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Bubble and crystal textures evolve during magma ascent, altering properties that control ascent such as permeability and viscosity. Eruption style results from feedbacks between ascent, bubble nucleation and growth, microlite crystallization, and gas loss, all processes recorded in pyroclasts. We show that pyroclasts of the mafic Curacautín ignimbrite of Llaima volcano, Chile, record a history of repeated autobrecciation, fusing, and crystallization. We identified pyroclasts with domains of heterogeneous vesicle textures in sharp contact with one another that are overprinted by extensive microlite crystallization. Broken crystals with long axes (l) >10 μm record fragmentation events during the eruption. A second population of unbroken microlites with l ≤10 μm overprint sutures between fused domains, suggesting the highly crystalline groundmass formed at shallow depths after autobrecciation and fusing. Nearly all pyroclasts contain plutonic and ancestral Llaima lithics as inclusions, implying that fusing occurs from a few kilometers depth to as shallow as the surface. We propose that Curacautín ignimbrite magma autobrecciated during ascent and proto-pyroclasts remained melt rich enough to fuse together. Lithics from the conduit margins were entrained into the proto-pyroclasts before fusing. Autobrecciation broke existing phenocrysts and microlites; rapid post-fusing crystallization then generated the highly crystalline groundmass. This proposed conduit process has implications for interpreting the products of mafic explosive eruptions. 

Mafic volcanic activity is dominated by effusive to mildly explosive eruptions. Plinian and ignimbrite-forming mafic eruptions, while rare, are also possible; however, the conditions that promote such explosivity are still being explored. Eruption style is determined by the ability of gas to escape as magma ascends, which tends to be easier in low-viscosity, mafic magmas. If magma permeability is sufficiently high to reduce bubble overpressure during ascent, volatiles may escape from the magma, inhibiting violent explosive activity. In contrast, if the permeability is sufficiently low to retain the gas phase within the magma during ascent, bubble overpressure may drive magma fragmentation. Rapid ascent may induce disequilibrium crystallization, increasing viscosity and affecting the bubble network with consequences for permeability, and hence, explosivity. To explore the conditions that promote strongly explosive mafic volcanism, we combine microlite textural analyses with synchrotron x-ray computed microtomography of 10 pyroclasts from the 12.6 ka mafic Curacautín Ignimbrite (Llaima Volcano, Chile). We quantify microlite crystal size distributions (CSD), microlite number densities, porosity, bubble interconnectivity, bubble number density, and geometrical properties of the porous media to investigate the role of magma degassing processes at mafic explosive eruptions. We use an analytical technique to estimate permeability and tortuosity by combing the Kozeny-Carman relationship, tortuosity factor, and pyroclast vesicle textures. The groundmass of our samples is composed of up to 44% plagioclase microlites, > 85% of which are < 10 µm in length. In addition, we identify two populations of vesicles in our samples: (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). Computed permeability ranges from 3.0 × 10⁻¹³to 6.3 × 10⁻¹²m², which are lower than the similarly explosive mafic eruptions of Tarawera (1886; New Zealand) and Etna (112 BC; Italy). The combination of our CSDs, microlite number densities, and 3D vesicle textures evidence 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 permeable by highly tortuous network of vesicles. Using the bubble number densities for the isolated vesicles (0.1-3⁻³ × 10⁴ bubbles per mm³), we obtain a minimum average decompression rate of 1.4 MPa/s. Despite the textural evidence that the Curacautín magma reached the percolation threshold, we propose that rapid ascent suppressed outgassing and increased bubble overpressures, leading to explosive fragmentation. 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 control the explosivity of mafic eruptions more generally.

 

A Cascadia Subduction Zone (CSZ) earthquake will cause widespread damage along the Pacific Northwest (PNW) coast of the United States. It is, therefore, crucial to understand how to reduce future impacts across this region and assess current levels of household preparedness. Here, we examine whether decades of risk and preparedness campaigns have established protective knowledge and promoted hazard adjustments for residents of the Portland, Oregon metropolitan (PDX) region, an area where risk and preparedness campaigns have been increasing over the last decade. We also explore how PDX preparedness levels compare to those in other PNW regions using analogous data from multiple surveys over a 20-year time interval and assess PDX respondent’s perceptions of risk information sources. We find that 63% of PDX residents do not intend to “Drop, Cover, and Hold On” (DCHO) when earthquake shaking starts and that, although residents are generally aware of earthquake hazards in the area, they are less aware of the specific risks for their homes. Furthermore, PDX residents’ preparedness levels seem to be comparable to those recently surveyed in Washington, but somewhat less than those surveyed two decades ago in California, though more comprehensive data are needed to verify these results. We also find that people rate risk information provided by emergency managers and their websites as more accurate, understandable, relevant, and new than other sources. Our results suggest that significant gaps remain in translating broad awareness of the CSZ earthquake into personal knowledge and preparedness. This work provides guidance to PDX emergency educators for more targeted messaging and provides methods to measure preparedness variables in other regions for future comparisons. By paying close attention to preparedness gaps, local officials can use their limited resources more effectively to develop strategies to inform their communities and improve preparedness before a major earthquake strikes.