首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
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   

2.
Reconnaissance mapping and 40Ar/39Ar age determinations establish an eruptive chronology for Koniuji Island in the central Aleutian island arc. Koniuji is a tiny 0.95 km2 island that rises only 896 ft above the Bering Sea. Previous accounts describe Koniuji as a mostly submerged, deeply eroded, dormant stratovolcano. However, new 40Ar/39Ar ages constrain the duration of subaerial eruptive activity from 15.2 to 3.1 ka. Furnace incremental heating experiments on replicate groundmass separates from two samples of a 30–50 m thick basaltic andesite flow at the southernmost point of the island gave a weighted mean 40Ar/39Ar age of 15.2 ± 5.0 (2σ). The next phase of eruptive activity includes a series of 5.8–4.6 ka basaltic andesitic to andesitic lava flows preserved along the western shoreline. The basal lavas contain numerous mafic enclaves and dioritic cumulates suggesting a major disturbance in the plumbing system during the initial stages of emplacement. The 5.8–4.6 ka lavas are truncated by an andesitic dome complex that includes hornblende-bearing domes, flows and pyroclastics which extruded into the center of the island and comprise the majority of the subaerial eruptive volume. An angular block from within the dome complex yielded 40Ar/39Ar age of 3.1 ± 1.9 ka, thereby making it one of the youngest island arc volcanics to be dated using the 40Ar/39Ar method. Overall, the 40Ar/39Ar data indicate that Koniuji is a nascent stratovolcano that has only recently emerged above sea level, not a glacially-eroded, long-lived volcanic complex like those found on many other central Aleutian Islands.  相似文献   

3.
The eruptive history of Kuju volcano on Kyushu, Japan, during the past 15,000 years has been determined by tephrochronology and 14C dating. Kuju volcano comprises isolated lava domes and cones of hornblende andesite together with aprons of pyroclastic-flow deposits on its flanks. Kuju volcano produced tephras at roughly 1000-yr intervals during the past 5000 years and 70% of the domes and cones have formed during the past 15,000 years. The youngest magmatic activity of Kuju volcano was the 1.6 km3 andesite eruption about 1600 years ago which emplaced a lava dome and block-and-ash flow. Kuju volcano shows a nearly constant long-term eruption rate (0.7–0.4 km3 for 1000 years) during the past 15,000 years. This rate is within the range of estimated average eruption rates of late Quaternary volcanoes in the Japanese Arc, but is about one order of magnitude higher than the eruption rate of Unzen volcano. Kuju volcano has been in phreatic eruption since October 1995. The late Quaternary history of Kuju indicates that it poses a significant volcanic hazard, primarily due to block-and-ash flows from collapsing lava domes.  相似文献   

4.
The Mascota volcanic field is located in the Jalisco Block of western Mexico, where the Rivera Plate subducts beneath the North American Plate. It spans an area of ∼ 2000 km2 and contains ∼ 87 small cones and lava flows of minette, absarokite, basic hornblende lamprophyre, basaltic andesite, and andesite. There are no contemporary dacite or rhyolite lavas. New 40Ar/39Ar ages are presented for 35 samples, which are combined with nine dates from the literature to document the eruptive history of this volcanic field. The oldest lavas (2.4 to 0.5 Ma) are found in the southern part of the field area, whereas the youngest lavas (predominantly < 0.5 Ma) are found in the northern portion. On the basis of these ages, field mapping, and the use of ortho aerial photographs and digital elevation models, it is estimated that a combined volume of 6.8 ± 3.1 km3 erupted in the last 2.4 Myr, which leads to an average eruption rate of ∼ 0.003 km3/kyr, and an average volume per eruptive unit of < 0.1 km3. The dominant lava type is andesite (2.1 ± 0.9 km3), followed by absarokite (1.6 ± 0.8 km3), basaltic andesite (1.2 ± 0.5 km3), basic hornblende lamprophyre (1.0 ± 0.4 km3), and minette (0.9 ± 0.5 km3). Thus, the medium-K andesite and basaltic andesite comprise approximately half (49%) of the erupted magma, with twice as much andesite as basaltic andesite, and they occur in close spatial and temporal association with the highly potassic, lamprophyric lavas. There is no time progression to the type of magma erupted. A wide variety of evidence indicate that the high-MgO (8–9 wt.% ) basaltic andesites (52–53% wt.% SiO2) were formed by H2O flux melting of the asthenopheric arc mantle wedge, whereas the mafic minettes and absarokites were formed by partial melting (induced by thermal erosion) of depleted lithospheric mantle containing phlogopite-bearing veins. There is only limited differentiation of the potassic magmas, with none more evolved than 55.4 wt.% SiO2 and 4.4 wt.% MgO. This may be attributable to rapid crystallization of the mantle-derived melts in the deep crust, owing to their low volumes. Thus, the andesites (58–63 wt.% SiO2) are notable for being both the most voluminous and the most evolved of all lava types in the Mascota volcanic field, which is not consistent with their extraction from extensively crystallized (60–70%), low-volume intrusions. Instead, the evidence supports the origin of the andesites by partial melting of amphibolitized, mafic lower crust, driven by the emplacement of the minettes, absarokites, and the high-Mg basaltic andesites.  相似文献   

