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1.
We analyze data from three seismic antennas deployed in Las Cañadas caldera (Tenerife) during May–July 2004. The period selected for the analysis (May 12–31, 2004) constitutes one of the most active seismic episodes reported in the area, except for the precursory seismicity accompanying historical eruptions. Most seismic signals recorded by the antennas were volcano-tectonic (VT) earthquakes. They usually exhibited low magnitudes, although some of them were large enough to be felt at nearby villages. A few long-period (LP) events, generally associated with the presence of volcanic fluids in the medium, were also detected. Furthermore, we detected the appearance of a continuous tremor that started on May 18 and lasted for several weeks, at least until the end of the recording period. It is the first time that volcanic tremor has been reported at Teide volcano. This tremor was a small-amplitude, narrow-band signal with central frequency in the range 1–6 Hz. It was detected at the three antennas located in Las Cañadas caldera. We applied the zero-lag cross-correlation (ZLCC) method to estimate the propagation parameters (back-azimuth and apparent slowness) of the recorded signals. For VT earthquakes, we also determined the S–P times and source locations. Our results indicate that at the beginning of the analyzed period most earthquakes clustered in a deep volume below the northwest flank of Teide volcano. The similarity of the propagation parameters obtained for LP events and these early VT earthquakes suggests that LP events might also originate within the source volume of the VT cluster. During the last two weeks of May, VT earthquakes were generally shallower, and spread all over Las Cañadas caldera. Finally, the analysis of the tremor wavefield points to the presence of multiple, low-energy sources acting simultaneously. We propose a model to explain the pattern of seismicity observed at Teide volcano. The process started in early April with a deep magma injection under the northwest flank of Teide volcano, related to a basaltic magma chamber inferred by geological and geophysical studies. The stress changes associated with the injection produced the deep VT cluster. In turn, the occurrence of earthquakes permitted an enhanced supply of fresh magmatic gases toward the surface. This gas flow induced the generation of LP events. The gases permeated the volcanic edifice, producing lubrication of pre-existing fractures and thus favoring the occurrence of VT earthquakes. On May 18, the flow front reached the shallow aquifer located under Las Cañadas caldera. The induced instability constituted the driving mechanism of the observed tremor.  相似文献   

2.
The Las Cañadas caldera of Tenerife (LCC) is a well exposed caldera depression filled with pyroclastic deposits and lava flows from the active Teide–Pico Viejo complex (TPVC). The caldera's origin is controversial as both the formation by huge lateral flank collapse(s) and multiple vertical collapses have been proposed. Although vertical collapses may have facilitated lateral slope failures and thus jointly contribute to the exposed morphology, their joint contribution has not been clearly demonstrated. Using results from 185 audiomagnetotelluric (AMT) soundings carried out between 2004 and 2006 inside the LCC, our study provides consistent geophysical constraints in favour of multiple vertical caldera collapse. One-dimensional modelling reveals a conductive layer at shallow depth (30–1000 m), presumably resulting from hydrothermal alteration and weathering, underlying the infilling resistive top layer. We present the resistivity distribution of both layers (resistivity images), the topography of the conductive layer across the LCC, as well as a cross-section in order to highlight the caldera's evolution, including the distribution of earlier volcanic edifices. The AMT phase anisotropy reveals the structural and radial characteristics of the LCC.  相似文献   

