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Geomorphologic analysis of submarine and subaerial surface features using a combined topographic/bathymetric digital elevation model coupled with onshore geological and geophysical data constrain the age and geometry of giant landslides affecting the north flank of Tenerife. Shaded relief and contour maps, and topographic profiles of the submarine north flank, permit the identification of two generations of post-shield landslides. Older landslide materials accumulated near the shore (<40-km) and comprise 700 km3 of debris. Thickening towards a prominent axis suggests one major landslide deposit. Younger landslide materials accumulated 40–70 km offshore and comprise the products of three major landslides: the La Orotava landslide complex, the Icod landslide and the East Dorsal landslide complex, each with an onshore scar, a proximal submarine trough, and a distal deposit lobe. Estimated lobe volumes are 80, 80 and 100 km3, respectively. The old post-shield landslide scar is an amphitheatre, 20–25 km wide, partly submarine, now completely filled with younger materials. Age–width relationships for Tenerife's coastal platform plus onshore geological constraints suggest an age of ca. 3 Ma for the old collapse. Young landslides are all less than 560 ka old. The La Orotava and Icod slides involved failures of slabs of subaerial flank to form the subaerial La Orotava and Icod valleys. Offshore, they excavated troughs by sudden loading and basal erosion of older slide debris. The onshore East Dorsal slide also triggered secondary failure of older debris offshore. The slab-like geometry of young failures was controlled by weak layers, deep drainage channels and flank truncation by marine erosion. The (partly) submarine geometry of the older amphitheatre reflects the absence of these features. Relatively low H/L ratios for the young slides are attributed to filling of the slope break at the base of the submarine edifice by old landslide materials, low aspect ratios of the failed slabs and channelling within troughs. Post-shield landslides on Tenerife correlate with major falls in sea level, reflecting increased rates of volcanism and coastal erosion, and reduced support for the flank. Landslide head zones have strongly influenced the pattern of volcanism on Tenerife, providing sites for major volcanic centres. 相似文献
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The latest cycle of volcanism on Tenerife has involved the construction of two stratovolcanoes, Teide and Pico Viejo (PV), and numerous flank vent systems on the floor of the Las Cañadas Caldera, which has been partially infilled by magmatic products of the basanite-phonolite series. The only known substantial post-caldera explosive eruption occurred 2 ka bp from satellite vents at Montaña Blanca (MB), to the east of Teide and at PV. The MB eruption began with extrusion of 0.022 km3 of phonolite lava (unit I) from a WNW-ESE fissure system. The eruption then entered an explosive subplinian phase. Over a 7–11 hour period, 0.25 km3 (DRE) of phonolitic pumice (unit II) was deposited from a 15 km high subplinian column, dispersed to the NE by 10 m/s winds. Pyroclastic activity also occurred from vents near PV to the west of Teide. Fire-fountaining towards the end of the explosive phase formed a proximal welded spatter facies. The eruption closed with extrusion of small volume domes and lavas (0.025 km3) at both vent systems. Geochemical, petrological data and Fe-Ti oxide geothermometry indicate the eruption of a chemically and thermally stratified magma system. The most mafic and hottest (875°C) unit I magma can yield the more evolved and cooler (755–825°C) phonolites of units II and III by between 7 and 11% fractional crystallization of an assemblage dominated by alkali feldspar. Analyses of glass inclusions from phenocrysts by ion microprobe show that the pumice was derived from the water-saturated roof zone of a chamber containing 3.0–4.5 wt.% H2O and abundant halogens (F0.35wt.%). Hotter, more mafic tephritic magma intermingled with the evolved phonolites in banded pumice, indicating the injection of mafic magma into the system during or just before eruption. Reconstruction ot the event indicates a small chamber chemically stratified by in situ (side-wall) crystallization at a depth of 3–4 km below PV. Although phonolite is the dominant product of the youngest activity of the Teide-PV system, there has been no eruption of phonolitic magma for at least 500 years from teide itself and for 2000 years from the PV system. Therefore there could be a large volume of highly evolved, volatile-rich magma accumulating in these magma systems. An eruption of fluorine-rich magma comparable with MB would have major damaging effects on the island. 相似文献
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R S J Sparks M I Bursik G J Ablay R M E Thomas S N Carey 《Bulletin of Volcanology》1992,54(8):685-695
A model for sedimentation from turbulent suspensions predicts that tephra concentration decreases exponentially with time in an ascending volcanic column and in the overlying umbrella cloud. For grain-size distributions typical of plinian eruptions application of the model predicts for thickness variations in good agreement with the exponential thinning observed in tephra fall deposits. The model also predicts a proximal region where fallout from the plume margins results in a more rapid decrease in thickness so that the deposit shows two segments on a thickness versus distance plot. Several examples of deposits with two segments are known. The distance at which the two segments intersect is a measure of eruption column height. The thickness half-distance ( equivalent to the dispersal index of Walker) is strongly correlated with column height, but is also weakly dependent on grain-size distribution of the ejecta. For a dispersal index of 500 km2 (the plinian/subplinian boundary of Walker) column heights between 14 and 18 km are calculated. For ultraplinian deposits with D>50000 km2 column heights of at least 45 km are implied. Model grain-size distributions of the deposits have sorting values comparable to those observed in tephra fall deposits formed from eruption columns in a weak or negligible cross-wind. Median diameter decreases exponentially with distance as is observed. Sorting () improves with distance as is observed in plinian deposits in a weak wind. However, tephra fall deposits formed in strong winds do not show improved sorting with distance and proximal deposits are typically somewhat better sorted than the model calculations. Differences are attributed to the influence of wind which disperses particles further than predicted in our model and which has an increasing influence as particle size decreases. 相似文献
