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1.
Michael O. Garcia J. M. Rhodes Frank A. Trusdell Aaron J. Pietruszka 《Bulletin of Volcanology》1996,58(5):359-379
The Puu Oo eruption has been remarkable in the historical record of Kilauea Volcano for its duration (over 13 years), volume
(>1 km3) and compositional variation (5.7–10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down
the east rift zone and the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous,
relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo eruption, episodes
48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983–1994). Nearly
aphyric to weakly olivine-phyric lavas were erupted during episodes 48 and 49. The variation in MgO content of Kupaianaha
lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled
by variations in Kilauea's magma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with
whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pauses in the
eruption, accumulated 2–4 vol.% olivine. The highest forsterite content of olivines (∼85%) in Kupaianaha lavas indicates that
the parental magmas for these lavas had MgO contents of ∼10 wt.%, which equals the highest observed value for lavas during
this eruption. The composition of the Puu Oo lavas has progressively changed during the eruption. Since early 1985 (episode
30), when mixing between an evolved rift zone magma and a more mafic summit reservoir-derived magma ended, the normalized
(to 10 wt.% MgO) abundances of highly incompatible elements and CaO have systematically decreased with time, whereas ratios
of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. These
results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10–20 km3 (assuming 5–10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible
elements and clinopyroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu
Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a–1 vs ∼0.03 a–1). This rapid change indicates that Puu Oo magmas did not mix thoroughly with magma in the summit reservoir. Thus, except
for variable amounts of olivine fractionation, the geochemical variation in these lavas is predominantly controlled by mantle
processes.
Received: 8 March 1996 / Accepted: 30 April 1996 相似文献
2.
Age spectra from 40Ar/39Ar incremental heating experiments yield ages of 298 ± 25 ka and 310 ± 31 ka for transitional composition lavas from two cones
on submarine Mahukona Volcano, Hawaii. These ages are younger than the inferred end of the tholeiitic shield stage and indicate
that the volcano had entered the postshield alkalic stage before going extinct. Previously reported elevated helium isotopic
ratios of lavas from one of these cones were incorrectly interpreted to indicate eruption during a preshield alkalic stage.
Consequently, high helium isotopic ratios are a poor indicator of eruptive stage, as they occur in preshield, shield, and
postshield stage lavas. Loihi Seamount and Kilauea are the only known Hawaiian volcanoes where the volume of preshield alkalic
stage lavas can be estimated.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
3.
The magma eruption rates of Merapi volcano form 1890 to 1992 are re-examined chronologically. For this volcano, movements of extruded lavas and domes as well as their extrusions are important because they control the modes of the subsequent activities and cause nuées ardentes and lahars. The monthly eruption rates varied widely, but the cumulative volume of lavas has increased linearly and is expressed as 0.1x106 m3/month. The magma production rate of this volcano may have been constant for these 100 years. Recurrent excessive effusion of lavas is tentatively interpreted by assuming a magma reservoir. The averaged eruption rate is small in comparison with other volcanoes such as Nyramuragia, Kilauea and Vesuvio. However, it is remarkable that the activity has been continuous for these 100 years and the total amount of lava discharged during this period reached more than 108 m3. A simple model for the formation of the 1992 lava dome is presented. The viscosity of the lavas is probably between 106 and 107 P and the length of the magma conduit is probably less than 10 km. 相似文献
4.
Pliocene–Recent volcanic outcrops at Seal Nunataks and Beethoven Peninsula (Antarctic Peninsula) are remnants of several
monogenetic volcanoes formed by eruption of vesiculating basaltic magma into shallow water, in an englacial environment. The
diversity of sedimentary and volcanic lithofacies present in the Antarctic Peninsula outcrops provides a clear illustration
of the wide range of eruptive, transportational and depositional processes which are associated with englacial Surtseyan volcanism.