3.
New age determinations from Tenerife, together with those previously published (93 in all), provide a fairly comprehensive picture of the volcanic evolution of the island. The oldest volcanic series, with ages starting in the late Miocene, are formed mainly by basalts with some trachytes and phonolites which appear in Anaga, Teno and Roque del Conde massifs. In Anaga (NE), three volcanic cycles occurred: one older than 6.5 Ma, a second one between 6.5 and 4.5 Ma, with a possible gap between 5.4 and 4.8 Ma, and a late cycle around 3.6 Ma. In Teno (NW), after some undated units, the activity took place between 6.7 and 4.5 Ma, with two main series separated by a possible pause between 6.2 and 5.6 Ma. In the zone of Roque del Conde (S), the ages are scattered between 11.6 and 3.5 Ma. Between 3.3 and 1.9 Ma, the whole island underwent a period of volcanic quiescence and erosion.The large Cañadas volcano, made up of basalts, trachytes and phonolites, was built essentially between 1.9 and 0.2 Ma. To the NE of this central volcano, linking it with Anaga, is a chain of basaltic emission centers, with a peak of activity around 0.8 Ma. The Cañadas Caldera had several collapse phases, associated with large ignimbrite emissions. There were, at least, an older phase more than 1 Ma old, on the western part of the volcano, and a younger one, less than 0.6 Ma old, in the eastern side. The two large “valleys” of Guimar and la Orotava were formed by large landslides less than 0.8 Ma ago, and probably before 0.6 Ma ago. The present Cañadas caldera was formed by another landslide, less than 0.2 Ma ago. This caldera was later filled by the huge Teide volcano, which has been active even in historic times. During the same period a series of small volcanoes erupted at scattered locations throughout the island.The average eruptive rate in Tenerife was 0.3 km3/ka, with relatively small variations for the different eruptive periods. This island and La Gomera represent a model of growth by discontinuous pulses of volcanic activity, separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years. The neighbouring Gran Canaria, on the other hand, had an initial, rapid “shield-building phase” during which more than 90% of the island was built, and a series of smaller pulses at a much later period.A comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation. Several gaps are present on each island, overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the Canaries.  相似文献   

4.
The ratio salic rocks/basalts is higher in Tenerife than in other Atlantic islands. It is also surprising the number of intermediate types between phonolites and trachytes and the basalts. The salic rocks of Tenerife have been grouped in to two large units, one related to the edifice of « Las Cañadas » and the other to « Teide-Pico Viejo ». The top of the former collapsed and the latter was built in the Caldera thus formed. Both units belong to a middle atlantic series, but the atlantic character of « Teide-Pico Viejo » is stronger. A clear alkalinitization can be observed during the whole evolution. Most of the materials which are related to the Cañadas edifice are near the saturation line, and they must be classed as phonolites and Na-trachytes. In these rocks a variation trend related to that of the former alkaline basalts can be observed. In the latest episodes of their evolution cutaxite and pumice emissions appeared with great intensity. The « Teide-Pico Viejo » lava-flows are always of phonolite types with high amounts of normative nepheline. These materials also represented the end of the differentiation trend of an alkaline basaltic series, which started after the Cañadas edifice was built. This second trend ended in less silica-rich rocks than those of the Cañadas series.  相似文献   

5.
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   

6.
Previous sonar surveys show that the north flank of Tenerife has been subject to at least four major landslides during the past 1 Ma. The youngest, Icod, affected the region to the north of the Teide-Pico Viejo complex, the world's third highest oceanic volcano. Recently, we obtained the first detailed acoustic images of Icod using a deep-tow side-scan sonar. The images suggest that Tenerife's north flank has experienced at least two types of flow deposit in the recent past. The older flow deposit, Icod I, is characterised by a 15- to 20-km-wide, >65-km-long, chaotic debris avalanche deposit which includes several very large blocks. We believe the deposit to be ~170 ka, and that it represents the mass-wasting products of the Cañadas edifice, remnants of which are now found in the Las Cañadas caldera wall. The younger flow deposit, Icod II, associated with a shute in its proximal part, appears to have produced a less chaotic deposit in its distal part which clearly preserves flow structures such as latitudinal boulder ridges and longitudinal shear structures. The sonar images cannot determine how much younger Icod II is than Icod I, although it is likely that they are a consequence of the same lateral collapse event. There is evidence from the shute area for erosional scour and sediment deposition since the Icod landslide. If this is correct, then it suggests that mass wasting is an ongoing process that has already started to modify the Teide-Pico Viejo complex itself.  相似文献   

