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1.
Palaeomagnetic data from lithic clasts collected at 46 sites within layers 1 and 2 of the 1.8-ka Taupo ignimbrite, New Zealand, have been used to determine the palaeotemperatures and thermal structure of the deposit on its emplacement. Equilibrium temperatures from sites less than 30–40 km from vent are 150–300 °C, whereas at greater distances site equilibrium temperatures increase up to 400–500 °C. This variation is seen in both layer 1 and 2 deposits, with values for layer 1 being somewhat cooler, and with its increase in temperature occurring at a greater distance from vent. A temperature maximum at ~50 km from vent coincides with a zone of pink thermal-oxidation colouration of pumices previously inferred to reflect higher emplacement temperatures. Additional palaeomagnetic data collected by us and others from pumice clasts show comparable temperature variations, but these temperature estimates are shown here to be due to a chemical remanence and unreliable for accurate temperature estimates. Cooler temperatures in proximal parts of the ignimbrite are consistent with admixture of >20% cold lithic clasts at source and interaction with the pre-eruption Lake Taupo. The similar, but offset, increases in equilibrium temperatures for medial and distal layers 1 and 2 are consistent with both layers being deposited from the same flow. However, any proximal deposits left by the later, hotter material must have been subsequently eroded, or be so thin that our collection failed to sample them. Radial asymmetries in equilibrium temperatures as well as other physical parameters suggest that the deposit emplacement temperature is primarily determined at source, rather than by interaction with air during transport. These data support previous interpretations that a concentrated basal flow played a dominant role in emplacement and deposition of the Taupo ignimbrite.Editorial responsibility: T. Druitt 相似文献
2.
The Pebble Creek Formation (previously known as the Bridge River Assemblage) comprises the eruptive products of a 2350 calendar
year B.P. eruption of the Mount Meager volcanic complex and two rock avalanche deposits. Volcanic rocks of the Pebble Creek
Formation are the youngest known volcanic rocks of this complex. They are dacitic in composition and contain phenocrysts of
plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. The eruption was episodic, and the
formation comprises fallout pumice (Bridge River tephra), pyroclastic flows, lahars and a lava flow. It also includes a unique
form of welded block and ash breccia derived from collapsing fronts of the lava flow. This Merapi-type breccia dammed the
Lillooet River. Collapse of the dam triggered a flood that flowed down the Lillooet Valley. The flood had an estimated total
volume of 109 m3 and inundated the Lillooet Valley to a depth of at least 30 m above the paleo-valley floor 5.5 km downstream of the blockage.
Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks (an older formation making up part of the Mount
Meager volcanic complex) underlie and overlie the primary volcanic units of the Formation. Both rock avalanches are unrelated
to the 2350 B.P. eruption, although the post-eruption avalanche may have its origins in the over-steepened slopes created
by the explosive phase of the eruption. Much of the stratigraphic complexity evident in the Pebble Creek Formation results
from deposition in a narrow, steep-sided mountain valley containing a major river.
Received: 20 January 1998 / Accepted: 29 September 1998 相似文献
3.
Lithic megablocks ranging from <1 to 50 m in diameter occur in the Yardea Dacite, a widespread (12,000 km2), thick (>200 m) felsic volcanic unit in the Mesoproterozoic Gawler Range Volcanic Province (GRV) of South Australia. Throughout
its vast extent, the Yardea Dacite shows typical lava-like features, in that it is massive, columnar jointed and evenly porphyritic
with 30–40% crystals in a spherulitic and granophyric groundmass. In addition, flow banding is present at many locations.
The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clasts are also scattered
throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence
of being confined to a particular stratigraphic level in the dacite. The most abundant and largest megablocks are granitoids
derived from older basement and from early-crystallised plutons of the Hiltaba Suite, which is broadly coeval and comagmatic
with the GRV. The granitoid megablocks have been partially melted, most likely prior to eruption when resident in the thermal
aureole of the Yardea Dacite magma chamber. The lithic megablock occurrences are unlike coarse pyroclastic breccias but are
similar in distribution and abundance to xenoliths in lavas, consistent with the lava-like character of the host dacite. Using
reasonable estimates of megablock density, magma density and magma viscosity, we show that the rise rate of the dacitic magma
exceeded the settling velocity of the megablocks, implying that they could have been entrained and erupted effusively. All
but the largest and least-melted megablocks would have remained suspended or else settled very slowly in the dacitic lava
during outflow. The rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered
disintegration of the thermally stressed wallrock surrounding the magma chamber, dislodging megablocks that were later entrained
and effusively erupted.
Received: 11 November 1998 / Accepted: 18 April 1999 相似文献
4.
