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1.
The geological evolution of Merapi volcano, Central Java, Indonesia   总被引：1，自引：0，他引：1  
Merapi is an almost persistently active basalt to basaltic andesite volcanic complex in Central Java (Indonesia) and often referred to as the type volcano for small-volume pyroclastic flows generated by gravitational lava dome failures (Merapi-type nuées ardentes). Stratigraphic field data, published and new radiocarbon ages in conjunction with a new set of 40K–40Ar and 40Ar–39Ar ages, and whole-rock geochemical data allow a reassessment of the geological and geochemical evolution of the volcanic complex. An adapted version of the published geological map of Merapi [(Wirakusumah et al. 1989), Peta Geologi Gunungapi Merapi, Jawa Tengah (Geologic map of Merapi volcano, Central Java), 1:50,000] is presented, in which eight main volcano stratigraphic units are distinguished, linked to three main evolutionary stages of the volcanic complex—Proto-Merapi, Old Merapi and New Merapi. Construction of the Merapi volcanic complex began after 170?ka. The two earliest (Proto-Merapi) volcanic edifices, Gunung Bibi (109?±?60?ka), a small basaltic andesite volcanic structure on Merapi’s north-east flank, and Gunung Turgo and Gunung Plawangan (138?±?3?ka; 135?±?3?ka), two basaltic hills in the southern sector of the volcano, predate the Merapi cone sensu stricto. Old Merapi started to grow at ~30?ka, building a stratovolcano of basaltic andesite lavas and intercalated pyroclastic rocks. This older Merapi edifice was destroyed by one or, possibly, several flank failures, the latest of which occurred after 4.8?±?1.5?ka and marks the end of the Old Merapi stage. The construction of the recent Merapi cone (New Merapi) began afterwards. Mostly basaltic andesite pyroclastic and epiclastic deposits of both Old and New Merapi (<11,792?±?90 14C years BP) cover the lower flanks of the edifice. A shift from medium-K to high-K character of the eruptive products occurred at ~1,900 14C years BP, with all younger products having high-K affinity. The radiocarbon record points towards an almost continuous activity of Merapi since this time, with periods of high eruption frequency interrupted by shorter intervals of apparently lower eruption rates, which is reflected in the geochemical composition of the eruptive products. The Holocene stratigraphic record reveals that fountain collapse pyroclastic flows are a common phenomenon at Merapi. The distribution and run-out distances of these flows have frequently exceeded those of the classic Merapi-type nuées ardentes of the recent activity. Widespread pumiceous fallout deposits testify the occurrence of moderate to large (subplinian) eruptions (VEI 3–4) during the mid to late Holocene. VEI 4 eruptions, as identified in the stratigraphic record, are an order of magnitude larger than any recorded historical eruption of Merapi, except for the 1872?AD and, possibly, the October–November 2010 events. Both types of eruptive and volcanic phenomena require careful consideration in long-term hazard assessment at Merapi.  相似文献   

2.
Monogenetic basaltic volcanoes are the most common volcanic landforms on the continents. They encompass a range of morphologies from small pyroclastic constructs to larger shields and reflect a wide range of eruptive processes. This paper reviews physical volcanological aspects of continental basaltic eruptions that are driven primarily by magmatic volatiles. Explosive eruption styles include Hawaiian and Strombolian (sensu stricto) and violent Strombolian end members, and a full spectrum of styles that are transitional between these end members. The end-member explosive styles generate characteristic facies within the resulting pyroclastic constructs (proximal) and beyond in tephra fall deposits (medial to distal). Explosive and effusive behavior can be simultaneous from the same conduit system and is a complex function of composition, ascent rate, degassing, and multiphase processes. Lavas are produced by direct effusion from central vents and fissures or from breakouts (boccas, located along cone slopes or at the base of a cone or rampart) that are controlled by varying combinations of cone structure, feeder dike processes, local effusion rate and topography. Clastogenic lavas are also produced by rapid accumulation of hot material from a pyroclastic column, or by more gradual welding and collapse of a pyroclastic edifice shortly after eruptions. Lava flows interact with — and counteract — cone building through the process of rafting. Eruption processes are closely coupled to shallow magma ascent dynamics, which in turn are variably controlled by pre-existing structures and interaction of the rising magmatic mixture with wall rocks. Locations and length scales of shallow intrusive features can be related to deeper length scales within the magma source zone in the mantle. Coupling between tectonic forces, magma mass flux, and heat flow range from weak (low magma flux basaltic fields) to sufficiently strong that some basaltic fields produce polygenetic composite volcanoes with more evolved compositions. Throughout the paper we identify key problems where additional research will help to advance our overall understanding of this important type of volcanism.  相似文献   

