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61.
Scott?K.?RowlandEmail author Andrew?J.?L.?Harris Martin?J.?Wooster Falk?Amelung Harold?Garbeil Lionel?Wilson Peter?J.?Mouginis-Mark 《Bulletin of Volcanology》2003,65(5):311-330
We have used a suite of remotely sensed data, numerical lava flow modeling, and field observations to determine quantitative characteristics of the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galápagos Islands. Flank lava flow areas, volumes, instantaneous effusion rates, and average effusion rates were all determined for these two eruptions, for which only limited syn-eruptive field observations are available. Using data from SPOT, TOPSAR, ERS-1, and ERS-2, we determined that the 1995 Fernandina flow covers a subaerial area of 6.5×106 m2 and has a subaerial dense rock equivalent (DRE) volume of 42×106 m3. Field observations, ATSR satellite data, and the FLOWGO numerical model allow us to determine that the effusion rate declined exponentially from a high of ~60–200 m3 s-1 during the first few hours to <5 m3 s-1 prior to ceasing after 73 days, with a mean effusion rate of 4–16 m3 s-1. Integrating the ATSR-derived, exponentially declining effusion rate over the eruption duration produces a total (subaerial + submarine) DRE volume of between 27 and 100×106 m3, the range in values being due to differing assumptions about heat loss characteristics; only values in the higher part of this range are consistent with the independently derived subaerial volume. Using SPOT, TOPSAR, ERS-1, and ERS-2 data, we determine that the 1998 Cerro Azul flow is 16 km long, covers 16 km2, and has a DRE volume of 54×106 m3. FLOWGO produces at-vent velocity and effusion rate values of 11 m s-1 and ~600 m3 s-1, respectively. The velocity value agrees well with the 12 m s-1 estimated in the field. The mean effusion rate (total DRE volume/duration) was 7–47 m3 s-1. Dike dimensions, fissure lengths, and pressure gradients along the conduit based on magma chamber depth estimates of 3–5 km produce mean effusion rates for the two eruptions that range over nearly four orders of magnitude, the range being due to uncertainty in the magma viscosity, dike dimensions, and pressure gradient between magma chamber and vent. Although somewhat consistent with mean effusion rates from other techniques, their wide range makes them less useful. The exponentially declining effusion rates during both eruptions are consistent with release of elastic strain being the driving mechanism of the eruptions. Our results provide independent input parameters for previously published theoretical relationships between magma chamber pressurization and eruption rates that constrain chamber volumes and increases in volume prior to eruption, as well as time constants of exponential decay during the eruption. The results and theoretical relationships combine to indicate that at both volcanoes probably 25–30% of the volumetric increase in the magma chamber erupted as lava onto the surface. In both eruptions the lava flow volumes are less than 1% of the magma chamber volume. 相似文献
62.
I.?Leost T.?Stachel G.?P.?BreyEmail author J.?W.?Harris I.?D.?Ryabchikov 《Contributions to Mineralogy and Petrology》2003,145(1):15-24
Mineral inclusions in diamonds from Namibia document a range of mantle sources, including eclogitic, websteritic and peridotitic parageneses. Based on unusual textural features a group of inclusions showing websteritic, peridotitic and transitional chemical features is assigned to an 'undetermined suite' (12% of the studied diamonds). The mutual characteristic of this group is the occurrence of lamellar intergrowths of clinopyroxene and orthopyroxene. In addition, the 'undetermined suite' is associated with a number of uncommon phases: in one diamond MgCO3 is enclosed by clinopyroxene. Other minerals that form touching inclusions with the pyroxene lamellae are (1) a SiO2 phase observed in three diamonds, together with CaCO3 in one of them, (2) phlogopite and a Cr-rich 'titanate' (probably lindsleyite). The inclusions document a metamorphic path of decreasing pressures and temperatures after entrapment in diamond. First, homogeneous low-Ca clinopyroxenes were entrapped at high temperatures. They subsequently exsolved orthopyroxene and probably also SiO2 (coesite) on cooling along a P,T trajectory that did not allow garnet to be exsolved as well. Phlogopite, carbonates and LIMA phases are the result of overprint of a peridotitic source rock by a carbon-rich agent. The resulting unusual, olivine-free mineral association and the host diamonds are interpreted as products of extensive carbonation of the peridotite. 相似文献
63.
