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1.
Young pumice deposits on Nisyros,Greece   总被引：1，自引：1，他引：1  
The island of Nisyros (Aegean Sea) consists of a silicic volcanic sequence upon a base of mafic-andesitic hyaloclastites, lava flows, and breccias. We distinguish two young silicic eruptive cycles each consisting of an explosive phase followed by effusions, and an older silicic complex with major pyroclastic deposits. The caldera that formed after the last plinian eruption is partially filled with dacitic domes. Each of the two youngest plinian pumice falls has an approximate DRE volume of 2–3 km3 and calculated eruption column heights of about 15–20 km. The youngest pumice unit is a fall-surge-flow-surge sequence. Laterally transitional fall and surge facies, as well as distinct polymodal grainsize distributions in the basal fall layer, indicate coeval deposition from a maintained plume and surges. Planar-bedded pumice units on top of the fall layer were deposited from high-energy, dry-steam propelled surges and grade laterally into cross-bedded, finegrained surge deposits. The change from a fall-to a surge/flow-dominated depositional regime coincided with a trend from low-temperature argillitic lithics to high-temperature, epidote-and diopside-bearing lithic clasts, indicating the break-up of a high-temperature geothermal reservoir after the plinian phase. The transition from a maintained plume to a surge/ash flow depositional regime occurred most likely during break-up of the high-temperature geothermal reservoir during chaotic caldera collapse. The upper surge units were possibly erupted through the newly formed ringfracture.  相似文献   

2.
Historical eruptions have produced lahars and floods by perturbing snow and ice at more than 40 volcanoes worldwide. Most of these volcanoes are located at latitudes higher than 35°; those at lower latitudes reach altitudes generally above 4000 m. Volcanic events can perturb mantles of snow and ice in at least five ways: (1) scouring and melting by flowing pyroclastic debris or blasts of hot gases and pyroclastic debris, (2) surficial melting by lava flows, (3) basal melting of glacial ice or snow by subglacial eruptions or geothermal activity, (4) ejection of water by eruptions through a crater lake, and (5) deposition of tephra fall. Historical records of volcanic eruptions at snow-clad volcanoes show the following: (1) Flowing pyroclastic debris (pyroclastic flows and surges) and blasts of hot gases and pyroclastic debris are the most common volcanic events that generate lahars and floods; (2) Surficial lava flows generally cannot melt snow and ice rapidly enough to form large lahars or floods; (3) Heating the base of a glacier or snowpack by subglacial eruptions or by geothermal activity can induce basal melting that may result in ponding of water and lead to sudden outpourings of water or sediment-rich debris flows; (4) Tephra falls usually alter ablation rates of snow and ice but generally produce little meltwater that results in the formation of lahars and floods; (5) Lahars and floods generated by flowing pyroclastic debris, blasts of hot gases and pyroclastic debris, or basal melting of snow and ice commonly have volumes that exceed 105 m3.The glowing lava (pyroclastic flow) which flowed with force over ravines and ridges...gathered in the basin quickly and then forced downwards. As a result, tremendously wide and deep pathways in the ice and snow were made and produced great streams of water (Wolf 1878).  相似文献   

