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1.
Neogene alkaline basaltic rocks in the western Pannonian Basin are eroded remnants of maars, tuff rings, tuff cones, scoria cones and lava fields. The erosion level of these volcanoes is deep enough to expose diatreme zones associated with the phreatomagmatic volcanoes. The erosion level is deeper yet in the west, exposing shallow dyke and sill swarms related to former intra-plate volcanoes. The basanitic sills are irregular in shape and their lateral extent is highly variable. Individual sills reach a thickness of a few tens of metres and they commonly form dome-like structures with rosette-like radial columnar joint patterns. The largest sill system identified in this region is traceable over kilometres, and forms a characteristic ridge running north-east to south-west. Elevation differences in the position of the basanitic sills within an otherwise undisturbed “layer cake-like” siliciclastic succession indicate emplacement of the basanite magma at multiple levels over kilometre-scale distances. The margins of sills in the system are irregular at a dm-to-mm-scale. Undulating contacts of the sills together with gentle thermal alteration in the host sediment over cm-to-dm distances indicate the soft, but not necessarily wet state of the host deposits at the time sills were intruded. Parts of the sill complex show a complicated relationship with the host sediment in form of peperitic zones and irregularly shaped, disrupted, peperite textures. This is interpreted to reflect inhomogenities in water content and rheology of the siliciclastic deposits during intrusion. The current summit of the longest continuous ridge preserves a small diatreme that seems to cut through an otherwise disk-like sill indicating of relationship between sill emplacement and phreatomagmatic explosive eruptions.  相似文献   

2.
The Golan Heights is a Plio-Pleistocene volcanic plateau. Cinder cones of Late Pleistocene age are very common in the eastern and northern Golan, while phreatomagmatic deposits are relatively rare and occur just in two structures — the maar of Birket Ram and the tuff ring of Mt. Avital. The complex of Mt. Avital includes two large cinder cones, a tuff ring with an elongated central depression and several basaltic flows, some of them breach the cinder cones. The (exposed) eruptive history of the complex includes (1) an early stage of basaltic lava flows, (2) strombolian activity and the buildup of the southern cinder cone, (3) a second stage of basaltic flows and the buildup of the northern cinder cone, and then a transition to (4) phreatomagmatic explosions. The phreatomagmatic deposits include surges, lapilli fallout deposits and coarse-grained lithic tuff breccias, which were found up to 200 m above the central depression. Basaltic and scoriaceous clasts are the main component of all deposits, while juvenile material is usually a minor component, almost absent in the lapilli deposits.It is suggested that the phreatomagmatic events in Mt. Avital were induced by the infiltration of water from a lake that existed in a nearby topographic low (Quneitra Valley). The lake was formed or significantly expanded at about 300 ka due to a lava flow that blocked the drainage of the valley to the west. The interlayering of tuff and scoria at the top of the northern cinder cone and the good preservation of a lava flow top breccia under the surges imply that the phreatomagmatic activity immediately followed and even coincided with the last stages of strombolian activity. It is suggested that the dry–wet transition was triggered by the effusion of the second stage lavas and the buildup of the northern cinder cone, which probably caused a reduction of pressure in the magmatic system and allowed the lake water an access to the magmatic system. The minimum age of the phreatomagmatic events is determined by a 54 ka Musterian site which lies directly on top of the tuff in the Quneitra Valley.  相似文献   

