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1.
Structures at calderas may form as a result of precursory tumescence, subsidence due withdrawal of magmatic support, resurgence, and regional tectonism. Structural reactivation and overprinting are common. To explore which types of structures may derive directly from subsidence without other factors, evidence is reviewed from pits caused by the melting of buried ice blocks, mining subsidence, scaled subsidence models, and from over 50 calderas. This review suggests that complex patterns of peripheral deformation, with multiple ring and arcuate fractures both inside and outside caldera rims, topographic embayments, arcuate graben, and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history. Downsag is a feature of many calderas and it does not indicate subsidence on an inward-dipping ring fault, as has been inferred previously. Where magmatic inflation is absent or slight, initial arcuate faults formed during collapse are likely to be multiple, and dip outwards to vertical. Associated downsag causes the peripheries of calderas undergo radial (centripetal) extension, and this accounts for some of the complex peripheral fractures, arcuate crevasses, graben, and some topographic moats. The structural boundary of a caldera, defined here as the outermost limits of subsidence and related deformation including downsag, commonly lies outside ring faults and outside the embayed topographic wall. It is likely to be funnel-shaped, i.e. inward-dipping, even though ring and arcuate fractures within it may dip outward. Inward-dipping arcuate normal faults at shallow levels and steep inward-dipping contacts between a caldera's fill and walls may both occur at a caldera that has initially subsided on outward-dipping ring faults. They arise due to peripheral surficial extension, gravitational spreading and scarp collapse. Topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag, respectively. These processes may occur in addition to surficial degradation of oversteep ring-fault scarps.  相似文献   

2.
 Diverse subsidence geometries and collapse processes for ash-flow calderas are inferred to reflect varying sizes, roof geometries, and depths of the source magma chambers, in combination with prior volcanic and regional tectonic influences. Based largely on a review of features at eroded pre-Quaternary calderas, a continuum of geometries and subsidence styles is inferred to exist, in both island-arc and continental settings, between small funnel calderas and larger plate (piston) subsidences bounded by arcuate faults. Within most ring-fault calderas, the subsided block is variably disrupted, due to differential movement during ash-flow eruptions and postcollapse magmatism, but highly chaotic piecemeal subsidence appears to be uncommon for large-diameter calderas. Small-scale downsag structures and accompanying extensional fractures develop along margins of most calderas during early stages of subsidence, but downsag is dominant only at calderas that have not subsided deeply. Calderas that are loci for multicyclic ash-flow eruption and subsidence cycles have the most complex internal structures. Large calderas have flared inner topographic walls due to landsliding of unstable slopes, and the resulting slide debris can constitute large proportions of caldera fill. Because the slide debris is concentrated near caldera walls, models from geophysical data can suggest a funnel geometry, even for large plate-subsidence calderas bounded by ring faults. Simple geometric models indicate that many large calderas have subsided 3–5 km, greater than the depth of most naturally exposed sections of intracaldera deposits. Many ring-fault plate-subsidence calderas and intrusive ring complexes have been recognized in the western U.S., Japan, and elsewhere, but no well-documented examples of exposed eroded calderas have large-scale funnel geometry or chaotically disrupted caldera floors. Reported ignimbrite "shields" in the central Andes, where large-volume ash-flows are inferred to have erupted without caldera collapse, seem alternatively interpretable as more conventional calderas that were filled to overflow by younger lavas and tuffs. Some exposed subcaldera intrusions provide insights concerning subsidence processes, but such intrusions may continue to evolve in volume, roof geometry, depth, and composition after formation of associated calderas. Received: 13 February 1997 / Accepted: 9 August 1997  相似文献   