7.
The island of Tenerife is volcanically complex, and its eruptive history predominantly reflects the processes and products of two different eruptive styles: (1) non-explosive effusions of basaltic lavas from fissure vents mostly aligned along two ridges; and (2) less frequent but explosive salic eruptions from central vents associated with the Las Cañadas volcanic edifice and associated summit caldera. We have taken into account this fundamental distinction to develop a volcanic-hazards zonation (for lava flows and ash fall only) that includes: definition of the principal hazards; identification of the areas that have higher probability of containing emission centres; and numerical modelling of the vulnerable areas to be affected by volcanic hazards. Not only does the volcanic-hazards zonation map provide emergency-management officials with an updated assessment of the volcanic hazards, but it also represents a starting point for the preparation of a volcanic risk map for Tenerife. Finally, the hazards-zonation map also furnishes the basis for the design of a proposed volcano surveillance network.  相似文献   

8.
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   

9.
Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the border between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K–Ar and new biotite laser 40Ar/39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (≤1.5 Ma), with a youngest date of 0.048±0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.94±0.13 Ma and 5.48±0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene–Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina.Oversteepening of the edifice by stacking of thick, viscous, dacitic lava flows resulted in collapse of its southeastern flank to form a large volcanic debris avalanche. Biotite 40Ar/39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after 0.15 Ma, and may have been triggered by extrusion of a dacitic flow similar to the one dated at 0.048±0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levées, longitudinal ridges, and megablocks up to 20-m diameter are observed.The avalanche descended 2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt flats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km2. Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km3. Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended 400 m where it broke against the western flank of Cerro Rosado, and a minimum flow velocity of 90 m s−1 can be calculated at this point; lower velocities of 45 m s−1 are calculated where distal toes ascend 200 m slopes.It is suggested that the remaining precipitous edifice has a high probability for further avalanche collapse in the event of renewed volcanism.  相似文献   

10.
A review of the general volcano-stratigraphy and geochronology of La Gomera, one of the lesser known Canary Islands, has led to the establishment of a new evolutionary model. The oldest edifice corresponds to the submarine stage built up between 20 and 15 Ma. The construction of the Submarine Edifice was followed by an important break in the activity (about 4 Ma) and deep erosion of the edifice. About 10.5 Ma ago, the main present-day edifice (the Old Edifice 10.5–6.4 Ma) emerged, which was also submarine in its initial phases. Two different main stages are distinguishable. The first stage was represented by a large, some 22 km wide basaltic shield volcano (the Lower Old Edifice). Several lateral collapse events (Tazo and San Marcos avalanches) occurred during this time and were responsible for the removal of an important part of its northern flank. In the second growth stage (the Upper Old Edifice), the activity migrated southwards. A 25-km wide composite volcano arose covering part of the remaining earlier shield volcano. The felsic (trachytic to phonolitic) activity occurring in two separate episodes formed a significant component of this composite volcano. Finally, one more recent large edifice (the Young Edifice) built up from 5.7 to 4 Ma. The lava flows of this younger edifice covered completely the centre and the south of the island and filled deep ravines in the north. More evolved magmas, including significant felsic magmas (the third and last felsic episode), occurred in this phase of activity.The growth of La Gomera was long-lasting, separated by an important gap in the activity in the Middle Miocene, with no Quaternary activity at all. At the same time on Tenerife (the nearest island east of La Gomera), three large edifices grew separately: Roque del Conde, Anaga and Teno (initially three separated islands). From the available data, it is inferred that the subaerial activity started earlier in the Roque del Conde Edifice, then on La Gomera and later in Teno in the NW and Anaga in NE of Tenerife, which is the youngest of all these edifices. These facts, together with the irregular general progress of the volcanic activity, support more complex views of the genesis for the Canary Islands than the simple hotspot model.  相似文献   