3.
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4.
 The Middle Jurassic Tuttle Lake Formation in the northern Sierra Nevada, California, comprises a thick volcaniclastic sequence deposited in a submarine island-arc setting and penetrated by numerous related hypabyssal intrusions. A composite andesite-diorite intrusive complex ≥4.5 km long and ≥1.5 km thick was emplaced while the host Tuttle Lake sediments were still wet and unconsolidated. Large parts of the intrusive complex consist of peperite formed where andesitic magma intruded and intermixed with tuff, lapilli-tuff and tuff-breccia. The southern half of the complex consists of augite-phyric andesite containing peperite in numerous small, isolated pockets and in more extensive, laterally continuous zones. The peperites comprise three main types recognized previously in other peperite studies. Fluidal peperite consists of small (≤30 cm), closely spaced, at least partly interconnected, globular to amoeboid andesite bodies enclosed by tuff. This peperite type developed during intrusion of magma into fine-grained wet sediment along unstable interfaces, and fluidization of the sediment facilitated development of complex intrusive geometries. Blocky peperite and mixed blocky and fluidal peperite formed where magma intruded coarser sediment and underwent variable degrees of brittle fragmentation by quenching and dynamic stressing of rigid margins, possibly aided by small steam explosions. The northern half of the intrusive complex consists predominantly of a different type of peperite, in which decimetre-scale plagioclase-phyric andesite clasts with ellipsoidal, elongate, or angular, polyhedral shapes are closely packed to widely dispersed within disrupted host sediment. Textural features suggest the andesite clasts were derived from conduits through which magma was flowing, and preserved remnants of the conduits are represented by elongate, sinuous bodies up to 30 m or more in length. Disruption and dispersal of the andesite clasts are inferred to have occurred at least partly by steam explosions that ripped apart a network of interconnected feeder conduits penetrating the host sediments. Closely packed peperite is present adjacent to mappable intrusions of coherent andesite, and along the margin of a large mass of coarse-grained diorite. These coherent intrusions are considered to be major feeders for this part of the complex. Examples of magma/wet sediment interaction similar in scale to the extensive peperites described here occur elsewhere in ancient island-arc strata in the northern Sierra Nevada. Based on these and other published examples, large-scale peperites probably are more common than generally realized and are likely to be important in settings where thick sediment sequences accumulate during active volcanism. Careful mapping in well-exposed terrains may be required to recognize large-scale peperite complexes of this type. Received: 8 June 1998 / Accepted: 4 December 1998  相似文献   

5.
Miocene successions in western Turkey are dominated by lacustrine, fluvial and evaporitic sedimentary deposits. These deposits include considerable amounts of volcaniclastic detritus derived from numerous NE-trending volcanic centres in western Turkey as well as in the Bigadiç region. Early Miocene syn-depositional NE-trending olivine basalt and trachyandesite bodies that formed as intrusions and lava flows occur within the Bigadiç borate basin. Olivine basalts occur as partly emergent intrusions, and trachyandesite dykes fed extensive lava flows emplaced in a semi-arid lacustrine environment.Peperites associated with the olivine basalt and trachyandesites appear to display contrasting textural features, although all the localities include a large variety of clast morphologies from blocky to fluidal. Fluidal clasts, mainly globular, ameboidal and pillow-like varieties, are widespread in the peperite domains associated with olivine basalts, apparently due to large-volume sediment fluidisation. In contrast, fluidal clasts related to trachyandesites are restricted to narrow zones near the margins of the intrusions and have commonly elongate and polyhedral shapes with digitate margins, rather than globular and equant varieties. Blocky and fluidal clasts in the olivine basalt peperite display progressive disintegration, suggesting decreasing temperature and increasing viscosity during fragmentation. Abundance of blocky clasts with respect to fluidal clasts in the trachyandesite peperite indicates that the fluidal emplacement and low-volume sediment fluidisation in the early stages were immediately followed by quench fragmentation due to the high viscosity of the magma.Size, texture and abundance of the blocky and fluidal clasts in the olivine basalt and trachyandesite peperites were mainly controlled by sediment fluidisation, pulsatory magma injection and magma properties such as composition, viscosity, vesicularity, and size, abundance and orientation of phenocrysts. Variously combining these contrasting features to varying degrees may form diverse juvenile clast shapes in peperitic domains.  相似文献   

