Showing papers on "Phreatomagmatic eruption published in 1993"

The Laki (Skaftár Fires) and Grímsvötn eruptions in 1783–1785

Abstract: The Laki (Skaftar Fires) fissure eruption in southern Iceland lasted for eight months during 1783 to 1784, and produced one of the largest basaltic lava flows in historic times (14.7±1.0 km3). In addition, neighboring Grimsvotn central volcano was frequently active during the period from May 1783 to May 1785. The combined activity is interpreted as having been the result of a two-year-long volcano-tectonic episode on the Grimsvotn volcanic system. Contemporary descriptions of the explosive activity make it possible to relate the tephra stratigraphy to the progress of the eruption on a weekly basis and show that activity on the fissures propagated to the NE with time, towards Grimsvotn. The eruption at Laki began on 8 June with a brief explosive event on a short fissure, and lava rapidly began to flow into the Skafta river gorge. It reached the lowlands, 35 km away, four days later and continued to flow, with variable discharge, until 7 February 1784. Approximately 90% of the lava was emplaced in the first five months of activity. The 27-km-long vent complex is composed of tenen echelon fissures distributed on both sides of the much older Laki hyaloclastite mountain. The surface expression of each fissure is a continuous row of vents consisting of scoria cones, spatter cones, and tuff cones. Six tephra fall units are positively identified; two units are completely compsed of phreatomagmatic tephra derived from two tuff cones and the others are Strombolian deposits. The volume of tephra, including ash fall that extended to mainland Europe, is 0.4 km3 dense rock equivalent volume, or 2.6% of the total erupted volume. Interpretation of contemporary descriptions of tephra falls, combined with the preserved stratigraphy, allow the identification of ten eruptive episodes during the eight months of activity on the Laki fissures. These eruptive episodes are inferred to have resulted from the unsteady flow of magma in the feeder system. In addition, at least eight eruption episodes occurred at Grimsvotn in 1783 to 1785, five in 1783, two in 1784, and one in 1785. Each episode at Laki began with a seismic swarm of increasing intensity that led to the formation of a new fissure, the opening of which was followed by short-lived phreatomagmatic activity caused by the high water table around the eruption site. Activity usually changed to violent Strombolian or sub-Plinian, followed by Hawaiian fire fountaining and effusive activity as the availability of groundwater dwindled. Thus, the explosive activity associated with the opening of each fissure was largely controlled by external watermagma interactions. Maximum effusion rates, occurring in the first two episodes, are estimated to have been 8.5x103 and 8.7x103 m3 s-1 from fissures totaling 2.2 and 2.8 km in length, respectively, and, in general, discharge gradually decreased over time. The highest rates are equivalent to 5.6x103 and 4.5x103 kg s-1 per meter length of fissure, values that could conceivably be similar to those that produced some flood basalt lava flows. Maximum fire fountain heights are estimated to have varied from 800 m to 1400 m and convecting eruption columns above the vents rose to a maximum altitude of about 15 km. The release of sulfur gases during fountaining produced an acid haze (aerosol) which spread widely and had a considerable environmental, and possibly climatic, impact on the Northern Hemisphere.

The Neapolitan Yellow Tuff — A large volume multiphase eruption from Campi Flegrei, Southern Italy

Abstract: the Neapolitan Yellow Tuff (NYT) (12 ka BP) is considered to be the product of a single eruption Two different members (A and B) have been identified and can be correlated around the whole of Campi Flegrei Member A is made up of at least 6 fall units including both ash and lapilli horizons The basal stratified ash unit (A1) is interpreted to be a phreatoplinian fall deposit, since it shows a widespread dispersal (>1000 km2) and a constant thickness over considerable topography The absence of many lapilli fall units in proximal and medial areas testifies to the erosive power of the intervening pyroclastic surges The overlying member B was formed by many pyroclastic flows, radially distributed around Campi Flegrei, that varied widely in their eruptive and emplacement mechanisms In some of the most proximal exposures coarse scoria and lithic-rich deposits, sometimes welded, have been identified at the base of member B Isopach and isopleth maps of fall-units, combined with the distribution of the coarse proximal facies, indicate that the eruptive vent was located in the NE area of Campi Flegrei It is considered that the NYT eruption produced collapse of a caldera approximately 10 km diameter within Campi Flegrei The caldera rim, located by geological and borehole evidence, is now largely buried by the products of more recent eruptions Initiation of caldera collapse may have been contemporaneous with the start of the second phase (member B) It is suggested that there was a single vent throughout the eruption rather than the development of multiple or ring vents Chemical data indicate that different levels of a zoned trachyte-phonolite magma chamber were tapped during the eruption The minimum volume of the NYT is calculated to be about 50 km3 (DRE), of which 35 km3 (∼70%) occurs within the caldera

