Document Type


Date of Degree

Spring 2019

Degree Name

MS (Master of Science)

Degree In


First Advisor

Ukstins, Ingrid A

First Committee Member

Peate, David W

Second Committee Member

Reagan, Mark K


Yasur is an active scoria cone volcano in the Siwi Caldera on the island of Tanna, Vanuatu, in the South Pacific. This volcano has been erupting continuously for the last 800 years and is the latest manifestation of episodic volcanic activity in this area dating back to the late Pliocene. Yasur eruptions consist of intermittent Strombolian-style explosions of pyroclastic debris with emissions of volatiles such as SO₂, HCl and HF. Other than CO2 and H2O, the most abundant gas emitted from Yasur is sulfur, and plume monitoring has confirmed the volcano as one of the largest point sources of sulfur on the planet with an average flux of 600-1400 tons/day. Fluorine poses a chronic environmental health risk on Tanna, so understanding long-term exposure rates as well as periodic increases in volcanic intensity will help to better quantify its risk. In this study we gauge compositional variation of magma using fresh pyroclastic bombs collected over a 3-month period from August to November, 2016. Our results suggest long-term broad compositional stability in both the whole-rock and groundmass glass and minerals. Our results show slight variation in volatile phases in both olivine-hosted melt inclusions and groundmass glass over an intensively sampled 3-month period, which suggests that the plumbing beneath Yasur harbors an open-system degassing environment. Volcanic eruptions are usually driven by magma mixing, however, our results show no compositional variation in phenocrysts. We show that Yasur is an excellent example of an inefficiently degassed volcano, and that volcanic activity is controlled by volatile flux. We also use cotectic compositional data to calculate pressure and temperature conditions within the magma chamber and assess fluxes of volatiles from the magma using melt inclusion analyses for S and Cl. Our study places new bounds on the vertical extent of the magma chamber and suggests differentiation from a basaltic trachyandesite at depths of up to 12 km.


xi, 73 pages


Includes bibliographical references (pages 49-53).


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Copyright © 2019 Tanner Hartsock

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