6.
In Central Anatolia, Quaternary olivine basalts overlie the Neogene calcalkaline and sialic post-orogenic volcanic series. Crater forms, cones and lava fields generally exhibit a very fresh morphology suggesting a sub-recent age. Near the town of Karapinar, eruptions took place through the shallow waters of a lake which occupied the present Konya-Eregli plain during Quaternary times. Depending on the varying influence of phreatomagmatic effects, hyaloclastitic tuff rings and maar craters, both rich in sideromelane and its palagonitic alteration products, or subaerial red cinder cones developed. The maar phase is characterized by a large development of base surge structures. The basalts are alkaline and, mostly, Ne-normative and are distinct for their low TiO2 content and low Fe/Mg ratio from the alkali olivine basalts from oceanic and stable continental areas.  相似文献   

7.
The ca. 8800 14C yrs BP Sulphur Creek lava flowed eastward 12 km from the Schriebers Meadow cinder cone into the Baker River valley, on the southeast flank of Mount Baker volcano. The compositionally-zoned basaltic to basaltic andesite lava entered, crossed and partially filled the 2-km-wide and > 100-m-deep early Holocene remnant of Glacial Lake Baker. The valley is now submerged beneath a reservoir, but seasonal drawdown permits study of the distal entrant lava. As a lava volume that may have been as much as 180 × 106 m3 entered the lake, the flow invaded the lacustrine sequence and extended to the opposite (east) side of the drowned Baker River valley. The volume and mobility of the lava can be attributed to a high flux rate, a prolonged eruption, or both. Basalt exposed below the former level of the remnant glacial lake is glassy or microcrystalline and sparsely vesicular, with pervasive hackly or blocky fractures. Together with pseudopillow fractures, these features reflect fracturing normal to penetrative thermal fronts and quenching by water. A fine-grained hyaloclastite facies was probably formed during quench fragmentation or isolated magma-water explosions. Although the structures closely resemble those developed in lava-ice contact environments, establishing the depositional environment for lava exhibiting similar intense fracturing should be confirmed by geologic evidence rather than by internal structure alone. The lava also invaded the lacustrine sequence, forming varieties of peperite, including sills that are conformable within the invaded strata and resemble volcaniclastic breccias. The peperite is generally fragmental and clast- or matrix-supported; fine-grained and rounded fluidal margins occur locally. The lava formed a thickened subaqueous plug that, as the lake drained in the mid-Holocene, was exposed to erosion. The Baker River then cut a 52-m-deep gorge through the shattered, highly erodible basalt.  相似文献   

8.
The Table Rock Complex (TRC; Pliocene–Pleistocene), first documented and described by Heiken [Heiken, G.H., 1971. Tuff rings; examples from the Fort Rock-Christmas Lake valley basin, south-central Oregon. J. Geophy. Res. 76, 5615-5626.], is a large and well-exposed mafic phreatomagmatic complex in the Fort Rock–Christmas Lake Valley Basin, south-central Oregon. It spans an area of approximately 40 km2, and consists of a large tuff cone in the south (TRC1), and a large tuff ring in the northeast (TRC2). At least seven additional, smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions. The first period of activity, TRC1, initiated with a Surtseyan-style eruption through a 60–70 m deep lake. The TRC1 deposits are dominated by multiple, 1-2 m thick, fining upward sequences of massive to diffusely-stratified lapilli tuff with intermittent zones of reverse grading, followed by a finely-laminated cap of fine-grained sediment. The massive deposits are interpreted as the result of eruption-fed, subaqueous turbidity current deposits; whereas, the finely laminated cap likely resulted from fallout of suspended fine-grained material through a water column. Other common features are erosive channel scour-and-fill deposits, massive tuff breccias, and abundant soft sediment deformation due to rapid sediment loading. Subaerial TRC1 deposits are exposed only proximal to the edifice, and consist of cross-stratified base-surge deposits. The eruption built a large tuff cone above the lake surface ending with an effusive stage, which produced a lava lake in the crater (365 m above the lake floor). A significant repose period occurred between the TRC1 and TRC2 eruptions, evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences. The TRC2 eruption was the last and most energetic in the complex. General edifice morphology and a high percentage of accidental material suggest eruption through saturated TRC1 deposits and/or playa lake sediments. TRC2 deposits are dominated by three-dimensional dune features with wavelengths 200–500 m perpendicular to the flow, and 20–200 m parallel to the direction of flow depending on distance from source. Large U-shaped channels (10–32 m deep), run-up features over obstacles tens of meters high, and a large (13 m) chute-and-pool feature are also identified. The TRC2 deposits are interpreted as the products of multiple, erosive, highly-inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions.  相似文献   