Moist convection and the injection of volcanic ash into the atmosphere

Abstract: If unsaturated water vapor is carried upward by a volcanic eruption column, it may eventually become saturated owing to the decrease in temperature of the column as it expands through decompression and transfers heat to entrained air. Heat released as a result of the subsequent condensation of water vapor causes the air within the column to expand. We show that this increases the buoyancy and therefore the total height of rise of the column. The increase in height is significant in relatively small sub-Plinian and Strombolian eruptions in which mass eruption rates lie in the range 103 to 106 kg/s. In such eruptions, the latent heat released as the entrained water vapor condenses may provide the main source of heat which drives the ash and clasts upward. The height of rise then becomes relatively insensitive to the mass flux erupted at the vent and depends primarily upon the vapor loading of the atmosphere. In a moist atmosphere, ash may rise several kilomtres higher than in an eruption of comparable strength in a dry environment. Moist convection leads to much wider ash dispersal, particularly from very small eruptions. Subsequently, rain flushing of ash from umbrella clouds may result from the water which forms through the condensation of entrained vapor; the ash provides natural condensation nuclei for some of this entrained vapor, whose mass may be much greater than that of the ash. In small columns ( 107 kg/s), the latent heat released by condensation of vapor is relatively small in comparison with the thermal energy provided by the hot clasts and therefore moisture has no significant effect upon the eruption column dynamics; furthermore, the mass of water vapor originating from the erupted volatiles is usually comparable to, or greater than, that entrained from the ambient air. Our model also shows that, if the erupting mixture becomes buoyant, then the eruption columns associated with phreatomagmatic eruptions ascend nearly as high as Plinian columns with the same mass eruption rate. This is because the water which is vaporized by the hot ash at the source, condenses higher in the column and thereby restores its latent heat to the ascending ash.

Recycling of magmatic clasts during explosive eruptions: estimating the true juvenile content of phreatomagmatic volcanic deposits

Abstract: The juvenile content of phreatomagmatic deposits contains both ‘first-cycle’ juvenile clasts derived from magma at the instant of eruption, and recycled juvenile clasts, which were fragmented and first ejected by earlier explosions during the eruption, but fell back or collapsed into the vent. Recycled juvenile clasts are similar to accessory and accidental lithics in that they contribute no heat to further magma: water interaction, but previously no effective criteria have been defined to separate them from ‘first-cycle’ juvenile clasts. We have investigated componentry parameters (vesicularity, clast morphology and extent of mud-coating) which, in specific circumstances, can distinguish between first-cycle juvenile clasts, involved in only one explosion, and such recycled juvenile clasts. Phreatomagmatic fall deposits commonly show gross grainsize and sorting characteristics identical to deposits of purely ‘dry’ or magmatic eruptions. However the abundance of non-juvenile clasts in pyroclastic deposits is a sensitive indicator of the involvement of external water. If this component is calculated including recycled juvenile clasts with accidental and accessory clasts the contrast is even more striking. Data from a Holocene maar deposit in Taupo Volcanic Zone, New Zealand, suggest that the first-cycle juvenile component of the deposits is less than one-third of that determined by simple juvenile:lithic:crystal componentry.