3.
Of 1.1 million people living on the flanks of the active Merapi volcano, 440,000 are at relatively high risk in areas prone to pyroclastic flows, surges, and lahars. For the last two centuries, the activity of Merapi has alternated regularly between long periods of viscous lava dome extrusion, and brief explosive episodes at 8–15 year intervals, which generated dome-collapse pyroclastic flows and destroyed part of the pre-existing domes. Violent explosive episodes on an average recurrence of 26–54 years have generated pyroclastic flows, surges, tephra-falls, and subsequent lahars. The 61 reported eruptions since the mid-1500s killed about 7000 people. The current hazard-zone map of Merapi (Pardyanto et al., 1978) portrays three areas, termed ‘forbidden zone’, ‘first danger zone’ and ‘second danger zone’, based on successively declining hazards. Revision of the hazard map is desirable, because it lacks details necessary to outline hazard zones with accuracy, in particular the valleys likely to be swept by lahars, and excludes some areas likely to be devastated by pyroclastic gravity-currents such as the 22 November 1994 surge. In addition, risk maps should be developed to incorporate social, technical, and economic factors of vulnerability.Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, on eruptive behavior and scenarios, and on existing models and preliminary numerical modeling. Firstly, the reconstructed eruptive activity, in particular for the past 7000 years and from historical accounts of eruptions, helps to define the extent and recurrence frequency of the most hazardous phenomena (Newhall et al., 2000; Camus et al., 2000). Pyroclastic flows traveled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down the radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Newhall et al., 2000; Camus et al., 2000). Secondly, four eruption scenarios are portrayed as hazardous zones on two maps and derived from the past eruptive behavior of Merapi and from the most affected areas in the past. Thirdly, simple numerical simulation, based on a Digital Elevation Model, a stereo-pair of SPOT satellite images, and one 2D-orthoimage helps to simulate pyroclastic and lahar flowage on the flanks and in radial valley channels, and to outline areas likely to be devastated.Three major threats are identified: (1) a collapse of the summit dome in the short-to mid-term, that can release large-volume pyroclastic flows and high-energy surges towards the south–southwest sector of the volcano; (2) an explosive eruption, much larger than any since 1930, may sweep all the flanks of Merapi at least once every century; (3) a potential collapse of the summit area, involving the fumarolic field of Gendol and part of the southern flank, which can contribute to moderate-scale debris avalanches and debris flows.  相似文献   

4.
A devastating pyroclastic surge and resultant lahars at Mount St. Helens on 18 May 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank. The tributaries channeled the flows to Smith Creek valley, which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank. Stratigraphy shows that the pyroclastic surge preceded the lahars; there is no notable “wet” character to the surge deposits. Therefore the lahars must have originated as snowmelt, not as ejected water-saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano. In stratigraphic order the Smith Creek valley-floor materials comprise (1) a complex valley-bottom facies of the pyroclastic surge and a related pyroclastic flow, (2) an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris, and (3) deposits of secondary pyroclastic flows. These units are capped by silt containing accretionary lapilli, which began falling from a rapidly expanding mushroom-shaped cloud 20 minutes after the eruption's onset. The Smith Creek valley-bottom pyroclastic facies consists of (a) a weakly graded basal bed of fines-poor granular sand, the deposit of a low-concentration lithic pyroclastic surge, and (b) a bed of very poorly sorted pebble to cobble gravel inversely graded near its base, the deposit of a high-concentration lithic pyroclastic flow. The surge apparently segregated while crossing the steep headwater tributaries of Smith Creek; large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge. The unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough, brown muddy sand matrix like that of lahar deposits upvalley. This unit contains irregular friable lenses and pods meters in diameter, blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier. The hummocky unit is the deposit of a high-viscosity debris flow which formed when lahars mingled with the pyroclastic materials on Smith Creek valley floor. Overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines-poor gas-escape pipes and containing charred wood. The deposits are thickest in topographic lows along margins of the hummocky diamict. Emplaced several minutes after the hot surge had passed, this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides.  相似文献   

5.
Pyroclastic flows from the 1991 eruption of Unzen volcano,Japan   总被引：1，自引：0，他引：1  
Pyroclastic flows from Unzen were generated by gravitational collapse of the growing lava dome. As soon as the parental lobe failed at the edge of the dome, spontaneous shattering of lava occurred and induced a gravity flow of blocks and finer debris. The flows had a overhanging, tongue-like head and cone- or rollershaped vortices expanding outward and upward. Most of the flows traveled from 1 to 3 km, but some flows reached more than 4 km, burning houses and killing people in the evacuated zone of Kita-kamikoba on the eastern foot of the volcano. The velocities of the flows ranged from 15 to 25 m/s on the gentle middle flank. Observations of the flows and their deposits suggest that they consisted of a dense basal avalanche and an overlying turbulent ash cloud. The basal avalanche swept down a topographic low and formed to tongue-like lobe having well-defined levees; it is presumed to have moved as a non-Newtonian fluid. The measured velocities and runout distances of the flows can be matched to a Bingham model for the basal avalanche by the addition of turbulent resistance. The rheologic model parameters for the 29 May flow are as follows: the density is 1300 kg/m3, the yield strength is 850 Pa, the viscosity is 90 Pa s, and the thickness of the avalanche is 2 m. The ash cloud is interpreted as a turbulent mixing layer above the basal avalanche. The buoyant portions of the cloud produced ash-fall deposits, whereas the dense portions moved as a surge separated from the parental avalanche. The ash-cloud surges formed a wide devastated zone covered by very thin debris. The initial velocities of the 3 June surges, when they detached from avalanches, are determined by the runout distance and the angle of the energy-line slope. A comparison between the estimated velocities of the 3 June avalanches and the surges indicates that the surges that extended steep slopes along the avalanche path, detached directly from the turbulent heads of the avalanches. The over-running surge that reached Kita-Kamikoba had an estimated velocity higher than that of the avalanche; this farther-travelled surge is presumed to have been generated by collapse of a rising ash-cloud plume.  相似文献   