9.
Small and large maars exist associated with small and large diatremes, respectively, their subsurface feeder structures. The problem of size and growth of maar-diatreme volcanoes is discussed from a phreatomagmatic point of view from field data, some geophysical data, and short-lived historic maar eruptions. A hydrostatic pressure barrier of usually about 20–30 bars is assumed to control the maximum depth level of explosive magma/groundwater interactions. Similar to the situation in submarine and subglacial volcanism, initial maar-forming water vapour explosions are therefore assumed to occur at shallow depth and to produce a small maar with a shallow diatreme. Because of limited availability of groundwater and ejection of groundwater in the form of steam, the confining pressure barrier is displaced downward. Consequently, water vapour explosions can take place at consecutively deeper levels with the result that the diatreme penetrates downward and grows in size. Since maars are collapse craters resulting from ejection of wallrocks fragmented by water vapour explosions at the level of the diatreme root zone, downward penetration of a diatreme not only results in increase in size of a diatreme but also in increase in size of the overlying maar. As availability of groundwater in limited amounts controls formation of diatremes and their downward penetration, lack of groundwater enables magma to rise within a diatreme and to form a scoria cone or lava lake within the maar, as is frequently found in volcanic fields such as the Eifel area in Germany. In contrast, availability of large amounts of water in near surface environments such as shallow marine, lake, water-rich coastal plains, or water-rich fluviatile gravel beds prevents formation of maars and deep diatremes but causes formation of tuff rings.  相似文献   

10.
The Fekete-hegy volcanic complex is located in the centre of the Bakony Balaton Highland Volcanic Field, in the Pannonian Basin, which formed from the late Miocene to Pliocene period. The eruption of at least four very closely clustered maar volcanoes into two clearly distinct types of prevolcanic rocks allows the observation and comparison of hard-substrate and soft-substrate maars in one volcanic complex. The analyses of bedding features, determination of the proportion of accidental lithic clasts, granulometry and age determination helped to identify and distinguish the two types of maar volcanoes. Ascending magma interacted with meteoric water in karst aquifers in Mesozoic carbonates, as well as in porous media aquifers in Neogene unconsolidated, wet, siliciclastic sediments. The divided basement setting is reflected by distinct bedding characteristics and morphological features of the individual volcanic edifices as well as a distinct composition of pyroclastic rocks. Country rocks in hard-substrate maars have a steep angle of repose, leading to the formation of steep sided cone-shaped diatremes. Enlargement and filling of these diatreme is mainly a result of shattering material by FCI related shock waves and wall-rock collapse during downward penetration of the explosion locus. Country rocks in soft-substrate maars have much shallower angles of repose, leading to the formation of broad, bowl shaped structures or irregular depressions. Enlargement and filling of these diatremes is mainly the result of substrate collapse, for example due to liquefaction of unconsolidated material by FCI-related shock waves, and its emplacement by gravity flows. The Fekete-hegy is an important example illustrating that the substrate of a volcanic edifice has to be taken into account as an important interface, which can have major control on phreatomagmatic eruptions and the resulting characteristics of the volcanic complex.  相似文献   