A facies interpretation of the eruption and emplacement mechanisms of the upper part of the Neapolitan Yellow Tuff, Campi Flegrei, southern Italy

Abstract: This study focuses on the upper part, Member B, of the Neapolitan Yellow Tuff (NYT). Detailed measurements of stratigraphic sections within the unlithified ‘pozzolana’ facies show that Member B is composed of at least six distinct depositional units which each record a complex fluctuation between different styles of deposition from pyroclastic density flows. Six lithofacies have been identified: (1) massive valleyponded facies, the product of non-turbulent flows; (2) inverse-graded facies formed by flows that were turbulent for the majority of transport but were deposited through a non-tubulent basal regime; (3) regressive sand-wave facies, the product of high-concentration, turbulent flows; (4) stratified facies, the product of deposition from turbulent, low-particle-concentration, flows; (5) particle aggregate and (6) vesicular ash lithofacies, both of which are considered to have formed by deposition from turbulent, low-concentration flows. Although the whole eruption may have been phreatomagmatic, facies 1–4 are interpreted to be the product of dry eruptive activity, whereas facies 5 and 6 are considered to be of wet phreatomagmatic eruptive phases. Small-scale horizontal variations between facies include inverse-graded lithofacies that pass laterally into regressive sand-wave structures and stratified deposits. This indicates rapid transition from non-turbulent to turbulent deposition within the same flow. Thin vesicular ash and particle aggregate layers pass laterally into massive valley-ponded vesicular lithofacies, suggesting contemporaneous wet pyroclastic surges and cohesive mud flows. Three common vertical facies relations were recognised. (1) Massive valley-ponded and inverse-graded facies are overlain by stratified facies, suggesting decreasing particle concentration with time during passage of a flow. (2) Repeated vertical gradation from massive up into stratified facies and back into massive beds, is indicative of flow fluctuating between non-turbulent and turbulent depositional conditions. (3) Vertical alternation between particle aggregates and vesicular facies is interpreted as the product of many flow pulses, each of which involved deposition of a single particle aggregate and vesicular ash layer. It is possible that the different facies record stages in a continuum of flow processes. The deposits formed are dependent on the presence, thickness and behaviour of a high-concentration, non-turbulent boundary layer at the base of the flow. The end members of this process are (a) flows that transported and deposited material from a non-turbulent flow regime and (b) flows that transported and deposited material from a turbulent flow regime.

Maars of the Westeifel, Germany

Abstract: Within the Westeifel Volcanic Field 27% of the 250 Quaternary eruptive centers are maars. Maars form as a result of a highly explosive interactive process between rising melt and groundwater. In the Westeifel, probably thermal water plays an important role for the productive phreatomagmatic interaction process and, con-sequently, the high number of maars. The Westeifel maars show all transitions to scoria cones. Only the youngest maars are filled by a maar lake or a raised bog, and are well preserved. The older maars show a low diameter to depth ratio. Nearly one third of the Westeifel maars were formed during the Weichselian glaciation period. The isostatic movements during the increasing and decreasing glaciation generated tectonic stress in front of the ice cap and, probably, caused the inten-sive volcanic activity during the last glaciation. This assumed to be the reason why for the last 10 000 years BP (Ulmen maar activity) no volcanic activity has happened.

Pyroclastic phases of a rhyolitic dome-building eruption: Puketarata tuff ring, Taupo Volcanic Zone, New Zealand

Abstract: The 14 ka Puketarata eruption of Maroa caldera in Taupo Volcanic Zone was a dome-related event in which the bulk of the 0.25 km3 of eruption products were emplaced as phreatomagmatic fall and surge deposits. A rhyolitic dike encountered shallow groundwater during emplacement along a NE-trending normal fault, leading to shallow-seated explosions characterised by low to moderate water/magma ratios. The eruption products consist of two lava domes, a proximal tuff ring, three phreatic collapse craters, and a widespread fall deposit. The pyroclastic deposits contain dominantly dense juvenile clasts and few foreign lithics, and relate to very shallow-level disruption of the growing dome and its feeder dike with relatively little involvement of country rock. The distal fall deposit, representing 88% of the eruption products is, despite its uniform appearance and apparently subplinian dispersal, a composite feature equivalent to numerous discrete proximal phreatomagmatic lapilli fall layers, each deposited from a short-lived eruption column. The Puketarata products are subdivided into four units related to successive phases of:(A) shallow lava intrusion and initial dome growth; (B) rapid growth and destruction of dome lobes; (C) slower, sustained dome growth and restriction of explosive disruption to the dome margins; and (D) post-dome withdrawal of magma and crater-collapse. Phase D was phreatic, phases A and C had moderate water: magma ratios, and phase B a low water: magma ratio. Dome extrusion was most rapid during phase B, but so was destruction, and hence dome growth was largely accomplished during phase C. The Puketarata eruption illustrates how vent geometry and the presence of groundwater may control the style of silicic volcanism. Early activity was dominated by these external influences and sustained dome growth only followed after effective exclusion of external water from newly emplaced magma.

