Rheological controls on the eruption potential and style of an andesite volcano: A case study from Mt. Ruapehu, New Zealand

Abstract

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. AbstractThe evolving magma rheology of three recent Ruapehu eruptions (1969, 1977, and 1995) is estimated using a combination of thermodynamic models and rheological calculations, supported by textural observations of the erupted scoria. We use a well-established thermodynamic model to determine the composition of these representative Ruapehu magmas from 300 MPa to ∼ 30MPa. The outputs of the model provide the changing crystal and bubble content in a closed system (assuming no gas loss), as well as the fractionating melt compositions. We calculate the melt viscosity, and the effect of bubbles and crystals, to quantify the rheology of the magma during ascent (under assumed equilibrium conditions). The moderately high phenocryst content of Ruapehu scoria (∼ 30 %) means that only a small amount of additional microlite crystallisation (∼ 5 %) would result in a yield strength, which may lead the magma to stall. However, if the strain rates are high enough, more crystallisation would be possible without developing a yield stress. This suggests that microlite-rich magmas are almost certain to stall unless they encounter significant fluid addition from a source such as a hydrothermal system, groundwater, or surface water (i.e., Ruapehu's Crater Lake).Ruapehu magmas are initially H 2 O-undersaturated and as a consequence, crystallisation and bubble growth were suppressed until the magma achieved saturation, at ∼ 100 to 50 MPa.From this analysis, we suggest that Ruapehu magmas are more likely to erupt compared to magmas of a similar composition that are H 2 O-saturated. This partly explains the regular, albeit small-volume eruptions at Ruapehu and the propensity for phreatomagmatic eruptions when the magma:water ratio is low.

Magmatic Triggering of Earthquakes on Distal Faults as a Potential Medium‐Term Warning Signal From Ruapehu Volcano

“…Unfortunately, there were no geodetic measurements made prior to the eruption, nor was there adequate gas monitoring (we might expect an increase in CO 2 flux during this period), which could confirm this hypothesis. In addition, one would normally expect a commensurate increase in volcano seismicity during dike ascent; however, Kilgour et al () show that magma ascent at Ruapehu is unlikely to generate any significant volume expansion due to gas exsolution until very shallow depths—possibly as shallow as 1 km beneath the vent area—due to the low H 2 O content (<2 wt %) of the magmas.…”

“…The two largest historical eruptions, in 1945 and 1995–1996, were each of the order of 0.1 km 3 of erupted material (Johnston et al, ), while other historical eruptions were significantly smaller, for example, 2007 (Kilgour et al, ). This means that small volumes of magma are able to ascend and erupt within the volcano, and clues to the controls on the high rate of activity are detailed in Kilgour et al (, ).…”

Coseismic and postseismic deformation associated with the 2016 Mw 7.8 Kaikoura earthquake, New Zealand: fault movement investigation and seismic hazard analysis

“…In addition, the tectonics of southwestern New Zealand are dominated by oblique convergence at the Puysegur subduction interface, where the Mw 7.8 Dusky Sound earthquake occurred on July 15, 2009 . New Zealand's tectonics are incredibly active, and interseismic accumulated strain is released by various mechanisms such as instantaneous earthquake ruptures Elliott et al 2012), volcanic explosions (Chardot et al 2015;Kilgour et al 2016), ongoing postseismic mechanisms (Motagh et al 2014), and aseismic slow-slip events (SSEs; McCaffrey et al 2008;Wallace and Beavan 2006).…”

Magma / Suspension Rheology

“…Note that under the condition of Φ ~ Φ m or Φ m = 1; Equation (6.65) reduces to the Phan-Thien and Pham (1997) model (see below). This approach was applied for example in Kilgour et al (2016) and Beckett et al (2014) where η l is the viscosity of the liquid phase and η r,c (Φ c ) is the relative viscosity of the crystal suspension and Φ b and Φ c are calculated with Equations (6.57) and (6.58). This approach was applied to estimate the viscosity of basaltic lava ows, using the crystal suspension as the effective medium (Rhéty et al 2017;Dietterich et al 2018;Chevrel et al 2019b).…”