Showing papers on "Phreatomagmatic eruption published in 1996"
TL;DR: In this paper, the results of a laboratory investigation of pyroclast formation following rapid decompression were reported, showing that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events.
Abstract: THE processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology. Observations of explosive eruptions1–3, as well as theoretical analyses of the underlying processes4, have raised the question of whether the rapid decompression of highly viscous, vesicular magma that results from the collapse of a lava dome or volcanic edifice can, in itself, produce explosive magma fragmentation and pyroclast formation. Here we report the results of a laboratory investigation of pyroclast formation following rapid decompression. Samples of magma from the 1980 eruption of Mount St Helens, when rapidly depressurized from initial pressures of up to 12 MPa (and at temperatures in the range 750–825 °C), fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions. Moreover, we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes, suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events.
295 citations
TL;DR: In this paper, the effects of impure coolants on the fuel-coolant interaction process were investigated and it was shown that impure cooling agents enhance the ability of magma to mix with large volumes of coolant and that the maximum unit-volume explosivity of fuel cooling is damped relative to interactions with pure water.
176 citations
TL;DR: A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the Pahvant volcano above lake level in the Pleistocene Lake Bonneville as mentioned in this paper.
Abstract: The subaqueous phases of an eruption initiated approximately 85 m beneath the surface of Pleistocene Lake Bonneville produced a broad mound of tephra. A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level. At the base of the volcano and very near inferred vent sites are fines-poor, well-bedded, broadly scoured beds of sideromelane tephra having local very low-angle cross-stratification (M1 lithofacies). These beds grade upward into lithofacies M3, which shows progressively better developed dunes and cross-stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level. Both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption. Intercalated lithofacies are weakly bedded tuff and breccia (M2), and nearly structureless units with coarse basal layers above strongly erosional contacts (M4). The former combines products of deposition from direct fall and moderate concentration sediment gravity flows, and the latter from progressively aggrading high-concentration sediment gravity flows. Early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous, unsteady, dilute density currents which produced the M1 lithofacies near the vent. Dunes and crossbeds which are better developed upward in M3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion-generated pressure waves and eruption jets impinged upon and occasionally breached the surface. Intermingling of (a) tephra emplaced after brief transport by tephra jets within a gaseous milieu and (b) laterally flowing tephra formed lithofacies M2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water-exclusion zones above a vent. M4 comprises mass flow deposits formed by disruption and remobilization of mound tephra. Intermittent, explosive magma–water interactions occurred from the outset of the Pahvant eruption, with condensation, entrainment of water and lateral flow marking the transformation from eruptive to "sedimentary" processes leading to deposition of the mound lithofacies.
127 citations
TL;DR: In this paper, it was shown that the changes in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of Taal in 1965 and Miyake-jima in 1983.
Abstract: When magma vents into the sea or a crater lake, the ensuing magma-water interaction can affect the style of eruption dramatically. If the mass of surface water incorporated into the erupting material is small, ( 108 kg/s, the height progressively decreases with surface water content. This occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column, so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air. The transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of Taal in 1965 and Miyake-jima in 1983 and for the changes in the eruptive activity at Surtsey in 1963–1964 as the access of seawater to the vent became more restricted. We also present calculations which suggest that the accretionary lapilli, which are often found in wet flow deposits, may result from condensation of vapor in both the cold, wet collapsing fountains and in the flows themselves.
118 citations
TL;DR: Joya Honda is a Quaternary maar of unusual type from the Mexican Basin and Range Province as discussed by the authors, which formed subaerially from the interaction of groundwater with rapidly ascending intraplate type basanitic magma carrying peridotitic mantle xenoliths.
80 citations
TL;DR: A series of alternating phreatomagmatic ("wet") and magmatic ("dry") basaltic pyroclastic deposits forming the Crater Hill tuff ring in New Zealand contains one unit (M1) which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system as discussed by the authors.
Abstract: A series of alternating phreatomagmatic ("wet") and magmatic ("dry") basaltic pyroclastic deposits forming the Crater Hill tuff ring in New Zealand contains one unit (M1) which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system. The principal characteristics of M1 are (a) rapid lateral changes in the thicknesses of, and proportions in juvenile components in individual beds, and (b) wide ranges of juvenile clast densities in every sample. M1 appears to have been associated with an elongate source of highly variable and fluctuating magma : water ratios and magma discharge rates. This contrasts with the only other documented mixed (wet and dry) basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred.