5.
Located at the volcanic front in the western Mexican arc, in the Colima Rift, is the active Volcán Colima, which lies on the southern end of the massive (∼450 km3) Colima-Nevado volcanic complex. Along the margins of this andesitic volcanic complex, is a group of 11 scoria cones and associated lavas, which have been dated by the 40Ar/39Ar method. Nine scoria cones erupted ∼1.3 km3 of alkaline magma (basanite, leucite-basanite, minette) between 450 and 60 ka, with >99% between 240 and 60 ka. Two additional cones (both the oldest and calc-alkaline) erupted <0.003 km3 of basalt (0.5 Ma) and <0.003 km3 of basaltic andesite (1.2 Ma), respectively. Cone and lava volumes were estimated with the aid of digital elevation models (DEMs). The eruption rate for these scoria cones and their associated lavas over the last 1.2 Myr is ∼1.2 km3/Myr, which is more than 400 times smaller than that from the andesitic Colima-Nevado edifice. In addition to these alkaline Colima cones, two other potassic basalts erupted at the volcanic front, but ∼200 km to the ESE (near the historically active Volcán Jorullo), and were dated at 1.06 and 0.10 Ma. These potassic suites reflect the tendency in the west-central Mexican arc for magmas close to the volcanic front to be enriched in K2O relative to those farther from the trench.Ferric-ferrous analyses on pristine samples from the alkaline cones adjacent to V. Colima and V. Jorullo indicate that their oxygen fugacities relative to the nickel-nickel oxide buffer are significantly higher (ΔNN0=2–4) than those for the calc-alkaline magma types (0–1.5). These ΔNNO values correlate positively with Ba concentrations and likely reflect the influence of a slab-derived fluid. As a result of the high oxidation states, the solubility of sulfur in these potassic magmas is enhanced. Indeed the sulfur content of both the whole rock and the apatite phenocrysts (and in olivine melt inclusions reported in the literature) suggest that part of their pre-eruptive sulfur gas (SO2) concentrations could have been discharged to the atmosphere in amounts comparable to the 1982 eruption of El Chichón, although over a prolonged period spanning thousands of years (not per eruption).Electronic Supplementary Material Supplementary material is available for this article at Editorial responsibility: J. Donnelly-Nolan  相似文献   

