Permeability During Magma Expansion and Compaction

Abstract

Plinian lapilli from the 1060 Common Era Glass Mountain rhyolitic eruption of Medicine Lake Volcano, California, were collected and analyzed for vesicularity and permeability. A subset of the samples were deformed at a temperature of 975°, under shear and normal stress, and postdeformation porosities and permeabilities were measured. Almost all undeformed samples fall within a narrow range of vesicularity (0.7–0.9), encompassing permeabilities between approximately 10−15 m2 and 10−10 m2. A percolation threshold of approximately 0.7 is required to fit the data by a power law, whereas a percolation threshold of approximately 0.5 is estimated by fitting connected and total vesicularity using percolation modeling. The Glass Mountain samples completely overlap with a range of explosively erupted silicic samples, and it remains unclear whether the erupting magmas became permeable at porosities of approximately 0.7 or at lower values. Sample deformation resulted in compaction and vesicle connectivity either increased or decreased. At small strains permeability of some samples increased, but at higher strains permeability decreased. Samples remain permeable down to vesicularities of less than 0.2, consistent with a potential hysteresis in permeability‐porosity between expansion (vesiculation) and compaction (outgassing). We attribute this to retention of vesicle interconnectivity, albeit at reduced vesicle size, as well as bubble coalescence during shear deformation. We provide an equation that approximates the change in permeability during compaction. Based on a comparison with data from effusively erupted silicic samples, we propose that this equation can be used to model the change in permeability during compaction of effusively erupting magmas.

Bubble Coalescence and Percolation Threshold in Expanding Rhyolitic Magma

“…We compare our experimental results with Plinian pumices compiled by Gonnermann et al (). They include samples from the following: (1) the explosive phase of the 1060 CE Glass Mountain eruption of Medicine Lake Volcano, California (Heiken, ); (2) the ∼55‐ka El Cajete member of Valles Caldera, New Mexico (Self et al, ); (3) Unit 5 of the 181 CE Taupo eruption, New Zealand (Houghton et al, ); and (4) Episode I of the 1912 eruption of Novarupta, Alaska (Hildreth & Fierstein, ).…”

“…Predictions and estimates of ϕ cr are based on percolation theory, experiments, and measurements on natural samples. They range from approximately 30% to 78% (Burgisser et al, ; Eichelberger et al, ; Garboczi et al, ; Gaonac'h et al, ; Gonnermann et al, ; Klug & Cashman, ; Lindoo et al, ; Namiki & Manga, ; Takeuchi et al, ).…”

“…Almost all samples from rhyolitic Plinian eruptions have porosities in the range 60–90%, and all are permeable (e.g., Colombier et al, ; Gonnermann et al, ; Mueller et al, , and references therein). Permeability‐porosity of such samples can be fit by a power law with a percolation threshold of approximately 60–70% (e.g., Nguyen et al, ; Rust & Cashman, ).…”

The Influence of Phenocrysts on Degassing in Crystal‐Bearing Magmas With Rhyolitic Groundmass Melts

“…The University of Alaska Fairbanks (UAF) permeameter has a detection limit of ~10 −15 m 2 , as compared to 10 −17 m 2 of Takeuchi et al (), which means our estimated ϕ c is a maximum. Significant magma degassing should occur when viscous ( k 1 ) permeability reaches 10 −15 m 2 (e.g., Clarke et al, ; Gonnermann et al, ; Takeuchi et al, ). We can approximate the relative effect of degassing at lower permeabilities using the characteristic timescale of permeable gas escape: where t is time in seconds, R is the length scale over which gas escapes in meters, μ gas is gas viscosity in pascal‐seconds (10 −5 ; Rust & Cashman, ), Δ P is the pressure gradient driving gas escape in pascals, and k is the viscous permeability in square meters (Rust & Cashman, ).…”

On the link between Earth tides and volcanic degassing

“…These enhancements can ultimately cause the percolation of the gas phase at a somewhat greater depth compared to the tide-free scenario. In consequence, the magma becomes gas-permeable at this greater depth potentially causing enhanced volcanic gas emissions (Rust and Cashman, 2011;Gonnermann et al, 2017). The additional contributions from this greater depth to the volcanic gas emissions may also slightly shift the chemical composition of the overall gas emissions towards the chemical composition of the gas phase at this greater 10 depth when compared to the tide-free scenario (Burton et al, 2007).…”