3.
Ambrym Island has an unusually large, well-preserved basaltic caldera 13 km across. The caldera occurs in the central region of an early broad composite cone, which formed a north-south line with three smailer volcanoes. Alter the caldera was formed volcanism occurred within it and along fissure lines running nearly east-west. Two volcanic cones are active almost continuously and historic fissure cruptions have been recorded. The caldera formed by quiet subsidence, or by subsidence accompanied by eruption of scoria lappili similar to that erupted prior and subsequent to caldera formation. The collapse was at least 600 metres and radiocarbon dating suggests it took place less than 2000 years ago. The caldera is detined by gravity anomalies 10 to 14 milligals lower than those at its rim suggesting predominantly ash infilling. Aeromagnetic anomalies show a prominent. nearly east-west lineation, with normally magnetised bipole anomalies over the centre of the caldera and over fissure lines east of it. The source of the present volcanic activity is believed to be located along dyke fissures, with a perched magma chamber beneath the caldera. The geophysical evidence on Ambrym, together with that of regional east trending magnetic anomalies and recent bathymetric results, suggests that the volcanic activity is localised by the intersection of an east-west fracture zone with the axis of the New Hebrides island are.  相似文献   

4.
The Garibaldi Complex is one of a chain of predominantly silicic volcanic cones along the centre of the Main Ethiopian Rift, which form part of the Pleistocene-Recent Aden Series (Mohr, 1962a). The present form of the Complex is largely a result of silicic conebuilding episodes, between which ignimbrites were erupted and areas collapsed to form calderas, and to a lesser extent of recent basalt eruptions. Comparisons are made with other areas of caldera collapse and attention drawn to the possible relationship between caldera complexes and plutonic ring structures.  相似文献   

5.
The authors have visited Suswa, a complex caldera-volcano situated thirty miles north-west of Nairobi, a feature surprisingly neglected by geologists. While they do not pretend to do more than present an introductory account of the general geology of this unique volcano, they are able to augment the brief references of earlier workers, Gregory, Spink and Richard. The principal rock types are described in general terms, and are found to include unusual rhomb-porphyry types of lava, vitrophyres of phonolitic composition (closely related to the kenytes, but devoid of modal nepheline). The earliest eruptions were of quite normal lava type, phonolites of Kenya type, erupted over a wide area in central Kenya in Plio-Pleistocene times (not later than 1.7 m.y. ago), and the rhomb-porphyries are restricted to a secondary eruptive sequence, of probable Pleistocene age. There was a minor reactivation in recent times, represented by restricted, bare, fresh flows, of type at present unknown. Chemical analyses of representative specimens of the two major suites are provided, and are supported by modal analyses of related specimens. Two summit calderas have been recognised, both apparently subsidence structures related to cauldron subsidence in depth. The earlier and larger caldera covers about 40 square miles, and is interpreted as ofGlencoe type with weakly developedKrakatoan characteristics. The inner caldera covers seven square miles, and is interpreted as aGlencoe type structure: it is not a simple caldera but contains an island — block of four square miles extent — a feature which may perhaps be reasonably compared with island features within the Lake Toba cauldron, Sumatra and Nyamlagira caldera, Congo. The terminal eruptions of the first volcano seem to have largely stemmed from a ring feeder, analogous with a body reported from Crater Lake caldera, Oregon, U.S.A. The outer caldera is now partly obscured by products of later eruption, from a secondary cone eccentric to the first caldera — Ol Doinyo Nyukie — and from minor parasitic vents. Ol Doinyio Nyukie volcano possessed an axial pit-crater, nearly a mile in diameter, now transected by the boundary fault of the inner caldera: this might reasonably be regarded as a third,Kilauean, summit caldera, since it was apparently drained by low-level, adventive eruptions. Fumarolic activity is rife within Suswa at the present time: steam is being emitted, probably derived from meteoric water but charged with CO2 and probably nitrogen. Analogies between the Suswa pattern of calderas and certain lunar crater patterns are briefly mentioned.  相似文献   