J.-C. Thouret K. E. Abdurachman J.-L. Bourdier S. Bronto 《Bulletin of Volcanology》1998,59(7):460-480
In contrast to most twentieth-century eruptions of Kelud volcano (eastern Java), the 10 February 1990 plinian eruption was
not accompanied by lake-outburst lahars. However, at least 33 post-eruption lahars occurred between 15 February and 28 March
1990. They swept down 11 drainage systems and travelled as far as 24 km at an estimated mean peak velocity in the range of
4–11 m s–1. The deposits (volume ≥30 000 000 m3) were approximately 7 m thick 2 km from vent, and 3 m thick 10 km from vent, on the volcaniclastic apron surrounding the
volcano. Subtle but significant sedimentological differences in the deposits relate to four flow types: (a) Early, massive
deposits are coarse, poorly sorted, slightly cohesive, and commonly inversely graded. They are inferred to record hot lahars
that incorporated pumice and scoria from pyroclastic-flow deposits, probably by rapid remobilization of hot proximal pyroclastic
flow deposits by rainfall runoff. Sedimentary features, such as clasts subparallel to bedding and thick, reversely to ungraded
beds, suggest that these flows were laminar. (b) Abundant, very poorly sorted deposits include non-cohesive, clast-supported,
inversely graded beds and ungraded, finer-grained, and cohesive matrix-supported beds. These beds display layering and vertical
segregation/density stratification, suggesting unsteady properties of pulsing debris flows. They are interpreted as deposited
from segments of flow waves at a middle distance downstream that incorporated pre-eruption sediments. Sedimentological evidence
suggests unsteady flow properties during progressive aggradation. (c) Fine-grained, poorly sorted and ungraded deposits are
interpreted as recording late hyperconcentrated streamflows that formed in the waning stage of an overflow and transformed
downcurrent into streamflows. (d) Ungraded, crudely stratified deposits were emplaced by flows transitional between hyperconcentrated
flows and streamflows that traveled farther downvalley (as far as 27 km from the vent). At Kelud, the transformation of flow
and behavior occurs within only 10 km of the source, at the apex of the alluvial fans. The rapid change of flow behavior is
attributed to the low fines content and to the unsteady flow regime, which may be due to: (a) the rapid deposition of bedload,
owing to the break in channel gradient close to the vent and to changes in channel cross-section and roughness; and (b) the
very low silt+clay content in the non-cohesive deposits. These deposits mix with water to produce streamflows.
Received: 27 June 1997 / Accepted: 5 January 1998 相似文献
5.
The massive unit of a lava flow from Porri volcano (Salina, Aeolian Islands) displays many unusual structures related to
the physical interaction between two different magmas. The magma A represents approximately 80% of the exposed lava surface;
it has a crystal content of 51 vol.% and a dacitic glass composition (SiO2=63–64 wt.%). The magma B has a basaltic-andesite glass composition (SiO2=54–55 wt.%) and a crystal content of approximately 18 vol.%. It occurs as pillow-like enclaves, banding, boudin-like and
rolling structures which are hosted in magma A. Structural analysis suggests that banding and boudin-like structures are the
result of the deformation of enclaves at different shear strain. The linear correlation between strain and stratigraphic height
of the measured elements indicates a single mode of deformation. We deduce that the component B deformed according to a simple
shear model. Glass analyses of the A–B boundary indicate that A and B liquids mix together at high shear strain, whereas only
mingling occurs at low shear strain. This suggests that the amount of deformation (i.e. forced convection) plays an important
role in the formation of hybrid magmas. High shear strain may induce stretching, shearing and rolling of fluids which promote
both forced convection and dynamical diffusion processes. These processes allow mixing of magmas with large differences in
their physical properties.
Received: 15 July 1995 / Accepted: 30 May 1996 相似文献
6.
The relationships between soil gas emissions and both tectonic and volcano-tectonic structures on Mt. Etna have been studied.
The investigation consisted of soil CO2 flux measurements along traverses orthogonal to the main faults and eruptive fissures of the volcano. Anomalous levels of
soil degassing were found mainly in coincidence with faults, whereas only 49% of the eruptive fissures were found to produce
elevated CO2 soil fluxes. This result suggests that only zones of strain are able to channel deep gases to the surface. According to this
hypothesis, several previously unknown structures are suggested. Based on our geochemical data, new structural maps of different
areas of Etna are proposed. The soil CO2 fluxes observed in this study are higher than those measured in a 1987 study, and they are consistent with the higher level
of volcanic unrest during the current study.
Received: 20 March 1998 / Accepted: 17 June 1998 相似文献
7.
Guido Ventura 《Bulletin of Volcanology》1998,59(6):394-403
A basaltic andesite lava flow from Porri Volcano (Salina, Southern Tyrrhenian Sea) is composed of two different magmas. Magma
A (51 vol.% of crystals) has a dacitic glass composition, and magma B (18 vol.% of crystals), a basaltic glass composition.
