Fall-out and deposition of volcanic ash during the 1979 explosive eruption of the soufriere of St. Vincent
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Lapilli
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Many timed observations make it possible to subdivide the 9‐hour Plinian eruption of Mount St. Helens on May 18, 1980, into six phases, defined by eruption style. The phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits. The suite of pyroclastic deposits indicates that the eruption became more pumice‐rich and compositionally diverse with time, perhaps owing to concurrent eruption of less evolved, gas‐poor parts of the magma body with the more evolved, gas‐rich parts. The paroxysmal phase I (0832–0900) consisted of landslides, lithic pyroclastic flows of a lateral blast and other explosions, and a weak pre‐Plinian column. Phase I pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer. The early Plinian phase II (0900–1215) consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks (1010–1035), a brief period of lithic ash ejection (1035–1100), and a pumice‐rich pulse that accompanied growth in height of the eruption column (1100–1215). Deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer, the lower lithic ash layer, and the middle pumice lapilli layer, all of which consist of evolved white dacitic pumice (63–64% SiO 2 ). During the early ash flow phase III (1215–1500) the height of the eruption column decreased, vertical ejection of tephra ceased, and pyroclastic flows were fed from intermittent fountains. Phase III deposits consist of a poorly exposed sequence (.≤12 m) of ash flow tuff that consist of many thin flow units (≤2 m each) containing pumiceous white dacite (63–64% SiO 2 ) and denser, gray silicic andésite (61–62% SiO 2 ), and fine‐grained ash cloud deposits interbedded with a nongraded middle pumice ash layer. The climactic phase IV (1500–1715) developed in two stages: fountain‐fed pyroclastic flows, followed by a short pulse (1625–1715) of vigorous vertical ejection of tephra. These stages were accompanied by the peak seismic energy release and peak eruption column height, respectively. Climactic deposits consist of a thick (≤35 m) sequence of thick, lapilli‐rich ash flow sheets (4–12 m each) with white and gray pumice, and streaky scoria bands (60% SiO 2 ) in pumice breccia clasts, and the reversely‐graded, upper pumice lapilli layer that is interbedded with fine‐grained ash cloud deposits. During the late ash flow phase V (1715–1815) eruption intensity waned but included a brief episode of small pyroclastic flows (1745–1815). Phase V deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice, and minor fine‐ash deposits. Phase VI activity (1815 to May 19, 1980) consisted of a low‐energy ash plume, with transient increases in intensity, while seismicity continued at depth. Sustained vertical discharge of phase II prodeced evolved dacitewith high S/Cl ratios. Ash flow activity of phase III is attributed to decreases in gas content, indicated by reduced S/Cl ratios and increased clast density of the less evolved, gray pumice. Climactic events are attributed to vent clearing and exhaustion of the evolved dacite.
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Explosive activity at Chaitén Volcano in May 2008 and subsequent dome collapses over the following nine months triggered multiple, small-volume pyroclastic density currents (PDCs). The explosive activity triggered PDCs to the north and northeast, which felled modest patches of forest as far as 2 km from the caldera rim. Felled trees pointing in the down-current direction dominate the disturbance zones. The PDC on the north flank of Chaitén left a decimeters-thick, bipartite deposit having a basal layer of poorly sorted, fines-depleted pumice-and-lithic coarse ash and lapilli, which transitions abruptly to fines-enriched pumice-and-lithic coarse ash. The deposit contains fragments of mostly uncharred organics near its base; vegetation protruding above the deposit is uncharred. The nature of the forest disturbance and deposit characteristics suggest the PDC was dilute, of relatively low temperature (<200°C), and to first approximation had a dynamic pressure of about 2-4 kPa and velocity of about 30-40 ms-1. It was formed by directionally focused explosions through the volcano's prehistoric, intracaldera lava dome. Dilute, low-temperature PDCs that exited the caldera over a low point on the east-southeast caldera rim deposited meters-thick fill of stratified beds of pumice-and-lithic coarse ash and lapilli. They did not fell large trees more than a few hundreds of meters from the caldera rim and were thus less energetic than those on the north and northeast flanks. They likely formed by partial collapses of the margins of vertical eruption columns. In the Chaitén River valley south of the volcano, several-meterthick deposits of two block-and-ash-flow (BAF) PDCs are preserved. Both have a coarse ash matrix that supports blocks and lapilli predominantly of lithic rhyolite dome rock, minor obsidian, and local bedrock. One deposit was emplaced by a BAF that traveled an undetermined distance downvalley between June and November 2008, apparently triggered by partial collapse of a newly effused lava dome that started growing on 12 May. A second, and larger, BAF related to another collapse of the new lava dome on 19 February 2009 traveled to within 3 km of the village of Chaitén, 10 km downstream of the volcano. It deposited as much as 8-10 m of diamict having sedimentary characteristics very similar to the previous BAF deposit. Charred trees locally encased within the BAF deposits suggest that the flows were of moderate temperature, perhaps as much as 300°C. Erosion of the BAF deposits filling the Chaitén River channel has delivered substantial sediment loads downstream, contributing to channel instability and challenged river management.