Early-formed pillow lava and glassy breccia, representing a pillow volcano stage of construction, are draped by tephra erupted
explosively during a tuff cone stage. The tephra was resedimented around the volcano flanks, mainly by coarse-grained sediment
gravity flows. Fine-grained lithofacies are rare, and fine material probably bypassed the main volcanic edifice, accumulating
in the surrounding englacial basin. The pattern of sedimentation records variations in eruption dynamics. Products of continuous-uprush
eruptions are thought to be represented by stacks of poorly bedded gravelly sandstone, whereas better bedded, lithologically
more diverse sequences accumulated during periods of quiescence or effusive activity. Evidence for volcano flank failure is
common. In Seal Nunataks, subaerial lithofacies (mainly lavas and cinder cone deposits) are volumetrically minor and occur
at a similar stratigraphical position to pillow lava, suggesting that glacial lake drainage may have occurred prior to or
during deposition of the subaerial lithofacies. By contrast, voluminous subaerial effusion in Beethoven Peninsula led to the
development of laterally extensive stratified glassy breccias representing progradation of hyaloclastite deltas.
Received: 5 February 1996 / Accepted: 17 January 1997 相似文献
5.
Eruptive style of the young high-Mg basaltic-andesite Pelagatos scoria cone,southeast of México City 总被引:1,自引:1,他引:0
The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico
City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long
lava flow, and continued with a phase of alternating and/or simultaneous explosive and effusive activity that built a 50-m-high
scoria cone on the western end of the fissure and formed a compound lava flow-field near the vent. The eruption ended with
the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater. Products
consist of a microlite-rich high-Mg basaltic andesite. Samples were analyzed to determine the magma’s initial properties as
well as the effects of degassing-induced crystallization on eruptive style. Although distal ash fallout deposits from this
eruption are not preserved, a recent quarry exposes a large section of the scoria cone. Detailed study of exposed layers allows
us to elucidate the mode of cone-building activity. Petrological and textural data, combined with models calibrated by experimental
work and melt-inclusion analyses of similar magmas elsewhere, indicate that the magma was initially hot (>1,200°C), gas-rich
(up to 5 wt.% H2O), crystal-poor (~10 vol.% Fo90 olivine phenocrysts) and thus poorly viscous (40–80 Pa s). During the early phase, low magma ascent velocity at the fissure
vent allowed low-viscosity magma to degas and crystallize during ascent, producing lava flows with elevated crystal contents
at T < 1,100°C, and blocky surfaces. Later, the closure of the fissure by cooling dikes focused the magma flow at a narrow section
of the fissure. This led to an increased magma ascent velocity. Rapid and shallow degassing (<3 km deep) triggered ~40 vol.%
microlite crystallization. Limited times for gas-escape and higher magma viscosity (6 × 105–4 × 106 Pa s) drove strong explosions of highly (60–80 vol.%) and finely vesicular magma. Coarse clasts broke on landing, which implies
brittle behavior due to complete solidification. This requires sufficient time to cool and in turn implies ejection heights
of over 1 km, which is much higher than “normal” Strombolian activity. Hence, magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics
of shallow degassing. The long-lasting eruptions of Jorullo and Paricutin, which produced similar magmas in western México,
were more explosive. This can be related to higher magma fluxes and total erupted volumes. Implications of this study are
important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the Trans-Mexican Volcanic Belt. 相似文献
6.