3.
The Croscat pyroclastic succession has been analysed to investigate the transition between different eruptive styles in basaltic monogenetic volcanoes, with particular emphasis on the role of phreatomagmatism in triggering Violent Strombolian eruptions. Croscat volcano, an 11 ka basaltic complex scoria cone in the Quaternary Garrotxa Volcanic Field (GVF) shows pyroclastic deposits related both to magmatic and phreatomagmatic explosions.Lithofacies analysis, grain size distribution, chemical composition, glass shard morphologies, vesicularity, bubble-number density and crystallinity of the Croscat pyroclastic succession have been used to characterize the different eruptive styles. Eruptions at Croscat began with fissural Hawaiian-type fountaining that rapidly changed to eruption types transitional between Hawaiian and Strombolian from a central vent. A first phreatomagmatic phase occurred by the interaction between magma and water from a shallow aquifer system at the waning of the Hawaiian- and Strombolian-types stage. A Violent Strombolian explosion then occurred, producing a widespread (8 km2), voluminous tephra blanket. The related deposits are characterized by the presence of wood-shaped, highly vesicular scoriae. Glass-bearing xenoliths (buchites) are also present within the deposit. At the waning of the Violent Strombolian phase a second phreatomagmatic phase occurred, producing a second voluminous deposit dispersed over 8.4 km2. The eruption ended with a lava flow emission and consequent breaching of the western-side of the volcano. Our data suggest that the Croscat Violent Strombolian phase was related to the ascent of deeper, crystal-poor, highly vesicular magma under fast decompression rate. Particles and vesicles elongation and brittle failure observed in the wood-shaped clasts indicate that fragmentation during Violent Strombolian phase was enhanced by high strain-rate of the magma within the conduit.  相似文献   

4.
Measurements of the sulfur dioxide (SO2) emission rate from three Guatemalan volcanoes provide data which are consistent with theoretical and laboratory studies of eruptive and shallow magma chamber processes. In particular, unerupted magma makes a major contribution to the measured SO2 emission rates at Santiaguito, a continuously erupting dacitic volcanic dome. Varying shallow magma convection rates can explain the variations in SO2 emission rates at Santiaguito. At Fuego, a basaltic volcano currently in repose, SO2 emission rate measurements are consistent with a high level magma body that is crystallizing and releasing volatiles. At Pacaya, a continuously erupting basaltic volcano, recent SO2 emission rate measurements support laboratory simulation studies of strombolian eruptions; these studies indicate that the majority of gas escapes during eruptions and little gas escapes between eruptions.Average SO2 emission rates over the last 20 years for Santiaguito, Fuego and Pacaya are 80, 160 and 260 Mg/d, respectively. On a global scale, these three volcanoes account for 1% of the annual global volcanic output of SO2. Santiaguito and Pacaya, together, emit 6% of the total annual SO2 emitted by continuously erupting volcanoes.Even though SO2 measurements at these volcanoes have been made infrequently and by different investigators, the collective data help to establish a useful baseline by which to judge future changes. A more complete record of SO2 emission rates from these volcanoes could lead to a better understanding of their eruption mechanisms and reduce the impact of their future eruptions on Guatemalan society.  相似文献   