6.
Mount Hasan is a double-peaked stratovolcano, located in Central Anatolia, Turkey. The magmas erupted from this multi-caldera complex range from basalt to rhyolite, but are dominated by andesite and dacite. Two terminal cones (Big Mt. Hasan and Small Mt. Hasan) culminate at 3253 m and 3069 m respectively. There are four evolutionary stages in the history of the volcanic complex (stage 1: Kecikalesi volcano, 13 Ma, stage 2: Palaeovolcano, 7 Ma, stage 3: Mesovolcano and stage 4: Neovolcano). The eruptive products consist of lava flows, lava domes, and pyroclastic rocks. The later include ignimbrites, phreatomagmatic intrusive breccias and nuées ardentes, sometimes reworked as lahars. The total volume is estimated to be 354 km3, the area extent 760 km2. Textural and mineralogical data suggest that both magma mixing and fractional crystallization were involved in the generation of the andesites and dacites. The magmas erupted from the central volcanoes show a transition with time from tholeite to calc-alkaline. Three generations of basaltic strombolian cones and lava flows were emplaced contemporaneously with the central volcanoes. The corresponding lavas are alkaline with a sodic tendency.  相似文献   

7.
More than 5000 km3 of magmatic material was erupted in Pliocene-Pleistocene times in a volcano-tectonic depression, i. e., the Hohi volcanic zone (HVZ) in central Kyushu, Japan. The eruptive deposits consist mainly of andesite lava flows and large-scale pyroclastic-flow deposits. Their eruptions were accompanied by the formation of an EW-oriented graben (70 km × 45 km) under regional NS extensional stress. Pre-Tertiary basement rocks are absent on the surface of the graben but occur at depth, having subsided up to 3 km. Radiometric ages of volcanic rocks on the surface show zoned isochrons from 5 Ma at the margin to 0.3 Ma in the center of the HVZ. The youngest center of age zonation coincides with a 30 mgal negative Bouguer gravity anomaly. Radiometric ages of rocks from drill cores are older toward the bottom of the graben, reaching a maximum of at least 4 Ma. Volcanic activity concentrated over time toward the center of the graben and buried successively erupted material. Areas of active volcanism in the HVZ became smaller and changed in style during the 5-Ma history of activity. Volcanism of the early stage (5-2 Ma) was characterized by voluminous eruptions of andesitic lava flows that formed lava plateaus and were intruded by EW-oriented feeder dikes, perhaps related to fissure eruptions. In contrast, late-stage volcanism (2-0 Ma) resulted primarily in andesitic to dacitic lava domes with features of monogenetic volcanoes produced at low eruption rates. The HVZ shows unimodal volcanism dominated by andesitic and dacitic lavas with a small amount of rhyolite and only traces of basalt; these characteristics differ from those that typify volcanism in most other extensional areas. Erupted material in the HVZ is of the calc-alkali and high-alkali tholeiite series and shows no significant chemical changes over 5 Ma, except for an increase in K2O after 1.6 Ma. The net horizontal displacement along normal faults indicates that the HVZ widened by about 10%–20% across the graben at an average rate of 0.1 cm/yr. I interpret the HVZ to be neither a pull-apart structure of the pre-Tertiary basement nor the result of propagation of the Okinawa Trough, but rather the earliest stage of rifting when vertical subsidence caused by normal faulting is compensated by filling with volcanic material.  相似文献   