61 citations
TL;DR: Using tephrochronostratigraphy of these eruptions and their magnitude, and the frequency of explosive volcanism at Witori and Dakataua, the authors in this article established the chronostratigraphic evidence of these events and their frequency.
53 citations
TL;DR: A record of the volcano-tectonic events leading to formation of this continental flood-basalt province is provided by strata underlying and only slightly predating the Kirkpatrick lavas as mentioned in this paper.
Abstract: The Middle Jurassic Kirkpatrick flood basalts and comagmatic Ferrar intrusions in the Transantarctic Mountains represent a major pulse of tholeiitic magmatism related to early stages in the breakup of Gondwana. A record of the volcano-tectonic events leading to formation of this continental flood-basalt province is provided by strata underlying and only slightly predating the Kirkpatrick lavas. In the central Transantarctic Mountains, the lavas rest on widespread (≥7500 km2) tholeiitic pyroclastic deposits of the Prebble Formation. The Prebble Formation is dominated by lahar deposits and is an unusual example of a regionally developed basaltic lahar field. Related, partly fault-controlled pyroclastic intrusions cut underlying strata, and vents are represented by the preserved flanks of two small tephra cones associated with a volcanic neck. Lahar and air-fall deposits typically contain 50–60% accidental lithic fragments and sand grains derived from underlying Triassic – Lower Jurassic strata in the upper part of the Beacon Supergroup. Juvenile basaltic ash and fine lapilli consist of nonvesicular to scoriaceous tachylite, sideromelane, and palagonite, and have characteristics indicating derivation from hydrovolcanic eruptions. The abundance of accidental debris from underlying Beacon strata points to explosive phreatomagmatic interaction of basaltic magma with wet sediment and groundwater, which appears to have occurred in particular where rising magma intersected upper Beacon sand aquifers. Composite clasts in the lahar deposits exhibit complex peperitic textures formed during fine-scale intermixing of basaltic magma with wet sand and record steps in subsurface fuel-coolant interactions leading to explosive eruption. The widespread, sustained phreatomagmatic activity is inferred to have occurred in a groundwater-rich topographic basin linked to an evolving Jurassic rift zone in the Transantarctic Mountains. Coeval basaltic phreatomagmatic deposits of the Mawson and Exposure Hill Formations, which underlie exposures of the Kirkpatrick Basalt up to 1500 km to the north along strike in Victoria Land, appear to represent other parts of a regional, extension-related Middle Jurassic phreatomagmatic province which developed immediately prior to rapid outpouring of the flood basalts. This is consistent with models which assign an important role to lithospheric stretching in the generation of flood-basalt provinces.
48 citations
TL;DR: In this article, extensive phreatomagmatic ash deposits (Secche di Lazzaro pyroclastics) overlying the lavas of the Neostromboli period (13000 to ca. 5000 years B.P.) have been identified at Strombolis volcano.
Abstract: Widespread phreatomagmatic ash deposits (Secche di Lazzaro pyroclastics) overlying the lavas of the Neostromboli period (13000 to ca. 5000 years B.P.) have been identified at Stromboli volcano. The deposits consist of accretionary lapilli-rich ash-fall layers and debris flows. Ash-fall layers originated by phreatomagmatic explosive activity from a vent probably located in the uppermost part of the sector-collapse scarp (Sciara del Fuoco) which formed at the end of Neostromboli period. Stratigraphic position, composition of fresh lapilli identical to the Neostromboli lavas, and an unusually large scale of explosions suggest that the Secche di Lazzaro pyroclastics were erupted in connection with the Sciara del Fuoco slope failure. Characteristics of the deposits are consistent with phreatomagmatic/hydrothermal explosivity related to catastrophic decompression of the plumbing/hydrothermal system. Occurrence of multiple ash-fall episodes, coupled with variation of nature of lithics lithology, is suggestive of repeated explosions eventually triggered by multiple, backward-migrating block detachments.
47 citations
TL;DR: In this article, it was shown that large clasts, with diameters in excess of 1-10 cm, separate from the main jet of fine ash and may be ejected from the crater rim with speeds as large as 300-400 m/s.
