The products of primary magma fragmentation finally revealed by pumice agglomerates

Following rapid decompression in the conduit of a volcano, magma breaks into ash- to block-sized fragments, powering explosive sub-Plinian and Plinian eruptions that may generate destructive pyroclastic falls and flows. It is thus crucial to assess how magma breaks up into fragments. This task is difficult, however, because of the subterranean nature of the entire process and because the original size of pristine fragments is modified by secondary fragmentation and expansion. New textural observations of sub-Plinian and Plinian pumice lapilli reveal that some primary products of magma fragmentation survive by sintering together within seconds of magma break-up. Their size distributions reflect the energetics of fragmentation, consistent with products of rapid decompression experiments. Pumice aggregates thus offer a unique window into the previously inaccessible primary fragmentation process and could be used to determine the potential energy of fragmentation.

Supplemental Material: The products of primary magma fragmentation finally revealed by pumice agglomerates

Additional information on (1) the four eruptions studied and sample collection, (2) lapilli selection, (3) measurements of volume and porosity, (4) analysis by X-Ray computed tomography and scanning electron microscopy, and (5) calculation of protopyroclasts size distributions.<br>

Supplemental Material: The products of primary magma fragmentation finally revealed by pumice agglomerates

Additional information on (1) the four eruptions studied and sample collection, (2) lapilli selection, (3) measurements of volume and porosity, (4) analysis by X-Ray computed tomography and scanning electron microscopy, and (5) calculation of protopyroclasts size distributions.<br>

Low-Energy Fragmentation Dynamics at Copahue Volcano (Argentina) as Revealed by an Infrasonic Array and Ash Characteristics

Ash-rich eruptions represent a serious risk to the population living nearby as well as at thousands of kilometers from a volcano. Volcanic ash is the result of extensive magma fragmentation during an eruption, and it depends upon a combination of magma properties such as rheology, vesicularity and permeability, gas overpressure and the possible involvement of external fluids during magma ascent. The explosive process generates infrasonic waves which are directly linked to the outflow of the gas-particle mixture in the atmosphere. The higher the overpressure in the magma, the higher should be the exit velocity of the ejected material and the acoustic pressure related to this process. During violent eruptions, fragmentation becomes more efficient and is responsible for the extensive production of ash which is dispersed in the atmosphere. We show that the phase of intense ash emission that occurred during March 2016 at Copahue volcano (Argentina) generated a very low (0.1 Pa) infrasonic amplitude at 13 km, raising a number of questions concerning the links among acoustic pressure, gas overpressure and efficiency of magma fragmentation. Infrasound and direct observations of the eruptive plume indicate that the large quantity of ash erupted at Copahue was ejected with a low exit velocity. Thus, it was associated with eruptive dynamics driven by a low magma overpressure. This is more evident when infrasonic activity at Copahue is compared to the moderate explosive activity of Villarrica (Chile), recorded by the same array, at a distance of 193 km. Our data suggest a process of rigid fragmentation under a low magma overpressure which was nearly completely dissipated during the passage of the erupting mixture through the granular, ash-bearing crater infilling. We conclude that ash released into the atmosphere during low-energy fragmentation dynamics can be difficult to monitor, with direct consequences for the assessment of the related hazard and management of eruptive crises.

Highly explosive basaltic eruptions: magma fragmentation induced by rapid crystallisation

