Externally triggered renewed bubble nucleation in basaltic magma: The 12 October 2008 eruption at Halema‘uma‘u Overlook vent, Kīlauea, Hawai‘i, USA

Abstract

until present, dozens of small impulsive explosive eruptions occurred from the Overlook vent on the southeast side of Halema'uma'u Crater, at Kīlauea volcano, USA. These eruptions were triggered by rockfalls from the walls of the volcanic vent and conduit onto the top of the lava column. Here we use microtextural observations and data from clasts erupted during the well-characterized 12 October 2008 explosive eruption at Halema'uma'u to extend existing models of eruption triggering. We present a potential mechanism for this eruption by combining microtextural observations with existing geophysical and visual data sets. We measure the size and number density of bubbles preserved in juvenile ejecta using 2D images and X-ray microtomography. Our data suggest that accumulations of large bubbles with diameters of >50 mm to at least millimeters existed at shallow levels within the conduit prior to the 12 October 2008 explosion. Furthermore, a high number density of small bubbles <50 mm is measured in the clasts, implying very rapid nucleation of bubbles. Visual observations, combined with preexisting geophysical data, suggest that the impact of rockfalls onto the magma free surface induces pressure changes over short timescales that (1) nucleated new additional bubbles in the shallow conduit leading to high number densities of small bubbles and (2) expanded the preexisting bubbles driving upward acceleration. The trigger of eruption and bubble nucleation is thus external to the degassing system.

Dynamics of an open basaltic magma system: The 2008 activity of the Halema‘uma‘u Overlook vent, Kīlauea Caldera

“…The very unusual textures observed in the JF pumice -at least three vesicle size populations (mm-, 100's of μm-and 10's of μm-sized vesicles) ( Table 2 and Fig. 2) -indicate that the parent magma experienced two or three nucleation events (Carey et al, 2012) prior to eruption. The last of them was short and inferred to be synchronous with the eruption, producing the population of very small and spherical vesicles (10's of μm) which, in some particles, occurs as a thin halo at the moulded surface of the particles (Fig.…”

“…Since the onset of the eruptive activity in March 2008, a persistent gas plume has been rising from the vent carrying juvenile and lithic pyroclasts which settle near the crater Houghton et al, 2013;Patrick et al, 2011;Swanson et al, 2009;Wooten et al, 2009). Weak explosions, triggered by vent wall and rim collapses (Carey et al, 2012;Houghton et al, 2011Houghton et al, , 2013Orr et al, 2013), have periodically interrupted this background activity of outgassing and minor production of pyroclasts. In addition to observational and geophysical data recorded remotely by the Hawaiian Volcano Observatory since 2008, a neardaily downwind sampling of the pyroclasts via a network of ash collectors (Swanson et al, 2009), has supplied an exceptional record of the ejecta since 2008.…”

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

“…The Halema'uma'u crater at the time of the eruption contained an active lava lake, periodically disturbed by rockfalls from the overhanging crater wall. One particularly large rockfall caused wholesale disruption of the crust of the lake, resulting in decompression of a temporarily overpressured, vesicle-rich lava body, and very rapid additional exsolution of a high proportion of fine gas (mostly water) bubbles, triggering an explosive eruption (Carey et al 2012). This combination of processes-crust destruction and degassing-could give rise to the circumstances recorded by the Murphy Well dendrite layer.…”

“…Given the low viscosity of the 32 wt% MgO liquid, the volatiles would have escaped rapidly and efficiently (e.g. Carey et al 2012), leaving the whole flow in an undercooled state. In the lower portion of the flow (porphyritic peridotite and basal zone), phenocrysts provided nucleation sites, but in the uppermost layer (amygdaloidal and transition zones), nucleation was homogenous and crystals started to grow throughout the entire layer.…”

From magma ascent to ash generation: investigating volcanic conduit processes by integrating experiments, numerical modeling, and observations

“…Textural measurements on samples Textural data provide information on magma pressure and thermal histories in conduits and plumes. For example, vesicle number densities (VNDs; i.e., the number of vesicles per unit bulk volume) provide estimates of magma decompression rates [Toramaru 2006, Shea et al 2011, Wright et al 2012b] and eruption intensities Cashman 2011, Alfano et al 2012], while crystal size distributions (CSDs) and vesicle size distributions (VSDs) (i.e., the number of crystals or vesicles in each size class per unit bulk volume) provide information on crystallization and differentiation in magma reservoirs and conduits [Fornaciai et al 2009, Shea et al 2009, Brugger and Hammer 2010, Arzilli and Carroll 2013, and on magma vesiculation [Bai et al 2008, Gurioli et al 2008, Colò et al 2010, Shea et al 2010, Carey et al 2012, permeability [Mueller et al 2008, Bouvet de Maissoneuve et al 2009, Polacci et al 2014, Heap et al 2014, Kendrick et al 2016, Colombier et al 2017, and degassing/outgassing [Burton et al 2007, Degruyter et al 2010a, 2012. Furthermore, VSDs, CSDs, melt chemistry and volatile content feed into magma rheology estimates [Mader et al 2013, Vona et al 2013.…”