Date of Award

12-1990

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology and Geophysics

Abstract

Galeras volcano is located in southwestern Colombia, rising 1600 m above Pasto the capital city (population of 300,000) of the department of Nariño. The most recent activity at Galeras volcano(< 4500 years) included six major eruptions before the reactivation of 1988-present. The activity at the volcano has been characterizated by eruptions of vulcanian type. Gravitational column collapse is the most common origin of the fines-depleted co-ignimbrite breccias and pyroclastic flows. Commonly, a large part of the material ejected with each explosion is not juvenile but includes country rock as accessory lithics. In the initial explosions at Galeras, the country rock appear to be more involved that any other material. Later in the eruption, the products become more juvenile-rich deposits. The total volume of deposits related to Galeras volcano has been calculated to be 0.71 km3

Galeras volcano has had a fairly constant chemical composition, characterized by high silica, medium potassium andesites. Products at Galeras are mainly composed of zoned plagioclase, hypersthene, augite, titanomagnetite, ilmenite and traces of apatite. Amphibole and olivine may be also present. A shallow, small magma ( « 0.5 km3) chamber is consistent with the data for Galeras Volcano. The magmatic conditions in this reservoir prior to eruptions have been determined using an oxide-pair geothermometer and a glass geothermometer. Two main sets of magmas are identified: a degassed magma erupted at temperatures from 850-900 °C (corresponding to lava flows) and a higher water-content magma (up to 4 % water) at temperatures from 890 to 950 °C, generally found in the pyroclastic flows. The present activity at Galeras confirms the idea of the recharge of new magma from deeper levels into a shallow, small magma chamber.

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