&lt;p&gt;Basaltic eruptions are the most common form of volcanism on Earth and planetary bodies. The low viscosity of basaltic magmas generally favours effusive and mildly explosive volcanic activity. Highly explosive basaltic eruptions occur less frequently and their eruption mechanism still remains subject to debate, with implications for the significant hazard associated with explosive basaltic volcanism. Particularly, highly explosive eruptions require magma fragmentation, yet it is unclear how basaltic magmas can reach the fragmentation threshold.&lt;/p&gt;&lt;p&gt;In volcanic conduits, the crystallisation kinetics of an ascending magma are driven by degassing and cooling. So far, the crystallisation kinetics of magmas have been estimated through ex situ crystallization experiments. However, this experimental approach induces underestimation of crystallization kinetics in silicate melts. The&amp;#160;&amp;#160; crystallization experiments reported in this study were performed in situ at Diamond Light Source (experiment EE12392 at the I12 beamline), Harwell, UK, using basalt from the 2001 Etna eruption as the starting material. We combined a bespoke high-temperature environmental cell with fast synchrotron X-ray microtomography to image the evolution of crystallization in real time. After 4 hours at sub-liquidus conditions (1170 &amp;#176;C and 1150 &amp;#176;C) the system was perturbed through a rapid cooling (0.4 &amp;#176;C/s), inducing a sudden increase of undercooling. Our study reports the first in situ observation of exceptionally rapid plagioclase and clinopyroxene crystallisation in trachybasaltic magmas. We combine these constraints on crystallisation kinetics and viscosity evolution with a numerical conduit model to show that exceptionally rapid syn-eruptive crystallisation is the fundamental process required to trigger basaltic magma fragmentation under high strain rates. Our in situ experimental and natural observations combined with a numerical conduit model allow us to conclude that pre-eruptive temperatures &lt;1,100&amp;#176;C can promote highly explosive basaltic eruptions, such as Plinian volcanism, in which fragmentation is induced by fast syn-eruptive crystal growth under high undercooling and high decompression rates. This implies that all basaltic systems on Earth have the potential to produce powerful explosive eruptions.&lt;/p&gt;

Morpho-textural and dynamic analysis of ash particles and ash aggregates at Sakurajima volcano (Japan)

&lt;p&gt;&lt;span&gt;Morphological, textural and granulometric studies of volcanic ash particle provides important insights into the mechanisms of fragmentation, transport and deposition in the context of low-to-mid intensity activity, and particularly during those eruptions showing high-transients in the style of activity. A comprehensive study of volcanic ash from Vulcanian activity of variable intensity at Sakurajima volcano (Japan) is here presented together with a detailed analysis of ash aggregates collected and filmed during the same eruptive sequences. Bulk tephra deposits from different events (July-August 2013, October 2014 and November 2019) and high-speed video of falling ash aggregates were collected directly during the fallout. Tephra samples, resulting from the different phases of activity, were analyzed using an optical particle analyzer which allowed to characterized the grain size distribution and to quantify the shape of a large set of particles. A set of objective parameters were used to constrain the shape of ash grains. This helped to better characterize different phases of activity also in the light of the magma fragmentation process and to evaluate the role played by the fragmentation process in controlling the eruption dynamics. SEM analyses of representative ash grains allowed distinguishing four principal types of ash fragments basing on morphological, surface and groundmass features: Blocky Irregular (BI), Blocky Regular (BR), Vesicular (V). A comprehensive textural analysis of grains belonging to either the different classes and phases of activity was provided in order to better resolve the complex relationships between the processes occurring before and during magma fragmentation and secondary processes affecting ash characteristics, like the intra-crateric recycling of ash. This helped also to shed light on the different processes of ash production and link them with the resulting dynamics of activity in the context of unsteady eruptions. On the other hand, the analysis of the high-speed video depicting ash aggregates, and aggregates collected during the same eruptive periods revealed important information about the influence of ash aggregation in controlling the depositional dynamics of Vulcanian eruptions. Three main types of ash aggregates were recognized to occur into all the Sakurajima samples: Ash Clusters, Coated Particles, Cored Clusters. Using image analysis techniques of SEM images, collected aggregates were characterized in terms of dimension, grain size of the aggregating ash, and shape features of the aggregated ash, pointing out important differences between the different types. Analysis of high-resolution, High-speed Camera video recordings, allowed finally to collect an important set of measurements of terminal velocity, bulk density, and size of a large number of observed falling aggregates. The resulting data reveal the strong influence of aggregation processes in controlling ash deposition processes at Sakurajima, and also represent a valuable dataset useful for validation and calibration of numerical models.&lt;/span&gt;&lt;/p&gt;

Magma fragmentation and timing of water-magma interaction of La Joya de Yuriria maar volcano and La Sanabria-San Roque tuff ring complex, Mexico

