Effect of temperature on the permeability of lava dome rocks from the 2004–2008 eruption of Mount St. Helens

Abstract

As magma ascends to shallow levels in the volcanic conduit, volatile exsolution can produce a dramatic increase in the crystal content of the magma. During extrusion, low porosity, highly crystalline magmas are subjected to thermal stresses which generate permeable microfracture networks. How these networks evolve and respond to changing temperature has significant implications for gas escape and hence volcano explosivity. Here, we report the first laboratory experimental study on the effect of temperature on the permeability of lava dome rocks under environmental conditions designed to simulate the shallow volcanic conduit and lava dome. Samples were collected for this study from the 2004-2008 lava dome eruption of Mount St. Helens (Washington State, USA). We show that the evolution of microfracture networks, and their permeability, depends strongly on temperature changes. Our results show that permeability decreases by nearly four orders of magnitude as temperature increases from room temperature to 800°C. Above 800°C, the rock samples become effectively impermeable. Repeated cycles of heating leads to sample compaction and a reduction in fracture density and therefore a decrease in permeability. We argue that changes in eruption regimes from effusive to explosive activity can be explained by strongly decreasing permeability caused by repeated heating of magma, conduit walls and volcanic plugs or domes. Conversely, magma becomes more permeable as it cools, which will reduce explosivity.

Permeability Evolution in Variably Glassy Basaltic Andesites Measured Under Magmatic Conditions

“…While thermal cracking in those rocks acted to combat permeability decrease, water‐facilitated precipitation‐driven crack sealing acted to further reduce permeability at T < 600°C. We conclude that the microstructural changes proposed by Gaunt et al () do very little to modify the permeability of the samples in the present study. Indeed, permeability of our volcanic materials is largely unaffected by changes in temperature below the calorimetric glass transition.…”

“…The permeability of volcanic products has been extensively studied under ambient laboratory conditions (e.g., Bernard et al, ; Farquharson et al, ; Klug & Cashman, ; Kushnir et al, ; Rust & Cashman, ; Saar & Manga, ; Wright et al, ) and while such measurements can provide critical post mortem insight into the processes governing permeability creation and destruction, they only provide a snapshot of a moment in the structural evolution of volcanic materials. Further, the permeability of erupted volcanic products may be modified by postemplacement processes—for example, cooling (Gaunt et al, ; Vinciguerra et al, ), crystallization (Kushnir et al, ; Wright et al, ), and transport (Siebert, )—that play important roles in the upper edifice and dome but may be less appropriate in constraining fluid movement deeper within the conduit. Thus, the expansion of these studies to conditions appropriate to the middle‐ to lower‐conduit requires the implementation of high temperature techniques.…”

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

“…For instance, cooling results in contraction, which may generate macroscopic thermal cracks to promote fluid flow (Lamur et al, ), yet it may also precipitate secondary mineralization that can, through time, block otherwise permeable pathways (Heap et al, ). In contrast, the impact of temperature increase (e.g., from magma intrusion) on the resultant rock permeability remains poorly constrained (Gaunt et al, ; Kushnir et al, ) as it has received noticeably less attention than the influence of changes in pressures on permeability (e.g., Cant et al, ; Eggertsson et al, ; Gudmundsson, , ; Heap, Baud, et al, ; Heap et al, ; Lamur et al, ). Understanding how the permeability of reservoir rock changes in response to thermal fluctuation is critical to elaborate solutions to maintain the economic potential of hydrothermal resources.…”

Cooling‐dominated cracking in thermally stressed volcanic rocks

“…The aim is to increase reservoir permeability and fracture surface area and thereby enhance the efficiency of the geothermal system. In addition to these processes, it is likely that cooling-related fractures increase the permeability and rate of degassing of magma in shallow conduits [Tuffen and Dingwell, 2005;Tuffen et al, 2003] as well as at the surface in viscous domes and lava flows [Cabrera et al, 2011;Gaunt et al, 2016].…”