8.
New investigations of the geology of Crater Lake National Park necessitate a reinterpretation of the eruptive history of Mount Mazama and of the formation of Crater Lake caldera. Mount Mazama consisted of a glaciated complex of overlapping shields and stratovolcanoes, each of which was probably active for a comparatively short interval. All the Mazama magmas apparently evolved within thermally and compositionally zoned crustal magma reservoirs, which reached their maximum volume and degree of differentiation in the climactic magma chamber 7000 yr B.P.The history displayed in the caldera walls begins with construction of the andesitic Phantom Cone 400,000 yr B.P. Subsequently, at least 6 major centers erupted combinations of mafic andesite, andesite, or dacite before initiation of the Wisconsin Glaciation 75,000 yr B.P. Eruption of andesitic and dacitic lavas from 5 or more discrete centers, as well as an episode of dacitic pyroclastic activity, occurred until 50,000 yr B.P.; by that time, intermediate lava had been erupted at several short-lived vents. Concurrently, and probably during much of the Pleistocene, basaltic to mafic andesitic monogenetic vents built cinder cones and erupted local lava flows low on the flanks of Mount Mazama. Basaltic magma from one of these vents, Forgotten Crater, intercepted the margin of the zoned intermediate to silicic magmatic system and caused eruption of commingled andesitic and dacitic lava along a radial trend sometime between 22,000 and 30,000 yr B.P. Dacitic deposits between 22,000 and 50,000 yr old appear to record emplacement of domes high on the south slope. A line of silicic domes that may be between 22,000 and 30,000 yr old, northeast of and radial to the caldera, and a single dome on the north wall were probably fed by the same developing magma chamber as the dacitic lavas of the Forgotten Crater complex. The dacitic Palisade flow on the northeast wall is 25,000 yr old. These relatively silicic lavas commonly contain traces of hornblende and record early stages in the development of the climatic magma chamber.Some 15,000 to 40,000 yr were apparently needed for development of the climactic magma chamber, which had begun to leak rhyodacitic magma by 7015 ± 45 yr B.P. Four rhyodacitic lava flows and associated tephras were emplaced from an arcuate array of vents north of the summit of Mount Mazama, during a period of 200 yr before the climactic eruption. The climactic eruption began 6845 ± 50 yr B.P. with voluminous airfall deposition from a high column, perhaps because ejection of 4−12 km3 of magma to form the lava flows and tephras depressurized the top of the system to the point where vesiculation at depth could sustain a Plinian column. Ejecta of this phase issued from a single vent north of the main Mazama edifice but within the area in which the caldera later formed. The Wineglass Welded Tuff of Williams (1942) is the proximal featheredge of thicker ash-flow deposits downslope to the north, northeast, and east of Mount Mazama and was deposited during the single-vent phase, after collapse of the high column, by ash flows that followed topographic depressions. Approximately 30 km3 of rhyodacitic magma were expelled before collapse of the roof of the magma chamber and inception of caldera formation ended the single-vent phase. Ash flows of the ensuing ring-vent phase erupted from multiple vents as the caldera collapsed. These ash flows surmounted virtually all topographic barriers, caused significant erosion, and produced voluminous deposits zoned from rhyodacite to mafic andesite. The entire climactic eruption and caldera formation were over before the youngest rhyodacitic lava flow had cooled completely, because all the climactic deposits are cut by fumaroles that originated within the underlying lava, and part of the flow oozed down the caldera wall.A total of 51−59 km3 of magma was ejected in the precursory and climactic eruptions, and 40−52 km3 of Mount Mazama was lost by caldera formation. The spectacular compositional zonation shown by the climactic ejecta — rhyodacite followed by subordinate andesite and mafic andesite — reflects partial emptying of a zoned system, halted when the crystal-rich magma became too viscous for explosive fragmentation. This zonation was probably brought about by convective separation of low-density, evolved magma from underlying mafic magma. Confinement of postclimactic eruptive activity to the caldera attests to continuing existence of the Mazama magmatic system.  相似文献   

9.
Acid rain and ongoing eruptive activity at Rincón de la Vieja volcano in northwestern Costa Rica have created a triangular, deeply eroded “dead zone” west-southwest of the Active Crater. The barren, steep-walled canyons in this area expose one of the best internal stratigraphic profiles of any active or dormant volcano in Costa Rica. Geologic mapping along the southwestern flank of the volcano reveals over 300 m of prehistoric volcanic stratigraphy, dominated by tephra deposits and two-pyroxene andesite lavas. Dense tropical forests and poor access preclude mapping elsewhere on the volcano. In the “dead zone” four stratigraphic groups are distinguished by their relative proportions of lava and tephra. In general, early volcanism was dominated by voluminous lava emissions, with explosive plinian eruptions becoming increasingly more dominant with time. Numerous phreatic eruptions have occurred in historic times, all emanating from the Active Crater. The stratigraphic sequence is capped by the Río Blanco tephra deposit, erupted at approximately 3500 yr B.P. Approximately 0.25 km3 (0.1 km3 DRE) of tephra was deposited in a highly asymmetrical dispersal pattern west-southwest of the source vent, indicating strong prevailing winds from the east and east-northeast at the time of the eruption. Grain-size studies of the deposit reveal that the eruption was subplinian, attaining an estimated column height of 16 km. A qualitative hazards assessment at Rincón de la Vieja indicates that future eruptions are likely to be explosive in style, with the zone of greatest hazard extending several kilometers north from the Active Crater.  相似文献   

