Experimental constraints on phreatic eruption processes at Whakaari (White Island volcano)

Abstract

f Kakapo Disaster Resilience Trust, 2/48 Brockworth Place, Riccarton, Christchurch 8011, New Zealand a b s t r a c tVigorous hydrothermal activity interspersed by sequences of phreatic and phreatomagmatic eruptions occur at Whakaari (White Island volcano), New Zealand. Here, we investigate the influence of sample type (hydrothermally altered cemented ash tuffs and unconsolidated ash/lapilli) and fragmentation mechanism (steam flashing versus gas expansion) on fragmentation and ejection velocities as well as on particle-size and shape. Our rapid decompression experiments show that fragmentation and ejection speeds of two ash tuffs, cemented by alunite and amorphous opal, increase with increasing porosity and that both are significantly enhanced in the presence of steam flashing. Ejection speeds of unconsolidated samples are higher than ejection speeds of cemented tuffs, as less energy is consumed by fragmentation. Fragmentation dominated by steam flashing results in increased fragmentation energy and a higher proportion of fine particles. Particle shape analyses before and after fragmentation reveal that both steam flashing and pure gas expansion produce platy or bladed particles from fracturing parallel to the decompression front. Neither fragmentation mechanisms nor sample type show a significant influence on the shape. Our results emphasize that, under identical pressure and temperature conditions, eruptions accompanied by the process of liquid water flashing to steam are significantly more violent than those driven simply by gas expansion. Therefore, phase changes during decompression and cementation are both important considerations for hazard assessment and modeling of eruptions in hydrothermally active environments.

Reconstruction of the 2014 eruption sequence of Ontake Volcano from recorded images and interviews

“…(4) Finally, muddy hot water, which did not transform to flash steam, overflowed from the craters. Considering the process of liquid water flashing to steam (e.g., Hedenquist and Henley 1985;Mayer et al 2015) and the conditions of PDCs that result from a phreatic eruption (e.g., Yamamoto et al 1999), our observations suggest that this model is consistent with the proposed sequence of events at Ontake.…”

“…The 2014 eruption was a phreatic one in origin as it did not contain evidence of juvenile magmatism (e.g., Minami et al 2015). One of the dominant mechanisms of a phreatic eruption involves sudden decompression of superheated water, followed by an explosion because of the generation of a large amount of steam (flash steam) (e.g., Browne and Lawless 2001;Hedenquist and Henley 1985;Mayer et al 2015;Thiery and Mercury 2009;Yamamoto et al 1999). In engineering, similar explosive phenomena are referred to as BLEVE (boiling liquid expanding vapor explosion) (Abbasi and Abbasi 2007).…”

Opal-A in Glassy Pumice, Acid Alteration, and the 1817 Phreatomagmatic Eruption at Kawah Ijen (Java), Indonesia

“…Any new intrusion of magma will therefore disturb a hydrothermal system that is already at elevated temperature such that additional heat will destabilize hydrous minerals, and engender vaporization of near-boiling fluids. The latter process, also known as steam-flashing, is known to increase fragmentation energy (Mayer et al, 2015). Moreover, acid alteration weakens rocks, and increases their porosity and permeability, making them much more prone to fragmentation (Mayer et al, 2016).…”

“…Amorphous silica can also form during decompression and quenching of hot hydrous fluids in magmatic and hydrothermal environments at temperatures up to 400 • C (Williamson et al, 2002;Tanner et al, 2015) or during cooling of silica-saturated waters in hot-spring environments (Fournier and Rowe, 1966;Rodgers et al, 2004). Occasionally, acid-altered rocks bearing amorphous silica are expelled directly from the subvolcanic environment during phreatomagmatic eruptions (Christenson and Wood, 1993;Wood, 1994;Christenson et al, 2010;van Hinsberg et al, 2010a;Mayer et al, 2015). In a study of erupted lake sediments from the Ruapehu (NZ) crater lake, Christenson et al (2010) noted the presence of opal-A as replacement of plagioclase.…”

Geological study of phreatic eruptions

“…H 2 O H 2 O mud volcano Bates and Jackson, 1987;, 1997 Phreatic eruption phreatic Barberi et al, 1992 Phreatic eruption Hydro-explosion; Steam-blast eruption; Hydrothermal explosion or eruption Phreatic eruption Barberi et al, 1992Barberi et al 1992 Hydromagmatic eruption Phreatomagmatic eruption , Barberi et al, 199220092015Geshi et al, 2016Mayer et al, 2015, Muffler et al, 1971Hedenquest and Henley, 1985;Yamamoto et al, 1999;Browne and Lawless, 2001;, 2011;Mayer et al, 2015;, 2016a;Oikawa et al, 2016;Maeno et al, 2016 BLEVE Boiling liquid expanding vapor explosion Abbasi and Abbasi, 2007;, 1985 Maeno et al, 2016;Takarada et al, 2016Takarada et al, 1995Takarada et al, , 2002 1 1 , 1995, 2011, 2002, 2002, 2011, , , 2017, Fig. 2 2010, 1980, , 1982, 1994, 1985, 14 C Okuno et al, 1997, 2011AMS 1 g , , 200320 2011201150 18 , 2017, 2007…”