5.
The small- to moderate-volume, Quaternary, Siwi pyroclastic sequence was erupted during formation of a 4 km-wide caldera on the eastern margin of Tanna, an island arc volcano in southern Vanuatu. This high-potassium, andesitic eruption followed a period of effusive basaltic andesite volcanism and represents the most felsic magma erupted from the volcano. The sequence is up to 13 m thick and can be traced in near-continuous outcrop over 11 km. Facies grade laterally from lithic-rich, partly welded spatter agglomerate along the caldera rim to two medial, pumiceous, non-welded ignimbrites that are separated by a layer of lithic-rich, spatter agglomerate. Juvenile clasts comprise a wide range of densities and grain sizes. They vary between black, incipiently vesicular, highly elongate spatter clasts that have breadcrusted pumiceous rinds and reach several metres across to silky, grey pumice lapilli. The pumice lapilli range from highly vesicular clasts with tube or coalesced spherical vesicles to denser finely vesicular clasts that include lithic fragments.Textural and lithofacies characteristics of the Siwi pyroclastic sequence suggest that the first phase of the eruption produced a base surge deposit and spatter-poor pumiceous ignimbrite. A voluminous eruption of spatter and lithic pyroclasts coincided with a relatively deep withdrawal of magma presumably driven by a catastrophic collapse of the magma chamber roof. During this phase, spatter clasts rapidly accumulated in the proximal zone largely as fallout, creating a variably welded and lithic-rich agglomerate. This phase was followed by the eruption of moderately to highly vesiculated magma that generated the most widespread, upper pumiceous ignimbrite. The combination of spatter and pumice in pyroclastic deposits from a single eruption appears to be related to highly explosive, magmatic eruptions involving low-viscosity magmas. The combination also indicates the coexistence of a spatter fountain and explosive eruption plume for much of the eruption.Editorial responsibility: R. Cioni  相似文献   

6.
Scoria cones are common volcanic features and are thought to most commonly develop through the deposition of ballistics produced by gentle Strombolian eruptions and the outward sliding of talus. However, some historic scoria cones have been observed to form with phases of more energetic violent Strombolian eruptions (e.g., the 1943–1952 eruption of Parícutin, central Mexico; the 1975 eruption of Tolbachik, Kamchatka), maintaining volcanic plumes several kilometers in height, sometimes simultaneous with active effusive lava flows. Geologic evidence shows that violent Strombolian eruptions during cone formation may be more common than is generally perceived, and therefore it is important to obtain additional insights about such eruptions to better assess volcanic hazards. We studied Irao Volcano, the largest basaltic monogenetic volcano in the Abu Monogenetic Volcano Group, SW Japan. The geologic features of this volcano are consistent with a violent Strombolian eruption, including voluminous ash and fine lapilli beds (on order of 10?1 km3 DRE) with simultaneous scoria cone formation and lava effusion from the base of the cone. The characteristics of the volcanic products suggest that the rate of magma ascent decreased gradually throughout the eruption and that less explosive Strombolian eruptions increased in frequency during the later stages of activity. During the eruption sequence, the chemical composition of the magma became more differentiated. A new K–Ar age determination for phlogopite crystallized within basalt dates the formation of Irao Volcano at 0.4?±?0.05 Ma.  相似文献   

7.
We report the stratigraphic sequence of the 2005 eruption of Ilamatepec volcano together with sedimentological and chemical analyses of its products.Structural and textural characteristics of the deposits indicate that the eruption was driven by a small-volume rhyolitic intrusion at shallow levels, which resulted first in the collapse of the existing hydrothermally altered fan of previous deposits inside the crater lake, driving phreatic explosions with launching of blocks on ballistic trajectories; later the magma interacted with lake waters producing several hydromagmatic pyroclastic density currents (PDCs). These flows were energetic enough to knock down pine trees up to distances of 1.8 km from the crater in the E-NE sector of the volcano. Finally, ejection of ballistic blocks that landed on previously emplaced, wet pyroclastic density current deposits, caused the generation of a lahar that flowed down the steep eastern flank toward the El Jabillal gully. Subsequent lahars occurred as a result of intense rain caused by hurricane Stan.Radiocarbon ages on paleosols and charcoal fragments, separating previous volcanogenic sequences, indicate that similar eruptions have occurred more frequently in the past centuries, than previously thought.The new data confirms that Ilamatepec volcano is one of the most active volcanoes in El Salvador. Nevertheless, more detailed studies of the eruptive sequence of Ilamatepec volcano are mandatory to establish future eruptive patterns.  相似文献   