6.
The active Karthala volcano is found on Grande Comore, the most westerly of four volcanic islands comprising the Comores Archipelago, between northern Madagascar and Mozambique. The caldera, roughly elliptical in outline, is 4 km long and 3 km wide, with outer walls around 100 m high. It is dominated by a large central pit crater, Chahale, which is 1300 m long, 800 m wide, and 300 m deep. A smaller cylindrical pit crater 250 m in diameter and 30 m deep, Changomeni, is found one km north of Chahale. The vertical walls of both pit craters show excellent sections of the ponded flows which form the caldera floor, and the minor faults and intrusions which affected these flows. The youngest lava on the island was produced on July 12th, 1965, as single aa basalt flow emitted from a fissure halfway between the two pit craters. Small fumaroles are still active on this flow, as well as in the pit craters and at several small cinder cones in the caldera. Alignment of pyroclastic cones and fissure eruptions forms a radial pattern centering on Chahale pit crater, suggesting that these radial fissures are locally controlled. Location of the caldera at the intersection of two regional fissure systems implies that its location is controlled by regional stresses. The present size and form of the caldera is a result of the coalescence of at least four smaller calderas. Although the visible walls of these smaller calderas do not show any outward dip, the theoretical considerations ofRobson andBarr (1964), if applicable, require that at depth these are outward-dipping ring dyke type of fractures.  相似文献   

7.
Aoba is a basalt volcano situated in the northern part of a chain containing all the active volcanoes in the New Hebrides. The chain extends the length of the New Hebrides. Growing from a depth of 2,400 meters on the sea floor, the volcano probably emerged above sea level in the late Pliocene or early Pleistocene. The age of the oldest exposed rocks is unknown. Relatively fluid lavas with autobrecciated surfaces probably issued from tissures, initiating a shield-building stage as the volcano emerged. Airfall pyroclastics increase towards the top of these lavas and are overlain by agglomerates marking a more explosive episode. Activity continued with the effusion of picrite basalt, accompanied by spasms of ash emission that formed crystal tuff. Subsequently a more explosive episode produced agglomerate and tuff with occasional tongues of lava. The two oval summit calderas are apparently related to deep-seated subsidence. Lack of pumice deposits, and the basic nature of the magma suggest that the foundering of the calderas was a quiet event, possibly due to massive outpourings of lava at a lower level, although a substantial volume also erupted from the summit volcanoes at this time. A broad pyroclastic cone, which was still growing 360 years ago, occupies the centre of the inner caldera. It is surmounted by a wide crater, or possibly small caldera, containing a lake in which palagonite tuff cones have formed. The western end of the inner caldera is occupied by an explosion crater, and the eastern end by a semicircular lake. A thermal area containing a solfatara on the southeast shore of the eastern lake, and staining in the crater lake suggestive of fumarole activity, are the only evidence of vulcanicity at the present time. It is difficult to correlate events at the centre of the volcano with those at the lateral fissures. Later episodes at the centre are probably broadly contemporaneous with activity along the fissures, the inner ends of which are mantled by younger deposits of the central volcano. Accumulation of material about this axial fiissure system, marked by no less than 64 cruptive foci, mainly spatter cones, and phreatic explosion craters where they intersect the coast, has extended the island to the northeast and southwest, producing the present oval shape. Numerous flows spilled from these fissures, the last reaching the sea at N’dui N’dui only 300 years ago according to local legend. Abundant ash was emitted from both the summit calderas and flank fissures at a late stage, forming a tuff mantle with layers of accretionary lapilli. The last volcanic event was the formation of a lahar which destoyed a village on the northeast slope of the volcano about 100 years ago. No consistent variation with time is evident in the composition of the magma, although plagiophyric and aphyric lava erupted during the later stages. All the rocks are basaltic, and differ only in the presence or absence of phenocryst-forming minerals, and the proportions in which they occur. Picrite basalt and ankaramite erupted from the central volcano and flank fissures, respectively.  相似文献   