&lt;p&gt;La Joya de Yuriria maar volcano and La Sanabria-San Roque tuff ring complex manifests at the southern and northern extreme of the NNW-SSE trending clusters of phreatomagmatic vents of Valle de Santiago volcanic field, which forms the NE part of the famous Michoacan-Guanajuato Volcanic Field ( (MGVF), central Mexico. La Sanabria-San Roque complex is located in the south of the town of Irapuato and is composed of three tuff rings namely San Joaquin (SJ), La Sanabria (LS) and San Roque (SR). Their tephra deposits were studied at 7 different active quarries, which suggests that the San Joaquin tuff ring was formed before La Sanabria-San Roque tuff ring complex. San Joaquin is composed of medium-size lapilli flow (Mdphi=-2.05 to -3.90, &amp;#963;phi=2.00 to 2.58) and fine ash surge units and contains different types of lithics and juvenile fragments (50-68 vol.%.). About four types of lithics were identified namely: grey-colored vesicular basaltic andesites (9-27 vol.%), grey-colored non-vesicular basaltic andesites (17-19 vol.%), white lithics (sediments 0-1 vol.%), red-colored lithics (volcanic breccias 1-3 wt.%) along with few plagioclase crystals (0.54-0.66 vol.%) that are exposed at quarries 1, 3. La Sanabria-San Roque tuff ring complex tephra deposits are exposed at quarries 2, 5 and 8 and are composed of intercalated flow (Mdphi=-1.65 to -2.15, &amp;#963;phi=1.00-1.83) and fallout (Mdphi=-2.00 to -6.10, &amp;#963;phi=2.00) units with juvenile content from 41-87 vol.% and four different types of lithic fragments: grey-colored vesicular lithics (1- 20 vol.%), grey-colored compact lithics (2-6 vol.%), which is considerably lower than the amount encountered within SJ deposits. Further-more, white-colored lithics, mostly sediments (0-10 vol.%) and red-colored lithics (rhyolites and/or volcanic breccias) around 0-3 vol.%.&lt;/p&gt;&lt;p&gt;La Joya de Yuriria is currently located on the southern margin of the artificial lake of Yuriria and its tephra sequence is composed of mostly fallout units (Md&amp;#966;=-4.45 to -4.60, &amp;#963;&amp;#966;=1.88 to 2.55), followed by flow units (Md&amp;#966;=-2.95 to -3.800, &amp;#963;&amp;#966;=1.93 to 2.05) that are separated with both indurated, fine-ash wet and dry surge units of which a very particular fine-ash dry surge unit ( Md&amp;#966;=-0.95, &amp;#963;&amp;#966;=2.03), yellowish in color (due to oxidation?), may represent a short-term break within the phreatomagmatic activity. It is also composed of flow units (Md&amp;#966;=-1.50 to -2.95, &amp;#963;&amp;#966;=1.40 to 3.43) that are clast supported, friable and contains medium to coarse lapilli size fragments that are rich in accidental lithics with very juvenile clasts (&lt;33 vol.%) of basaltic andesite (SiO&lt;sub&gt;2&lt;/sub&gt;= 54.4 wt%, Na&lt;sub&gt;2&lt;/sub&gt;O+K&lt;sub&gt;2&lt;/sub&gt;O= 5.21 wt%) with very few juvenile content (5-37 wt.%), except at VS-1741-P7 (85 vol.%) and abundance of light grey colored angular lithics that were classified as vesicular (4.51 vol.%) and non-vesicular (1-66 vol.%) with few reworked lithics (1-5 vol.%) and altered lithics (1-5 vol.%).&lt;/p&gt;&lt;p&gt;Vesicularity index on 2741 juvenile clasts from these vents was utilized to determine the magma fragmentation and the timing of magma-water interactions (especially exsolution of volatiles before or during mag-water interaction). To corroborate this, Bubble Nucleation Density and crystal texture of primary vesicles within glass shards were also performed to validate the interpretations made.&lt;/p&gt;