11.
The relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented. Our field data, aerial image analysis, and 40Ar/39Ar chronology from Anaga basaltic shield volcano on Tenerife, Canary Islands, support a rift zone—flank instability relationship. A single rift zone dominated the early stage of the Anaga edifice (~6–4.5 Ma). Destabilization of the northern sector led to partial seaward collapse at about ~4.5 Ma, resulting in a giant landslide. The remnant highly fractured northern flank is part of the destabilized sector. A curved rift zone developed within and around this unstable sector between 4.5 and 3.5 Ma. Induced by the dilatation of the curved rift, a further rift-arm developed to the south, generating a three-armed rift system. This evolutionary sequence is supported by elastic dislocation models that illustrate how a curved rift zone accelerates flank instability on one side of a rift, and facilitates dike intrusions on the opposite side. Our study demonstrates a feedback relationship between flank instability and intrusive development, a scenario probably common in ocean island volcanoes. We therefore propose that ocean island rift zones represent geologically unsteady structures that migrate and reorganize in response to volcano flank instability.Editorial responsibility: T. DruittThis revised version was published online in February 2005 with typographical corrections and a changed wording.  相似文献   

12.
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13.
Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0–6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (δ18O, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6–5.3, 3.5–3.6, and 0–1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajökull volcano (Thórsmörk, ca. 54.5 ka) and Katla volcano (Sólheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K2O/SiO2>0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab–Q–Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nb and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sør Jan in the south of the island.  相似文献   

14.
The Active Crater at Rincón de la Vieja volcano, Costa Rica, reaches an elevation of 1750 m and contains a warm, hyper-acidic crater lake that probably formed soon after the eruption of the Rio Blanco tephra deposit approximately 3500 years before present. The Active Crater is buttressed by volcanic ridges and older craters on all sides except the north, which dips steeply toward the Caribbean coastal plains. Acidic, above-ambient-temperature streams are found along the Active Crater's north flank at elevations between 800 and 1000 m. A geochemical survey of thermal and non-thermal waters at Rincón de la Vieja was done in 1989 to determine whether hyper-acidic fluids are leaking from the Active Crater through the north flank, affecting the composition of north-flank streams.Results of the water-chemistry survey reveal that three distinct thermal waters are found on the flanks of Rincón de la Vieja volcano: acid chloride–sulfate (ACS), acid sulfate (AS), and neutral chloride (NC) waters. The most extreme ACS water was collected from the crater lake that fills the Active Crater. Chemical analyses of the lake water reveal a hyper-acidic (pH0) chloride–sulfate brine with elevated concentrations of calcium, magnesium, aluminum, iron, manganese, copper, zinc, fluorine, and boron. The composition of the brine reflects the combined effects of magmatic degassing from a shallow magma body beneath the Active Crater, dissolution of andesitic volcanic rock, and evaporative concentration of dissolved constituents at above-ambient temperatures. Similar cation and anion enrichments are found in the above-ambient-temperature streams draining the north flank of the Active Crater. The pH of north-flank thermal waters range from 3.6 to 4.1 and chloride:sulfate ratios (1.2–1.4) that are a factor of two greater than that of the lake brine (0.60). The waters have an ACS composition that is quite different from the AS and NC thermal waters that occur along the southern flank of Rincón de la Vieja.The distribution of thermal water types at Rincón de la Vieja strongly indicates that formation of the north-flank ACS waters is not due to mixing of shallow, steam-heated AS water with deep-seated NC water. More likely, hyper-acidic brines formed in the Active Crater area are migrating through permeable zones in the volcanic strata that make up the Active Crater's north flank. Dissolution and shallow subsurface alteration of north-flank volcanoclastic material by interaction with acidic lake brine, particularly in the more permeable tephra units, could weaken the already oversteepened north flank of the Active Crater. Sector collapse of the Active Crater, with or without a volcanic eruption, represents a potential threat to human lives, property, and ecosystems at Rincón de la Vieja volcano.  相似文献   

15.
During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km3, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano.  相似文献   

16.
We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on “growing” the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years.  相似文献   