Postshield volcanism and catastrophic mass wasting of the Waianae Volcano, Oahu, Hawaii 总被引:2,自引:2,他引:0
The 3.9- to 2.9-Ma Waianae Volcano is the older of two volcanoes making up the island of Oahu, Hawaii. Exposed on the volcanic
edifice are tholeiitic shield lavas overlain by transitional and alkalic postshield lavas. The postshield "alkalic cap" consists
of aphyric hawaiite of the Palehua Member of the Waianae Volcanics, overlain unconformably by a small volume of alkalic basalt
of the Kolekole Volcanics. Kolekole Volcanics mantle erosional topography, including the uppermost slopes of the great Lualualei
Valley on the lee side of the Waianae Range. Twenty new K–Ar dates, combined with magnetic polarity data and geologic relationships,
constrain the ages of lavas of the Palehua member to 3.06–2.98 Ma and lavas of the Kolekole Volcanics to 2.97–2.90 Ma. The
geochemical data and the nearly contemporaneous ages suggest that the Kolekole Volcanics do not represent a completely independent
or separate volcanic event from earlier postshield activity; thus, the Kolekole Volcanics are reduced in rank, becoming the
Kolekole Member of the Waianae Volcanics. Magmas of the Palehua and Kolekole Members have similar incompatible element ratios,
and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pressures below
the edifice. However, lavas of the Kolekole Member are less fractionated and appear to have evolved at greater depths than
the earlier Palehua hawaiites. Postshield primary magma compositions of the Palehua and Kolekole Members are consistent with
formation by partial melting of mantle material of less than 5–10% relative to Waianae shield lavas. Within the section of
Palehua Member lavas, an increase with respect to time of highly incompatible to moderately incompatible element ratios is
consistent with a further decrease in partial melting by approximately 1–2%. This trend is reversed with the onset of eruption
of Kolekole Member lavas, where an increase in extent of partial melting is indicated. The relatively short time interval
between the eruption of Palehua and Kolekole Member lavas appears to date the initial formation of Lualualei Valley, which
was accompanied by a marked change in magmatic conditions. We speculate that the mass-wasting event separating lavas of the
Palehua and Kolekole Members may be related to the formation of a large submarine landslide west and southwest of Waianae
Volcano. Enhanced decompression melting associated with removal of the equivalent volume of this landslide deposit from the
edifice is more than sufficient to produce the modeled increase of 1–2% in extent of melting between the youngest Palehua
magmas and the posterosional magmas of the Kolekole Member. The association between magmatic change and a giant landsliding
event suggests that there may be a general relationship between large mass-wasting events and subsequent magmatism in Hawaiian
volcano evolution.
Received: 1 September 1996 / Accepted: 26 November 1996 相似文献
7.
Crystal size distributions and other quantitative textural measurements in lavas and tuff from Egmont volcano (Mt. Taranaki), New Zealand 总被引:2,自引:0,他引:2
Michael D. Higgins 《Bulletin of Volcanology》1996,58(2-3):194-204
The size, shape and orientation of plagioclase crystals have been quantified in a tuff and series of andesite lavas from
the active Egmont volcano (Mt. Taranaki), New Zealand. Linear crystal size distributions (CSDs) show that if the magma had
several components, then only one provided the crystals. The slope of the CSD indicates that the earliest lavas measured had
a residence time of ∼50 years in the magma chamber for a growth rate of 10–11 cm/s. Subsequent lavas had slightly longer residence times (50–75 years), but the following series returned to 50-year residence
times. The youngest magmas, from both Egmont summit and the parasitic Fantham's Peak, have the shortest residence times of
∼30 years. Variations in residence time may reflect changes in the magma chamber shape or depth, or the temperature of the
surrounding rocks. Crystal shapes and zonation suggest that crystallization occurred in a bottle-shape magma chamber, and
not in a narrow conduit. If future eruptions use the same magma chamber as the most recent eruptions, then a delay of approximately
30 years can be expected between refilling and eruption.
Received: 25 October 1995 / Accepted: 19 April 1996 相似文献
8.
The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of
the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this
event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products
of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or
lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations
in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative
atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have
generated ∼250 Mt of H2SO4 aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements.
Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine
concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and
15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried
by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development
of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki
are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years.
The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved
in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over
Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements
of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of
aerosols estimated by other methods.
Received: 30 May 1995 / Accepted: 19 April 1996 相似文献
9.