6.
Abstract The Himeji–Yamasaki region in the Inner Zone of southwest Japan is underlain mainly by Late Cretaceous volcanic rocks called the Ikuno Group or the Hiromine and Aioi Groups. A new stratigraphic and geochronological study shows that the volcanic rocks in this area consist of 15 eroded caldera volcanoes between 82 and 65 Ma; they are, in order of decreasing age, the Hiromine, Hoden, Ibo, Okawachi, Seppikosan, Hayashida, Shinokubi, Fukusaki, Kurooyama, Ise, Fukadanigawa, Nagusayama, Matobayama, Yumesaki and Mineyama Formations. These calderas vary in diameter from 1 to 20 km and are bounded by steep unconformities; they coalesce and overlap each other. The individual caldera fills are composed mainly of single voluminous pyroclastic flow deposits, which are often interleaved with debris avalanche deposits and occasionally underlie lacustrine deposits. The intracaldera pyroclastic flow deposits are made up of massive, welded or non‐welded tuff breccia to lapilli tuff, and are characterized by their great thickness. The debris avalanche deposits are ill‐sorted breccia, generated by the collapse of the caldera wall toward the caldera floor during the pyroclastic‐flow eruption. The large calderas that are more than 10 km in diameter contain original values of approximately 100 km3 of intracaldera pyroclastic flow deposits. These large calderas are similar to the well‐known Valles‐type calderas in their dimensions, although it is uncertain whether their caldera floors are coherent plates or incoherent pieces. Conversely, the small calderas have diatreme‐like subsurface structures. The variety of the caldera volcanoes in this area is caused by the difference in the volume of caldera‐forming pyroclastic eruptions, as the large and small calderas coexisted. The caldera‐forming eruption rates in Late Cretaceous southwest Japan, including the studied area, were similar to those in late Cenozoic central Andes and northeast Honshu arc, Japan, but obviously smaller than those of late Cenozoic intracratonic caldera clusters in western North America and the Quaternary extensional volcanic arcs in Taupo, New Zealand. The widespread Late Cretaceous felsic igneous rocks in southwest Japan were generated by a long‐term accumulation of low‐rate granitic magmatism at the eastern margin of the Eurasian Plate.  相似文献   

7.
Ischia is an active volcanic island in the Gulf of Naples whose history has been dominated by a caldera-forming eruption (ca. 55 ka) and resurgence phenomena that have affected the caldera floor and generated a net uplift of about 900 m since 33 ka. The results of new geomorphological, stratigraphical and textural investigations of the products of gravitational movements triggered by volcano-tectonic events have been combined with the information arising from a reinterpretation of historical chronicles on natural phenomena such as earthquakes, ground deformation, gravitational movements and volcanic eruptions. The combined interpretation of all these data shows that gravitational movements, coeval to volcanic activity and uplift events related to the long-lasting resurgence, have affected the highly fractured marginal portions of the most uplifted Mt. Epomeo blocks. Such movements, mostly occurring since 3 ka, include debris avalanches; large debris flows (lahars); smaller mass movements (rock falls, slumps, debris and rock slides, and small debris flows); and deep-seated gravitational slope deformation. The occurrence of submarine deposits linked with subaerial deposits of the most voluminous mass movements clearly shows that the debris avalanches impacted on the sea. The obtained results corroborate the hypothesis that the behaviour of the Ischia volcano is based on an intimate interplay among magmatism, resurgence dynamics, fault generation, seismicity, slope oversteepening and instability, and eruptions. They also highlight that volcano-tectonically triggered mass movements are a potentially hazardous phenomena that have to be taken into account in any attempt to assess volcanic and related hazards at Ischia. Furthermore, the largest mass movements could also flow into the sea, generating tsunami waves that could impact on the island’s coast as well as on the neighbouring and densely inhabited coast of the Neapolitan area.  相似文献   