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The structure and volcanic stratigraphy of the Pico Teide–Pico Viejo (PT–PV) formation, deriving from the basanite–phonolite stratovolcanoes PT and PV, and numerous flank vent systems, are documented in detail based on new field and photogeologic mapping, geomorphologic analysis, borehole data, and petrological and geochemical findings. Results provide insight into the structure and evolution of the PT–PV magma system, and the long-term, cyclic evolution of Tenerife's post-shield volcanic complex. The PT–PV formation comprises products of central volcanism, mainly emplaced into the Las Cañadas caldera (LCC), and contemporaneous products from adjacent rifts. PT–PV central volcanic products become more differentiated up-section with felsic lavas dominating the recent output of the system. This is attributed to the evolution of a shallow magma reservoir beneath PT that was emplaced early in the PT–PV cycle on the intra-caldera segment of Tenerife's post-shield rift system. The rift axis has been the focus of PT–PV intrusive and eruptive activity, and has controlled the location of the stratocones. The current geometry of the rifts reflects a major structural reorganisation defining the start of the PT–PV cycle at 0.18 Ma, namely the truncation of the north side of the LCC/LCE by the giant Icod landslide. The internal stratigraphy of the PT–PV formation suggests that PT developed early, with PV developing as a satellite vent. Activity has since alternated between PT and PV due to episodes of vent blockage or chamber sealing. These processes have allowed significant volumes of phonolitic magmas to develop and accumulate within the PT chamber, which have vented through radial dike systems during tumescence episodes and from the rift system, which has permitted lateral magma transport. The PT–PV magma system is a potentially hazardous source of future, felsic eruptive activity on Tenerife. 相似文献
5.
M. Wilding Sharon Webb D. Dingwell Giray Ablay Joan Marti 《Contributions to Mineralogy and Petrology》1996,125(2-3):151-160
Silicate melts form glasses in a variety of geological environments. The relaxation (equilibration) of the frozen glass structure
provides a means of investigating the quench rates of natural glasses, and this cooling history provides an important constraint
for models of melt dynamics. Phonolite glasses from the central volcanic edifice of Tenerife, Canary Islands indicate a range
of five orders of magnitude cooling rate, determined by modeling the relaxation of the structure-dependent property, enthalpy
(H) across the glass transition. The relaxation of enthalpy is determined by heat capacity (c
p
= ΔH/ΔT) measurement of natural glass samples by differential scanning calorimetry (DSC). Upon heating, the heat capacity curve in
the vicinity of the glass transition has a geometry characteristic of the previous cooling rate. A series of thermal treatments
applied to each individual sample results in a set of sample-specific parameters which are used to model the heat capacity
curve of the naturally cooled glass. The cooling rate is then derived. The equivalence of shear and enthalpic relaxation enables
the relaxation of enthalpy for these volcanic samples to be described by a general term for the evolution of fictive temperature.
Quench rates for thirty-one glasses are calculated to be within the range 10°C s–1 to 7°C per day. The cooling rates quoted are linear approximations across the glass transition. Within different volcanic
facies cooling rates depend on several factors. The most rapidly cooled glasses occur where samples lose heat by radiation
from the surface. Our analyses indicate that in certain environments, a natural annealing process results in slow quench rates.
This is interpreted as either a slow initial cooling process or the reheating of a glass to an annealing temperature within
the glass transition interval. The latter results in relaxation to a lower temperature structure. Controls on these processes
include the initial temperature and dissipation of thermal energy from the volcanic body. Our results are consistent with
an influence of volatiles on quench rates in volcanic bombs where glass adjacent to vesicular layers is relatively rapidly
quenched. We interpret this as a rapid quench rate frozen into the glass resulting from a change in viscosity due to volatile
degassing. In lava flows, the conduction of heat from the hot flow interior controls the cooling process and diminishes the
effect of volatile exsolution. Relaxation geospeedometry can be applied to glass samples from a variety of geological environments
where cooling rates cannot be measured directly. Such measurements provide a means of determining cooling rates for a variety
of volcanic processes, an independent calibration for existing temperature and time data and a means for testing cooling-rate-dependent
models.
Received: 9 January 1996 / Accepted: 13 May 1996 相似文献
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