Wendell A. Duffield Robert L. Christiansen Robert Y. Koyanagi Donald W. Peterson 《Journal of Volcanology and Geothermal Research》1982,13(3-4)
The magmatic plumbing system of Kilauea Volcano consists of a broad region of magma generation in the upper mantle, a steeply inclined zone through which magma rises to an intravolcano reservoir located about 2 to 6 km beneath the summit of the volcano, and a network of conduits that carry magma from this reservoir to sites of eruption within the caldera and along east and southwest rift zones. The functioning of most parts of this system was illustrated by activity during 1971 and 1972. When a 29-month-long eruption at Mauna Ulu on the east rift zone began to wane in 1971, the summit region of the volcano began to inflate rapidly; apparently, blockage of the feeder conduit to Mauna Ulu diverted a continuing supply of mantle-derived magma to prolonged storage in the summit reservoir. Rapid inflation of the summit area persisted at a nearly constant rate from June 1971 to February 1972, when a conduit to Mauna Ulu was reopened. The cadence of inflation was twice interrupted briefly, first by a 10-hour eruption in Kilauea Caldera on 14 August, and later by an eruption that began in the caldera and migrated 12 km down the southwest rift zone between 24 and 29 September. The 14 August and 24–29 September eruptions added about 107 m3 and 8 × 106 m3, respectively, of new lava to the surface of Kilauea. These volumes, combined with the volume increase represented by inflation of the volcanic edifice itself, account for an approximately 6 × 106 m3/month rate of growth between June 1971 and January 1972, essentially the same rate at which mantle-derived magma was supplied to Kilauea between 1952 and the end of the Mauna Ulu eruption in 1971.The August and September 1971 lavas are tholeiitic basalts of similar major-element chemical composition. The compositions can be reproduced by mixing various proportions of chemically distinct variants of lava that erupted during the preceding activity at Mauna Ulu. Thus, part of the magma rising from the mantle to feed the Mauna Ulu eruption may have been stored within the summit reservoir from 4 to 20 months before it was erupted in the summit caldera and along the southwest rift zone in August and September.The September 1971 activity was only the fourth eruption on the southwest rift zone during Kilauea's 200 years of recorded history, in contrast to more than 20 eruptions on the east rift zone. Order-of-magnitude differences in topographic and geophysical expression indicate greatly disparate eruption rates for far more than historic time and thus suggest a considerably larger dike swarm within the east rift zone than within the southwest rift zone. Characteristics of the historic eruptions on the southwest rift zone suggest that magma may be fed directly from active lava lakes in Kilauea Caldera or from shallow cupolas at the top of the summit magma reservoir, through fissures that propagate down rift from the caldera itself at the onset of eruption. Moreover, emplacement of this magma into the southwest rift zone may be possible only when compressive stress across the rift is reduced by some unknown critical amount owing either to seaward displacement of the terrane south-southeast of the rift zone or to a deflated condition of Mauna Loa Volcano adjacent to the northwest, or both. The former condition arises when the forceful emplacement of dikes into the east rift zone wedges the south flank of Kilauea seaward. Such controls on the potential for eruption along the southwest rift zone may be related to the topographic and geophysical constrasts between the two rift zones. 相似文献
10.
Richard J. Brown S. Manya I. Buisman G. Fontana M. Field C. Mac Niocaill R. S. J. Sparks F. M. Stuart 《Bulletin of Volcanology》2012,74(7):1621-1643
The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic 3He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30?Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40?m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >102 to 106?Pa?s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma). 相似文献
11.