43 citations
TL;DR: Macauley Island (3 km2) is the tiny emergent part of the large submarine Macauley volcano (c 380 km2at the 900 m isobath) on the Kermadec Ridge.
Abstract: Macauley Island (3 km2) is the tiny emergent part of the large submarine Macauley volcano (c 380 km2at the 900 m isobath) on the Kermadec Ridge It is composed mainly of arc tholeiite basalts, with a single interbedded dacite tephra The oldest rocks seen are subaerial aa flows (North Cliff Lavas), overlain by basaltic tephra deposits (Boulder Beach Formation) Continued eruption of thin basalt flows (Annexation Lavas) built a large shield volcano, at least 150 m above sea level, with a crater in the vicinity of what is now Mt Haszard A large eruption of dacite tephra (Sandy Bay Tephra) caused collapse of the flanks of the submarine volcano, to form a caldera immediately northwest of the present island Renewed basaltic volcanism produced scoria cones, flows, and tephra (Haszard Formation), and the final stage of this eruptive phase was associated with the collapse of the northwest edge of the island into the caldera The freshness of the exposed rocks on Macauley Island indicates a late Quater
TL;DR: Three basaltic tuff-rings (Pupuke, Motukorea, and Orakei) in the Auckland volcanic field, show a range of eruption mechanisms from dry magmatic to dry and wet phreatomagmatic as mentioned in this paper.
Abstract: Three basaltic tuff‐rings (Pupuke, Motukorea, and Orakei) in the Auckland volcanic field, show a range of eruption mechanisms from dry magmatic to dry and wet phreatomagmatic. A complex mix of physical processes resulted in a variety of deposits within each tuff‐ring. Motukorea tuff‐ring shows a drying‐upward sequence from matrix‐supported facies at the base to clast‐supported facies in transitional and upper units. Pupuke tuff‐ring shows an unusual sequence for the Auckland volcanic field from clast‐supported facies at its base to matrix‐supported facies at the top. Orakei tuff‐ring deposits are dominantly matrix‐supported facies. Clast‐supported facies comprise black, incipiently vesicular lapilli and blocks with little ash, resulting from minor magmatic activity energised by steam. Clast‐supported facies are interpreted to have been deposited by fall or dry surges from explosions that involved a limited amount of water at the vent. Matrix‐supported facies are dominated by ash, poor sorting, ro...
TL;DR: In this article, the authors reveal the detailed syn-and post-caldera eruptive history of Izu Oshima Volcano, Japan, by tephra and loess stratigraphy.
Abstract: We reveal the detailed syn-and post-caldera eruptive history of Izu Oshima Volcano, Japan, by tephra and loess stratigraphy. Twenty-four tephra layers, which overlie the slope outside the caldera, show that 24 eruptions occurred since the formation of the caldera (about 1, 450 years ago). These eruptions are separated by 10-200 years clear dormant periods, which can be identified by eolian dust (loess) interbedded with tephra layers. The 24 eruptions can be classified into three types : 1) eruption with scoria and ash falls (12 eruptions), 2) eruption only with scoria falls (7 eruptions), and 3) eruption only with ash falls (5 eruptions). While tephra discharge mass of the type 1 is generally large (1.5×1010 to 7×1011 kg), that of the type 2 or 3 is small (0.6 ×109 to 1 ×1011 kg). The 1986 eruption is classified into the type 2. Debris avalanches, which occurred just before the caldera formation and covered almost all of the Izu Oshima island, demonstrate that the present caldera wall was formed by slope failure of an old edifice. The tephra-discharge stepdiagram, which shows a relationship between time and cumulative discharge volume / mass of magma, shows : 1) the average tephra-discharge rate is constant (92 kg/ s before the N1.0 eruption and 25 kg/s after the N1.0 eruption), showing an abrupt decrease of the rate at about the time of the N1.0 eruption, which occurred about 900 years ago and was the most voluminous eruption for the past 1, 450 years, 2) both before and after the N1.0 eruption, the type 1 eruption shows volume-predictability, that is, the discharge volume / mass of a next type 1 eruption can be predicted, 3) a type 1 eruption should occur sometime in the future again, and when it occurs, the discharge mass of tephra should attain to as much as 2×1011 kg or more.
