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1.
K. Jónasson 《Bulletin of Volcanology》1994,56(6-7):516-528
At the Krafla central volcano in north-east Iceland, two main phases of rhyolite volcanism are identified. The earlier phase (last interglacial) is related to the formation of a caldera, whereas the second phase (last glacial) is related to the emplacement of a ring dike. Subsequently, only minor amounts of rhyolite have been erupted. The volcanic products of Krafla are volumetrically bimodal. Geochemically, there is a series of basaltic to basalto-andesitic rocks and a cluster of rhyolitic rocks. Rocks of intermediate to silicic composition (icelandites and dacites) show clear signs of mixing. The rhyolites are Fe-rich (tholeiitic), and aphyric to slightly porphyritic (plagioclase, augite, pigeonite, fayalitic olivine and magnetite). They are minimum melts on the quartz-plagioclase cotectic plane in the granite system (Qz-Or-Ab-An). The rhyolites at Krafla were produced by near-solidus, rather than nearliquidus fractionation. They are interpreted as silicic minimum melts of hydrothermally altered crust, mainly of basaltic composition. They were primarily generated on the peripheries of an active basaltic magma chamber or intrusive domain, where sufficient volumes of crust were subjected to temperatures favorable for rhyolite genesis (850–950° C). The silicic melts were extracted crystal-free from their source in response to crustal deformation. 相似文献
2.
Bjrn Gunnarsson Bruce D. Marsh Hugh P. Taylor Jr. 《Journal of Volcanology and Geothermal Research》1998,83(1-2)
The Torfajökull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (225 km3). Within SW-Torfajökull, postglacial mildly alkalic to peralkalic silicic lavas and lava domes (67–74 wt.% SiO2) have erupted from a family of fissures 1–2.5 km apart within or just outside a large caldera (12×18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas are of low crystallinity (0–7 vol.%) and contain phenocrysts in the following order of decreasing abundance: plagioclase (An10-40), Na-rich anorthoclase (<Or23), clinopyroxene (Fs37-20), FeTi oxides (Usp32-60; Ilm93-88), hornblende (edenitic–ferroedenitic) and olivine (Fo22-37), with apatite, pyrrhotite and zircon as accessory phases. The phenocryst assemblage (0.2–4.0 mm) consistently exhibits pervasive disequilibrium with the host melt (glass). Xenoliths include sparse, disaggregated, and partially fused leucocratic fragments as well as amphibole-bearing rocks of broadly intermediate composition. The
values of the silicic lavas are in the range 3.6–4.4, and these are lower than the
values of comagmatic, contemporaneous basaltic extrusives within SW-Torfajökull, implying that the former can not be derived from the latter by simple fractional crystallization. FeTi-oxide geothermometry reveals temperatures as low as 750–800°C. To explain the fissure-dependent chemical variations,
depletions, low FeTi-oxide temperatures and pervasive crystal-melt disequilibrium, we propose the extraction and collection of small parcels of silicic melt from originally heterogeneous basaltic crustal rock through heterogeneous melting and wall rock collapse (solidification front instability, SFI). The original compositional heterogeneity of the source rock is due to (1) silicic segregations, in the form of pods and lenses characteristically formed in the upper parts of gabbroic intrusives, and (2) extreme isostatic subsidence of the earlier, less differentiated lavas of the Torfajökull central volcano. Ridge migration into older crustal terranes, coupled with establishment of concentrated volcanism at central volcanoes like Torfajökull due to propagating regional fissure swarms, supplies the heat source for this overall process. Continued magmatism in these fissures promotes extensive prograde heating of older crust and the progressive vitality and rise of the central volcano magmatic system that leads to, respectively, SFI and subsidence melting. The ensuing silicic melts (with relict crystals) are extracted, collected and extruded before reaching complete internal equilibrium. Chemically, this appears as a two-stage process of crystal fractionation. In general, the accumulation of high-temperature basaltic magmas at shallow depths beneath the Icelandic rift zones and major central volcanoes, coupled with unique tectonic conditions, allows large-scale reprocessing and recycling of the low-
, hydrothermally altered Icelandic crust. The end result is a compositionally bimodal proto-continental crust. 相似文献
3.