In 1874 and 1875 the fissure swarm of Askja central volcano was activated during a major rifting episode. This rifting resulted in a fissure eruption of 0.3 km3 basaltic magma in Sveinagja graben, 50 to 70 km north of Askja and subsequent caldera collapse forming the Oskjuvatn caldera within the main Askja caldera. Five weeks after initial collapse, an explosive mixed magma eruption took place in Askja. On the basis of matching chemistry, synchronous activity and parallels with other rifted central volcanoes, the events in Askja and its lissure swarm are attributed to rise of basaltic magma into a high-level reservoir in the central volcano, subsequent rifting of the reservoir and lateral flow magma within the fissure swarm to emerge in the Sveinagja eruption. This lateral draining of the Askja reservoir is the most plausible cause for caldera collpse. The Sveinagja basalt belong to the group of evolved tholejites characteristie of several Icelandic central volcanoes and associated fissure swarms. Such tholeiites, with Mgvalues in the 40 to 50 tange, represent magmas which have suffered extensive fractional crystallization within the crust. The 12% porphyritic Sveinagja basalt contains phenocrysts of olivine (Fo62–67), plagioclase (An57–62), clinopyroxene (Wo38En46Wo16) and titanomagnetite. Extrusion temperature of the lava, calculated on the basis of olivine and plagioclase geothermometry, is found to be close to 1150°C. 相似文献
12.
Alain Demant Patrick Lestrade Ruananza T Lubala Ali B Kampunzu Jacques Durieux 《Bulletin of Volcanology》1994,56(1):47-61
Three major phases are distinguished during the growth of Nyiragongo, an active volcano at the western limit of the Virunga
Range, Zaire. Lavas erupted during phase 1 are strongly undersaturated melilitites characterized by the presence of kalsilite
phenocrysts, perovskite, and the abundance of calcite in the matrix. Such lavas crop out mainly on the inner crater wall and
progressively evolve toward more aphyric melilite nephelinites well represented on the flanks of the volcano. Adventive vents
lying at the base of the cone developed along radial fracture systems and erupted olivine and/or clinopyroxene – rich melilitites
or nephelinites. Stage 2 lavas are melilite-free nephelinites. Clinopyroxene is the main phenocryst and feldspathoids are
abundant in the lavas exposed on the crater wall. These flows result from periodic overflowing of a magma column from an open
crater. Extensive fissure flows which erupted from the base of the cone at the end of this stage are related to widespread
draining out of magma which in turn induces the formation of the summit pit crater. Magmas erupted during stage 3 are relatively
aphyric melilite nephelinites and the main volcanological characteristic is the permanent lava lake observed into the pit
crater until the 1977 eruption. Fluctuations of the level of the lava lake was responsible for the development of the inner
terraces. Periodic overflowing of the lava lake from the central pit formed the nepheline aggregate lava flows. Petrography
and major element geochemistry allow the determination of the principal petrogenetic processes. Melilitites and nephelinites
erupted from the summit crater are lavas derived, via clinopyroxene fractionation, from a more primitive melt. The abundance
of feldspathoids in these lavas is in keeping with nepheline flotation. Aphyric melilite nephelinites covering the flanks
and the extensive fissure flows have a homogeneous chemical composition; rocks from the historical lava lake are slightly
more evolved. All these lavas differentiated in a shallow reservoir. Lavas erupted from the parasitic vents are mainly olivine
and/or clinopyroxene-phyric rocks. Rushayite and picrites from Muja cone are peculiar high-magnesium lavas resulting from
the addition of olivine xenocrysts to melilitic or nephelinitic melts. Fluid and melt inclusions in olivine and clinopyroxene
phenocrysts indicate a crystallization depth of 10–14 km. A model involving two reservoirs located at different depths and
periodically connected is proposed to explain the petrography of the lavas; this hypothesis is in accordance with geophysical
data.
Received: July 8, 1993/Accepted: September 10, 1993 相似文献
13.
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.
Received: 28 August 1995 / Accepted: 18 April 1996 相似文献
14.
The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main
deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced
varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these
units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and
scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial
age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller
Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together
cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as
well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated
with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial
lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding
lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y.
Received: 24 June 1996 / Accepted: 6 December 1996 相似文献
15.