11.
Pahoehoe flows interbedded with sediments have been identified in the superior portion of Paraná Continental Flood Basalts (PCFB), west portion of Paraná State, southern Brazil. In the study area peperites are generated by the interaction between lava flows and wet lacustrine sediments (silt and clay). Evidence that the sediments were unconsolidated or poorly consolidated and wet when the lava flowed over them includes vesiculated sediment, sediment in vesicles and fractures in lava flow and in juvenile clasts in the peperite and soft sediment deformation. Hydrodynamic mingling of lava and wet sediments (coarse mingling) is predominant and volcanic rocks and textures related to explosive phase of Molten Fuel Coolant Interaction (MFCI) are not observed in study area. Locally centimeter-sized areas display direct contact between ash-sized juvenile clasts and sediments formed by the collapse of a vapor film. The textures of fluidal peperites in the central PCFB indicate that the relevant factors that led to a coarse mingling between lava/sediment are (1) lava properties (low viscosity); (2) fine grained, unconsolidated or poorly consolidated wet sediment; and (3) a single episode of interaction between lava flows and sediment.  相似文献   

12.
Peperites formed by mixing of magma and wet sediment are well exposed along Punta China, Baja California, Mexico, where two sills intrude a section of lava flows, limestones, and volcaniclastic rocks. Irregular lobes and dikes extend from the sills several meters into host sediments, including highly comminuted flow top breccias (lithic lapilli tuff breccias) and shelly micrites, whereas intrusive contacts with lava flows are sharp and planar. Where one sill intruded both coarse-grained volcaniclastic rock and fine-grained limestone, textural differences between the hosts produced strikingly different styles of peperite. Blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias; the blocks consequently underwent in situ fragmentation as they were rapidly quenched. The high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse-grained permeable host. Elutriation of fine-grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter-scale fluidization of the host. The contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous, immiscible fluids (fluidal peperite). Intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off grain by grain as magma advanced into the host. Thin-section-scale elutriation pipes formed. Microglobular peperite represents a frozen example of a fuel-coolant interaction (FCI) between basaltic magma and fluidized micrite host. The intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma-vapor-host interface. Such intimate intermixing of magma and water-bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism.  相似文献   

13.
The first volcanic eruption in the recorded history of Marion Island (46°54′S, 37°45′E) occurred between February and October 1980 at a locality on the west coast. It was a minor event that passed unnoticed at the meteorological station 20 km distant. The discovery was made on November 4, by five expedition members who walked around the island. When examined in more detail on November 25, the lava was still warm in places and numerous fumaroles existed. Three blocky flows emanated from two adjacent cinder cones built-up on a pre-existing phreatomagmatic tuff cone known as Kaalkoppie. The largest flow covers an area of about seven hectares and a further two hectares have been inundated by ash. Another flow poured seawards to form a new beach front, blocking access to what was previously the largest elephant seal wallowing ground on the island. No earth tremors were felt and the activity seems to have ended for the time being.  相似文献   

14.
The evaluation of volcanic hazard in the Roman hinterland related to the quiescent Colli Albani Volcano has recently been the subject of renewed attention and several interpretations by many authors. However, very little was known of the recent history of the volcano, making such interpretations rather speculative. The most recent activity of Colli Albani Volcano originated from the Albano polygenetic maar lake, which erupted several phreatomagmatic units, the most recent of which, the Peperino Albano ignimbrite, has been dated at around 25 ka. An area of several square kilometers centered around Albano Lake is presently the site of shallow and frequent seismic activity and gaseous emission as well as hydrothermal activity and is therefore considered the most prone to geologic hazards. This paper presents new stratigraphic and geomorphologic data as well as age determinations that allow rejuvenation of the most recent activity of the Colli Albani Volcano, and particularly the Albano maar lake, to the Holocene. This study allows for the first time to identify a potential hazard related to the Albano maar lake withdrawal interpreted to be related to endogenous causes, namely CO2 emission. The main results of the study are: (1) the Peperino Albano is not, as is generally believed, the last phreatomagmatic eruption from the Colli Albani Volcano; a previously unrecognized phreatomagmatic surge deposit has been identified overlying the paleosol at the top of the Peperino Albano and related lahar deposits; (2) two lahar deposits separated by paleosols top the stratigraphic succession and are dispersed only to the NW, corresponding to the lowest point of the maar rim, indicating that catastrophic hydrologic events occurred at the Albano Lake in recent times; rapid and substantial lake-level variations and lake withdrawal are reported by Roman historians and recorded by the stratigraphy of the Albano Lake lacustrine sediments; (3) microfracturing related to seismic energy release is linked to sudden variation of CO2 flow and upwelling of hydrothermal fluids. These occurrences across the lake are the likely causes that triggered during Holocene several episodes of lake withdrawal, rising the water table and probably triggering convective rollover of the lake water.  相似文献   

