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1.
The Las Cañadas caldera wall and the outer slopes of the caldera provide three-dimensional exposures of numerous proximal-welded fallout deposits and have been mapped in detail. As a result, some parts of the Ucanca and Guajara Formations of the stratigraphy of Martí et al. (1994) have been divided into members that correspond to individual eruptions. Mapping has also revealed the occurrence of conduit-vent structures associated with proximal-welded fallout deposits. Conduit-vent structures consist of an upper flaring area and a lower narrow conduit. Conduit-vent geometry and dimensions include cylindrical plugs and eruptive fissures steeply dipping towards the caldera depression and elongated vents. The flaring area can be rather asymmetric and is usually filled by down-vent rheomorphic flow of the proximal fallout deposit. The lower conduits are filled by lava plug, agglutination of juveniles onto conduit walls and dyke intrusion with eventual dome extrusion. The eruption dynamics of welded fallout deposits and magma fragmentation within the conduit are consistent with an evolution from explosive to effusive. In this context conduit flow regimes evolve from turbulent to annular flow in which the conduit is progressively choked, and laminar flow leading to the final conduit closure. 相似文献
2.
Chris B. Folkes Shanaka L. de Silva Heather M. Wright Raymond A. F. Cas 《Bulletin of Volcanology》2011,73(10):1455-1486
By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components,
we show that the Cerro Galán volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr.
The Cerro Galán system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km3 (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO2). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides,
apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro
Galán ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite
contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide
and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially
hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice
is interpreted to represent the parental magmas to the Galán system emplaced into the upper crust from a deeper storage zone.
Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from
40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and
Yb between the Galán ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent
magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km3) and Real Grande Ignimbrite (∼390 km3) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km3), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute
this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like
magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume
ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galán
system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts.
A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance
of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly
identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma
source in a subduction zone volcanic setting. At Cerro Galán, we propose that there is a zone between mantle magma input and
upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures.
This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation,
storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve
only to slightly modify the geochemistry of the magmas. 相似文献
3.
Carles Soriano Daniele Giordano Inés Galindo Marcel Hürlimann Paola Ardia 《Bulletin of Volcanology》2009,71(8):919-932
During rheomorphism subsequent to fallout deposition, a portion of the densely welded fallout of the La Grieta Member flowed back into the vent from where it was erupted, while the rest of it flowed down the outer slopes of the Las Cañadas caldera in Tenerife. The welded fallout and conduit-vent structure are physically connected and constitute a rare example of this type of deposits rooted to its feeder conduit and exposed in the caldera wall. The lower part of the vent-filling rheomorphic rocks shows gas bubbles and cavities that increase in size (up to 4 m) down vent. Bubbles are deformed against other bubbles, against the steep vent walls, flattened parallel to the flow foliation planes, and elongated parallel to the flow lineation and flow fold axes. The preservation of such giant bubbles, rather than their formation, seems to be a pretty unique feature of the phonolitic products investigated here and it is likely the result of the combination of factors that acted to preserve, in the surrounding of the glass transition interval, the sealing and the late stage cooling of a pressurized system. In addition, strain drop at the base of the vent-filling rheomorphic flow caused by flow stopping against vertical vent walls may have promoted rapid gas exsolution and the formation of large bubbles. 相似文献
4.
The sea surface cannot be used as reference for Major Vertical Datum definition because its deviations from the ideal equipotential surface are very large compared to rms in the observed quantities. The quasigeoid is not quite suitable as the surface representing the most accurate Earth's model without some additional conditions, because it depends on the reference field. The normal Earth's model represented by the rotational level ellipsoid can be defined by the geocentric gravitational constant, the difference in the principal Earth's inertia moments, by the angular velocity of the Earth's rotation and by the semimajor axis or by the potential (U
0
) on the surface of the level ellipsoid. After determining the geopotential at the gauge stations defining Vertical Datums, gravity anomalies and heights should be transformed into the unique vertical system (Major Vertical Datum). This makes it possible to apply Brovar's (1995) idea of determining the reference ellipsoid by minimizing the integral, introduced by Riemann as the Dirichlet principle, to reach a minimum rms anomalous gravity field. Since the semimajor axis depends on tidal effects, potential U
0
should be adopted as the fourth primary fundamental geodetic constant. The equipotential surface, the actual geopotential of which is equal to U
0
, can be adopted as reference for realizing the Major Vertical Datum. 相似文献
5.
Kim I. S. Alexeeva I. V. Suyunova E. Z. Popov V. V. Osokin A. R. Mironova I. V. 《Geomagnetism and Aeronomy》2018,58(8):1073-1080
Geomagnetism and Aeronomy - A brief comparative analysis determining the magnetic field in optical and radio range prominences is presented; a tendency to increase averaged magnetic fields two to... 相似文献
6.