8.
 Akutan Volcano is one of the most active volcanoes in the Aleutian arc, but until recently little was known about its history and eruptive character. Following a brief but sustained period of intense seismic activity in March 1996, the Alaska Volcano Observatory began investigating the geology of the volcano and evaluating potential volcanic hazards that could affect residents of Akutan Island. During these studies new information was obtained about the Holocene eruptive history of the volcano on the basis of stratigraphic studies of volcaniclastic deposits and radiocarbon dating of associated buried soils and peat. A black, scoria-bearing, lapilli tephra, informally named the "Akutan tephra," is up to 2 m thick and is found over most of the island, primarily east of the volcano summit. Six radiocarbon ages on the humic fraction of soil A-horizons beneath the tephra indicate that the Akutan tephra was erupted approximately 1611 years B.P. At several locations the Akutan tephra is within a conformable stratigraphic sequence of pyroclastic-flow and lahar deposits that are all part of the same eruptive sequence. The thickness, widespread distribution, and conformable stratigraphic association with overlying pyroclastic-flow and lahar deposits indicate that the Akutan tephra likely records a major eruption of Akutan Volcano that may have formed the present summit caldera. Noncohesive lahar and pyroclastic-flow deposits that predate the Akutan tephra occur in the major valleys that head on the volcano and are evidence for six to eight earlier Holocene eruptions. These eruptions were strombolian to subplinian events that generated limited amounts of tephra and small pyroclastic flows that extended only a few kilometers from the vent. The pyroclastic flows melted snow and ice on the volcano flanks and formed lahars that traveled several kilometers down broad, formerly glaciated valleys, reaching the coast as thin, watery, hyperconcentrated flows or water floods. Slightly cohesive lahars in Hot Springs valley and Long valley could have formed from minor flank collapses of hydrothermally altered volcanic bedrock. These lahars may be unrelated to eruptive activity. Received: 31 August 1998 / Accepted: 30 January 1999  相似文献   

9.
Six years after the 1991 Mt. Pinatubo eruption, deep erosion incisions into the pyroclastic deposits accumulated around the volcano enabled us to investigate the stratigraphy of the climactic deposits both in valley bottoms and on contiguous ridges. Stratigraphic relationships between fall, flow, and surge deposits in the Marella drainage system indicate that during the climactic eruption a progressive shift occurred from an early convective regime, to a transitional regime feeding both the plinian convective column and mostly dilute density currents, to a fully collapsing regime producing mostly dense pyroclastic flows. Syn-plinian dilute density currents (surges) propagated up to ~10 km from the crater, both along valley bottoms and on contiguous ridges of the Marella Valley, whereas post-plinian pyroclastic flows had greater runout (~13 km), were confined to valleys and were not associated with significant surges. Stratigraphic study and grain-size analyses allow the identification of three types of intra-plinian deposits: (a) lower and often coarse-grained surge deposits, emplaced during the accumulation of the coarsest portion of the fallout bed at time intervals of ~16-24 min; (b) upper fine-grained surge deposits, interstratified with the fine-grained portion of the fall bed and emplaced at shorter time intervals of ~3-13 min; and (c) small-volume, channel-confined, massive pumiceous flow deposits interbedded with the upper surges in the upper fine-grained fall bed. Maximum clast size isopleths of 1.6 and 0.8 cm for lithics (ML) and 2.0 and 4.0 cm for pumices (MP) show almost symmetrical distribution around the vent, indicating that the passing of the typhoon Yunya during the climactic eruption had little effect on trajectories of high-Reynold-number clasts. Significant distortion was, however, observed for the 3.2-cm ML and 6.0-cm MP proximal isopleths, whose patterns were probably influenced by the interaction of the clasts falling from column margins with the uprising co-ignimbrite ash plumes. Application of the Carey and Sparks (1986) model to the undisturbed isopleths generated by the umbrella cloud yields a maximum column height of ~42 km, in good agreement with satellite measurements. Systematic stratigraphic and vertical grain-size studies of the plinian fall deposit in the Marella Valley, combined with satellite data and eyewitness accounts, reveal that the carrying capacity of the convective column and related fallout activity peaked in the early phase of the eruption, beginning slightly before 13:41 and gradually declined until its cessation 3 h later. Most of the pumiceous pyroclastic flow deposits were emplaced after the end of the fallout activity at ~16:30 but before the summit caldera collapse at approximately 19:11. Only a small volume of pumiceous flow deposits accumulated after the final caldera collapse. In contrast to the previous reconstruction of Holasek et al. (1996), which interpreted the progressive lowering of the column, documented by satellite data, as due to a decreasing mass eruption rate, we suggest that a progressive shift from a plinian column to a large co-ignimbrite column could also account for such a variation.  相似文献   