15.
Split Butte is a volcanic crater of Quaternary age consisting of a tephra ring which at one time retained a lava lake. The tephra is thinly bedded and is composed of partially palagonitized sideromelane clasts and subordinate lithic fragments. The beds typically dip radially away from the center of the crater, but locally dip toward the crater center. The tephra ring resulted from phreatomagmatic eruptions as a result of interaction of groundwater with rising basaltic magma, evidenced by glassy and granulated pyroclastic debris, the presence of abundant palagonite and other secondary minerals, numerous armored lapilli, and plastically deformed ash layers below ejecta blocks. Statistical analysis of the grain size distribution of the ash also indicates a phreatomagmatic origin of Split Butte tephra. In addition, the analysis reveals that the stratigraphically lowest tephra was deposited primarily by pyroclastic flow mechanisms while the upper tephra layers, comprising the bulk of the deposits, were deposited dominantly by airfall and pyroclastic surge. The lava lake and four en echelon basalt dikes were emplaced when phreatomagmatic activity at the vent ceased. Subsequent collapse caused a broad, shallow pit crater to form in the laval lake, and minor spattering occurred at one point along the pit crater scarp. Partial erosion of the tephra, deposition of aeolian sediments and encroachment of the Butte by later lava flows completed the development of Split Butte.  相似文献   

16.
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   

17.
张雯倩  李霓 《地震地质》2021,43(1):105-122
高温岩浆在上升过程中遇到地下水或地表水发生水岩相互作用,产生大量水蒸汽导致的爆炸式喷发作用,可称为射汽岩浆喷发作用,是一种较为特殊的火山活动,主要产物为低平火山口和基浪堆积物。国内外许多火山学家对射汽岩浆喷发作用的喷发过程和产物开展了岩相学、沉积学、火山物理学和地球化学综合研究,通过实验、计算机模拟等方法探究了射汽岩浆喷发过程的影响因素。文中介绍了国内外研究人员的相关研究成果,以便更好地了解射汽岩浆喷发这种特殊的火山作用形式,以期能将其应用于现代火山灾害预防和监测工作中,保护人们的生命和财产安全。  相似文献   

18.
The Atexcac maar is located in the central part of the Serdán–Oriental lacustrine/playa basin in the eastern Mexican Volcanic Belt. It is part of a dispersed and isolated monogenetic field consisting of maar volcanoes, basaltic cinder cones and rhyolitic domes. Atexac is a maar volcano excavated into pyroclastic deposits, basaltic lava flows and the flanks of a cinder cone cluster, which itself was built on a topographic high consisting of limestone. It has an ENE-trending elliptical shape with beds, mostly unconsolidated deposits that dip outward at 16–22°. The Atexcac crater was formed from vigorous phreatomagmatic explosions in which fluctuations in the availability of external water, temporal migration of the locus of the explosion, and periodic injection of new magma were important controls on the evolution of the maar crater. Variations in grain sizes and component proportions of correlated deposits from the different sections suggest a migration of the locus of explosions, producing different eruptive conditions with fluctuating water–magma interactions. Deposits rich in large intrusive and limestone blocks are associated with a matrix enriched in small andesitic lapilli. This could suggest differential degrees of fragmentation due to inherited (previously acquired) fragmentation and/or relative distance to the locus of explosions. Initial short-lived phreatic explosions started at the southwest part of the crater and were followed by an ephemeral vertical column and the influx of external water that led to relatively shallow explosive interactions with the ascending basaltic magma. Drier explosions progressed downward and/or laterally northward, sampling subsurface rock types, particularly intrusive, limestone and andesitic zones as well as localized altered zones (N-NE), caused by repetitive injection of basaltic magma. A final explosive phase involved a new injection of magma and a new influx of external water producing wetter conditions at the end of the maar formation. We infer the aquifer was formed by fractured rocks, predominantly andesitic lava flows and limestone rocks. Andesitic accessory clasts dominate in all stratigraphic levels but these rocks are not exposed in the nearby area. These local hydrogeological conditions contrast with those at nearby maar volcanoes, where the water for the magma/water interactions apparently mostly came from a dominantly unconsolidated tuffaceous aquifer, producing tuff rings with a much lower profile than Atexcac.  相似文献   