D. H. Richter E. J. Moll-Stalcup T. P. Miller M. A. Lanphere G. B. Dalrymple R. L. Smith 《Bulletin of Volcanology》1994,56(1):29-46
Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral
Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression
of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and
250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive
activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to
rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where
extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top
and a part of the south flank of the volcano, produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities
and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and
72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks,
but have higher MgO contents at a given SiO2, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range
of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small,
but significant, range in 87Sr/86Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites
and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence
of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages.
In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents
that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas.
Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic
andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by
fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial
melts of underlying volcanic rocks.
Received March 13, 1993/Accepted September 10, 1993 相似文献
4.
Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0–6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (δ18O, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6–5.3, 3.5–3.6, and 0–1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajökull volcano (Thórsmörk, ca. 54.5 ka) and Katla volcano (Sólheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K2O/SiO2>0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab–Q–Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nb and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sør Jan in the south of the island. 相似文献
5.
Tectonics — Between 15° and 13° N, Afar northern apex’ tectonics are determined essentially by sets of fractures with a NNW trend. This faulting is made of open tension fissures and normal faults, that form a graben with a narrow central trench. This trench is clearly visible over approx. 30 km NNW from lake Giulietti. It is then hidden below volcanic piles of the Erta Ale Range that mark the central trench. Further north the graben is concealed below very thick (several thousand m) evaporite deposits of the Salt Plain; but the central trench is still marked there by a line of varied accidents such as salt domes (including the potash dome of Dalol), phreatic explosion craters, and CO2 charged springs. North of the Salt Plain, the NNW trend is marked by the Mara’a-Alid Range. As a set-off, no lineaments have been observed in the field that could back the hypothesis of a N-S active zone (Wonji Fault Belt) being the most important feature of Afar Depression; this is definitively not the case, at least north of latitude 13°. Neither transverse transcurrent nor transform faults have been found. The assertion the huge scarp bordering Afar to the West being only an erosional feature superimposed over a large downwarp of the Ethiopian Plateau stratas is contradicted by several facts (actual normal faults, magnetic data, volcanoes close to the scarp, etc.). The conclusion of this first approach is that Northern Afar is a graben structure «en échelon» with the main Red Sea Rift, taking the place of the latter exactly at the latitude where it dies out. According to us, northern Afar is definitively a Red Sea structure and not, as previously proposed, a funnel shaped widening of the Main Ethiopian (Est African) Rilt. Volcanology — The volcanism of the Erta Ale Range is typically fissural in its southermost third; northwards shield volcanoes appear in the central part of the range; eventually, strato volcanoes, with trachytic and rhyolitic lavas, are heaped over the fissure basalts in the northern third of the chain. All the 7 volcanoes of the Range are active (either eruptive or fumarolic activity). The volcanoes over the 13th parallel are, from East to West, Barawli-Franca, a complexe acid center; Afdera, a dormant strato volcano; Alayta a big half fissure, half shield volcano; and Pierre Pruvost complex, with basaltic lava fields, a caldera strato volcano with related ignimbritic sheets, and a big cluster of active rhyolitic domes with rhyolitic lava flows. Peirology and magmatology — Samples analysed to date show five types of rocks outcropping in the surveyed area: 1) basalts; 2) andesine basalts; 3) dark trachytes; 4) oversatured trachytes; 5) soda rhyolites and pantellerites. In the Erta Ale range, acid rocks appear in the northern half and their quantity, relative to basic ones, increases northwards though remaining always quite subordinate. These acid rocks are always emitted by the central crater itself or through a nearby point. In the complex volcanoes of the 13th parallel lineament, trachytes and rhyolites are more generally concentrated on one side of the related basic strato-volcano. Current studies allow to detect the existence of an evolutive series of alkaline character: 1) initial fissure activity emits olivine alkali basalts; 2) a second stage is characterized by abundant andesine basalts; 3) a third stage generates either dark, femic trachytes, and (or) soda rhyolites. The inter-relationships between the basalt-dark trachytes series on the one hand and the oversatured trachytes — soda rhyolites series on the other hand, is one of the main problems of the northern Alar magmatology. 相似文献
6.