8.
Within the neovolcanic zones of Iceland many volcanoes grew upward through icecaps that have subsequently melted. These steep-walled and flat-topped basaltic subglacial volcanoes, called tuyas, are composed of a lower sequence of subaqueously erupted, pillowed lavas overlain by breccias and hyaloclastites produced by phreatomagmatic explosions in shallow water, capped by a subaerially erupted lava plateau. Glass and whole-rock analyses of samples collected from six tuyas indicate systematic variations in major elements showing that the individual volcanoes are monogenetic, and that commonly the tholeiitic magmas differentiated and became more evolved through the course of the eruption that built the tuya. At Herdubreid, the most extensively studies tuya, the upward change in composition indicates that more than 50 wt.% of the first erupted lavas need crystallize over a range of 60°C to produce the last erupted lavas. The S content of glass commonly decreases upward in the tuyas from an average of about 0.08 wt.% at the base to < 0.02 wt.% in the subaerially erupted lava at the top, and is a measure of the depth of water (or ice) above the eruptive vent. The extensive subsurface crystallization that generates the more evolved, lower-temperature melts during the growth of the tuyas, apparently results from cooling and degassing of magma contained in shallow magma chambers and feeders beneath the volcanoes. Cooling may result from percolation of meltwater down cracks, vaporization, and cycling in a hydrothermal circulation. Degassing occurs when progressively lower pressure eruption (as the volcanic vent grows above the ice/water surface) lowers the volatile vapour pressure of subsurface melt, thus elevating the temperature of the liquidus and hastening liquid-crystal differentiation.  相似文献   

9.
Merapi Volcano (Central Java, Indonesia) has been frequently active during Middle to Late Holocene time producing basalts and basaltic andesites of medium-K composition in earlier stages of activity and high-K magmas from 1900 14C yr BP to the present. Radiocarbon dating of pyroclastic deposits indicates an almost continuous activity with periods of high eruption rates alternating with shorter time spans of distinctly reduced eruptive frequency since the first appearance of high-K volcanic rocks. Geochemical data of 28 well-dated, prehistoric pyroclastic flows of the Merapi high-K series indicate systematic cyclic variations. These medium-term compositional variations result from a complex interplay of several magmatic processes, which ultimately control the periodicity and frequency of eruptions at Merapi. Low eruption rates and the absence of new influxes of primitive magma from depth allow the generation of basaltic andesite magma (56–57 wt% SiO2) in a small-volume magma reservoir through fractional crystallisation from parental mafic magma (52–53 wt% SiO2) in periods of low eruptive frequency. Magmas of intermediate composition erupted during these stages provide evidence for periodic withdrawal of magma from a steadily fractionating magma chamber. Subsequent periods are characterised by high eruption rates that coincide with shifts of whole-rock compositions from basaltic andesite to basalt. This compositional variation is interpreted to originate from influxes of primitive magma into a continuously active magma chamber, triggering the eruption of evolved magma after periods of low eruptive frequency. Batches of primitive magma eventually mix with residual magma in the magmatic reservoir to decrease whole-rock SiO2 contents. Supply of primitive magma at Merapi appears to be sufficiently frequent that andesites or more differentiated rock types were not generated during the past 2000 years of activity. Cyclic variations also occurred during the recent eruptive period since AD 1883. The most recent eruptive episode of Merapi is characterised by essentially uniform magma compositions that may imply the existence of a continuously active magma reservoir, maintained in a quasi-steady state by magma recharge. The whole-rock compositions at the upper limit of the total SiO2 range of the Merapi suite could also indicate the beginning of another period of high eruption rates and shifts towards more mafic compositions.  相似文献   