19.
Detailed gravity and aeromagnetic data over maars in the Auckland volcanic field reveal contrasting anomalies, even where surface geology is similar. Pukaki and Pukekiwiriki, almost identical maars marked by sediment-filled craters and tuff rings, have gravity and magnetic anomalies of − 6 g.u. and 20 nT, and 8 g.u. and 160 nT, respectively. The Domain and Waitomokia maars, with similar tuff rings but each with a small central scoria cone, have gravity and magnetic anomalies of 32 g.u. and 300 nT, and 21 g.u. and 310 nT, respectively. These differences in geophysical expression are attributed to varying volumes of dense, magnetic basalt in the form of shallow bowl-shaped bodies up to several hundreds of metres in diameter and up to 140 m thick beneath the maar centres. These bodies are interpreted as solidified magma that ponded into early-formed phreatomagmatic explosion craters. Where magma supply was limited relative to groundwater availability, no residual subsurface basalt occurs (as at Pukaki); continued magma supply, but limited groundwater, resulted in ponding (e.g. at Pukekiwiriki) and eventually the building of a scoria cone (as at Domain and Waitomokia). There is no evidence in these geophysical data for diatreme structures below the maars or for shallow and/or extensive feeder dykes associated with these maars. If diatreme structures do occur, their lack of geophysical signature must be a consequence of either their small geophysical contrast with host Miocene sediments and/or masking by the stronger anomalies associated with the subsurface basalt. In addition, any magma conduits appear to be confined centrally beneath the maars, at least to shallow depths (upper 100 m).  相似文献   

20.
Clinker is a term used to describe massive or scoriaceous fragments commonly associated with ‘a‘ā lava flows. Clinker is generally considered to form by fragmentation of an upper vesiculated crust, due to an increase in apparent viscosity and/or to an increase in shear strain rate. Surface clinker is considered to be transported to the flow front and incorporated at the base by caterpillar motion. Clinker that we have observed on a variety of lava flows has very variable textures, which suggests several different mechanisms of formation. In order to study clinker formation, we examined several lava flows from the Chaîne des Puys Central France, where good sections, surface morphology and surface textures are widespread and clearly visible. We observed basal and surface ‘a‘ā clinker that has fragmentation textures similar to those observed in ash formed in eruptions under dry conditions. In two pāhoehoe flows we have observed basal clinker that formed in-situ. Two other flows display clinker features identical to those commonly observed in phreatomagmatic ash, such as adhering particles, blocky shapes, spherical glass and attached microphenocrysts. Another pāhoehoe flow has a flakey, angular basal breccia, with microfaulted and abraded clasts. These were probably formed at a cooled lava base by large amounts of simple shear and consequent intra-lava brittle faulting. Using these observations we propose three different ways of fragmentation. (1) Clinker can form at the surface and eventually produce roll-over basal breccia. (2) Water/lava interactions can form basal clinker by phreatomagmatic fragmentation. Water/lava ratio variations may produce different clinker structures, in a manner similar to observed textural changes in phreatomagmatic eruptions. (3) Clinker can be formed by brittle brecciation during basal simple shear. The different clinker can provide information about the mechanisms and environmental conditions during lava flow emplacement.  相似文献   

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