It is shown that the size-frequency distribution (SFD) of a time-averaged projectile population derived from the lunar crater SFD of Neukum and Ivanov (in Hazards Due to Comets and Asteroids (T. Gehrels, Ed.), 1994, pp. 359-416, Univ. of Arizona Press, Tucson) provides a convincing fit to the SFD of the current near-Earth asteroid (NEA) population, as deduced from the results of asteroid search programs. Our results suggest that the shape of the SFD of the impactor flux has remained in a steady state since the late heavy bombardment, so that the current NEA population can be viewed as a snapshot of the flux of impactors on the Moon. The number of bodies in the projectile population with diameters of 1 km or more is 700±130, which is in good agreement with recent estimates of the total number of NEAs in this size range. Our results imply that the contribution to the projectile flux from comets is small for diameters below 10 km. 相似文献
64.
A short length of channel on Pico Partido volcano, Lanzarote, provides us the opportunity to examine the dynamics of lava flowing in a channel that extends over a sudden break in slope. The 1–2-m-wide, 0.5–2-m-deep channel was built during the 1730–1736 eruptions on Lanzarote and exhibits a sinuous, well-formed channel over a steep (11° slope) 100-m-long proximal section. Over-flow units comprising smooth pahoehoe sheet flow, as well as evidence on the inner channel walls for multiple (at least 11) flow levels, attest to unsteady flow in the channel. In addition, superelevation is apparent at each of the six bends along the proximal channel section. Superelevation results from banking of the lava as it moves around the bend thus causing preferential construction of the outer bank. As a result, the channel profile at each bend is asymmetric with an outer bank that is higher than the inner bank. Analysis of superelevation indicates flow velocities of ~8 m s–1. Our analysis of the superelevation features is based on an inertia-gravity balance, which we show is appropriate, even though the down-channel flow is in laminar flow. We use a viscosity-gravity balance model, together with the velocities calculated from superelevation, to obtain viscosities in the range 25–60 Pa s (assuming that the lava behaved as a Newtonian liquid). Estimated volume fluxes are in the range 7–12 m3 s–1. An apparent down-flow increase in derived volume flux may have resulted from variable supply or bulking up of the flow due to vesiculation. Where the channel moves over a sharp break in slope and onto slopes of ~6°, the channel becomes less well defined and widens considerably. At the break of slope, an elongate ridge extends across the channel. We speculate that this ridge was formed as a result of a reduction in velocity immediately below the break of slope to allow deposition of entrained material or accretion of lava to the channel bed as a result of a change in flow regime or depth. 相似文献
65.
66.
Synergy of multiple geophysical approaches to unravel explosive eruption conduit and source dynamics - A case study from Stromboli 总被引:1,自引:0,他引:1
An effective approach to understanding the dynamics of explosive volcanic eruptions and the conduit systems that drive them is through synergy of multiple data sets. Three data sets that lend themselves to ease of integration are seismic, infrasonic and thermal. Although approaches involving these data have been used to record volcanological phenomena since 1862, 1955 and 1965, respectively, their integrated use has only developed since 1999. When combined, these three data sets allow constraint of shallow system geometry and the dynamics of the explosive events that occur within that system. Using Stromboli volcano (Italy) as a case study, we review the complete range of geochemical and geophysical studies that can be applied. In doing so, we aim to show how integration of these diverse studies allows insights into a plumbing system and the dynamics of the eruptive activity that the system feeds. When combined at Stromboli, these data provide constraint of multiple system parameters including chamber depths, gas and magma fluxes, shallow system magma residence times, explosion source depths, and the rise/ejection velocities of ascending gas slugs and ejecta. In turn, these results allow various conduit and eruption dynamic models to be applied and tested.The persistent and repeated mildly explosive events that characterize Stromboli have been modeled in terms of the coalescence of gas within the magma to form large gas slugs that ascend the remaining portion of the conduit to burst at the free surface. Our integrated seismic, infrasonic and thermal data sets indicate that gas coalescence occurs at a depth of ∼260 m, with a typical event frequency of ∼9/h. Infrasonic and thermal data show the explosion source to be located 20-220 m below the vent. Thermal data give emission velocities for the ejected fragments of 8-20 m/s, which converts to gas jet velocities of 23-39 m/s. Tracking these parameters in space and time shows that, although eruptions at Stromboli can be grouped into two characteristic types (simple and complex-each of which characterizes a particular crater, NE and SW, respectively), events within each type show significant short-term variability. The system does, however, appear robust, maintaining its characteristic strombolian eruption style after significant effusive phases and more energetic explosive events. 相似文献
67.