10.
Contrary to general belief, not all large igneous provinces (LIPs) are characterised by rocks of basaltic composition. Silicic-dominated LIPs, such as the Whitsunday Volcanic Province of NE Australia, are being increasingly recognised in the rock record. These silicic LIPs are consistent in being: (1) volumetrically dominated by ignimbrite; (2) active over prolonged periods (40–50 m.y.), based on available age data; and (3) spatially and temporally associated with plate break-up. This silicic-dominated LIP, related to the break-up of eastern continental Gondwana, is also significant for being the source of >1.4×106 km3 of coeval volcanogenic sediment preserved in adjacent sedimentary basins of eastern Australia.The Whitsunday Volcanic Province is volumetrically dominated by medium- to high-grade, dacitic to rhyolitic lithic ignimbrites. Individual ignimbrite units are commonly between 10 and 100 m thick, and the ignimbrite-dominated sequences exceed 1 km in thickness. Coarse lithic lag breccias containing clasts up to 6 m diameter are associated with the ignimbrites in proximal sections. Pyroclastic surge and fallout deposits, subordinate basaltic to rhyolitic lavas, phreatomagmatic deposits, and locally significant thicknesses of coarse-grained volcanogenic conglomerate and sandstone are interbedded with the ignimbrites. The volcanic sequences are intruded by gabbro/dolerite to rhyolite dykes (up to 50 m in width), sills and comagmatic granite. Dyke orientations are primarily from NW to NNE.The volcanic sequences are characterised by the interstratification of proximal/near-vent lithofacies such as rhyolite domes and lavas, and basaltic agglomerate, with medial to distal facies of ignimbrite. The burial of these near-vent lithofacies by ignimbrites, coupled with the paucity of mass wastage products such as debris-flow deposits indicates a low-relief depositional environment. Furthermore, the volcanic succession records a temporal change in: (1) eruptive styles; (2) the nature of source vents; and (3) erupted compositions. An early explosive dacitic pyroclastic phase was succeeded by a later mixed pyroclastic-effusive phase producing an essentially bimodal suite of lavas and rhyolitic ignimbrite. From the nature and distribution of volcanic lithofacies, the volcanic sequences are interpreted to record the evolution of a multiple vent, low-relief volcanic region, dominated by several large caldera centres.  相似文献   

11.
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   

12.
 The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y. Received: 24 June 1996 / Accepted: 6 December 1996  相似文献   

13.
The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3 s–1. After this peak, the effusion rate declined rapidly to 10–20 m3 s–1, then more slowly to 1 m3 s–1, and remained at 1–12 m3 s–1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.  相似文献   

14.
Volcán Alcedo is one of the seven western Galápagos shields and is the only active Galápagos volcano known to have erupted rhyolite as well as basalt. The volcano stands 4 km above the sea floor and has a subaerial volume of 200 km3, nearly all of which is basalt. As Volcán Alcedo grew, it built an elongate domal shield, which was partly truncated during repeated caldera-collapse and partial-filling episodes. An outward-dipping sequence of basalt flows at least 250 m thick forms the steepest (to 33°) flanks of the volcano and is not tilted; thus a constructional origin for the steep upper flanks is favored. About 1 km3 of rhyolite erupted late in the volcano's history from at least three vents and in 2–5 episodes. The most explosive of these produced a tephra blanket that covers the eastern half of the volcano. Homogeneous rhyolitic pumice is overlain by dacite-rhyolite commingled pumice, with no stratigraphic break. The tephra is notable for its low density and coarse grain size. The calculated height of the eruption plume is 23–30 km, and the intensity is estimated to have been 1.2x108 kg/s. Rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase. The age of the rhyolitic eruptions is 120 ka, on the basis of K-Ar ages. Between ten and 20 basaltic lava flows are younger than the rhyolites. Recent faulting resulted in a moat around part of the caldera floor. Alcedo most resently erupted sometime between 1946 and 1960 from its southern flank. Alcedo maintains an active, transient hydrothermal system. Acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional-scale earthquake.  相似文献   