10.
Apoyo caldera, near Granada, Nicaragua, was formed by two phases of collapse following explosive eruptions of dacite pumice about 23,000 yr B.P. The caldera sits atop an older volcanic center consisting of lava flows, domes, and ignimbrite (ash-flow tuff). The earliest lavas erupted were compositionally homogeneous basalt flows, which were later intruded by small andesite and dacite flows along a well defined set of N—S-trending regional faults. Collapse of the roof of the magma chamber occurred along near-vertical ring faults during two widely separated eruptions. Field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast, followed by eruption column collapse, which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse. A period of quiescence was marked by the eruption of scoria-bearing tuff from the nearby Masaya caldera and the development of a soil horizon. Violent plinian eruptions then resumed from a vent located within the caldera. A second phase of caldera collapse followed, accompanied by the effusion of late-stage andesitic lavas, indicating the presence of an underlying zoned magma chamber. Detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume. Mapping of the Apoyo airfall and ignimbrite deposits gives a volume of 17.2 km3 within the 1-mm isopach. Crystal concentration studies show that the true erupted volume was 30.5 km3 (10.7 km3 Dense Rock Equivalent), approximately the volume necessary to fill the caldera. A vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions. This vent area is controlled by N—S-trending precaldera faults related to left-lateral motion along the adjacent volcanic segment break. Fractional crystallization of calc-alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at Apoyo. Prior to caldera collapse, highly atypical tholeiitic magmas resembling low-K, high-Ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber. The injection of tholeiitic magmas may have contributed to the paroxysmal caldera-forming eruptions.  相似文献   

11.
Tsunami mitigation, preparedness and early warning initiatives have begun at the global scale only after the tragic event of Sumatra in 2004. Turkey, as a country with a history of devastating earthquakes, has been also affected by tsunamis in its past. In this paper we present the Tsunami Hazard in the Eastern Mediterranean and its connected seas (Aegean, Marmara and Black Sea) by providing detailed information on historically and instrumentally recorded significant tsunamigenic events surrounding Turkey, aiming to a better understanding of the Tsunami threat to the Turkish coasts. In addition to the review of the Tsunami hazard, we have studied a possible Tsunami source area between Rhodes and SW of Turkey using Tsunami numerical model NAMI DANCE-two nested domains. We have computed a maximum positive amplitude of 1.13 m and maximum negative amplitude of −0.5 m at the Tsunami source by this study. The distribution of maximum positive amplitudes of the water surface elevations in the selected Tsunami forecast area and time histories of water level fluctuations near selected locations (Marmaris, Dalaman, Fethiye and Kas towns) indicate that the maximum positive amplitude near the coast in the selected forecast area exceeds 3.5 m. The arrival time of maximum wave to Marmaris, Dalaman, is 10 min, while that of Fethiye and Kas towns is 15–20 min. The maximum positive amplitudes near the shallow region of around 10 m depth are 3 m (Marmaris), 1 m (Dalaman), 2 m (Fethiye) and 1 m (Kas). Maximum positive amplitudes of water elevations in the duration of 4 h simulation of the Santorini-Minoan Tsunami in around 1600 BC in the Aegean Sea are also calculated based on a simulation performed using 900 m grid resolution of Aegean sea bathymetry with a 300 m collapse of 10 km diameter of Thera (Santorini) caldera. We have also presented the results of the Tsunami modeling and simulation for Marmara Sea obtained from a previous study. Last part of this paper provides information on the establishment of a Tsunami Warning Center by KOERI, which is expected to act also as a regional center under the UNESCO Intergovernmental Oceanographic Commission – Intergovernmental Coordination Group for the Tsunami Early Warning and Mitigation System in the North-Eastern Atlantic, the Mediterranean and Connected Seas (ICG/NEAMTWS) initiative, emphasizing on the challenges together with the future work needed to be accomplished.  相似文献   