Volcán Las Navajas, a Pliocene-Pleistocene volcano located in the northwestern portion of the Mexican volcanic belt, erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite, and is the only volcano in mainland Mexico known to have erupted pantellerites. Las Navajas is located near the northwestern end of the Tepic-Zacoalco rift and covers a 200-m-thick pile of alkaline basaltic lavas, one of which has been dated at 4.3 Ma. The eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation. During the first stage a broad shield volcano made up of alkali basalts, mugearites, benmoreites, trachytes, and peralkaline rhyolites was constructed. Eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera. The second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls. This stage ended about 200 000 years ago with the eruption of pumice falls and ash flows, which led to the collapse of the southern portion of the volcano to form the second caldera. During the third stage, two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano. Finally, the calc-alkaline volcano Sanganguey was built on the southern flank of Las Lavajas. Alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along NW-trending fractures through the area. Although other mildly peralkaline rhyolites are found in the rift zones of western Mexico, only Las Navajas produced pantellerites. Greater volumes of basic alkaline magma have erupted in the Las Navajas region than in the other areas of peralkaline volcanism in Mexico, a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions. 相似文献
7.
The Western Volcanic Zone in Iceland (64.19° to 65.22° N) has the morphological characteristics of a distinct Mid-Atlantic ridge segment. This volcanic zone was mapped at a scale of 1:36.000, and 258 intraglacial monogenetic volcanoes from the Late Pleistocene (0.01–0.78?Ma) were identified and investigated. The zone is characterized by infrequent comparatively large volcanic eruptions and the overall volcanic activity appears to have been low throughout the Late Pleistocene. Tholeiitic basaltic rocks dominate in the Western Volcanic Zone with about 0.5?vol.?% of intermediate and silicic rocks. The basalts divide into picrites, olivine tholeiites, and tholeiites. Three main eruptive phases can be distinguished in the intraglacial volcanoes: an effusive deep-water lava phase producing basal pillow lavas, an explosive shallow-water phase producing hyaloclastites and an effusive subaerial capping lava phase. Three evolutionary stages therefore charcterize these volcanoes; late dykes and irregular minor intrusions could be added as the fourth main stage. These intrusions are potential heat sources for short-lived hydrothermal systems and may play an important role in the final shaping of the volcanoes. Substantial parts of the hyaloclastites of each unit are proximal sedimentary deposits. The intraglacial volcanoes divide into two main morphological groups, ridge-shaped volcanoes, i.e., tindars (including pillow lava ridges) and subrectangular volcanoes, i.e., tuyas and hyaloclastite or pillow lava mounds. The volume of the tuyas is generally much larger than that of the tindars. The largest tuya, Eiríksj?kull, is about 48?km3 and therefore the largest known monogenetic volcano in Iceland. Many of the large volcanoes, both tuyas and tindars, show a similar, systematic range in geochemistry. The most primitive compositions were erupted first and the magmas then changed to more differentiated compositions. The ridge-shaped tindars clearly erupted from volcanic fissures and the more equi-dimensional tuyas mainly from a single crater. It is suggested that the morphology and structure of the intraglacial volcanos mainly depends on two factors, (a) tectonic control and (b) availability of magma at the time of eruption. 相似文献
8.
This study aims to contribute a possible explanation for magma migration within volcanoes located in contractional tectonic settings, based on field data and physically-scaled experiments. The data demonstrate the occurrence of large stratovolcanoes in areas of coeval reverse faulting, in spite of the widely accepted idea that volcanism can develop only in extensional/transcurrent tectonic settings. The experiments simulate the propagation of deformation from substrate reverse faults with different attitudes and locations into volcanoes. The substrate fault splits into two main shear zones within the volcano: A shallow-dipping one, with reverse motion, propagates towards the lower volcano flank, and a steeper-dipping one, with normal motion, propagates upwards. In plan view, the reverse fault zone is arcuate and convex outwards, whereas the normal fault zone is rectilinear. Structural field surveys at volcanoes located in contractional settings show similar features: The Plio–Quaternary Trohunco and Los Cardos–Centinela volcanic complexes (Argentina) lie above Plio–Quaternary reverse faults. The Late Pleistocene–Holocene El Reventador volcano (Ecuador) is also located in a coeval contractional tectonic belt. These volcanoes show curvilinear reverse faults along one flank and rectilinear extensional fracture zones across the crater area, consistent with the experiments. These data consistently suggest that magma migrates along the substrate reverse fault and is channelled along the normal fault zone across the volcano. 相似文献
9.