Magma-water interaction in closed systems and application to lava tunnels and volcanic conduits

Abstract: The magma-water interaction in closed systems was studied by a lumped-parameter model which employs the conservation of mass and balance of energy equations for magma, water, and gas (water vapor). The resulting system of ordinary differential equations models the distributions of system pressure, gas volume, water droplet size, and magma, water, and gas temperatures as a function of time and system parameters. The system parameters include the characteristics of the surrounding rocks and degrees of magma and water fragmentation as specified by the initial water and magma drop sizes. The resulting model was applied to the rupturing of lava tunnels at Etna and magma-water interaction in the magmatic conduit of Vesuvius during the gray eruption phase in A.D. 79. The results demonstrate that very rapid system pressurization times of the order of a tenth of a second are possible when the water and magma are highly fragmented. The magma-water interaction within the conduit of Vesuvius was associated with the water inflow into the conduit due to the decrease of the magmatic pressure below the hydrostatic pressure of the surrounding aquifer and fracturing of the conduit wall. The predicted times of pressure increase within the conduit and the high pressure achieved are consistent with observations of phreatomagmatic eruptions and characteristics of the deposits which suggest that these eruptions are characterized by a series of eruptive pulses associated with the transition between magmatic and hydromagmatic phases.

Volcanology and Petrology of Irazú Volcano, Costa Rica

Abstract: Irazu volcano, the highest (3432 m a.s.l.) and one of the largest volcanoes (600 km3) at the southern end of the Central American Volcanic Front (CA VF), is a basaltic andesite shield volcano located in the Cordillera Central of Costa Rica. The eruptive history of Irazu is reconstructed through mapping and stratigraphic studies of the summit area and south flank. The rocks consist in high-K basaltic andesite to andesite and rare dacites (Si(h= 53-64%, FeQJMgO = 1.46, Cr= 18-145 ppm, Ni= 9-111 ppm) and high-Mg basalts (Si(h = 50-53%, FeOr'Mg 0= 1, Cr= 216-411, Ni = 115-211). The basaltic andesite lava flows and minor associated tephras a.re volumetrically dominant. The basaltic lava flows and tephra deposits are subordinate but very important in being the most prumuve magmas yet recogmzed m the Quaternary Costa Rican Volcanic Front (c;&VF). Irazu lava contain phenocrysts of plagioclase (A1140-s1) with variable roning, clinopyroxene (augite,W03344E1141.wss.1s), weakly zoned orthopyroxene (enstatite, En69-78), olivine (Fo,2.90_5) with Cr-spin el inclusions, and Petri-oxides (titanomagnetite and chromian magnetite). Biotite, apatite and hornblende phenocrysts and microphenocrysts a.re uncommon. Pre-eruption temperatures of the magmas, as determined by two-pyroxene thermometry, range from > 920°C to I 185°C for hornblende andesites to basalts, respectively. Irazu lavas have high REE abundances and radiogenic isotopic compositions (Sr and Nd) anomalous compared to the rest of the CA VF but are similar to other lavas in the Cordillera Central of Costa Rica. Basaltic lavas are interpreted to have formed by low degrees of melting of an enriched mantle source (E-type MORB or OIB-lik:e). Mostly high-K basaltic andesites and andesites are in general consistent with derivation from a calc-alkaline magma by fractional crystalliz.ation at different crustal levels. There is, however, good evidence that mixing of these magmas at different crustal levels played an important role in the petrogenesis of these lavas. Banded tephras are common and indicate incomplete 'low-pressure mixing' (p s9 in some cases coated by pyroxene and plagioclase, or with picotite inclusions in basaltic andesites, two or more generations of plagioclase and pyroxene phenocrysts, and plagioclase and augite with reverse roning record a complex history of magma mixing. Homogeous 'high-pressure mixing' events between high-Mg basalts and basaltic andesites (p � 3 kb) produced hybrid calcalkaline lavas with anomalous geochemical trends (e.g., high MgO, Cr, and Ni contents). Other processes, such as in situ crystallization and assimilation