Garima Lohani

Publication Date

2018

Document Type

Dissertation/Thesis

First Advisor

Walker, James Allen, 1952-

Degree Name

M.S. (Master of Science)

Legacy Department

Department of Geology and Environmental Geosciences

LCSH

Geology

Abstract

The Masaya caldera complex is a basaltic shield volcano that has produced rare basaltic Plinian eruptions (BPE) but is characterized by effusive activity in the last 6000 years. More than two million people live within a 20 km radius of the caldera. Magma viscosity plays an important role in determining the eruption style of a volcano. In this thesis, I provide the first viscosity measurements on an erupted magma composition and examine the role of magma viscosity in determining eruptive style at Masaya and in promoting BPE's. Viscosity experiments were conducted for a Masaya lava sample at superliquidus, subliquidus and near glass transition temperatures. The measured viscosity values ranged from 5.6 Pa s to 2.61 x 10¹² Pa s at temperatures from 1551°C to 720°C. Results from the superliquidus viscosity experiments suggest that the melt deviates from Newtonian to non-Newtonian behavior at 1352°C. Rheological changes from Newtonian to non-Newtonian generally suggests the presence of crystals. The quenched glass samples from the viscosity experiments did not provide any evidence for presence of macro-or micro- crystals but Raman spectroscopy data suggests presence of nanolites. The oxidized conditions during the experiments lead to an increase in melt polymerization thereby increasing melt viscosity. Comparison of the viscosity of a Masaya lava to viscosity measurements of basaltic lavas from Pacaya, Cima, Holuhraun and Etna volcanoes indicates that Masaya's viscosity was the highest. However, the viscosity measurements for the Masaya lava, at eruption temperatures, are not high enough to produce explosive eruptions. Instead several factors, including elevated magma viscosity, volatile content, ascent rate and bubble overpressure likely combined to produce the BPE's at Masaya.

Comments

Committee members: Carpenter, Philip J.; Frank, Mark R.||Advisor: Walker, James A.||Includes illustrations and maps.||Includes bibliographical references.

Extent

72 pages

Language

eng

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

Media Type

Text

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