8.
Lava flows spanning the eruptive record of Graciosa Island (Azores archipelago) and a gabbro xenolith were dated by 40Ar/39Ar in order to constrain the Pleistocene and Holocene volcanic evolution of the island. The results range from 1.05 Ma to 3.9 ka, whereas prior published K–Ar and 14C ages range from 620 to 2 ka. The formation of the Serra das Fontes shield volcano started at minimum 1.05 Ma, and the magmatic system was active for ca. 600 ky, as suggested by the formation of the gabbro xenolith by magmatic differentiation. Evolved magmas making up the Serra das Fontes–Serra Branca composite volcano were generated at ca. 450 ka. After a period of ca. 110 ky of volcanic inactivity and erosion of volcanic edifices, volcanism was reactivated with the formation of the Vitória Unit NW platform. Later, the development of the Vulcão Central Unit started with the formation of monogenetic cones located to the south of the Serra das Fontes–Serra Branca–Vitória Unit. This volcanism became progressively more evolved and was concentrated in a main eruptive center, forming the Vulcão Central stratovolcano with an age older than 50 ka. The caldera related to this stratovolcano is older than 47 ka and was followed by effusion of basaltic magmas into the caldera, resulting in the formation of a lava lake, which ultimately spilled over the caldera rim at ca. 11 ka. The most recent eruptions on Graciosa formed two small pyroclastic cones within the caldera and the Pico do Timão cone within the Vitória Unit at ca 3.9 ka.  相似文献   

9.
Erosion calderas: origins, processes, structural and climatic control   总被引：1，自引：0，他引：1  
 The origin and development of erosion-modified, erosion-transformed, and erosion-induced depressions in volcanic terrains are reviewed and systematized. A proposed classification, addressing terminology issues, considers structural, geomorphic, and climatic factors that contribute to the topographic modification of summit or flank depressions on volcanoes. Breaching of a closed crater or caldera generated by volcanic or non-volcanic processes results in an outlet valley. Under climates with up to ∼2000–2500 mm annual rainfall, craters, and calderas are commonly drained by a single outlet. The outlet valley can maintain its dominant downcutting position because it quickly enlarges its drainage basin by capturing the area of the primary depression. Multi-drained volcanic depressions can form if special factors, e.g., high-rate geological processes, such as faulting or glaciation, suppress fluvial erosion. Normal (fluvial) erosion-modified volcanic depressions the circular rim of which is derived from the original rim are termed erosion craters or erosion calderas, depending on the pre-existing depression. The resulting landform should be classed as an erosion-induced volcanic depression if the degradation of a cluster of craters produces a single-drained, irregular-shaped basin, or if flank erosion results in a quasi-closed depression. Under humid climates, craters and calderas degrade at a faster rate. Mostly at subtropical and tropical ocean-island and island-arc volcanoes, their erosion results in so-called amphitheater valleys that develop under heavy rainfall (>∼2500 mm/year), rainstorms, and high-elevation differences. Structural and lithological control, and groundwater in ocean islands, may in turn preform and guide development of high-energy valleys through rockfalls, landsliding, mudflows, and mass wasting. Given the intense erosion, amphitheater valleys are able to breach a primary depression from several directions and degrade the summit region at a high rate. Occasionally, amphitheater valleys may create summit depressions without a pre-existing crater or caldera. The resulting, negative landforms, which may drain in several directions and the primary origin of which is commonly unrecognizable, should be included in erosion-transformed volcanic depressions. Received: 4 January 1998 / Accepted: 18 January 1999  相似文献   

