Ignimbrite lithofacies analysis can be used to interpret the number and behaviour of pyroclastic density currents (PDCs) generated during a Plinian eruption, through identification of flow units in the rock record. However, pyroclastic stratigraphic successions are rarely complete and without breaks around a volcano, complicating regional analysis of hiatus markers. This study uses entrachron correlation to reconcile conflicting proximal and distal lithofacies architecture of the 273 ka Poris Formation, Tenerife, to reveal a coherent eruption history. The novel correlation illustrates that hiatus in PDC activity can vary spatially and rapidly due to regional-scale unsteadiness and non-uniformity in PDC dynamics. Distal stratigraphy may not accurately record the number of PDCs generated during an eruption and breaks in ignimbrite deposition do not necessarily imply discontinuous PDC development. These findings bear importantly on hazard inferences derived from ignimbrite lithofacies successions, and have significance for numerical and experimental modelling of unsteadiness in granular flows.
First posted August 2, 2017 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025 This field trip will provide an introduction to several fascinating features of Mount St. Helens. The trip begins with a rigorous hike of about 15 km from the Johnston Ridge Observatory (9 km north-northeast of the crater vent), across the 1980 Pumice Plain, to Windy Ridge (3.6 km northeast of the crater vent) to examine features that document the dynamics and progressive emplacement of pyroclastic flows. The next day, we examine classic tephra outcrops of the past 3,900 years and observe changes in thickness and character of these deposits as we traverse their respective lobes. We examine clasts in the deposits and discuss how the petrology and geochemistry of Mount St. Helens deposits reveal the evolution of the magmatic system through time. We also investigate the stratigraphy of the 1980 blast deposit and review the chronology of this iconic eruption as we travel through the remains of the blown-down forest. The third day is another rigorous hike, about 13 km round trip, climbing from the base of Windy Ridge (elevation 1,240 m) to the front of the Crater Glacier (elevation 1,700 m). En route we examine basaltic andesite and basalt lava flows emplaced between 1,800 and 1,700 years before present, a heterolithologic flow deposit produced as the 1980 blast and debris avalanche interacted, debris-avalanche hummocks that are stranded on the north flank and in the crater mouth, and shattered dacite lava domes that were emplaced between 3,900 and 2,600 years before present. These domes underlie the northern part of the volcano. In addition, within the crater we traverse well-preserved pyroclastic-flow deposits that were emplaced on the crater floor during the summer of 1980, and a beautiful natural section through the 1980 deposits in the upper canyon of the Loowit River.Before plunging into the field-trip log, we provide an overview of Mount St. Helens geology, geochemistry, petrology, and volcanology as background. The volcano has been referred to as a "master teacher." The 1980 eruption and studies both before and after 1980 played a major role in the establishment of the modern U.S. Geological Survey Volcano Hazards Program and our understanding of flank collapses, debris avalanches, cryptodomes, blasts, pyroclastic density currents, and lahars, as well as the dynamics of magma ascent and eruption.
Summary A volcanic episode, initiated in the late Tremadoc and persisting into the lower Llanvirn, is represented around Rhobell Fawr (SH 7825) by basaltic lavas and their breccias (the Rhobell Volcanic Group), by dolerite, microdiorite and microtonalite intrusions, by a rhyolite tuff and by the basic and intermediate constituents of certain sedimentary rocks. Initially basalts were erupted, at a variable rate, subaerially from fissures. This effusive phase was followed by eruptions of andesite and dacite, which, in turn, were followed by explosive eruption of rhyolite. A dyke regime in the west of the area represents the sub- and intravolcanic parts of a major site of eruption and was the source of the volcanic group. The rocks of the Rhobell episode are genetically related by amphibole-dominated fractional crystallisation of mantle-derived basaltic magma and are similar to those of modern destructive plate margin environments. The Rhobell fractionation series, from silica-undersaturated to quartz-normative, is closely paralleled by that at Grenada, at the southern end of the Lesser Antilles island arc. The tectonovolcanic evolution of the area is consistent with the development of a N-S basement fracture situated beneath the eastern margin of the Harlech Dome.
Abstract The Glaramara tuff presents extensive exposures of the medial and distal deposits of a large tuff ring (original area >800 km 2 ) that grew within an alluvial to lacustrine caldera basin. Detailed analysis and correlation of 21 sections through the tuff show that the eruption involved phreatomagmatic to magmatic explosions resulting from the interaction of dacitic magma and shallow‐aquifer water. As the eruption developed to peak intensity, numerous, powerful single‐surge pyroclastic density currents reached beyond 8 km from the vent, probably >12 km. The currents were strongly depletive and deposited coarse lapilli (>5 cm in diameter) up to 5 km from source, with only fine ash and accretionary lapilli deposited beyond this. As the eruption intensity waned, currents deposited fine ash and accretionary lapilli across both distal and medial regions. The simple wax–wane cycle of the eruption produced an overall upward coarsening to fining sequence of the vertical lithofacies succession together with a corresponding progradational to retrogradational succession of lithofacies relative to the vent. Various downcurrent facies transitions record transformations of the depositional flow‐boundary zones as the depletive currents evolved with distance, in some cases transforming from granular fluid‐based to fully dilute currents primarily as a result of loss of granular fluid by deposition. The tuff‐ring deposits share several characteristics with (larger) ignimbrite sheets formed during Plinian eruptions and this underscores some overall similarities between pyroclastic density currents that form tuff rings and those that deposit large‐volume ignimbrites. Tuff‐ring explosive activity with such a wide area of impact is not commonly recognized, but it records the possibility of such currents and this should be factored into hazard assessments.
Abstract: Calc-alkaline magmatism in the Grampian Terrane started at c . 430 Ma, after subduction of the edge of continental Avalonia beneath Laurentia, and it then persisted for at least 22 Ma. Isotope dilution thermal ionization mass spectrometry U–Pb zircon dating yields 425.0 ± 0.7 Ma for the Lorn Lava Pile, 422.5 ± 0.5 Ma for Rannoch Moor Pluton, 419.6 ± 5.4 Ma for a fault-intrusion at Glencoe volcano, 417.9 ± 0.9 Ma for Clach Leathad Pluton in Glencoe, and, in the Etive Pluton, 414.9 ± 0.7 Ma for the Cruachan Intrusion and 408.0 ± 0.5 Ma for the Inner Starav Intrusion. The Etive Dyke Swarm was mostly emplaced during 418–414 Ma, forming part of the plumbing of a large volcano (≥2000 km 3 ) that became intruded by the Etive Pluton and was subsequently removed by erosion. During the magmatism large volumes (thousands of km 3 ) of high Ba–Sr andesite and dacite were erupted repeatedly, but were mostly removed by contemporaneous uplift and erosion. This volcanic counterpart to the ’Newer Granite' plutons has not previously been fully recognized. The intermediate magmas forming both plutons and volcanoes originated mainly by partial melting of heterogeneous mafic to intermediate lowermost crust that had high Ba–Sr derived from previous melting of large ion lithophile element (LILE)-enriched mantle, possibly at c . 1.8 Ga. This crustal recycling was induced by heat and volatiles from underplated small-degree melts of LILE- and light REE-enriched lithospheric mantle (appinite–lamprophyre magmas). The post-collision magmatism and uplift resulted from breakoff of subducted oceanic lithosphere and consequent rise of asthenosphere. Supplementary material: Geochronological methods, tabulated data and additional figures are available at http://www.geolsoc.org.uk/SUP18343 .
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