Brittle fragmentation, generating small pyroclasts from magma, is a key process determining eruptive style. How low-viscosity magma fragments within a rising fountain in a brittle manner, however, is not well understood. Here we describe a fragmentation process in Hawaiian fountains on the basis of observations from the 2018 lower East Rift Zone eruption of Kīlauea Volcano, Hawai’i. The dominant fragmentation mechanism is inertia driven and produces a population of large fluidal pyroclasts. However, when sufficient volcanic gas is released in the fountain, a subpopulation of smaller and more vesicular pyroclasts is generated and entrained into the gas-dominant convective plume. The size distribution of these pyroclasts is similar to that of brittlely fragmented solid materials. The erupted high-vesicularity pyroclasts sometimes preserve a deformed shape. These observations suggest that late-stage rapid expansion lowers the gas temperature adiabatically and cools the outer surface of liquid pyroclasts below the glass transition temperature. The rigid crust fragments as the hot interior attempts to expand due to further volatile diffusion from the melt into bubbles. Adiabatic expansion of volcanic gas occurs in all eruptions. Brittle fragmentation induced by rapid adiabatic cooling may be a widespread process, although of varying importance, in explosive eruptions.
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To create Figs. 2 and 4 and Extended Data Fig. 7, we used MATLAB 9.5. The MATLAB scripts used for the calculations are available from the corresponding author upon request. The CT data in Fig. 3 was analysed using Fiji Is Just ImageJ (https://imagej.net/Fiji) with the Trainable Weka Segmentation plug-in (https://imagej.net/Trainable_Weka_Segmentation).
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We thank M. Ramsey for allowing us to use his FLIR camera, M. Benage for internal review at the USGS, L. DeSmither for sharing her data for fountain height, the field crews from the USGS for their help during the field work, D. Churchill for taking the SEM images in Extended Data Fig. 3 and Y. Tanaka for measuring the vesicularity in Extended Data Table 1. X-ray microtomography was enabled by access to the Lawrence Berkeley National Lab Advanced Light Source on beamline 8.3.2. A.N. was supported by JSPS Kakenhi grant numbers 17KK0092 and 19H00721. M.M. was supported by NSF grant number 1615203. B.F.H. was supported by NSF grant number 1829188. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Laura Spina and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely.
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a,b, Panoramic view from the location marked by the red circle in (c) taken on September 16th, 2018. The view angle is denoted by characters α − χ in (c). c, Photo around the F8 cone downloaded from Google Earth in June 2020. The bluish region is the overlay of the digital elevation map around the F8 cone created from thermal images acquired during a helicopter overflight on August 15th, 2018. The yellow square and green square with white rim are the location where the photos in Figs. 1a, 2, and 1b were taken, respectively. The red circle and blue plus indicate the sites where samples in Fig. 3a,b-c are collected, respectively. d, Pictures of the ground surface in a region 0.5 m × 75 mm taken in August 2018. The numbers correspond to the locations denoted by × in (c). The location close to the vent ( × 1 in c) is covered by silvery spatter. Sites close to the cone ( × 2-16) were initially covered by lava flows, then covered by broken reticulite, Pele’s hairs, and tears. The typical size of the pyroclasts is smaller than 50 mm. The locations west of number 17 are covered by large ( > 50 mm) reticulite with a black rind that erupted during the fountaining. The volume ratio of the widespread tephra/cone for F8 has not yet been reported but based on the 1959 Kīlauea Iki cone with similar dimensions to the F8 cone, the volume fraction of F8 widespread tephra is as low as 2%.
A large spatter clast close to the vent ( × 1 in Extended Data Fig. 1c).
Back-Scatter Electron (BSE) Scanning Electron Microscope (SEM) images of pyroclasts from F8 shown in Fig. 3b. Images were collected at 20kV on a Zeiss EVO-10 Variable Vacuum SEM at the University of California, Berkeley. a, A wide field of view shows that crystallinity is low except for the region with clusters of phenocrysts (for example, blue rectangle). The area fraction of crystals in the blue rectangle is 25 %. The outer rim of the sample outlined by the red curve, which experienced fragmentation, contains few crystals. b, Magnified image shows the microlite free glass between bubbles.
Same as Fig. 2, but for other time spans. On 4 and 6 June (19:47-20:00), the fountain shows similar trends observed in Fig. 2a-c, 6 June (20:01-20:12). On 31 May, (a-c and blue curve in j), we used another lens (FOL36mm lens, 28∘), and the spatial resolution is 0.19 m/pixel. Thus it is difficult to compare with other data directly. However, the areal ratio Sh/Sl is lower than on 4 and 6 June, but higher than 8 June.
The right and left parts were initially connected. To show the interior, we broke a pyroclast entirely coated by a black glassy rind. The internal brown region is made of small bubbles. A large bubble exists in the middle of the pyroclast. This photograph is taken on August 9th at the location denoted by blue plus in Extended Data Fig. 1.
A collection of photographs of the high vesicularity pyroclasts including large bubbles ( ~ 20-30 mm). The size of each picture is approximately 0.1 × 0.1 m2. The edge color of each panel indicates the image locations of samples; blue and whites correspond to blue + and around the white × in Extended Data Fig. 1, and black indicates 1.6 km west-southwest from the F8 vent. The photographs are taken from July - September 2018.
a, The vent pressure estimated from fountain height and magma vesicularity. Yellow lines identify the observed range of fountain heights, 30-80 m. b, The estimated gas temperature following adiabatic expansion by the pressure reduction from (a). The blue region indicates the glass transition temperature of 680-730 ∘C53,57. c, Estimated vesicularity range from the solubility of water in a basaltic magma62. The initial volatile fraction of the LERZ magma is not yet known, but that for other similar fountains is estimated to be 0.55-0.75 wt.%63, which we used. The blue curves and region show exsolved volatile fraction and vesicularity at the vent, respectively. The red curve shows the remaining volatiles in the melt. We here do not consider CO2, because the estimated CO2 fraction of 170-400 ppm63 is sufficiently smaller than water, and CO2 bubbles exsolved deeper may behave differently from steam bubbles. d, Vesicularity of pyroclasts. The blue and red curves show the vesicularity change from ϕo to ϕ by adiabatic and constant temperature decompression, respectively, from 106 Pa to 105 Pa. We here use 106 Pa as a typical estimate based on (a). The black dashed and dot-dashed curves show the porosity increases by the additional exsolution of 0.01 wt.% and 0.1 wt.% water, respectively. The green shaded region indicates the measured porosity of fall deposited pyroclasts. Estimated vesicularities in (c,d) and pressure in (a) suggest that the magma in the fountain can have a porosity > 0.6, if the separation of gas and melt was not efficient. See Methods for details.
The sampling sites indicate the markers denoted in Extended Data Fig. 1. To obtain the vesicularity, we measured the mass and volume of pyroclasts. To measure the volume, we coated the pyroclasts with a paraffin film and immersed within water. The volume of the solid part is calculated by the mass of the pyroclasts and the density of the solid part, which was obtained by weighing and measuring the volume of the powdered pyroclasts using a pycnometer64.
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Namiki, A., Patrick, M.R., Manga, M. et al. Brittle fragmentation by rapid gas separation in a Hawaiian fountain. Nat. Geosci. 14, 242–247 (2021). https://doi.org/10.1038/s41561-021-00709-0
Nature Geoscience (2021)