12.
The initial explosions at Mount St. Helens, Washington, on the moring of 18 May 1980 developed into a huge pyroclastic surge that generated catastrophic floods off the east and west flanks of the volcano. Near-source surge deposits on the east and west were lithic, sorted, lacking in accretionary lapilli and vesiculated ash, not plastered against upright obstacles, and hot enough to char wood — all attributes of dry pyroclastic surge. Material deposited at the surge base on steep slopes near the volcano transformed into high-concentration lithic pyroclastic flows whose deposits contain charred wood and other features indicating that these flows were hot and dry. Stratigraphy shows that even the tail of the surge had passed the east and west volcano flanks before the geomorphically distinct floods (lahars) arrived. This field evidence undermines hypotheses that the turbulent surge was itself wet and that its heavy components segregated out to transform directly into lahars. Nor is there evidence that meters-thick snow-slab avalanches intimately mixed with the surge to form the floods. The floods must have instead originated by swift snowmelt at the base of a hot and relatively dry turbulent surge. Impacting hot pyroclasts probably transferred downslope momentum to the snow surface and churned snow grains into the surge base. Melting snow and accumulating hot surge debris may have moved initially as thousands of small thin slushflows. As these flows removed the surface snow and pyroclasts, newly uncovered snow was partly melted by the turbulent surge base; this and accumulating hot surge debris in turn began flowing, a self-sustaining process feeding the initial flows. The flows thus grew swiftly over tens of seconds and united downslope into great slushy ejecta-laden sheetfloods. Gravity accelerated the floods to more than 100 km/h as they swept down and off the volcano flanks while the snow component melted to form great debris-rich floods (lahars) channeled into valleys.  相似文献   

13.
The general characteristics of seismic signals produced by pyroclastic flows (generated by either the collapse of a lava dome or an eruptive column) and lahars at Volcán de Colima, México are discussed. The paper concentrates on the 2004–2006 activity associated with and following the extrusion of andesitic block-lava in October–November 2004. It is shown that the duration of the broad-band seismic records of pyroclastic flows lasts a few minutes while the duration of seismic records of lahars continues for tens of minutes or hours. The spectra of seismic records produced by pyroclastic flows are characterized by lower peak frequencies (around 3–4 Hz) than for lahars (around 6–8 Hz). This difference in the frequency content together with the difference in the duration of seismic signals allows early diagnostic of the events in real time.  相似文献   

14.
 The evolution of the Somma-Vesuvius caldera has been reconstructed based on geomorphic observations, detailed stratigraphic studies, and the distribution and facies variations of pyroclastic and epiclastic deposits produced by the past 20,000 years of volcanic activity. The present caldera is a multicyclic, nested structure related to the emptying of large, shallow reservoirs during Plinian eruptions. The caldera cuts a stratovolcano whose original summit was at 1600–1900 m elevation, approximately 500 m north of the present crater. Four caldera-forming events have been recognized, each occurring during major Plinian eruptions (18,300 BP "Pomici di Base", 8000 BP "Mercato Pumice", 3400 BP "Avellino Pumice" and AD 79 "Pompeii Pumice"). The timing of each caldera collapse is defined by peculiar "collapse-marking" deposits, characterized by large amounts of lithic clasts from the outer margins of the magma chamber and its apophysis as well as from the shallow volcanic and sedimentary units. In proximal sites the deposits consist of coarse breccias resulting from emplacement of either dense pyroclastic flows (Pomici di Base and Pompeii eruptions) or fall layers (Avellino eruption). During each caldera collapse, the destabilization of the shallow magmatic system induced decompression of hydrothermal–magmatic and hydrothermal fluids hosted in the wall rocks. This process, and the magma–ground water interaction triggered by the fracturing of the thick Mesozoic carbonate basement hosting the aquifer system, strongly enhanced the explosivity of the eruptions. Received: 24 November 1997 / Accepted: 23 March 1999  相似文献   