10.
The late Pleistocene San Venanzo maar and nearby Pian di Celle tuff ring in the San Venanzo area of Umbria, central Italy, appear to represent different aspects of an eruptive cycle accompanied by diatreme formation. Approximately 6x106 m3 of mostly lapillisized, juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted. The stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions. The pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion, roughly stratified lapilli tuff deposited by concentrated pyroclastic surge, chaotic scoriaceous pyroclastic flow and inverse graded grain-flow deposits. The key feature of the pyroclastics is the presence of concentric-shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits. The rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite, phonolite and calcitic melilitite pyroclasts. Juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin. Pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds. The San Venanzo maar-forming event is believed to have been trigered by fluid-rich carbonatite-phonolite magma. The eruptive centre the moved to the Pian di Celle tuff ring, where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area. In both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of CO2 which, flowing through the diatremes, produced the peculiar intrusive pyroclastic facies and triggered explosions.  相似文献   

11.
The 274 ka “Basalt-Trachytic Tuff of Tuoripunzoli” (TBTT) from Roccamonfina volcano (Roman Region, Italy) consists of a basaltic scoria lapilli fall (Unit A) overlain by a trachytic sequence formed by a surge (Unit B), repetitive pumice lapilli and ash-rich layers both of fallout origin (Unit C) and a pyroclastic flow deposit (Unit D). The TBTT is widespread (40 km2) in the northern sector of the volcano, but limited to a small area on the southern slopes of the main cone. Interpolation between the northern deposits and the latter one yields a minimum depositional area of 123 km2, and an approximate bulk volume of 0.2-0.3 km3. Isopach and isopleth maps are consistent with a source vent within the main caldera of Roccamonfina.Unit A shows a fairly good sorting and a moderate grain size; glass fragments are cuspate and vesicular. Unit B is fine grained and poorly sorted; shards are blocky and nonvesicular. Pumice lapilli of Unit C are moderately sorted and moderately coarse grained. Glass shards are equant and vesicular. Lithic clasts are strongly comminuted to submillimetric sizes. By contrast, the ash-rich internal divisions are very fine grained and poorly sorted. They consist of a mixture of equant shards which are prevailingly blocky and poorly vesicular. Unit D is a massive, poorly sorted, moderately coarse-grained deposit. Glass fragments are nearly equant and slightly or nonvesicular.The TBTT is interpreted as due to eruption of a basaltic magma followed in rapid succession by one trachyte magma. Unit A formed by Subplinian fallout of a moderate, purely magmatic column. Interaction between a trachyte magma and water resulted in eruption of surge Unit B. A high-standing eruption column erupted alternating fallout pumice lapilli and fallout ashes. Pumice lapilli originated prevailingly from the inner part of the eruption column, whereas magma-water interaction on the external parts of the column resulted in ash fallout. The uppermost pyroclastic flow Unit D is interpreted as due to final collapse of the eruption column.  相似文献   

12.
A new stratigraphy for bimodal Oligocene flood volcanism that forms the volcanic plateau of northern Yemen is presented based on detailed field observations, petrography and geochemical correlations. The >1 km thick volcanic pile is divided into three phases of volcanism: a main basaltic stage (31 to 29.7 Ma), a main silicic stage (29.7 to 29.5 Ma), and a stage of upper bimodal volcanism (29.5 to 27.7 Ma). Eight large-volume silicic pyroclastic eruptive units are traceable throughout northern Yemen, and some units can be correlated with silicic eruptive units in the Ethiopian Traps and to tephra layers in the Indian Ocean. The silicic units comprise pyroclastic density current and fall deposits and a caldera-collapse breccia, and they display textures that unequivocally identify them as primary pyroclastic deposits: basal vitrophyres, eutaxitic fabrics, glass shards, vitroclastic ash matrices and accretionary lapilli. Individual pyroclastic eruptions have preserved on-land volumes of up to ∼850 km3. The largest units have associated co-ignimbrite plume ash fall deposits with dispersal areas >1×107 km2 and estimated maximum total volumes of up to 5,000 km3, which provide accurate and precisely dated marker horizons that can be used to link litho-, bio- and magnetostratigraphy studies. There is a marked change in eruption style of silicic units with time, from initial large-volume explosive pyroclastic eruptions producing ignimbrites and near-globally distributed tuffs, to smaller volume (<50 km3) mixed effusive-explosive eruptions emplacing silicic lavas intercalated with tuffs and ignimbrites. Although eruption volumes decrease by an order of magnitude from the first stage to the last, eruption intervals within each phase remain broadly similar. These changes may reflect the initiation of continental rifting and the transition from pre-break-up thick, stable crust supporting large-volume magma chambers, to syn-rift actively thinning crust hosting small-volume magma chambers.Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