David J. Kratzmann Steven Carey Roberto Scasso Jose-Antonio Naranjo 《Bulletin of Volcanology》2009,71(4):477-439
The August 1991 eruptions of Hudson volcano produced ~2.7 km3 (dense rock equivalent, DRE) of basaltic to trachyandesitic pyroclastic deposits, making it one of the largest historical
eruptions in South America. Phase 1 of the eruption (P1, April 8) involved both lava flows and a phreatomagmatic eruption
from a fissure located in the NW corner of the caldera. The paroxysmal phase (P2) began several days later (April 12) with
a Plinian-style eruption from a different vent 4 km to the south-southeast. Tephra from the 1991 eruption ranges in composition
from basalt (phase 1) to trachyandesite (phase 2), with a distinct gap between the two erupted phases from 54–60 wt% SiO2. A trend of decreasing SiO2 is evident from the earliest part of the phase 2 eruption (unit A, 63–65 wt% SiO2) to the end (unit D, 60–63 wt% SiO2). Melt inclusion data and textures suggest that mixing occurred in magmas from both eruptive phases. The basaltic and trachyandesitic
magmas can be genetically related through both magma mixing and fractional crystallization processes. A combination of observed
phase assemblages, inferred water content, crystallinity, and geothermometry estimates suggest pre-eruptive storage of the
phase 2 trachyandesite at pressures between ~50–100 megapascal (MPa) at 972 ± 26°C under water-saturated conditions (log fO2 –10.33 (±0.2)). It is proposed that rising P1 basaltic magma intersected the lower part of the P2 magma storage region between
2 and 3 km depth. Subsequent mixing between the two magmas preferentially hybridized the lower part of the chamber. Basaltic
magma continued advancing towards the surface as a dyke to eventually be erupted in the northwestern part of the Hudson caldera.
The presence of tachylite in the P1 products suggests that some of the magma was stalled close to the surface (<0.5 km) prior
to eruption. Seismicity related to magma movement and the P1 eruption, combined with chamber overpressure associated with
basalt injection, may have created a pathway to the surface for the trachyandesite magma and subsequent P2 eruption at a different
vent 4 km to the south-southeast.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
16.
A significant number of volcano-tectonic (VT) earthquake swarms, some of which are accompanied by ground deformation and/or
volcanic gas emissions, do not culminate in an eruption. These swarms are often thought to represent stalled intrusions of
magma into the mid- or shallow-level crust. Real-time assessment of the likelihood that a VT swarm will culminate in an eruption
is one of the key challenges of volcano monitoring, and retrospective analysis of non-eruptive swarms provides an important
framework for future assessments. Here we explore models for a non-eruptive VT earthquake swarm located beneath Iliamna Volcano,
Alaska, in May 1996–June 1997 through calculation and inversion of fault-plane solutions for swarm and background periods,
and through Coulomb stress modeling of faulting types and hypocenter locations observed during the swarm. Through a comparison
of models of deep and shallow intrusions to swarm observations, we aim to test the hypothesis that the 1996–97 swarm represented
a shallow intrusion, or “failed” eruption. Observations of the 1996–97 swarm are found to be consistent with several scenarios
including both shallow and deep intrusion, most likely involving a relatively small volume of intruded magma and/or a low
degree of magma pressurization corresponding to a relatively low likelihood of eruption. 相似文献
17.
M. J. Rossi 《Bulletin of Volcanology》1996,57(7):530-540
Postglacial Icelandic shield volcanoes were formed in monogenetic eruptions mainly in the early Holocene epoch. Shield volcanoes vary in their cone morphology and in the areal extent of the associated lava flows. This paper presents the results of a study of 24 olivine tholeiite and 7 picrite basaltic shield volcanoes. For the olivine tholeiitic shields the median slope is 2.7°, the median height 60 m, the median diameter 3.6 km, the median aspect ratio (height against diameter) 0.019, and the median cone volume 0.2 km3. The picritic shield volcanoes are considerably steeper and smaller. A shield-volcano cone forms from successive lava lake overflows which are of shelly-type pahoehoe. A widespread apron surrounding the cone forms from tube-fed P-type pahoehoe. The slopes of the cones have (a) a planar or slightly convex form, (b) a concave form, or (c) a convex-concave form. A successive stage of a shield volcano is determined on the basis of cone morphology and lava assemblages. A shield-producing eruption has alternating episodes of lava lake overflows and tube-fed delivery to the distal parts of the flow field. In the late stages of eruption, the cone volume increases in response to the increased amount of rootless outpouring on the cone flanks. Normally, only a small percentage of the total erupted volume of a shield volcano, sometimes as little as 1–3%, is in the shield volcano cone itself, the main volume being in the apron of the shield. 相似文献
18.
