The eruption of basaltic magmas dominates explosive volcanism on Earth and other planets within the Solar System. The mechanism through which continuous magma fragments into volcanic particles is central in governing eruption dynamics and the ensuing hazards. However, the mechanism of fragmentation of basaltic magmas is still disputed, with both viscous and brittle mechanisms having been proposed. Here we carry out textural analysis of the products of ten eruptions from seven volcanoes by scanning electron microscopy. We find broken crystals surrounded by intact glass that testify to the brittle fragmentation of basaltic magmas during explosive activity worldwide. We then replicated the natural textures of broken crystals in laboratory experiments where variably crystallized basaltic melt was fragmented by rapid deformation. The experiments reveal that crystals are broken by the propagation of a network of fractures through magma, and that afterwards the fractures heal by viscous flow of the melt. Fracturing and healing affect gas mobility, stress distribution, and bubble and crystal size distributions in magma. Our results challenge the idea that the grain size distribution of basaltic eruption products reflects the density of fractures that initially fragmented the magma and ultimately indicate that brittle fracturing and viscous healing of magma may underlie basaltic explosive eruptions globally.
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A selection of 317 microphotographs detailing pyroclast textures related to the fracturing and healing of basaltic magmas in explosive volcanic eruptions and in fragmentation experiments is provided as Supplementary Information and is available at Mendeley Data, V1, https://doi.org/10.17632/h5ynspf336.1, while the entire dataset of more than 2,100 microphotographs is available upon request from J.T. The data used in Fig. 4 and Extended Data Fig. 1 and Extended Data Table 1 are available at Mendeley Data, V1, https://doi.org/10.17632/38rss8f2yb.1 and presented in the source data provided with this paper.
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M. Nazzari provided essential support during the FESEM analyses. Partial financial support from INGV (project UNO) to J.T., Deutsche Forschungsgemeinschaft project CI 254/2-1 to C.C. and DGAPA-UNAM to H.D.-G. is acknowledged. B. Zimanowski, R. Büttner and I. Sonder are acknowledged for the fragmentation experiments.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editors: Rebecca Neely.
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Extended Data Fig. 1 Spatial distribution and fracture orientation of broken crystals within one pyroclast.
a, Bi-dimensional fracture number density (number of fractures per mm2, in red), and percent of plagioclase crystal fragments over the total number of plagioclase crystals (fragments plus unbroken, in black) within the individual images of the mosaic. Note the higher fracture density and percent of broken crystals along the margins of the particle despite uniform crystallinity all through the clast. b, Direction of extension (parallel to red dash) for broken crystal with extensional features (see, for example, Fig. 1k). No preferred orientation is visible. In the inset, the mosaic of the whole pyroclast, with the outline of the investigated area marked in white.
Extended Data Fig. 2 Scanning electron micrographs of volcanic particles with additional features of broken crystals and healing fractures.
a-c, Ash-sized pyroclasts fringed by broken crystals, reflecting the propagation of large, clast-forming fractures and smaller ones that healed after breaking the crystals. d-f, Transitions between clast- and crystal-cutting fractures. Open fractures (d) surrounded by broken crystals with glass-filled fractures. e,f, A fracture almost dividing an entire clast, except for a narrow, viscously healed portion with vesicles and broken crystals. g, Flow banding of Fe-rich glass around broken crystals. h-j, Dispersal of crystal fragments marks post-fracture melt flow direction, with a rotational flow component (i) or shearing similar to boudinage (j). k-o, Features of broken crystals in microcrystalline groundmass are identical to those in glassy groundmass (see previous figures). Note the intermingling of the two types of groundmass (k,n), and the presence of nano-scale Fe-oxide crystals in the crystal-cutting fracture infill (o). Red boxes are enlarged in the subsequent panel.
Extended Data Fig. 3 Scanning electron micrographs of experimental products showing additional transitions between fractures, vesicles, and broken crystals.
a-c, Progressive zooms into the terminal part of a large fracture (same as in Fig. 3a), with viscous healing isolating vesicles of variable size. d-i, Transitions between small, fracture-derived vesicles (d,e) and broken crystals with open (f-h) and glass-filled fractures (g,i). j-k, Train of vesicles within one experimental product (j). Vesicle alignment and the surrounding broken crystals (k) reveal their fracture-related origin. l, Wisps of Fe-rich glass marking sutured fractures. m, Viscous healing dispersing crystal fragments and forming flow banding of Fe-rich melt. n,o, Flow banding of Fe-rich melt induced by syn-experimental deformation of the melt after fragmentation, with entrained broken crystals (o). Red boxes are enlarged in the subsequent panel.
Extended Data Table 1 Crystallinity measurements of the experimental products and of natural pyroclasts from selected samples.
Crystallinity is calculated on a vesicle-free basis.
Raw measurements of fracture parameters in volcanic and experimental products.
Point count measurements of intact crystals, crystal fragments, and vesicle-free area for all the 30 images of the mosaic in Extended Data Fig. 1.
Point count and image segmentation crystallinity for all measured images of natural and experimental products.
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Taddeucci, J., Cimarelli, C., Alatorre‑Ibargüengoitia, M.A. et al. Fracturing and healing of basaltic magmas during explosive volcanic eruptions. Nat. Geosci. 14, 248–254 (2021). https://doi.org/10.1038/s41561-021-00708-1
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