17.
Gravity and magnetic methods have been applied to the Tenerife Island, to provide new information about its internal structure. For this study, 365 gravity stations covering the central part of the island have been selected. The anomalous density maps at different depths were obtained by means of an inversion global adjustment, on fixed density contrast, to describe the three-dimensional (3D) geometry of the anomalous bodies. On the other hand, several analysis techniques, such as reduction to the pole, spectral analysis, low-pass filtering, terrain correction and forward modelling, were applied to process the high-resolution data obtained in an aeromagnetic survey, completed with marine and terrestrial data.The joint analysis of gravity and magnetic anomalies has shown tectonic and volcanic features that define some fundamental aspects of the structural framework and volcanic evolution of the island. A strong gravity anomaly produced by a large and deep source has been associated with an uplifted block of the basement beneath the southern part of Tenerife. The sources of the observed gravity highs from 8 km b.s.l. may be associated with the growth of the submarine shield stage that was clearly controlled by regional tectonic.The long-wavelength magnetic anomalies reveal highly magnetic sources, interpreted as gabbro-ultramafic cumulates associated with the root zone of a large dyke swarm. This intrusive body could be topped by the emplacement zone of magma chambers that correlate with a magnetic horizon at 5.7±0.8 km depth. Rooted in this highly magnetic zone, two dike–like structures can be associated with the magmatic feeding system of large recent basaltic volcanoes. A shallow magnetic horizon (1.4 km a.s.l.) can be correlated with the bottom phonolites of the Las Cañadas Edifice.In the central part of the island the coincidence of some gravity and magnetic lows is consistent with the presence of low-density and low-magnetic materials, that infill a collapsed caldera system. The structures close to the surface are characterised by low-density areas connected with the recent volcanism, in particular the minimum over the Teide volcano. Hydrothermal alteration is assumed to be the cause of a short-wavelength magnetic low over the Teide volcano.  相似文献   

18.
Ubinas volcano has had 23 degassing and ashfall episodes since A.D. 1550, making it the historically most active volcano in southern Peru. Based on fieldwork, on interpretation of aerial photographs and satellite images, and on radiometric ages, the eruptive history of Ubinas is divided into two major periods. Ubinas I (Middle Pleistocene >376 ka) is characterized by lava flow activity that formed the lower part of the edifice. This edifice collapsed and resulted in a debris-avalanche deposit distributed as far as 12 km downstream the Rio Ubinas. Non-welded ignimbrites were erupted subsequently and ponded to a thickness of 150 m as far as 7 km south of the summit. These eruptions probably left a small collapse caldera on the summit of Ubinas I. A 100-m-thick sequence of ash-and-pumice flow deposits followed, filling paleo-valleys 6 km from the summit. Ubinas II, 376 ky to present comprises several stages. The summit cone was built by andesite and dacite flows between 376 and 142 ky. A series of domes grew on the southern flank and the largest one was dated at 250 ky; block-and-ash flow deposits from these domes filled the upper Rio Ubinas valley 10 km to the south. The summit caldera was formed between 25 and 9.7 ky. Ash-flow deposits and two Plinian deposits reflect explosive eruptions of more differentiated magmas. A debris-avalanche deposit (about 1.2 km3) formed hummocks at the base of the 1,000-m-high, fractured and unstable south flank before 3.6 ka. Countless explosive events took place inside the summit caldera during the last 9.7 ky. The last Plinian eruption, dated A.D.1000–1160, produced an andesitic pumice-fall deposit, which achieved a thickness of 25 cm 40 km SE of the summit. Minor eruptions since then show phreatomagmatic characteristics and a wide range in composition (mafic to rhyolitic): the events reported since A.D. 1550 include many degassing episodes, four moderate (VEI 2–3) eruptions, and one VEI 3 eruption in A.D. 1667. Ubinas erupted high-K, calc-alkaline magmas (SiO2=56 to 71%). Magmatic processes include fractional crystallization and mixing of deeply derived mafic andesites in a shallow magma chamber. Parent magmas have been relatively homogeneous through time but reflect variable conditions of deep-crustal assimilation, as shown in the large variations in Sr/Y and LREE/HREE. Depleted HREE and Y values in some lavas, mostly late mafic rocks, suggest contamination of magmas near the base of the >60-km-thick continental crust. The most recently erupted products (mostly scoria) show a wide range in composition and a trend towards more mafic magmas.Recent eruptions indicate that Ubinas poses a severe threat to at least 5,000 people living in the valley of the Rio Ubinas, and within a 15-km radius of the summit. The threat includes thick tephra falls, phreatomagmatic ejecta, failure of the unstable south flank with subsequent debris avalanches, rain-triggered lahars, and pyroclastic flows. Should Plinian eruptions of the size of the Holocene events recur at Ubinas, tephra fall would affect about one million people living in the Arequipa area 60 km west of the summit.Editorial responsibility: D Dingwell  相似文献   

