Date of Award

Spring 4-6-1990

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology and Geophysics

First Advisor

Williams, Stanley

Second Advisor

Byerly, Gary

Third Advisor

McCabe, Chad

Abstract

The northernmost active Andean volcano, Cerro Bravo is a small, young, and very explosive stratovolcano. Cerro Bravo has produced voluminous tephra deposits, that are found> 30 km away, as well as, pumice flow deposits, block and ash flow deposits, and high-aspect lava flows, which are found proximally to the volcano. There have been eight episodes of activity during the past 6250±110 years (Herd, 1982), with the most recent <200 years ago and the present being a period of quiescence with no visible activity or thermal manifestations. Stratigraphic relationships suggest the occurrence of an initial explosive phase and a concluding effusive phase of activity during individual episodes. The products of volcanic activity at Cerro Bravo are a chemically and mineralogically monotonous suite of medium-K dacite and high-silica andesite (59.2- 67.5% SiO2 pumices and lavas. The dominant phenocrysts present are plagioclase and hornblende (oxyhomblende in lavas) with lesser quantities of orthopyroxene, titanomagnetite, and rare augite and biotite also present. Petrology and eruption dynamics suggest the existence of a frequently mixing, zoned magma chamber, with 4 wt. % H2O and temperatures of 975-850°C, which is getting smaller with time. Future activity will likely be smaller volumetrically, but with greater frequency. The potential hazards of Cerro Bravo are great. Air fall would likely be the source of the greatest mortality and damage due to the remote location of the volcano. Cerro Bravo has consistently deposited pumice on the city of Manizales, 20 km to the west, totaling in excess of 3 m of tephra. Due to the extended nature of activity at the volcano, the wide airfall distribution, and the massive potential modification of the hydrologic system, renewed activity at Cerro Bravo would result in a major long-term impact on the region.

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