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
10.
Hideo Hoshizumi Kozo Uto Kazunori Watanabe 《Journal of Volcanology and Geothermal Research》1999,89(1-4)
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
11.
Andreas Klügel Stefanie Schwarz Paul van den Bogaard Kaj A. Hoernle Cora C. Wohlgemuth-Ueberwasser Jana J. Köster 《Bulletin of Volcanology》2009,71(6):671-685
Ponta de São Lourenço is the deeply eroded eastern end of Madeira’s east–west trending rift zone, located near the geometric intersection of the Madeira rift axis with that of the Desertas Islands to the southeast. It dominantly consists of basaltic pyroclastic deposits from Strombolian and phreatomagmatic eruptions, lava flows, and a dike swarm. Main differences compared to highly productive rift zones such as in Hawai’i are a lower dike intensity (50–60 dikes/km) and the lack of a shallow magma reservoir or summit caldera. 40Ar/39Ar age determinations show that volcanic activity at Ponta de São Lourenço lasted from >5.2 to 4 Ma (early Madeira rift phase) and from 2.4 to 0.9 Ma (late Madeira rift phase), with a hiatus dividing the stratigraphy into lower and upper units. Toward the east, the distribution of eruptive centers becomes diffuse, and the rift axis bends to parallel the Desertas ridge. The bending may have resulted from mutual gravitational influence of the Madeira and Desertas volcanic edifices. We propose that Ponta de São Lourenço represents a type example for the interior of a fading rift arm on oceanic volcanoes, with modern analogues being the terminations of the rift zones at La Palma and El Hierro (Canary Islands). There is no evidence for Ponta de São Lourenço representing a former central volcano that interconnected and fed the Madeira and Desertas rifts. Our results suggest a subdivision of volcanic rift zones into (1) a highly productive endmember characterized by a central volcano with a shallow magma chamber feeding one or more rift arms, and (2) a less productive endmember characterized by rifts fed from deep-seated magma reservoirs rather than from a central volcano, as is the case for Ponta de São Lourenço. 相似文献
12.
Geochronology and magmatic character of the pliocene-pleistocene volcanism in Sardinia (Italy) 总被引：2，自引：0，他引：2
L. Beccaluva M. Deriu G. Macciotta C. Savelli G. Venturelli 《Bulletin of Volcanology》1977,40(3):153-168
The Pliocene-Pleistocene subaerial volcanic activity of the island of Sardinia developed from about five million years ago to the Pleistocene. Volcanism was mainly fissural, related to rifting of the Sardinian crustal block and connected to intraplate tensional tectonics involving at the same time the area of the Tyrrhenian Sea. Areally the most abundant rocks are basic, ranging in serial character from alkaline to subalkaline types. In some areas intermediate and salic lavas also occur; trachytic and phonolitic rocks are mainly associated with basalts of alkaline affinity, whereas rhyolites and dacites are mainly related to subalkalic basalts. K/Ar data show that lavas of different serial character (from alkalic to subalkalic) are produced on the island within the same time range, from about four to two million years; it is to be noted, however, that the early products (about 5 million years) are mildly alkalic in character whereas most of the youngest products (0.6–0.14 million years) are strongly alkalic. 相似文献
13.
The Erta Ale range is the main volcanic unit in the Danakil rift. It is located along the axis of the northern part of the Depression, in a zone clearly related to the Red Sea rift. The detailed study of the tectonics, volcanism and petrographic succession of the whole range allows one to draw the following conclusions.
- A compl3te, tectonically controlled evolution is observed in the subaerial volcanism from simple fissure activity, to small shield volcanoes built along open fissures, and to more complex central volcanoes. The emitted products form a complete differentiation series from basalts to rhyolites, with a remarkably regular volume decrease from basic to silicic terms.
- The close relationships existing between petrological and volcanological evolution suggest that the magma fractionation has been realized at relatively shallow depth, at the emplacement level of magmatic reservoirs. The importance of a time factor in the differentiation processes in volcanic conditions is clearly stressed.