19.
Since the March–April 1982 eruption of El Chichòn volcano, intense hydrothermal activity has characterised the 1-km-wide summit crater. This mainly consists of mud and boiling pools, fumaroles, which are mainly located in the northwestern bank of the crater lake. During the period 1998–2000, hot springs and fumaroles discharging inside the crater and from the southeastern outer flank (Agua Caliente) were collected for chemical analyses. The observed chemical fluctuations suggest that the physico-chemical boundary conditions regulating the thermodynamic equilibria of the deep rock/fluid interactions have changed with time. The chemical composition of the lake water, characterised in the period 1983–1997 by high Na+, Cl−, Ca2+ and SO42− contents, experienced a dramatic change in 1998–1999, turning from a Na+–Cl−- to a Ca2+–SO42−-rich composition. In June 2000, a relatively sharp increase in Na+ and Cl− contents was observed. At the same time, SO2/H2S ratios and H2 and CO contents in most gas discharges increased with respect to the previous two years of observations, suggesting either a new input of deep-seated fluids or local variations of the more surficial hydrothermal system. Migration of gas manifestations, enhanced number of emission spots and variations in both gas discharge flux and outlet temperatures of the main fluid manifestations were also recorded. The magmatic-hydrothermal system of El Chichòn is probably related to interaction processes between a deep magmatic source and a surficial cold aquifer; an important role may also be played by the interaction of the deep fluids with the volcanic rocks and the sedimentary (limestone and evaporites) basement. The chemical and physical changes recorded in 1998–2000 were possibly due to variations in the permeability of the conduit system feeding the fluid discharges at surface, as testified by the migration of gas and water emanations. Two different scenarios can be put forward for the volcanic evolution of El Chichòn: (1) build-up of an infra-crater dome that may imply a future eruption in terms of tens to hundreds of years; (2) minor phreatic–phreatomagmatic events whose prediction and timing is more difficult to constrain. This suggests that, unlike the diminished volcanic activity at El Chichòn after the 1982 paroxistic event, the volcano-hydrothermal fluid discharges need to be more constantly monitored with regular and more frequent geochemical sampling and, at the same time, a permanent network of seismic stations should be installed.  相似文献   

20.
A geochronological study utilized the unspiked potassium–argon (K–Ar) technique to obtain ages from the two main volcanic members of the shield stage of the Waianae Volcano, HI. These new dates are further constrained using a combination of stratigraphic relationships, magnetostratigraphy and major element geochemistry. Exposed shield lavas encompass 0.85 Ma, with reliably dated tholeiitic lavas from the main shield ranging from 3.93±0.08 to 3.54±0.04 Ma, and a later shield stage ranging in age from 3.57±0.04 to 3.08±0.04 Ma. These data suggest that the total extent of Waianae shield activity was significantly more than 1 Ma. The age of faulting in two flank zones is constrained to be about 3.4 Ma. Preliminary estimates of lava accumulation rates vary from about 0.3 to 2.0 mm/a; calculated rates show no systematic variation with location in the volcano or with time.  相似文献   

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