Joachim Höpfner 《Surveys in Geophysics》2000,21(5-6):521-566
The study of Polar Motion (PM) is associated withthe work of the InternationalLatitude Service (ILS), the International PolarMotion Service (IPMS) and theBureau International de l'Heure (BIH), andthen the International Earth RotationService (IERS). After reviewing briefly thehistory of early PM research, thescientific objective of the ILS including itsrealizing, and the activities of all thecited services and giving information on thePM time series available from mid-19thcentury to the present, important details onthe events and results are presentedover the period until 1922. It is shown, inwhich way the Geodetic Institute Potsdamcontributed to the foundation of the ILS andtook part in its activities through thework of Helmert, Albrecht, Wanach and Mahnkopf.Based on international scientificcooperation, the results were the rectangularcoordinates of the Polar Motion from1890.0 to 1922.7 at 0.1 year intervals, inparticular those derived from the latitudeobservations at independent stations from 1890.0to 1899.8, and those derived fromthe latitude observations at the ILS stationsfrom 1899.9 to 1922.7. 相似文献
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8.
Sheppard C 《Marine pollution bulletin》2004,49(7-8):525-526
9.
Tiziana Rossetto 《Bulletin of Earthquake Engineering》2018,16(4):1657-1660
10.
The discovery of the radiocarbon(14C)dating technique in the mid-twentieth century by Willard Libby and colleagues(Libby et al.,1949)revolutionised such fields as archaeology and palaeoclimatology that require robust chronological information to inform their study.Any sample yielding sufficient quantities of carbon could be dated in this manner,with the older age limit of the method(currently circa 50 to 60 thousand years ago)having been pushed back significantly since its inception. 相似文献
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14.
Andy Campbell 《Bulletin of Earthquake Engineering》2016,14(5):1333-1336
15.
In order to improve the accuracy of the spatial distribution of earthquake foci in the principal Novy Kostel focal zone, refined focal locations of about 1500 micro-earthquakes of the 1991 – 1997 period were determined using the relative Master-Event location method. To estimate the reduction in the scatter of located hypocentres, the results were compared with those obtained by routinely used FASTHYPO method and cluster analysis (the nearest-neighbour method) was applied to the located foci to evaluate the spatial distribution of the foci. Based on the results of refined location and of the cluster analysis, a concept of seismic energy release in space and time in the main focal zone was developed. Especially the January 1997 earthquake swarm was studied in detail: 946 events were located with the Master-Event location method, and the dimensions and geometry of focal clusters were determined. Type analysis was applied to waveforms to divide approximately 800 located events into eight multiplet groups to each of which a characteristic source mechanism was assigned. The spatial distribution of the foci as well as of the eight types of source mechanisms was revealed in this way and also the planes fitting the clusters of foci with two predominant source mechanisms were determined fairly well. 相似文献
16.
The Seismicity and Seismological Work of Myanmar 总被引：1，自引：0，他引：1
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18.
This paper deals with the summit eruptions of 2015–2016, as well as with the 2016 subterminal eruption of Klyuchevskoi. We estimate the dimensions of the depression that was produced by a landfall in the southeastern trough of the volcano. We estimated the volume and area of landfall deposits. The observed volumes of landfalls during the terminal eruptions of 1944?1945, 1985, and 2016 can vary within 0.006?0.140 km3. The theoretical volumes can reach 4?8 km3. We discuss the leading factors that cause landfalls on Klyuchevskoi. These include irreversible creep at depth, the influence of cracks and fissuring in the volcanic cone, as well as the constant intrusive activity of the volcano. Geodetic measurements revealed that the rates of sliding for several individual patches on the slopes varied between 6.7 cm/yr and 19.4 cm/yr. Video and photographic observations were used to estimate the thermal power of stable steam–gas and ash jets, volume of pyroclastics, and the volume of the erupted lava. The thermal power of the steam–gas jets for 2015 was approximately 122 × 106 kW, that of the gas–ash jets was 5.9 × 106 kW. The volume of discharged pyroclastic material was V = 0.00007 km3 for 2015 and V = 0.0003 km3 for 2016. 相似文献
19.
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20.
The Navier–Stokes-α equation is a regularised form of the Euler equation that has been employed in representing the sub-grid scales in large-eddy simulations. Determined efforts have been made to place it on a secure deductive foundation. This requires two steps to be completed. The first is fundamental and consists of establishing from the equations governing the fluid flow, a relationship between two velocities called by Holm (Chaos, 2002a, 12, 518) the “filtered” and “unfiltered” velocities. The second consists of the relation between these two velocities. Until now, the preferred route to the first objective has been variational, by varying the action using Hamilton's principle. Soward and Roberts (J. Fluid Mech., 2008, 604, 297) followed that variational route and established the existence of an important but unwelcome term omitted by Holm in his derivation. It is shown here that the Soward and Roberts result may be derived from Euler's equation by a direct approach with considerably greater efficiency. Holm achieved the second objective by making a “Taylor hypothesis”, which we use here to evaluate the unwelcome term missing from his analysis of the first step. The resulting model equations differ from those of Holm's α model, and the attractive mean Kelvin's circulation theorem that follows from his α equations is no longer valid. For that reason, we call the term omitted by Holm unwelcome. 相似文献