10.
Physical volcanology of the submarine Mariana and Volcano Arcs   总被引：17，自引：0，他引：17  
Narrow-beam maps, selected dredge samplings, and surveys of the Mariana and Volcano Arcs identify 42 submarine volcanos. Observed activity and sample characteristics indicate 22 of these to be active or dormant. Edifices in the Volcano Arc are larger than most of the Mariana Arc edifices, more irregularly shaped with numerous subsidiary cones, and regularly spaced at 50–70 km. Volcanos in the Mariana Arc tend to be simple cones. Sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110° counterclockwise from that trend, suggesting a strong fault control on the distribution of arc magmas. Volcanos in the Mariana Arc are generally developed west of the frontal arc ridge, on rifted frontal arc crust or new back-arc basin crust. Volcanos in the central Mariana Arc are usually subaerial, large (> 500 km3), and spaced about 50–70 km apart. Those in the northern and southern Marianas are largely submarine, closer together, and generally less than 500 km3 in volume. There is a shoaling of the arc basement around Iwo Jima, accompanied by the appearance of incompatible-element enriched lavas with alkalic affinities. The larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos. If the magma supply (estimated at 10–20 km3/km of arc per million years at 18° N) has been relatively constant along the Mariana Arc, we can infer a possible evolutionary sequence for arc volcanos from small, irregularly spaced edifices to large (over 1000 km3) edifices spaced at 50–70 km. The volcano distribution and basal depths are consistent with the hypothesis of back-arc propagation into the Volcano Arc.  相似文献   

11.
Geology of the peralkaline volcano at Pantelleria,Strait of Sicily   总被引：1，自引：1，他引：1  
Situated in a submerged continental rift, Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka. Its eruptive behavior, edifice morphologies, and complex, multiunit geologic history are representative of strongly peralkaline centers. It is dominated by the 6-km-wide Cinque Denti caldera, which formed ca. 45 Ka ago during eruption of the Green Tuff, a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls, surges, and pyroclastic flows. Soon after collapse, trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it. Progressive chemical evolution of the chamber between 45 and 18 Ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera. Beginning about 18 Ka ago, inflation of the chamber opened old ring fractures and new radial fractures, along which recently differentiated pantellerite constructed more than 25 pumice cones and shields. Continued uplift raised the northwest half of the intracaldera trachyte cone 275 m, creating the island's present summit, Montagna Grande, by trapdoor uplift. Pantellerite erupted along the trapdoor faults and their hingeline, forming numerous pumice cones and agglutinate sheets as well as five lava domes. Degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18-3-Ka episode led to entrainment of subjacent, crystal-rich, pantelleritic trachyte magma as crenulate inclusions. Progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0.82 km3 of magma erupted during the episode.The 45-Ka-old caldera is nested within the La Vecchia caldera, which is thought to have formed around 114 Ka ago. This older caldera was filled by three widespread welded units erupted 106, 94, and 79 Ka ago. Reactivation of the ring fracture ca. 67 Ka ago is indicated by venting of a large pantellerite centero and a chain of small shields along the ring fault. For each of the two nested calderas, the onset of postcaldera ring-fracture volcanism coincides with a low stand of sea level.Rates of chemical regeneration within the chamber are rapid, the 3% crystallization/Ka of the post-Green Tuff period being typical. Highly evolved pantellerites are rare, however, because intervals between major eruptions (averaging 13–6 Ka during the last 190 Ka) are short. Benmoreites and mugearites are entirely lacking. Fe-Ti-rich alkalic basalts have erupted peripherally along NW-trending lineaments parallel to the enclosing rift but not within the nested calderas, suggesting that felsic magma persists beneath them. The most recent basaltic eruption (in 1891) took place 4 km northwest of Pantelleria, manifesting the long-term northwestward migration of the volcanic focus. These strongly differentiated basalts reflect low-pressure fractional crystallization of partial melts of garnet peridotite that coalesce in small magma reservoirs replenished only infrequently in this continental rift environment.  相似文献   

