During shearing in geological environments, frictional processes, including the wear of sliding rock surfaces, control the nature of the slip events. Multiple studies focusing on natural samples have investigated the frictional behaviour of a large suite of geological materials. However, due to the varied and heterogeneous nature of geomaterials, the individual controls of material properties on friction and wear remain unconstrained. Here, we use variably porous synthetic glass samples (8, 19 and 30% porosity) to explore the frictional behaviour and development of wear in geomaterials at low normal stresses (≤1 MPa). We propose that porosity provides an inherent roughness to material which wear and abrasion cannot smooth, allowing material at the pore margins to interact with the slip surface. This results in an increase in measured friction coefficient from <0.4 for 8% porosity, to <0.55 for 19% porosity and 0.6–0.8 for 30% porosity for the slip rates evaluated. For a given porosity, wear rate reduces with slip rate due to less asperity interaction time. At higher slip rates, samples also exhibit slip weakening behaviour, either due to evolution of the slipping zone or by the activation of temperature-dependent microphysical processes. However, heating rate and peak temperature may be reduced by rapid wear rates as frictional heating and wear compete. The higher wear rates and reduced heating rates of porous rocks during slip may delay the onset of thermally triggered dynamic weakening mechanisms such as flash heating, frictional melting and thermal pressurisation. Hence porosity, and the resultant friction coefficient, work, heating rate and wear rate, of materials can influence the dynamics of slip during such events as shallow crustal faulting or mass movements.
Even modest ash-rich volcanic eruptions can severely impact a range of human activities, especially air travel. The dispersal of ash in these eruptions depends critically on aggregation and sedimentation processes - however these are difficult to quantify in volcanic plumes. Here, we image ash dynamics from mild explosive activity at Santiaguito Volcano, Guatemala, by measuring the depolarisation of scattered sunlight by non-spherical ash particles, allowing the dynamics of diffuse ash plumes to be investigated with high temporal resolution (>1 Hz). We measure the ash settling velocity downwind from the main plume, and compare it directly with ground sampled ash particles, finding good agreement with a sedimentation model based on particle size. Our new, cost-effective technique leverages existing technology, opening a new frontier of integrated ash visualisation and ground collection studies which could test models of ash coagulation and sedimentation, leading to improved ash dispersion forecasts. This will provide risk managers with improved data quality on ash location, reducing the economic and societal impacts of future ash-rich eruptions.
Long-term eruptive activity at the Santiaguito lava dome complex, Guatemala, is characterised by the regular occurrence of small-to-moderate size explosions from the active Caliente dome. Between November 2014 and December 2018, we deployed a seismo-acoustic network at the volcano, which recorded several changes in the style of eruption, including a period of elevated explosive activity in 2016. Here, we use a new catalogue of explosions to characterise changes in the eruptive regime during the study period. We identify four different phases of activity based on changes in the frequency and magnitude of explosions. At the two ends of the spectrum of repose times we find pairs of explosions with near-identical seismic and acoustic waveforms, recorded within 1–10 min of one another, and larger explosions with recurrence times on the order of days to weeks. The magnitude-frequency relationship for explosions at Santiaguito is well described by a power-law; we show that changes in b-value between eruptive regimes reflect temporal and spatial changes in rupture mechanisms, likely controlled by variable magma properties. We also demonstrate that the distribution of inter-explosion repose times between and within phases is well represented by a Poissonian process. The Poissonian distribution describing repose times changes between and within phases as the source dynamics evolve. We find that changes in source properties restrict the extrapolation of explosive behaviour to within a given eruptive phase, limiting the potential for long-term assessments of anticipated eruptive behaviour at Santiaguito.
Earth history is punctuated by massive magmatic events.Resultant mafic large igneous provinces (LIPs) provide the strongest evidence that at certain times in the past, energy transfer from the Earth's interior to its surface has occurred in a manner substantially different from modern plate tectonic processes.Cretaceous time, in particular, is marked by voluminous and episodic basaltic magmatic events generated from the mantle, and these events appear to correlate with extreme states or rapid changes in the oceans, atmosphere, and biosphere.The Kerguelen Plateau/Broken Ridge, one of two giant oceanic plateaus formed in Cretaceous time, is a prime target for investigating (1) mantle processes resulting in LIPs; (2) mechanisms of growth, emplacement, and post-constructional deformation of LIPS; and (3) environmental consequences of voluminous mafic magmatism.Ocean Drilling Program (ODP) Leg 183 will penetrate igneous basement to depths of ~150 to 200 m at several morphologically and tectonically diverse locations on the ~2 x 10 6 km 2 LIP formed by the Kerguelen Plateau/Broken Ridge in the Southeast Indian Ocean.This leg will build on results obtained by basement drilling at four ODP sites on the Central and Southern Kerguelen Plateau during Legs 119 and 120.A major objective of Leg 183 is to determine the magmatic chronology of the Kerguelen Plateau/Broken Ridge LIP by determining the eruption ages of the uppermost igneous crust at several locations.Studies of basement basalt obtained from dredges and drill cores from Legs 119 and 120 show that much of the Southern Kerguelen Plateau formed at 110 to 115 Ma, whereas the Central Kerguelen Plateau and parts of Broken Ridge are younger (~85 Ma).However, ages of basement from major morphological features, such as Elan Bank and the submarine Northern Kerguelen Plateau, are unknown because they have not been previously sampled.During evolution of a LIP, it is likely that hydrothermal and metamorphic processes differ from those occurring in a spreading ridge environment.Therefore, another objective is to use cores of the basement and overlying sediments to assess the interaction between LIP magmatism and the surficial environment.Episodes of high magma flux during formation of a LIP may have significant impact on the Earth's hydrosphere, atmosphere, and biosphere.Additional goals of Leg 183 are to determine the mechanism of LIP growth and the tectonic history of the plateau by integrating seismic data with studies of the sedimentary and igneous cores (i.e., seismic volcanostratigraphy). Specifically, these cores will be used to address the following issues: the timing and extent of initial uplift, the relative roles of subaerial and submarine volcanism, the Leg 183 Scientific Prospectus Page 4 cooling and subsidence into a submarine environment, and the multiple episodes of postemplacement deformation.A unique aspect of this LIP is its clear association with a long linear volcanic ridge, i.e., the Ninetyeast Ridge.Dating of basement basalt from the seven Deep Sea Drilling Project (DSDP) and ODP drill holes that penetrated the igneous basement of the Ninetyeast Ridge established a systematic south to north progression of ages from 38 to ~82 Ma along this hot spot track.In addition, the Kerguelen Archipelago and Heard Island, constructed on the Northern and Central Kerguelen Plateau, respectively, have a volcanic record from ~38 Ma to the present.Studies of subaerial lavas from these islands and submarine lavas recovered by drilling provide a 115 m.y.record of volcanism that can be used to evaluate the hypothesis that the Kerguelen Plateau/Broken Ridge system is related to decompression melting of a plume head and that the subsequent Ninetyeast Ridge and oceanic island volcanism are related to partial melting of the following plume tail.The Kerguelen plume is particularly important because it is a source of an "enriched isotopic component" that forms an end-member in the isotopic arrays defined by ocean island basalts, and it may have been important in creating the distinctive isotopic characteristics of Indian Ocean ridge basalts.Determination of spatial and temporal variations in geochemical characteristics of the basalts forming the Kerguelen Plateau and Broken Ridge are essential for understanding the early history of the Kerguelen plume.
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