eruption, lahars inundated large portions of the lower flanks, and erosion of deposits and creation of new channels triggered rapid alluviation. Pre-1835 eruptions are poorly dated; however, scoria-fall, pyroclastic-flow, and lahar deposits record a penultimate eruption of smaller magnitude than that of 1835. It occurred a few centuries earlier—perhaps in the fifteenth century. An undated sequence of thick tephra-fall deposits on the west flank of the volcano records tens of eruptions, some of which were greater in magnitude than that of 1835. Weathering evidence suggests this sequence is at least several thousand years old. The wide extent of pyroclastic flows and thick tephra fall during 1835, the greater magnitude of some previous Holocene eruptions, and the location of Cosigüina on a peninsula limit the options to reduce risk during future unrest and eruption.