The rate of sediment supply from erosional catchment to depositional basin depends primarily upon climate, relief, catchment slope and lithology. It varies in both time and space. Spatial changes in erosion rates due to variations in lithology are illustrated by contrasting rates of drainage divide migration away from faults of known ages. Time variations in relative sediment supply are extremely complex and vary widely according to the direction and magnitude of climate change. In many parts of the Great Basin and south-western USA, glacial maximum climates were characterized by higher effective moisture and the altitudinal downward spread of woods and forests. Sparse data from alluvial fans indicate reduced sediment supply, despite the increased runoff evident from higher lake levels. The situation in Mediterranean areas is less clear, with rival climatic scenarios for vegetation ecotypes predicting contrasting runoff. In order to test these latter we run Cumulative Seasonal Erosion Potential [CSEP] experiments for present-day and a variety of full-glacial Mediterranean candidate climates. The results indicate the likelihood of enhanced sediment supply and runoff compared to the present day during full-glacial times for a cool wet winter climate and a reduction in sediment supply and runoff for a full-glacial cool dry winter climate. We then explore the consequences of such phase differences in sediment supply, and sea and lake levels for the stratigraphy of sedimentary basins. Highstands and lowstands of sea or lake may be accompanied by greater or lesser sediment and water supply, as determined by the regional climate and the direction of climatic change. Thus marine lowstands are not necessarily periods of great transfer of coarse clastic sediments to shelves and deep water basinal environments. Unsteady sediment supply has greatest implications for alluvial systems, in particular the effect that changing relative supplies of water and sediment have upon river and fan channel incision. 相似文献
68.
Dynamics of the turbidity maximum zone in a micro-tidal estuary: Hawkesbury River, Australia 总被引:3,自引:0,他引:3
Bed sediment, velocity and turbidity data are presented from a large (145 km long), generally well-mixed, micro-tidal estuary in south-eastern Australia. The percentage of mud in the bed sediments reaches a maximum in a relatively narrow zone centred ≈30–40 km from the estuary mouth. Regular tidal resuspension of these bed sediments produces a turbidity maximum (TM) zone in the same location. The maximum recorded depth-averaged turbidity was 90 FTU and the maximum near-bed turbidity was 228 FTU. These values correspond to suspended particulate matter (SPM) concentrations of roughly 86 and 219 mg l?1, respectively. Neither of the two existing theories that describe the development and location of the TM zone in the extensively studied meso- and macro-tidal estuaries of northern Europe (namely, gravitational circulation and tidal asymmetry) provide a complete explanation for the location of the TM zone in the Hawkesbury River. Two important factors distinguish the Hawkesbury from these other estuaries: (1) the fresh water discharge rate and supply of sediment to the estuary head is very low for most of the time, and (2) suspension concentrations derived from tidal stirring of the bed sediments are comparatively low. The first factor means that sediment delivery to the estuary is largely restricted to short-lived, large-magnitude, fluvial flood events. During these events the estuary becomes partially mixed and it is hypothesized that the resulting gravitational circulation focuses mud deposition at the flood-determined salt intrusion limit (some 35 km seaward of the typical salt intrusion limit). The second factor means that easily entrained high concentration suspensions (or fluid muds), typical of meso- and macro-tidal estuaries, are absent. Maintenance of the TM zone during low-flow periods is due to an erosion-lag process, together with a local divergence in tidal velocity residuals, which prevent the TM zone from becoming diffused along the estuary axis. 相似文献
69.
Semi-batch flotation tests were performed using different narrow size fractions of three different xanthate-conditioned sulphides floated singly and together in all possible combinations. To ensure proper selective flotation conditions a gangue mineral was included in all tests. Within the reproducibility of the experimentation no significant interaction among the mineral species is indicated, a result which has useful implications in distributed rate coefficient models of flotation kinetics.The conditions under which the present findings may be valid in industrial flotation are discussed, and suggestions are made for further work. 相似文献
70.
Centropages typicus was collected at two hour intervals over a diel cycle at a station near the Chesapeake Bay mouth. The species migrated upward at dusk and downward at dawn. Neither light, temperature, salinity, nor density appeared to cue the migration. Attention is drawn to the need for information on the extent and regularity of migration by individual organisms, and how the lack of such information may lead to poor interpretations of migration data. 相似文献