15.
One active and ten extinct Quaternary volcanoes are described from the Cape Hoskins area, on the north coast of New Britain. They are mostly strato volcanoes built up of lava flows, lava domes, pyroclastic flows, lahars, tephra, and derived alluvial sediments. The volcanic products range in composition from basalt to rhyolite, but basaltic andesite and andesite predominate. Much of the area is covered by tephra, several metres thick, consisting mainly of rhyolitic pumice. The active volcano, Pago, is built up of several glacier-like lava flows, the last of which was formed during an eruption in 1914–18. Pago lies within a well-preserved caldera forming the central part of a broad low-angle cone, named Witori, which consists largely of welded and unwelded pyroclastic flow deposits. C-14 dates obtained on charcoal indicate that the caldera eruption occurred about 2500 years B. P. Another caldera of similar age lies south of Witori. Of the other eight volcanoes described four are relatively well-preserved steep-sided cones formed mainly of lava flows, one is a remnant of a low-angle cone with a caldera, and three are deeply eroded cones which have none of their constructional surfaces preserved.  相似文献   

16.
Volcanological analysis of the 10 000 yr –1538 explosive activity at Campi Flegrei shows that the most common explosive eruptions are characterized by the emplacement of flow or surge deposits, originating from the interaction between magma and shallow and/or sea water. The minimum volumes of pyroclastic products range between 0.04 and 0.7 km3; the proximal areas covered by these products range from 3–4 to 40–50 km2. The pyroclastic flow and surge deposits occurring inside the caldera have been strongly controlled by pre-existent morphology; because of this, the area of present Napoli city was blanketed by approximately 5 m of pyroclastic deposits, during the last 5000 yr.Previous analysis suggests that the presence of even very low topographic obstacles may influence pyroclastic density current run out such that future eruptive deposits would mainly be confined inside the caldera rim. We suggest that a future eruption at Campi Flegrei would not seriously involve the urbanized area of Napoli city located on the hills. On the contrary, the plains located on the eastern side of the caldera (Fuorigrotta, Bagnoli) would be the most damaged area.  相似文献   

17.
The majority of tephra generated during the paroxysmal 1883 eruption of Krakatau volcano, Indonesia, was deposited in the sea within a 15-km radius of the caldera. Two syneruptive pyroclastic facies have been recovered in SCUBA cores which sampled the 1883 subaqueous pyroclastic deposit. The most commonly recovered facies is a massive textured, poorly sorted mixture of pumice and lithic lapilli-to-block-sized fragments set in a silty to sandy ash matrix. This facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the Krakatau islands on the basis of grain size and component abundances. A less common facies consists of well-sorted, planarlaminated to low-angle cross-bedded, vitric-enriched silty ash. Entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated, deflated components of pyroclastic flows as they traveled over water. The basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction, high temperature of emplacement, and lack of traction structures, and no significant hydraulic sorting of components. The laminated facies was most likely deposited from low-concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater. The Krakatau deposits are the first well-documented example of true submarine pyroclastic flow deposition from a modern eruption, and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences.  相似文献   