12.
The Campi Flegrei caldera is a restless structure affected by general subsidence and ongoing resurgence of its central part. The persistent activity of the system and the explosive character of the volcanism lead to a very high volcanic hazard that, combined with intense urbanization, corresponds to a very high volcanic risk. One of the largest sources of uncertainty in volcanic hazard/risk assessment for Campi Flegrei is the spatial location of the future volcanic activity. This paper presents and discusses a long-term probability hazard map for vent opening in case of renewal of volcanism at the Campi Flegrei caldera, which shows the spatial conditional probability for the next vent opening, given that an eruption occurs. The map has been constructed by building a Bayesian inference scheme merging prior information and past data. The method allows both aleatory and epistemic uncertainties to be evaluated. The probability map of vent opening shows that two areas of relatively high probability are present within the active portion of the caldera, with a probability approximately doubled with respect to the rest of the caldera. The map has an immediate use in evaluating the areas of the caldera prone to the highest volcanic hazard. Furthermore, it represents an important ingredient in addressing the more general problem of quantitative volcanic hazards assessment at the Campi Flegrei caldera.  相似文献   

13.
The Vatukoula caldera is semi-elliptical in shape with the long axis trending north-easterly and occupies about 14 square miles of an undulating topographical basin located near the central north coast of Viti Levu, the largest island of the Fiji Group. The caldera formed when Tertiary basalts collapsed after prolonged explosion from a central vent area. The ensuing subsidence, which appears to have been cyclic, was accompanied by the deposition of andesitic volcanic material to form 5,000 to 7,000 feet of rhythmic tuffs, breccias and agglomerates partly under lacustrine conditions. The peripheral basalts were shattered during the stages of collapse forming a ring fault zone around the caldera. The depositional and subsidence stages were followed by an intrusive augite andesitic one from which extensive cone sheets formed in the caldera rocks. Radial and tangential dykes formed around the caldera in the peripheral basalts. After a time interval, the comparatively shallow central depression of the caldera received biotite andesitic pyroclastics and flows. Biotite andesite dykes followed a similar structural pattern to the augite andesitic ones. Finally, plug like bodies of porphyrite and monzonite intruded into the highly fractured zones, particularly the ring fault zone in the peripheral basalts. An important younger structural development with economic significance was the formation of a north-westerly shear system across the caldera. Flatly dipping structures formed in the peripheral basalts from the resettling of major blocks around the caldera. After the monzonite intrusions, epithermal mineralisers were liberated with economic amounts of gold in the form of telluride and auriferous pyrite. The mineralisers favoured the north-westerly shear system and, in the peripheral basalts, the accompanying flatly dipping structures. Thermal spring activity appears to mark the last phase of volcanicity.  相似文献   

14.
 The Woods Mountain volcanic center is a well-exposed, mildly alkaline volcanic center that formed during the Miocene in southeastern California. Detailed geologic mapping and geochemical studies have distinguished three major volcanic phases: precaldera, caldera forming, and postcaldera. Geologic mapping indicates that caldera formation occurred incrementally during eruptions of three large ignimbrites and continued into a period of voluminous intracaldera lava-flow eruptions. Rhyolitic ignimbrites and lava flows within the caldera are associated with large amplitude, circular gravity, and magnetic minima that are among the most prominent gravity and magnetic anomalies in southeastern California. Analysis of a Bouguer gravity anomaly map, reduced-to-the-pole magnetic intensity map, and three-dimensional gravity and magnetic models indicates that there is a single, funnel- to bowl-shaped caldera approximately 4 km thick and approximately 10 km wide at the surface. This model is consistent with other siliceous, pyroclastic-filled calderas on continental crust, except that most siliceous volcanic centers associated with more than one eruption are characterized by more than one caldera. Received: 20 December 1997 / Accepted: 15 October 1998  相似文献   