- All collected data strongly support the subcrustal origin of the whole volcanic series and the hypothesis of crustal separation with formation of new oceanic crust along the axis of the Northern Danakil Depression.
14.
Bimodal volcanism at the Katla subglacial caldera,Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas 总被引：1，自引：0，他引：1
C. Lacasse H. Sigurdsson S. N. Carey H. Jóhannesson L. E. Thomas N. W. Rogers 《Bulletin of Volcanology》2007,69(4):373-399
The Katla subglacial caldera is one of the most active and hazardous volcanic centres in Iceland as revealed by its historical
volcanic activity and recent seismic unrest and magma accumulation. A petrologic and geochemical study was carried out on
a suite of mid-Pleistocene to Recent lavas and pyroclastic rocks originated from the caldera. The whole series is characterised
by a bimodal composition, including Fe-Ti transitional alkali basalts and mildly alkalic rhyolites. Variations in trace-element
composition amongst the basalts and rhyolites show that their chemical differentiation was mainly controlled by fractional
crystallisation and possible assimilation. The petrology and chemistry of the few intermediate extrusive rocks show that they
were derived from magma mingling or hybridisation. The absence of extrusive rocks of true intermediate magmatic composition
and the occurrence of amphibole-bearing felsic xenoliths support the hypothesis of partial melting of the hydrated basalt
crust as the main process leading to the generation of rhyolites. The 143Nd/144Nd and 87Sr/86Sr values of Katla volcanic rocks fit the general isotopic array defined by late Quaternary to Recent lavas from Iceland.
A few rock specimens are distinguished by low 143Nd/144Nd values suggesting assimilation and mixing of much older crustal material. Despite their similar whole-rock chemical compositions,
the postglacial rhyolitic extrusives differ from the felsic xenoliths by their glass composition and the absence of amphibole.
This, together with the general chemical trend of volcanic glasses, indicates that the postglacial rhyolitic extrusives were
probably derived by a process involving late reheating and partial melting of crustal material by intrusion of basaltic magmas. 相似文献
15.
16.
Tyatya Volcano, southwestern Kuril arc: Recent eruptive activity inferred from widespread tephra 总被引：2，自引：0，他引：2
MITSUHIRO NAKAGAWA YOSHIHIRO ISHIZUKA † TAKASHI KUDO MITSUHIRO YOSHIMOTO ‡ WATARU HIROSE YOSHIO ISHIZAKI NOBUO GOUCHI YOSHIO KATSUI ALEXANDER W. SOLOVYOW GENRIKH S. STEINBERG ARSLAN I. ABDURAKHMANOV 《Island Arc》2002,11(4):236-254
Abstract Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low‐ and medium‐K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and caad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short‐lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub‐Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km3), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions. 相似文献
17.
Mechanically, many volcanoes may be regarded as elastic inclusions, either softer (with a lower Young's modulus) or stiffer (with a higher Young's modulus) than the host-rock matrix. For example, many central volcanoes (stratovolcanoes, composite volcanoes) are composed of rocks that are softer than the crustal segments that host them. This is particularly clear in Iceland where central volcanoes are mostly made of soft rocks such as rhyolite, pyroclastics, hyaloclastites, and sediments whereas the host rock is primarily stiff basaltic lava flows. Most active central volcanoes also contain fluid magma chambers, and many have collapse calderas. Fluid magma chambers are best modelled as cavities (in three dimensions) or holes (in two dimensions), entire calderas as holes, and the ring faults themselves, which commonly include soft materials such as breccias, as soft inclusions. Many hyaloclastite (basaltic breccias) mountains partly buried in the basaltic lava pile also function as soft inclusions. Modelling volcanoes as soft inclusions or holes, we present three main numerical results. The first, using the hole model, shows the mechanical interaction between all the active central volcanoes in Iceland and, in particular, those forming the two main clusters at the north and south end of the East Volcanic Zone (EVZ). The strong indication of mechanical interaction through shared dykes and faults in the northern cluster of the EVZ is supported by observations. The second model, using a soft inclusion, shows that the Torfajökull central volcano, which contains the largest active caldera in Iceland, suppresses the spreading-generated tensile stress in its surroundings. We propose that this partly explains why the proper rift zone northeast of Torfajökull has not managed to propagate through the volcano. Apparently, Torfajökull tends to slow down the rate of southwest propagation of the rift-zone part of the EVZ. The third model, again using a soft inclusion, indicates how the lateral propagation of a segment of the 1783 Laki fissure became arrested in the slopes of the hyaloclastite mountain Laki. 相似文献
18.