18.
Geology of a submarine volcanic caldera in the Tonga Arc: Dive results   总被引：2，自引：0，他引：2  
A submersible dive conducted on Volcano #1 located near 21° 09′S–175° 45′W on the Tonga Arc showed that the volcanic edifice with a caldera floor area of 30 km2 located at and 450 m deep (b.s.l.=below sea level) was constructed recently during episodic volcanism. The sequential volcanic events are recorded along a faulted terrain formed in response to the collapse of the caldera wall. The post-caldera events are marked by occasional eruptions that have built scoriaceous cones associated with low-temperature hydrothermal venting and localized small-scale collapse features. The stratigraphy of the caldera wall indicates that the volcano was built by explosive volcanism alternating with quieter eruptive events. The repeated, violent explosive events formed ≤ 20 m thick sequences composed of alternating fine-grained ash beds and sand- to boulder-sized pyroclastic layers. During quieter volcanic events, dykes and massive flows intruded and/or accompanied the eruption of the volcaniclastic deposits throughout the sections of the wall explored. Massive columnar-jointed flows consist of viscous, silica-rich lavas forming tabular and giant radial-jointed (GRJ) flows formed in large (> 8 m in diameter) conduits and extruded onto the sea floor. In addition, massive lava flows forming sill-like complexes were observed underneath and near the giant radial-jointed columnar flows. Also, an intermittent quiet type of eruption produced vesicular lava flows, which are interbedded within the pyroclastic layered deposits. The massive and vesicular lavas consist of andesites and dacites with Ca-depleted (pigeonite) and Ca-enriched (salite) pyroxene, and intermediate (andesine-labradorite) to calcic (bytownite) plagioclase. They are depleted in total alkalis (Na2O + K2O < 3%), K2O (< 1%), Zr/Y (< 1.8), Nb/Zr (< 0.01) and light Rare Earth Elements. We interpret that these andesite–dacite series were erupted after undergoing crystal-liquid fractionation in a magma chamber located underneath the caldera floor.  相似文献   

19.
The November 13, 1985 eruption of Nevado del Ruiz produced a series of pyroclastic flows and surges that eroded channels on the surface of the summit glacier and generated lahars which descended down most of the rivers that drain the volcano. The stratigraphy of the proximal pyroclastic deposits indicates that there were at least four episodes to the eruption. Episode I, deposited an unusual surge consisting of small pieces of ice mixed with ash and exhibiting planar stratification. Ballistically emplaced fragments are also intercalated with this unit. During Episode II, at least two pyroclastic flows were erupted. Their deposits contain the most evolved pumice of the entire eruption; SiO2 content of matrix glass ranges between 74.5 and 74.9%. Episode III is marked by the emplacement of a welded tuff with an average SiO2 content of about 66% in the matrix glass. The final Episode IV was characterized by the development of a high-altitude eruption column and the emplacement of several nonwelded pyroclastic flows. Banded pumice are common in the pyroclastic flow as well as in the pumice fall deposits. Co-existing dark and light pumice bands differ in SiO2 content by 3.5% and in general are similar to the composition of the welded pumice from Episode III.The compositional zonation of the pyroclastic deposits from Episode I to IV suggests that a nearsurface compositionally-stratified portion of the magma body was tapped during Episode II. During Episodes III and IV the main body of magma was involved although the coexistence of the compositionally distinct pumice clasts at similar stratigraphic levels argues for mixing of magma from different levels in the chamber during the eruptive process.  相似文献   

20.
Sumisu volcano was the site of an eruption during 30–60 ka that introduced ∼48–50 km3 of rhyolite tephra into the open-ocean environment at the front of the Izu-Bonin arc. The resulting caldera is 8 × 10 km in diameter, has steep inner walls 550–780 m high, and a floor averaging 900 m below sea level. In the course of five research cruises to the Sumisu area, a manned submersible, two ROVs, a Deep-Tow camera sled, and dredge samples were used to study the caldera and surrounding areas. These studies were augmented by newly acquired single-channel seismic profiles and multi-beam seafloor swath-mapping. Caldera-wall traverses show that pre-caldera eruptions built a complex of overlapping dacitic and basaltic edifices, that eventually grew above sea level to form an island about 200 m high. The caldera-forming eruption began on the island and probably produced a large eruption column. We interpret that prodigious rates of tephra fallback overwhelmed the Sumisu area, forming huge rafts of floating pumice, choking the nearby water column with hyperconcentrations of slowly settling tephra, and generating pyroclastic gravity currents of water-saturated pumice that traveled downslope along the sea floor. Thick, compositionally similar pumice deposits encountered in ODP Leg 126 cores 70 km to the south could have been deposited by these gravity currents. The caldera-rim, presently at ocean depths of 100–400 m, is mantled by an extensive layer of coarse dense lithic clasts, but syn-caldera pumice deposits are only thin and locally preserved. The paucity of syn-caldera pumice could be due to the combined effects of proximal non-deposition and later erosion by strong ocean currents. Post-caldera edifice instability resulted in the collapse of a 15° sector of the eastern caldera rim and the formation of bathymetrically conspicuous wavy slump structures that disturb much of the volcano’s surface.  相似文献   

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