of subvolcanic rocks and deep-crustal igneous rocks could also have occurred. Generally, the main Holocene eruptive phases at the summit began with magmatic or "dry" eruptions followed by phreatomagmatic and/or phreatic eruptions. During these time tephras are predominantly of basaltic andesite composition. The deposits contain , however, evidence for the existence of basaltic to dacite magmas preserved in the mixing events. The earliest reported eruption of Irazu (phreatomagmatic and strombolian with rare phreatic phases) was in 1723. Other reports of volcanic eruptions during the 18th and 19th centuries are very doubtful and no deposits have been located. Several phreatomagmatic phases have occurred during this century (1917-1921, 1924, 1928, 1930, 1933, 1939-1940 and 1963-1965). The maximum Volcanic Explosivity Index CVEi Newhall and Self, 1982) was grade 3. A minimum of 0.5 km3 (DRE, dense rock equivalent) of basaltic andesite magma has been produced in historic time. According to historic documents, several volcanic explosions were preceded by earthquakes. The timing of explosive phases at Irazu in 1723, 1918, 1928, 1933, and 1963-1965 was comparated with earth tides (maximum or minimum tidal amplitude), but a correlation is not always evident. Historic strombolian, phreatic and phreatomagmatic eruptions generated ballistic fallout, pyroclastic surge and 'pyroclastic' flow deposits. Some thin ( < 1 m) pyroclastic-rich mass flow deposits and proximal, pyroclastic, "dry" and "wet" surge deposits were deposited from currents that flowed back into the crater. The explosive eruptions of lrazu and their products are governed by external ( erosion and collapse of the conduit walls, and recycling of tephras) and internal factors (magma discharge rate and rise rate of bubbles through the magma). The high degree of fragmentation of tephra at Irazu reflects the recycling tephras, and magma/water interaction during eruptions. The distribution of the pyroclastic deposits, the stratigraphy, and their relationships, together with eyewitness, grain siz.e analyses and SEM (scanning electron microscope) investigations support the interpretation of eruption slyle and the recycling of tephra. Volcaniclastic debris fans in the Reventado river, Cartago, have been the sites of repeated mass flows in prehistoric and historic time. Volcaniclastic mass flow events unrelated to eruptions have been triggered during high-intensity rain storms principally in October and November. In addition, very fine ash deposits from phreatomagmatic eruptions form an impermeable cover and cause the destruction of the forest resulting in unstable slope conditions (especially deep erosion and reactivation of landslides) in the Irazu drainage basins. This contributes to the formation of lahars during the rainy season (May to December). According to historic records, the recurrence interval of Irazu's main mass flows is 24.5 ± 12.5 years for the last 130 years, and an average of about 50 years from 1723 to 1965. Based on direct sediment concentration measurement of the 1963-1965 lahars and their comparison to the structure of the sediments, there is no strict correlation between the arbitrary rheological classification of Beverage and Culbertson (1964) and the sedimentological criteria of Smith (1986). Thus, the rheological behavior cannot always be inferred from the deposits, as can be seen from the 1963-1965-lahar historic events. The tectonic lineations at Irazu are likely to be the sites of future eruptions (summit and flank) or shallow (depth S 15 km) strong earthquakes (Modified Mercalli scale Intensity, MMI S VII; surface wave magnitude, Ms S 6.5). The greatest hazards at Irazu are related to thick fallout deposits, volcaniclastic mass flows (lahars), landslides and strong ground motion during earthquakes. The total estimated damage caused by eruptions of Irazu and mass flows that affect Cartago and other towns could be in the order of U.S. $ 100-200 million. This investigation presents information on the characteristic eruption styles at Irazu, patterns of its historic and prehistoric eruptive activity, and their effects (including socio-economical aspects) that are essential for an adequate short-term hazard assessment, and provide the basis for planning land use and emergency responses.

