Magma ascent during eruptions at Mt Ngauruhoe: Insights from xenolith fluid inclusions

Technical Abstract

Eruptions from Ngauruhoe volcano have occurred frequently in recent times. During most of these eruptions, the basement rock that exists many kilometres below the volcano is eroded and transported to the surface as foreign rock fragments called xenoliths. These xenoliths are almost entirely composed of quartz crystals. When the quartz crystals were growing under the volcano, small bubbles of mainly CO2 and H2O gas and liquid were trapped, recording the pressure, and therefore depth, at which a given quartz crystal grew.

This study used two standard geological techniques to measure the gas pressure of inclusions within quartz crystals. Using a Laser Raman facility at Geoscience Australia, Canberra and a fluid inclusion freezing/heating stage microscope at GNS Science, Taupo, we were able to accurately measure the pressure within CO2-rich inclusions. What we found was that all of the inclusions record a similar pressure of crystal growth. The results of this work indicate that a network of shallow magma chambers existed (and probably still exists) at a depth of between 1.5 and 3.5 km below the crater of Ngauruhoe. This is an important finding for improving the interpretation of periods of volcanic unrest at Ngauruhoe. Earthquakes are produced by magma movement either within a magma chamber or within a conduit on its way to the surface before erupting. Now that we have a better understanding of where the magma storage areas are located under the volcano, our ability to forecast the likely timing of future volcanic activity will be enhanced.

The second aspect to this study was to determine the speed at which magma moves to the surface (its ascent rate). First, we calculated the physical properties (e.g. density, viscosity) of the host magma and the xenoliths; then we obtained a minimum magma ascent rate of at least 1 m/hr using calculations of how fast the xenoliths would settle downwards in the magma through the effect of gravity. It is likely that the true magma ascent rate before an explosive eruption would be much faster, perhaps as high as 1 km/hr, and therefore there are a number of factors that increase the rate of magma rise, such as the formation of gas bubbles as the magma depressurises. The potential large difference between a simple settling calculation and a modelled ascent rate show that bubble and crystal formation within a magma are significant factors and modelling their effect is extremely complicated. If magma rises at a rate of 1 km/hr then the estimated time for magma to travel from a shallow storage chamber (1.5 to 3.5 km depth) is a matter of hours rather than days or months, suggesting that monitoring of magma recharge or activity in the shallow magma chamber is an important tool in predicting volcanic activity at Ngauruhoe.