Kazuhiko Kano 《Bulletin of Volcanology》1996,58(2-3):131-143
A subaqueous volcaniclastic mass-flow deposit in the Miocene Josoji Formation, Shimane Peninsula, is 15–16 m thick, and comprises
mainly blocks and lapilli of rhyolite and andesite pumices and non- to poorly vesiculated rhyolite. It can be divided into
four layers in ascending order. Layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0.3–6 m thick.
Layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6–11 m thick. This layer bifurcates laterally into
minor depositional units individually composed of a massive, lithic-rich lower part and a diffusely stratified, pumice-rich
upper part with inverse to normal grading of both lithic and pumice clasts. Layer 3 is 2.5–3 m thick, and consists of interbedded
fines-depleted pumice-rich and pumice-poor layers a few centimeters thick. Layer 4 is a well-stratified and well-sorted coarse
ash bed 1.5–2 m thick. The volcaniclastic deposit shows internal features of high-density turbidites and contains no evidence
for emplacement at a high temperature. The mass-flow deposit is extremely coarse-grained, dominated by traction structures,
and is interpreted as the product of a deep submarine, explosive eruption of vesicular magma or explosive collapse of lava.
Received: 10 January 1996 / Accepted: 23 February 1996 相似文献
19.
Results are presented from 11 microgravity surveys on Mt. Etna between 1987 and 1993, a period including the major 1989 and
1991–1993 flank eruptions and subordinate 1990 activity. Measurements were made with LaCoste and Romberg D-62 and D-157 gravity
meters along a network around the volcano between 1000 and 1900 m a.s.l. and, since 1992, a N–S summit profile. Gravity changes
of as much as 200 μGal were observed at scales from the size of the summit region to that of the volcano. None was associated
with significant changes in ground elevation. The data show an increase in gravity for 2 years before the 1989 eruption. The
increase is attributed to the accumulation of magma (0.25–1.7×109 m3) in an elongate zone, oriented NNW–SSE, between 2.5 and 6 km below sea level. Part of this magma was injected into the volcanic
pile to supply the 1989 and 1990 eruptions. It also probably fed the start of the 1991–1993 eruption, since this event was
not preceded by significant gravity changes. A large gravity increase (up to 140 μGal) detected across the volcano between
June and September 1992 is consistent with the arrival in the accumulation zone of 0.32–2.2×109 m3 of new magma, thus favoring continued flank effusion until 1993. A large gravity decrease (200 μGal) in the summit region
marked the closing stages of the 1991–1993 event and is associated with magma drainage from the upper levels of Etna's central
feeding system.
Received: 15 July 1995 / Accepted: 27 October 1997 相似文献
20.
D. H. Richter E. J. Moll-Stalcup T. P. Miller M. A. Lanphere G. B. Dalrymple R. L. Smith 《Bulletin of Volcanology》1994,56(1):29-46
Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral
Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression
of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and
250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive
activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to
rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where
extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top
and a part of the south flank of the volcano, produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities
and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and
72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks,
but have higher MgO contents at a given SiO2, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range
of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small,
but significant, range in 87Sr/86Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites
and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence
of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages.
In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents
that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas.
Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic
andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by
fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial
melts of underlying volcanic rocks.
Received March 13, 1993/Accepted September 10, 1993 相似文献