EVOLUTION AND ERUPTION OF GEOTHERMALLY COOLED MAGMA BODIES

Abstract

Magmatic geothermal systems extract heat and volatiles from deep magma bodies and transport these upward through the brittle crust. Sometimes, these same magma bodies evolve toward an unstable, overpressured state, precipitating a catastrophic caldera eruption. Understanding the deep coupling between magmatic and geothermal systems can help contextualise our near surface observations and plan for utilisation of the deep resource. In addition, insight into how the system transitions between convective and eruptive phases has implications for our understanding of volcanic hazard.
Here, we introduce a lumped parameter thermomechanical model of a magma body that is being cooled from above by geothermal systems, and recharged from below by deep magma sources. The model tracks the evolution of temperature, pressure and magma composition, and includes parameterised descriptions of eruption, volatile leakage across a viscoelastic shell, and overlying geothermal systems. We have used this model to explore generic eruptive styles and to develop an approximation of the eruptive record of the modern Taup? Volcanic Zone (TVZ).
Our findings suggest that magma bodies at 4 to 5 km depth in the TVZ are overlaid by a 500 m thick, high-temperature, partly permeable (10-19 -10-20 m2), viscoelastic shell. The timing and volume of eruptions are dominated by short intervals of sporadic magma recharge originating deeper in the crust. Long-term climatic modulation of rainfall driven geothermal systems has minimal impact on eruptive timing. However, under certain conditions, efficient geothermal systems can trigger an eruption by cooling a magma body too rapidly, resulting in gas exsolution that builds to a critical overpressure. These insights highlight the value of coupled magmatic-geothermal models, and suggest development of a magmatic reservoir simulator could shine new light on the complex physical interplays occurring deep in the TVZ.