Volcanism and sedimentation in part of a Late Archaean rift: the Hartbeesfontein basin, Transvaal, South Africa

Abstract: The Hartbeesfontein basin is one basin within the Late Archaean rift system of South Africa. This rift system has been recently compared to the Basin and Range province in western North America and may therefore be an ensialic extensional back-arc basin. Structurally, the Hartbeesfontein basin is a half-graben structure bounded to the south-east by a major, normal, listric fault and to the north-east and south-west by strike-slip (transfer?) fault zones. It is infilled by over 2000 m of diamictites, shales, lavas and chemical sediments. Initial basin formation appears to be accompanied by phreatomagmatic volcanic activity caused by the interaction between basic tholeiitic magmas rising along fractures and groundwater. Volcaniclastic debris from these eruptions was incorporated into laharic debris flows and deposited on basin marginal alluvial fans. At the same time a deep, permanent lake formed within the basin in which silts and muds accumulated. Major fissure eruptions of basic, tholeiitic lavas followed, their eruptive centres being apparently located along the strike-slip (transfer?) fault /ones. Initially, these fissure eruptions had high rates of magma discharge accompanied by intense fire fountaining that resulted in the rapid accumulation of aa type flows. Later lava discharge rates decreased and more quiescent pahoehoe type flows were erupted. Localized centres of acid volcanism within the basic lava pile were located along the south-western strike-slip fault zone. These acid volcanic rocks are interpreted as co-ignimbrite lag breccias and pyroclastic flow deposits and tuffs produced by the repeated formation and collapse of Plinian eruption columns. Towards the top of the basic lava pile, two breaks in volcanism permitted the formation of dolomitic playa lakes. Sedimentation in these lakes was terminated by further basic lava flows. At the top of the basin fill sequence is a thick, bedded chert interpreted as a magadiitic, alkaline playa lake fed by silica-rich hot springs located along the south-eastern edge of the basin. Quartzites and conglomerates deposited by braided rivers unconformably overlie the basin-fill sequence and probably represent a through flowing river system signifying termination of the Hartbeesfontein basin as a separate basin. The Hartbeesfontein basin and its fill demonstrate that a close relationship exists between fissure volcanism, sedimentation and basin evolution and that the strike-slip, transfer faults acted as the loci of volcanic activity.

Role of magma-water interaction in very large explosive eruptions

Abstract: An important class of explosive eruptions, involving large-scale magma-water interaction during the discharge of hundreds to thousands of cubic kilometers of magma, is discussed. Geologic evidence for such eruptions is summarized. Case studies from New Zealand, Australia, England, and the western United States are described, focusing on inferred eruption dynamics. Several critical problems that need theoretical and experimental research are identified. These include rates at which water can flow into a volcanic vent or plumbing system, entrainment of water by explosive eruptions through lakes and seas, effects of magma properties and gas bubbles on magma-water interaction, and hazards associated with the eruptions.

Magmatic Eruptions of Niigata-Yakeyama Volcano in Historic Times

Abstract: 新潟焼山火山(2,400.3m)は,妙 高火山群の北 端 に位置する ドーム状の小型成層火山で,有 吏以 降も断続的に活発な活動を続けている活火山であ る。1974年 の水蒸気爆発では,登 山者3人 が犠牲 になった。焼山は,爆 発的噴火によって火砕流や 火砕サージを噴出しやすい極めて危険な火山であ るとい うことが判明してお り,火 砕流が噴出され た場合には,地 形上その流走域 となることが確実 な北方の早川流域には,現在約5,000人 が定住 して いる。また,過 去の火砕サージの一一部は,約3万 人の人 口を有する新井市近郊にまで達 している。 本研究は,極 めて活動的な火山であり,噴 火 し た場合には大きな災害に結びつく可能性の高い新 潟焼山火山について,火 砕流 と火砕サージ堆積物 の年代 を決定し,こ れら堆積物 と噴火記録 との対 応関係を明らかにすることを目的としておこなわ れた。結果の詳細については,別 稿を準備中であ るので,こ こではその概要のみを記述する。