IAVCEI/CEV Webinar: The causes and consequences of mafic explosive volcanic eruptions

IAVCEI/CEV Webinar: The causes and consequences of mafic explosive volcanic eruptions

Dear colleagues,

We are happy to introduce Dr. Brittany Brand as our speaker for the next webinar on April 20, 2023, at 17:00 CEST.
Please see the abstract (by authors: Brittany Brand-primary speaker, Aaron Marshall, and Pedro Valdivia) below, and do not forget to register HERE!

The causes and consequences of mafic explosive volcanic eruptions: A case study from the mafic Curacautín Eruption at Volcan Llaima, Chile

Abstract: Mafic eruptions dominate volcanic activity on Earth. While the majority of these eruptions exhibit effusive to mildly explosive behavior, more violent explosive eruptions are possible. However, the causes of strongly explosive behavior in mafic eruptions remain less well understood relative to their high silica counterparts.
Here we explore the conditions that promote highly explosive, mafic volcanism using the Curacautín eruption at Volcan Llaima, Chile, which produced an extensive ignimbrite (~3.5–4.5 km3 DRE). We use pyroclast textural properties including porosity, bubble number density, bubble interconnectivity, tortuosity, microlite crystal size distribution (CSD), and microlite number densities to investigate the role of magma ascent and degassing processes that contributed to the Curacautín explosive eruption.  
The groundmass of Curacautín pyroclasts is composed of up to 44% plagioclase microlites with volumetric number densities between 8.21 × 106 and 1.84 × 107  mm−3. More than 85% of microlites are ≤10 μm in length. In addition, we identify two distinct populations of vesicles: (1) a convoluted interconnected vesicle network produced by extensive coalescence of smaller vesicles (> 99% of pore volume), and (2) a population of very small and completely isolated vesicles (< 1% of porosity). Finally, in all examined pyroclasts, heterogeneous vesicle textures are in sharp contact with one another and overprinted by ≤10 μm unbroken microlites.

The combination of our CSDs, microlite number densities, and 3D vesicle textures suggest that the Curacautín magma underwent a rapid ascent that induced high disequilibrium conditions, promoting rapid syn-eruptive crystallization of microlites within the shallow conduit. We interpret that microlite crystallization increased viscosity while simultaneously forcing bubbles to deform as they grew together, resulting in the convoluted interconnected vesicle network. . Further, despite the textural evidence that the Curacautín magma reached the percolation threshold, we propose that rapid ascent suppressed outgassing and increased bubble overpressures, and that second boiling and the overpressures within the small, isolated vesicle population led to brittle fragmentation. Finally, the heterogeneous vesicle textures in sharp contact with one another suggest repeated autobrecciation and fusing within the conduit. The overprinting of sutures with unbroken  ≤10 μm microlites indicates that the the highly-crystalline groundmass formed at shallow depths after autobrecciation and fusing.
Using the bubble number densities for the isolated vesicles, we obtain a minimum average decompression rate of 1.4 MPa/s (only applies to the shallow subsurface). Further, using the porosity and permeability of our samples, we estimated that a bubble overpressure > 5 MPa could have been sufficient to fragment the Curacautín magma. Other mafic explosive eruptions report similar disequilibrium conditions induced by rapid ascent rate, implying that syn-eruptive disequilibrium conditions may play a significant role in regulating the explosiveness  of mafic eruptions.

Related Publications:
Marshall, Aaron (2022) Mafic Explosive Volcanism at Llaima Volcano: 3D X-Ray Microtomography Reconstruction of Pyroclasts to Constrain Shallow Conduit Processes. [Doctoral Dissertation] Boise State University, Boise, Idaho.
Marshall, A. A., Manga, M., Brand, B. D., & Andrews, B. J. (2022). Autobrecciation and fusing of mafic magma preceding explosive eruptions. Geology, 50(10), 1177-1181.
Marshall, A.A, Brand, B.D., Martínez, V., Bowers, J.M., Walker, M., Wanless, V.D., Andrews, B.J., Manga, M., Valdivia, P., and Giordano, G., (2022), The mafic Curacautín ignimbrite of Llaima volcano, Chile. Journal of Volcanology and Geothermal Research, 421, 107418  [*Graduate Student Paper; Published with BSU Affiliation]
Valdivia, P., Marshall, A.A., Brand, B.D and Manga, M., (2022), Mafic explosive volcanism at Llaima Volcano: 3D x-ray microtomography reconstruction of pyroclasts to constrain shallow conduit processes. Bulletin of Volcanology, 84(1), 1-18.  [*Graduate Student Paper; Published with BSU Affiliation]
Valdivia Munoz, P. (2021) Mafic explosive volcanism at Llaima volcano (Chile): 3D X-ray microtomography reconstruction of pyroclast to constrain shallow conduit processes. [Masters Thesis] Boise State University, Boise, Idaho

We encourage IAVCEI members and undergraduate students worldwide to participate actively.
You need to register with the paid membership 2023, using your membership number for verification (to activate your status, please click HERE).

We are looking forward to seeing (virtually) you there!

Patrick Allard, IAVCEI President
Roberto Sulpizio, IAVCEI Secretary General

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