15.
The Taupo Volcanic Zone forms part of the Taupo-Hikurangi subduction system, and comprises five volcanic centres: Tongariro, Taupo, Maroa, Okataina and Rotorua. Tongariro Volcanic Centre is formed almost entirely of andesite while the other four centres contain predominantly rhyolitic volcanics and later fissure eruptions of high-Al basalt. Estimated total volume of each lava type are as follows: 2 km3 of high-Al basalt (< 0.1%); 260 km3 of andesite (< 2.5%); 5 km3 of dacite (< 0.1%); > 10,000 km3 of rhyolite and ignimbrite (> 97.4%).The location of the andesites and vent alignments suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island-arc andesites such as those of Tonga; in particular by having higher contents of the alkali elements, light REE and Sr and Pb isotopes. This suggests some crustal contamination, and it is considered that this may occur beneath the wide accretionary prism of the subduction system. Amphibolite of the subduction zone will break down between 80 and 100 km and a partial melt will rise. A multi-stage process of magma genesis is then likely to occur. High-Al basalts are thought to be derived from partial melting of a garnet-free peridotite near the top of the mantle wedge overlying the subduction zone, locations of the vents controlled largely by faults within the crust. Rhyolites and ignimbrites were probably derived from partial melting of Mesozoic greywacke and argillite under the Taupo Volcanic Zone. Initial partial melting may have been due to hydration of the base of the crust; the “water” having come from dehydration of the downgoing slab. The partial melts would rise to form granodiorite plutons and final release of the magma to form rhyolites and ignimbrites was allowed because of extension within the Taupo graben.Dacites of the Bay of Plenty probably resulted from mixing of andesitic magma with small amounts of rhyolitic magma, but those on the eastern side of the Rotorua-Taupo area were more likely formed by a higher degree of partial melting of the Mesozoic greywacke-argillite basement. This may be due to intrusion of andesite magma on this side of the Taupo volcanic zone.  相似文献   

16.
Eighty-nine basaltic lava flows from the northwest wall of Haleakala caldera preserve a concatenated paleomagnetic record of portions of the Matuyama-Brunhes (M-B) reversal and the preceding Kamikatsura event as well as secular variation of the full-polarity reversed and normal geomagnetic field. They provide the most detailed volcanic record to date of the M-B transition. The 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles (VGPs) that move from reverse to normal along the Americas, concluding with an oscillation in the Pacific Ocean to a cluster of VGPs east of New Zealand and back finally to stable polarity in the north polar region. All but one of the 16 Kamikatsura VGPs cluster in central South America. The full-polarity flows, with 40Ar/39Ar ages spanning a total of 680 kyr, pass a reversal test and give an average VGP insignificantly different from the rotation axis, with standard deviation consistent with that for other 0-5 Ma lava flows of similar latitude. Precise 40Ar/39Ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775.6±1.9 and 900.3±4.7 ka for the M-B and Kamikatsura transitional flows, respectively. This Matuyama-Brunhes age is ∼16 kyr younger than ages for M-B flows from the Canary Islands, Tahiti and Chile that were dated using exactly the same techniques and standards, suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals.  相似文献   

17.
Six new 40Ar/39Ar and three cosmogenic 36Cl age determinations provide new insight into the late Quaternary eruptive history of Erebus volcano. Anorthoclase from 3 lava flows on the caldera rim have 40Ar/39Ar ages of 23 ± 12, 81 ± 3 and 172 ± 10 ka (all uncertainties 2σ). The ages confirm the presence of a second, younger, superimposed caldera near the southwestern margin of the summit plateau and show that eruptive activity has occurred in the summit region for 77 ± 13 ka longer than previously thought. Trachyte from “Ice Station” on the eastern flank is 159 ± 2 ka, similar in age to those at Bomb Peak and Aurora Cliffs. The widespread occurrences of trachyte on the eastern flank of Erebus suggest a major previously unrecognized episode of trachytic volcanism. The trachyte lavas are chemically and isotopically distinct from alkaline lavas erupted contemporaneously in the summit region < 5 km away.  相似文献   