TL;DR: In this article, the authors used thermoluminescence dating of four volcanic samples (material of eruption ejecta) from the island of Yiali near the Nissyros volcano in the southeastern Aegean sea, Greece, to reveal, for the first time, a volcanic eruption which occurred during the 2nd millennium B.C.
Abstract: Thermoluminescence dating of four volcanic samples (material of eruption ejecta), from the island of Yiali near the Nissyros volcano in the southeastern Aegean sea, Greece, has revealed, for first time, a volcanic eruption which occurred during the 2nd millennium B.C. (c. 1460 B.C.), different from the known Santorini eruption. The interrelation between Yiali and the well-known Santorini eruption, as well as the implications of these eruptions regarding their potential impact to the Aegean prehistoric archaeology (demise of the Minoan civilization), are discussed. © 1996 John Wiley & Sons, Inc.
TL;DR: The 1677 volcanic eruption of the town of Fuencaliente at the south end of La Palma has been associated with the large volcanic cone of San Antonio, an emission centre showing relatively high energy phreatomagmatic phases as mentioned in this paper.
Abstract: The 1677 volcanic eruption, located close to the town of Fuencaliente at the south end of La Palma, has been associated with the large volcanic cone of San Antonio, an emission centre showing relatively high energy phreatomagmatic phases However, detailed geological mapping and a reinterpretation of available eye-witness accounts elearly prove the San Antonio emission centre to be a preexisting volcano related to an eruption that occurred several thousands years earlier The 1677 eruption, or Volcan de Fuencaliente is a low magnitude eruption composed of a small strombolian vent and a cluster of aligned spatter vents About 75-125 x 10 6 m 3 of lavas from these spatter vents covered an area of 45 x 10 6 m 2 and formed a wide coastal platform with 16 x 10 6 m 2 of new land gained from the sea This modest magnitude eruption is in better accord with the negligible damage caused to the area reported in the contemporary accounts This revision of the 1677 eruption and its magnitude is relevant for the precise reconstruction of the recent volcanism of La Palma and the correct definition of volcanic hazards in the island
TL;DR: Al Wahbah, on Harrat Kishb, is the most spectacular of several volcanic explosion craters found on the lava fields of western Saudi Arabia as mentioned in this paper, where a quaternary phreatic event drilled out a crater 2 km in diameter through Proterozoic basement rocks and Quaternary lava flows.
Abstract: Al Wahbah, on Harrat Kishb, is the most spectacular of several volcanic explosion craters found on the lava fields of western Saudi Arabia. A Quaternary phreatic event drilled out a crater 2 km in diameter through Proterozoic basement rocks and Quaternary lava flows. The crater is rimmed with a tuff ring of debris from the explosion, around which were diverted Holocene basaltic lavas.
TL;DR: A Classic Period agricultural village in El Salvador was partially destroyed and encased in pyroclastic debris during the eruption of Loma Caldera about A.D. 590 as mentioned in this paper.
Abstract: A Classic Period agricultural village in El Salvador was partially destroyed and encased in pyroclastic debris during the eruption of Loma Caldera about A.D. 590. The eruption was phreatomagmatic in nature, depositing alternating units of “muddy” pyroclastic surge beds and units of air fall lapilli, pumice, and volcanic bombs. This ephemeral eruptive left only a partially eroded collapsed tephra ring. The eruption began with earth tremors and possible steam explosions, giving enough warning to allow the inhabitants of the nearby village to flee, but violent enough that they left behind many of their most valuable personal items. The low temperature of wet ash surge units, which were likely emplaced as “ash hurricanes,” preserved much of the vegetation and other botanical remains surrounding the village. Analysis of the maturity of maize preserved in agricultural fields, and the presence or absence of blossoming and fruiting plants indicates that the eruption occurred in the mid-rainy season, probably late August or September. The placement of artifacts within buildings indicate that the eruption occurred in the early evening, after the inhabitants had returned from their agricultural fields and eaten an evening meal, but before retiring for the night. Although the exact year of the eruption can only be estimated within the uncertainty of radiocarbon dating, the season of the year and the time of day can be identified with unusual precision. © 1996 John Wiley & Sons, Inc.