Rift zones at the divergent plate boundary in Iceland consist of central volcanoes with swarms of fractures and fissures extending
away from them. Fissure swarms can display different characteristics, in accordance with their locations within the ∼50-km-wide
rift zones. To better discern the characteristics of fissure swarms, we mapped tectonic fractures and volcanic fissures within
the Kverkfj?ll volcanic system, which is located in the easternmost part of the Northern Volcanic Rift Zone (NVZ). To do this,
we used aerial photographs and satellite images. We find that rifting structures such as tectonic fractures, Holocene volcanic
fissures, and hyaloclastite ridges are unevenly distributed in the easternmost part of the NVZ. The Kverkfj?ll fissure swarm
extends 60 km north of the Kverkfj?ll central volcano. Holocene volcanic fissures are only found within 20 km from the volcano.
The Fjallgarear area, extending north of the Kverkfj?ll fissure swarm, is characterized by narrow hyaloclastite ridges indicating
subglacial volcanism. We suggest that the lack of fractures and Holocene volcanic fissures there indicates decreasing activity
towards the north in the easternmost part of the NVZ, due to increasing distance from the long-term spreading axis. We argue
that arcuate hyaloclastite ridges at the eastern boundary of the Northern Volcanic Rift Zone are mainly formed during deglaciations,
when three conditions may occur; firstly, eruption rate increases due to decompression of the mantle. Secondly, the high tensile
stresses accumulated during glaciations due to lack of magma supply may be relieved as magma supply increases during deglaciations.
Thirdly, faulting may occur during unloading due to differential movements between the thinner and younger Northern Volcanic
Rift Zone crust and the thicker and older crust to the east of it. 相似文献
19.
The south flank of Kilauea Volcano is unstable and has the structure of a huge landslide; it is one of at least 17 enormous catastrophic landslides shed from the Hawaiian Islands. Mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments. Slip on a low friction décollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest, or why this section contains a 300 km3 aseismic volume above the seismically defined décollement. It is proposed that this aseismic volume, adjacent to the caldera in the direction of flank slip, consists of olivine cumulates that creep outward, pushing the south flank seawards. Average primary Kilauea tholeiitic magma contains about 16.5 wt.% MgO compared with an average 10 wt.% MgO for erupted subaerial and submarine basalts. This difference requires fractionation of 17 wt.% (14 vol.%) olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates. Submarine-erupted Kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates. Deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas. The deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed, probably with as little as 5–10 vol.% intercumulus liquid, before entrainment of the xenocrysts. The olivine cumulates were at magmatic temperatures (>1100°C) when the xenocrysts were entrained. Olivine at 1100°C has a rheology similar to ice, and the olivine cumulates should flow down and away from the summit of the volcano. Flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank, leading to an extensional region that localizes deep intrusions behind the flank; these intrusions add to the seaward push. This mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and, therefore, limits catastrophic sector failure of Hawaiian volcanoes to their active growth phase, when the core of olivine cumulates is still hot enough to flow. 相似文献
20.
Seven Pliocene volcanoes, one of which is described in detail, occur in the northern part of the Kenya Rift. They have low-angle, shield like forms, and comprise lavas, pumice tuffs and ash-flow tuffs almost wholly of trachytic composition. Each volcano possesses a structurally complex source zone in which plugs, dykes and pumice tuffs are concentrated and in which clearly defined craters and calderas are uncommon. By contrast, the flank zones are stratiform with slopes of about 5° and are composed of lavas and ash-flow sheets erupted in a highly fluid condition. The volcanoes range up to 50 km in diameter and are elongated parallel to the general trend of the rift reflecting a tectonic control on the distribution of the vents and their products. This combination of morphological, structural and compositional features suggests that the volcanoes are of a type not described before. Notes on the petrography of the lavas are included and it is suggested that the trachytes are petrogenetically related to alkali basalts, compositionally similar to those which form the substrate to the trachyte volcanoes. 相似文献