15.
Along the south coast of Arabia, between Aden and the southern entrance to the Red Sea, there are six central vent volcanoes of probable Pliocene age. All are characterised by the interstratification of basic and acidic extrusives, the formation of large central calderas at a late stage in the volcanic cycle and the subsequent infilling of these calderas with horizontal acidic ignimbrites and basic lavas. Lying 60 miles to the west of Aden and of particular interest is Jebel Khariz, the largest and best preserved of the six volcanic centres, covering a roughly circular area of about 100 square miles and rising to a height of 2,766 feet. The volcanic sequence of Jebel Khariz is broadly divisible into two suites: a) alkali-rich rhyolites and trachytes which occur as flows and pyroclastic horizons and form about 80 per cent of the volume of the cone, and b) effusives of basaltic composition that occur in the caldera, locally on the south-east and south-west flanks and in a small parasitic cone on the northern flank. The alkali-rich acidic suite includes lavas, ash-flow and ash-fall rocks as well as vent and flow breccias, Generally, all rocks of this suite have phenocrysts of anorthoclase, and may contain phenocrysts of fayalitic olivine, aegirine-augite, magnetite and/or quartz. The fine grained matrix is composed of the same minerals with skeletal riebeckite and, in some cases, cossyrite. The basaltic suite is characteristically porphyritic, the phenocrysts being of calcic plagioclase, clinopyroxene, olivine and magnetite in a fine-grained mesostasis of plagioclase, olivine, clinopyroxene and ore. The plagioclase, on initial investigation, appears to lie in the labradorite-bytownite range, the olivine is commonly replaced by iddingsite and the clinopyroxene is most commonly a pale mauve titanaugite. Near the centre of the volcanic pile, as exposed in the caldera wall, masses of rhyolitic composition can be seen to form over half of the volcanic sequence. These masses are markedly lenzoid in cross-section normal to the flow direction and display intricate flow folding; they are considered to have been extruded as viscous lava. Further from the volcanic centre, these acidic extrusives become less markedly lenzoid until in the distal areas of individual units, some 5 miles from the caldera, they have spread out to form sheet-like masses covering as much as 10 square miles to a uniform thickness rarely exceeding 25 feet. The presence of agglomeratic bases, hard compact central sections and less compact upper divisions, together with the ubiquitous presence of columnar jointing and occasional shard textures suggest that these distal parts of each extrusive unit have been formed by an ash-flow/ash-fall mechanism. It is postulated that the majority of the Jebel Khariz volcanic pile was formed by emission of acidic material, effusive in the central area, but deposited mainly by an ash-flow mechanism around the flanks of the cone. This could be due to either the synchronous eruption of viscous lava from the central vent with ash flow eruptions on the flanks; or, more probably, to the progression of an individual volcanic episode through an initial ash-flow phase followed by the effusion of viscous lava, all emanating from the central vent.  相似文献   

16.
The Christmas Mountains caldera complex developed approximately 42 Ma ago over an elliptical (8×5 km) laccolithic dome that formed during emplacement of the caldera magma body. Rocks of the caldera complex consist of tuffs, lavas, and volcaniclastic deposits, divided into five sequences. Three of the sequences contain major ash-flow tuffs whose eruption led to collapse of four calderas, all 1–1.5 km in diameter, over the dome. The oldest caldera-related rocks are sparsely porphyritic, rhyolitic, air-fall and ash-flow tuffs that record formation and collapse of a Plinian-type eruption column. Eruption of these tuffs induced collapse of a wedge along the western margin of the dome. A second, more abundantly porphyritic tuff led to collapse of a second caldera that partly overlapped the first. The last major eruptions were abundantly porphyritic, peralkaline quartz-trachyte ash-flow tuffs that ponded within two calderas over the crest of the dome. The tuffs are interbedded with coarse breccias that resulted from failure of the caldera walls. The Christmas Mountains caldera complex and two similar structures in Trans-Pecos Texas constitute a newly recognized caldera type, here termed a laccocaldera. They differ from more conventional calderas by having developed over thin laccolithic magma chambers rather than more deep-seated bodies, by their extreme precaldera doming and by their small size. However, they are similar to other calderas in having initial Plinian-type air-fall eruption followed by column collapse and ash-flow generation, multiple cycles of eruption, contemporaneous eruption and collapse, apparent pistonlike subsidence of the calderas, and compositional zoning within the magma chamber. Laccocalderas could occur else-where, particularly in alkalic magma belts in areas of undeformed sedimentary rocks.  相似文献   