Magma B is hosted in A and consists of sub-spherical enclaves and boudin-like, banding and rolling structures (RS). Four types
of RS have been recognized: σ–type;δ–type; complex σ-δ–types and transitional structures between sub-spherical enclaves and rolling structures. An analysis of the RS has been performed
in order to reconstruct the flow kinematics and the mechanism of flow emplacement. Rolling structures have been selected in
three sites located at different distances from the vent. In all sites most RS show the same sense of shear. Kinematic analysis
of RS allows the degree of flow non-coaxiality to be determined. The non-coaxiality is expressed by the kinematic vorticity
number Wk, a measure of the ratio Sr between pure shear strain rate and simple shear strain rate. The values of Wk calculated from the measured shapes of microscopic RS increase with increasing distance from the vent, from approximately
0.5 to 0.9. Results of the structural analysis reveal that the RS formed during the early–intermediate stage of flow emplacement.
They represent originally sub-spherical enclaves deformed at low shear strain. At higher strain, RS deformed to give boudin-like
and stretched banding structures. Results of the kinematic analysis suggest that high viscosity lava flows are heterogeneous
non-ideal shear flows in which the degree of non-coaxiality increases with the distance from the vent. In the vent area, deformation
is intermediate between simple shear and pure shear. Farther from the vent, deformation approaches ideal simple shear. Lateral
extension processes occur only in the near-vent zone, where they develop in response to the lateral push of magma extruded
from the vent. Lateral shortening processes develop in the distal zone and record the gravity-driven movement of the lava.
The lava flow advanced by two main mechanisms, lateral translation and rolling motion. Lateral translation equals rolling
near the vent, while rolling motion prevailed in the distal zones.
Received: 6 November 1997 / Accepted: 29 November 1997 相似文献
8.
The behaviour of a discrete sub‐bank‐full flow event in a small desert stream in western NSW, Australia, is analysed from direct observation and sediment sampling during the flow event and from later channel surveys. The flow event, the result of an isolated afternoon thunderstorm, had a peak discharge of 9 m3/s at an upstream station. Transmission loss totally consumed the flow over the following 7·6 km. Suspended sediment concentration was highest at the flow front (not the discharge peak) and declined linearly with the log of time since passage of the flow front, regardless of discharge variation. The transmission loss responsible for the waning and eventual cessation of flow occurred at a mean rate of 13.2% per km. This is quite rapid, and is more than twice the corresponding figure for bank‐full flows estimated by Dunkerley (1992) on the same stream system. It is proposed that transmission losses in ephemeral streams of the kind studied may be minimized in flows near bank‐full stage, and be higher in both sub‐bank‐full and overbank flows. Factors contributing to enhanced flow loss in the sub‐bank‐full flow studied included abstractions of flow to pools, scour holes and other low points along the channel, and overflow abstractions into channel filaments that did not rejoin the main flow. On the other hand, losses were curtailed by the shallow depth of banks wetted and by extensive mud drapes that were set down over sand bars and other porous channel materials during the flow. Thus, in contrast with the relatively regular pattern of transmission loss inferred from large floods, losses from low flows exhibit marked spatial variability and depend to a considerable extent on streamwise variations in channel geometry, in addition to the depth and porosity of channel perimeter sediments. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
9.
To improve understanding of DOC dynamics in seasonal Mediterranean environments, rainfall, soil water, groundwater and stream water samples were taken during a 27-month period in the Can Vila catchment (northeast Spain). Using these data, we characterized DOC dynamics in the different hydrological compartments and analysed the factors affecting them. We also analysed DOC dynamics during storm events and the factors that control DOC delivery to the stream. The results show some seasonality in rainwater and soil water DOC concentrations, while no clear seasonality was observed in stream water and groundwater, where DOC dynamics were strongly related to discharge and water table variations. For storm events with several discharge peaks, the slope of the discharge–DOC concentration relationship was higher for the first peak. The rather similar dynamics of stream water DOC concentration in all floods contrast with the observed diversity of hydrological processes. This raises the question of the origin of the observed rapid DOC increase.
EDITOR M.C. AcremanASSOCIATE EDITOR K. Heal 相似文献
10.
Shape-preferred orientation and imbrication structures of crystals have been measured on samples representative of the base, centre and top of a highly viscous lava flow on Salina (Aeolian Islands, southern Tyrrhenian Sea). The data allow zones with different deformation patterns to be identified. In the base and top of the flow, deformation leads to the development of discrete preferred orientation and imbrication of the elongate crystals. The sense of shear is right-lateral at the base and left-lateral at the top of the flow. Shear strain can be estimated by the analysis of crystal preferred orientation. Deformation increases from the flow centre to the outer, more viscous boundary layers. Random orientation of crystals in the inner zone supports the presence of plug flow in a pseudoplastic lava. The textural features of the studied lava may be related to different mechanisms (i.e. lateral expansion). We conclude that the observed crystal alignments and imbrication structures may be related to a plug flow moving between two non-deforming walls. The walls are represented by the solidified, broken upper and basal crust of the flow. The low shear strain values calculated in the outer margins of the flow are indicative of the last deformation event. Crystal preferred orientation and imbrication structures may be related to the occurrence of velocity gradients existing between the inner zone of the flow and its solidus or near-solidus outer margins. 相似文献