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The January 1835 eruption of Volcán Cosigüina in northwestern Nicaragua was one of the largest and most explosive in Central America since Spanish colonization. We report on the results of reconnaissance stratigraphic studies and laboratory work aimed at better defining the distribution and character of deposits emplaced by the eruption as a means of developing a preliminary hazards assessment for future eruptions. On the lower flanks of the volcano, a basal tephra-fall deposit comprises either ash and fine lithic lapilli or, locally, dacitic pumice. An overlying tephra-fall deposit forms an extensive blanket of brown to gray andesitic scoria that is 35–60 cm thick at 5–10 km from the summit-caldera rim, except southwest of the volcano, where it is considerably thinner. The scoria fall produced the most voluminous deposit of the eruption and underlies pyroclastic-surge and -flow deposits that chiefly comprise gray andesitic scoria. In northern and southeastern sectors of the volcano, these flowage deposits form broad fans and valley fills that locally reach the Gulf of Fonseca. An arcuate ridge 2 km west of the caldera rim and a low ridge east of the caldera deflected pyroclastic flows northward and southeastward. Pyroclastic flows did not reach the lower west and southwest flanks, which instead received thick, fine-grained, accretionary-lapilli–rich ashfall deposits that probably derived chiefly from ash clouds elutriated from pyroclastic flows. We estimate the total bulk volume of erupted deposits to be ∼6 km3. Following the eruption, lahars inundated large portions of the lower flanks, and erosion of deposits and creation of new channels triggered rapid alluviation. Pre-1835 eruptions are poorly dated; however, scoria-fall, pyroclastic-flow, and lahar deposits record a penultimate eruption of smaller magnitude than that of 1835. It occurred a few centuries earlier—perhaps in the fifteenth century. An undated sequence of thick tephra-fall deposits on the west flank of the volcano records tens of eruptions, some of which were greater in magnitude than that of 1835. Weathering evidence suggests this sequence is at least several thousand years old. The wide extent of pyroclastic flows and thick tephra fall during 1835, the greater magnitude of some previous Holocene eruptions, and the location of Cosigüina on a peninsula limit the options to reduce risk during future unrest and eruption.
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In Tertiary basins of NE Slovenia, Upper Oligocene volcanic activity occurred in a submarine environment that experienced contemporaneous clastic Sedimentation. Pyroclastic deposits are essentially related to gas- and water-supported eruption-fed density currents. At Trobni Dol, the Laško Basin, an over 100 m thick deposit formed by a sigle sustained volcanic explosion that fed gas-supported pyroclastic flow. Diagnostic features are large matrixshard content, normal grading of pumice lapilli, collapsed pumice lapilli and the presence of charcoal. In the Smrekovec Volcanic Complex, several but only up to 5 m thick deposits related to eruption-fed gas-supported pyroclastic flows occur. Deposits settled from water-supported eruption-fed density currents form fining- and thinning-upward sedimentary units which resemble the units of volcaniclastic turbidites. Pyroclastic deposits related to gas- and water-supported density currents occur in an up to 1000 m thick succession composed of coherent volcanics, autoclastic, pyroclastic, reworked volcaniclastic and mixed volcaniclastic-siliciclastic deposits that indicate a complex explosive and depositional history of the Smrekovec Volcanic Complex.
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Pyroclastic fall
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Kasatochi volcano in the central Aleutian Islands erupted unexpectedly on 7–8 August 2008. Kasatochi has received little study by volcanologists and has had no confirmed historical eruptions. The island is an important nesting area for seabirds and a long‐term biological study site of the U.S. Fish and Wildlife Service. After a notably energetic preeruptive earthquake swarm, the volcano erupted violently in a series of explosive events beginning in the early afternoon of 7 August. Each event produced ash‐gas plumes that reached 14–18 km above sea level. The volcanic plume contained large amounts of SO 2 and was tracked around the globe by satellite observations. The cumulative volcanic cloud interfered with air travel across the North Pacific, causing many flight cancelations that affected thousands of travelers. Visits to the volcano in 2008–2009 indicated that the eruption generated pyroclastic flows and surges that swept all flanks of the island, accumulated several tens of meters of pyroclastic debris, and increased the diameter of the island by about 800 m. Pyroclastic flow deposits contain abundant accidental lithic debris derived from the inner walls of the Kasatochi crater. Juvenile material is crystal‐rich silicic andesite that ranges from slightly pumiceous to frothy pumice. Fine‐grained pyroclastic surge and fall deposits with accretionary lapilli cover the lithic‐rich pyroclastic flow deposits and mark a change in eruptive style from episodic explosive activity to more continuous ash emission with smaller intermittent explosions. Pyroclastic deposits completely cover the island, but wave erosion and gully development on the flanks have begun to modify the surface mantle of volcanic deposits.
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Current and ancestral lakes within the central Taupo Volcanic Zone (TVZ) provide depocentres for pyroclastic deposits, providing a reliable record of eruption history. These lakes can also be the source of explosive eruptions that directly feed pyroclast-rich density currents. The lithofacies characteristics of pyroclastic deposits allow discrimination between eruption-fed and resedimented facies. The most frequently recognised styles of subaqueous eruptions in the TVZ are shallow-water phreatomagmatic and phreatoplinian eruptions that form subaerial eruption columns. However, deeper source conditions (>150 m water depth) could generate subaqueous explosive eruptions that feed water-supported pyroclast-rich density currents, similar to neptunian eruptions. Such deep-water eruptions have not previously been recognised in the TVZ. Here we study a subsurface deposit, the middle Huka Falls Formation (MHFF), in the Wairakei–Tauhara Geothermal Fields (Wairakei–Tauhara), TVZ, which we interpret to be the product of a relatively deep-water pyroclastic eruption (150–250 m). The largely subsurface Huka Falls Formation records past sedimentary and volcaniclastic deposition in ancient Lake Huka. Deposits examined from eight drill cores reveal a lithic-rich lower unit, a middle volumetrically dominant pumice lapilli-tuff and an upper thinly bedded suspension-settled tuff unit. A coarse lithic lapilli-tuff within the lower unit is locally thick and coarse near well THM12, suggesting proximity to a source located beneath Lake Huka. This research provides an understanding of the origin of the MHFF deposit and offers insights for evaluating and interpreting the diversity of subaqueous volcanic lake deposits elsewhere.
Phreatomagmatic eruption
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