13.
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   

14.
New tephro-stratigraphic studies of the Tongariro Volcanic Centre (TgVC) on the North Island (New Zealand) allowed reconstruction of some of the largest, andesitic, explosive eruptions of Mt. Ruapehu. Large eruptions were common in the Late Pleistocene, before a transition to strombolian-vulcanian and phreatomagmatic eruptive styles that have predominated over the past 10,000?years. Considering this is the most active volcano in North Island of New Zealand and the uppermost hazard limits are unknown, we identified and mapped the pyroclastic deposits corresponding to the five largest eruptions since ~27?ka. The selected eruptive units are also characterised by distinctive lithofacies associations correlated to different behaviours of the eruptive column. In addition, we clarify the source of the ~10–9.7?ka Pahoka Tephra, identified by previous authors as the product of one of the largest eruptions of the TgVC. The most common explosive eruptions taking place between ~13.6 and ~10?ka?cal?years BP involved strongly oscillating, partially collapsing eruptive columns up to 37?km high, at mass discharge rates up to 6?×?108?kg/s and magnitudes of 4.9, ejecting minimum estimated volumes of 0.6?km3. Our results indicate that this volcano (as well as the neighbouring andesitic Mt. Tongariro) can generate Plinian eruptions similar in magnitude to the Chaitén 2008 and Askja 1875 events. Such eruptions would mainly produce pyroclastic fallout covering a minimum area of 1,700?km2 ESE of the volcano, where important touristic, agricultural and military activities are based. As for the 1995/1996 eruption, our field data indicate that complex wind patterns were critical in controlling the dispersion of the eruptive clouds, developing sheared, commonly bilobate plumes.  相似文献   

15.
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   

16.
Following an increase of eruptive activity at Stromboli summit craters in February 2004, we promptly carried out SEM-EDS microanalyses and textural observations on samples of lapilli to check the possible occurrence of Low Porphyritic magma (LP magma), a forerunner of hazardous paroxysmal eruptions. The acquired results suggest that all erupted glasses belong to the High Porphyritic magma (HP magma), which characterizes the typical mild explosive activity of the volcano.  相似文献   

17.
Cladistics is a systematic method of classification that groups entities on the basis of sharing similar characteristics in the most parsimonious manner. Here cladistics is applied to the classification of volcanoes using a dataset of 59 Quaternary volcanoes and 129 volcanic edifices of the Tohoku region, Northeast Japan. Volcano and edifice characteristics recorded in the database include attributes of volcano size, chemical composition, dominant eruptive products, volcano morphology, dominant landforms, volcano age and eruptive history. Without characteristics related to time the volcanic edifices divide into two groups, with characters related to volcano size, dominant composition and edifice morphology being the most diagnostic. Analysis including time based characteristics yields four groups with a good correlation between these groups and the two groups from the analysis without time for 108 out of 129 volcanic edifices. Thus when characters are slightly changed the volcanoes still form similar groupings. Analysis of the volcanoes both with and without time yields three groups based on compositional, eruptive products and morphological characters. Spatial clusters of volcanic centres have been recognised in the Tohoku region by Tamura et al. (Earth Planet Sci Lett 197:105–106, 2002). The groups identified by cladistic analysis are distributed unevenly between the clusters, indicating a tendency for individual clusters to form similar kinds of volcanoes with distinctive but coherent styles of volcanism. Uneven distribution of volcano types between clusters can be explained by variations in dominant magma compositions through time, which are reflected in eruption products and volcanic landforms. Cladistic analysis can be a useful tool for elucidating dynamic igneous processes that could be applied to other regions and globally. Our exploratory study indicates that cladistics has promise as a method for classifying volcanoes and potentially elucidating dynamic and evolutionary volcanic processes. Cladistics may also have utility in hazards assessment where spatial distributions and robust definitions of a volcano are important, as in locating sensitive facilities such as nuclear reactors and repositories.  相似文献   