18.
The volcanic rocks from the Hachimantai geothermal area, Northeast Japan, are divided into the following five groups: (1) rhyolite of the Tamagawa Welded Tuffs (about 500 km3, 2.0–1.4 Ma); (2) andesite related to the Tamagawa Welded Tuffs (about 10 km3, 1.5 Ma); (3) andesite of the Matsukawa Andesite (about 100 km3, 1.8-1.2 Ma); (4) dacite of the Kashinai Formation (about 100 km3, 1.1 Ma); and (5) andesite of the Young Volcanics (about 300 km3, younger than 1 Ma). ratios of less-altered samples of the five groups range from 0.7038 to 0.7046. There is no significant difference in ratios among the five groups in spite of differences in age and chemical composition. Therefore, fusion of or contamination by old acidic crust can be ruled out for the genesis of the voluminous acidic magma which produced the rhyolite of the Tamagawa Welded Tuffs and the dacite of the Kashinai Formation.The low ratios and the chemistry suggest two possibilities for the genesis of the acidic rocks. One is a low degree of partial melting of the basic crust, which has a low ratio, under high temperature. Another is a high degree of fractional crystallization from basic magma derived from the mantle.  相似文献   

19.
Among the youngest lava flows of the Pinacate volcanic field, Sonora, Mexico, is a large outpouring of tholeiite, the Ives flow. This tube-fed pahoehoe flow contrasts sharply with other Pinacate lavas in its great volume, alkali-poor composition and morphologic features, which include novel small structures named “spatter tubes.” Despite its K-poor character, young age, and the presence of excess 40Ar, we determined a 40Ar/39Ar age on samples of this flow at 13 ± 3 ka. Such an age determination is made possible via careful monitoring of the mass discrimination of the mass spectrometer and by stacking results from multiple incremental-heating experiments into a single, composite isochron. This age is among the youngest ever to be determined with such precision by the 40Ar/39Ar method on a K-poor tholeiite.  相似文献   

20.
Tungurahua, one of Ecuador's most active volcanoes, is made up of three volcanic edifices. Tungurahua I was a 14-km-wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series. Tungurahua II, mainly composed of acid andesite lava flows younger than 14,000 years BP, was partly destroyed by the last collapse event, 2955±90 years ago, which left a large amphitheater and produced a ∼8-km3 debris deposit. The avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for ∼15 km. A large lahar formed during this event, which was followed in turn by dacite extrusion. Southwestward, the damming of the Chambo valley by the avalanche deposit resulted in a ∼10-km-long lake, which was subsequently breached, generating another catastrophic debris flow. The eruptive activity of the present volcano (Tungurahua III) has rebuilt the cone to about 50% of its pre-collapse size by the emission of ∼3 km3 of volcanic products. Two periods of construction are recognized in Tungurahua's III history. From ∼2300 to ∼1400 years BP, high rates of lava extrusion and pyroclastic flows occurred. During this period, the magma composition did not evolve significantly, remaining essentially basic andesite. During the last ∼1300 years, eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition (andesite+dacite), and end with more basic andesite lava flows or crater plugs. This pattern is observed in the three historic eruptions of 1773, 1886 and 1916–1918. Given good age control and volumetric considerations, Tungurahua III growth's rate is estimated at ∼1.5×106 m3/year over the last 2300 years. Although an infrequent event, a sector collapse and associated lahars constitute a strong hazard of this volcano. Given the ∼3000 m relief and steep slopes of the present cone, a future collapse, even of small volume, could cover an area similar to that affected by the ∼3000-year-old avalanche. The more frequent eruptive episodes of each century, characterized by pyroclastic flows, lavas, lahars, as well as tephra falls, directly threaten 25,000 people and the Agoyan hydroelectric dam located at the foot of the volcano.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号