17.
The formation of ring faults yields important implications for understanding the structural and dynamic evolution of collapse calderas and potentially associated ash-flow eruptions. Caldera collapse occurred in 2000 at Miyakejima Island (Japan) in response to a lateral intrusion. Based on geophysical data it is inferred that a set of caldera ring faults was propagating upward. To understand the kinematics of ring-fault propagation, linkage, and interaction, we describe new laboratory sand-box experiments that were analyzed through Digital Image Correlation (DIC) and post-processed using 2D strain analysis. The results help us gain a better understanding of the processes occurring during caldera subsidence at Miyakejima. We show that magma chamber evacuation induces strain localization at the lateral chamber margin in the form of a set of reverse faults that sequentially develops and propagates upwards. Then a set of normal faults initiates from tension fractures at the surface, propagating downwards to link with the reverse faults at depth. With increasing amounts of subsidence, interaction between the reverse- and normal-fault segments results in a deactivation of the reverse faults, while displacement becomes focused on the outer normal faults. Modeling results show that the area of faulting and collapse migrates successively outward, as peak displacement transfers from the inner ring faults to later developed outer ring faults. The final structural architecture of the faults bounding the subsiding piston-like block is hence a consequence of the amount of subsidence, in agreement with other caldera structures observed in nature. The experimental simulations provide an analogy to the observations and seismic records of caldera collapse at Miyakejima volcano, but are also applicable to caldera collapse in general.  相似文献   

18.
The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nuées ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego.The oldest caldera, C1 (7–8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3–3.5 km wide), ended the evolution of Nevado I.Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4–5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters.The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls.The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II–III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano.  相似文献   

19.
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   

20.
In the geological record, the intrusion of substantial amounts of magma into circumferential faults and ring fractures is commonly observed. Finite element modelling is used here to investigate the strain field that may be expected from such intrusive events. Two simple vertical scenarios are explored, one for a caldera with a central block of thickness to diameter ratio of ~1:1 (similar to Rabaul) and one with a ratio much less than 1:1 (similar to the Valles type). Surface deformation in both cases is similar with central uplift, the development of a moat (or trough) like feature just outside of the intersection of the azimuth of the intruded ring fault and the free surface, and broader scale tumescence at a scale several times larger than the calderas radius. The response of the block and sub-caldera magma chamber for the two scenarios, however, is different. The blocks are in effect squeezed; the high aspect ratio one deforms upwards at the surface and downwards at its base, whereas the low aspect ration one experiences up arching (or bending) of the central part of the caldera block. Central uplift still occurs when only a short arc of a ring fracture system or a circumferential fault is intruded. In both models, tumescence in the centre of the caldera from single ring dyke intrusion can only account for decimetres to metres of surface uplift. Repeated intrusions over tens to hundreds of thousands of years, however, may cause incremental up doming of the caldera block leading to larger scale resurgent features. The amount of uplift possible due to squeezing of a high aspect ratio block is limited. It is proposed, however, that where bending of plate-like blocks occur above a decompressible and/or malleable magma body, ring fault intrusion may be a significant contributor to resurgence. In the simple conceptual models shown here, the amount of ring dyke-induced central uplift will be >40–50% of the width of the ring complex. In the geological record the accumulation of intrusions into some ring fractures has led to annular or arcuate plutons of hundreds of meters to several kilometres in thickness. At certain calderas such intrusions may be a control on the marked concentration of uplift within the restricted area defined by the caldera faults. The complex nature of the horizontal displacements associated with the intrusion of ring and arcuate dykes is also explored. Intrusion into ring fracture zones will tend to take place into those sectors of the annular zone which are perpendicular to the least compressive stress vector. This may be a factor in the observed difference for caldera evolution in extensional and compressional areas. The unrest at several modern calderas is tentatively related to circumferential fault intrusion.Editorial responsibility: J. Stix  相似文献   

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