18.
We studied the distribution of tephra deposits discharged by the basaltic (52–54% SiO2) explosive eruption of 1973 on Tyatya Volcano (Kunashir I., Kuril Islands). We made maps showing lines of equal tephra thickness (isopachs) and lines of maximum size of pyroclastic particles (isopleths). These data were used to find the parameters of explosive activity using the standard techniques for each of the two phases of this eruption separately. The first, phreatomagmatic, phase discharged 0.008 km3 of tephra during the generation of maars on the volcano’s northern slope. The tephra mostly consisted of fragmented host rocks with admixtures of fragments of low vesiculated juvenile basalt. The phase lasted 20 hours, the rate of pyroclastic discharge was 2 × 105 kg/s; the eruptive plume reached heights of 4–6 km with wind speeds within 10 m/s. The second, magmatic, phase discharged 0.07 km3 of tephra during the generation of the Otvazhnyi scoria cone on the volcano’s southeastern slope. The tephra mostly consisted of juvenile basaltic scoria. The highly explosive Plinian part of this phase lasted 36 hours, the rate of pyroclastic discharge was 8 × 105 kg/s; the eruptive plume reached heights of 6–8 km with wind speeds of 10–20 m/s. The total tephra volume discharged by the eruption was approximately 0.08 km3; the total amount of ejected pyroclastic material (including the resulting monogenic edifices) was 0.11 km3; the volume of erupted magma was 0.05 km3 (the conversion was based on 2800 kg/m3 density); the volcanic explosivity index, or VEI, was 3. The production rate of the Tyatya plumbing system is estimated as 3 × 105 m3 magma per annum.  相似文献   

19.
Fuji volcano is the largest active volcano in Japan, and consists of Ko-Fuji and Shin-Fuji volcanoes. Although basaltic in composition, small-volume pyroclastic flows have been repeatedly generated during the Younger stage of Shin-Fuji volcano. Deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1,300 and 2,000 m a.s.l., and have been stratigraphically divided into the Shin-Fuji Younger pyroclastic flows (SYP) 1 to 4. Downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits. The new14C ages for SYP1 to SYP4 are 3.2, 3.0, 2.9, and 2.5 ka, respectively, and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east. The SYP1 to SYP4 deposits consist of two facies: the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size, scoria lapilli, and fresh lithic basalt fragments supported in an ash matrix; the surge facies is represented by beds 1 to 15 cm thick, consisting mainly of ash with minor amount of fine lapilli. The bombs and scoria are 15 to 30% in volume within the massive facies. The ashes within the SYP deposits consist largely of comminuted basalt lithics and crystals that are derived from the Middle-stage lava flows exposed at the western flanks. SYP1 to SYP4 were only dispersed down the western flanks. The reason for this one-sided distribution is the asymmetric topography of the edifice; the western slopes of the volcano are the steepest (over 34 degrees). Most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees. Therefore, ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high-temperature granular flows. These flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom. Furthermore, the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts. The large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose. Taking the typical kinetic friction coefficient of small pyroclastic flows, such flows could descend the western flanks where scattered houses are below 1,000 m a.s.l. A similar type of pyroclastic flow could result if explosive summit eruptions occur in the future.Editorial responsibility: R Cioni  相似文献   

20.
We describe two small scoria cone volcanoes, Hidden Cone and Little Black Peak (ages between ~320–390 ka), in the Southwestern Nevada Volcanic Field and discuss their eruption mechanisms and inferences about their plumbing systems. Cone-forming pyroclastic deposits are consistent with eruptive styles ranging from Strombolian to violent Strombolian, and lavas emanated from near the bases of the cones. The volcanoes are monogenetic (rather than polycyclic, as allowed by previous geomorphic interpretations). Vents at each volcano appear to coincide with pre-existing normal faults, consistent with observations at older, deeply eroded volcanoes in the region. The existence of these two volcanoes on a topographically high area (particularly Hidden Cone) provides evidence for short feeder dike lengths (~500 m at the surface). We infer that this short length reflects the small length scale of the mantle source region that was tapped to feed each volcano. Editorial responsibility: J Stix  相似文献   

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