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1.
Marine shallow-water to emergent volcanoes have been described in detail, but comparable englacial centres are not well documented. Brown Bluff is a Pleistocene, shallow water, alkali basaltic volcano whose deposits were ponded within an englacial lake, enclosed by ice >400 m thick. Its evolution is divided chronologically into pillow volcano, hyalotuff cone, slope failure and hyaloclastite delta/subaerial stages. Seventeen lithofacies and five structural units (A-E) are recognised and described. The pillow volcano stage (Unit A) is similar to those of many submarine seamount volcanoes. It comprises extrusive and intrusive pillow lavas draped by slumped hyaloclastite. Units B and D define the hyalotuff cone stage, which was centred on a summit vent(s), and comprises slumped, poorly sorted hyalotuffs redeposited downslope by sediment gravity flows and ponded against an ice barrier. This stage also includes water-cooled subaerial lavas and massive hyalotuffs ponded within a crater. Cone construction was interrupted by drainage of the lake and slope failure of the northeast flank, represented by debris avalanche-type deposits (Unit C). Unit E represents the youngest stage and consists of a Gilbert-type hyaloclastite delta(s), which prograded away from a summit vent(s), and compound subaerial lavas. A second drainage episode allowed subaerial lavas to accumulate in the surrounding trough.  相似文献   

2.
New field observations with the submersible ALVIN and photographic evidence from a study of the summits of seamounts near the East Pacific Rise show that hyaloclastite deposits occur commonly. Hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m. These new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies. Most of the hyaloclastite samples are of hydrovolcanic eruptive origin, but a few show evidence of a predominantly sedimentary origin. Primarily from morphology, we identify several vent areas from which hyaloclastite presumably erupted. The surface appearance of the hyaloclastite deposits varies with distance to these vents, leading us to propose a facies model for deep-sea hyaloclastites on seamount summits. Hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding-parallel alignment of platy shards. They consist of blocky, sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment. The shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous. We propose that the shards form mainly by cooling-contraction granulation, but cannot rule out the possibility of limited steam explosivity. Hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate, promoting clastic-dominated versus flow-dominated eruptive behavior, is the dominant control on hyaloclastite formation. We propose that shard formation occurs during submarine lava fountaining. Gravitational instability of the resulting slurry of shards, sea water and possibly steam causes gravity flow that carries the shards outward from the vent. Further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism, eruption dynamics and deposition of deep-sea hyaloclastites.  相似文献   

3.
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   

4.
Hlöðufell is a familiar 1186 m high landmark, located about 80 km northeast of Reykjavík, and 9 km south of the Langkjökull ice-cap in south-west Iceland. This is the first detailed study of this well-exposed and easily accessible subglacial to emergent basaltic volcano. Eight coherent and eleven volcaniclastic lithofacies are described and interpreted, and its evolution subdivided into four growth stages (I–IV) on the basis of facies architecture. Vents for stages I, II, and IV lie along the same fissure zone, which trends parallel to the dominant NNE–SSW volcano-tectonic axis of the Western Volcanic Zone in this part of Iceland, but the stage III vent lies to the north, and is probably responsible for the present N–S elongation of the volcano. The basal stage (I) is dominated by subglacially erupted lava mounds and ridges, which are of 240 m maximum thickness, were fed from short fissures and locally display lava tubes. Some of the stage I lavas preserve laterally extensive flat to bulbous, steep, glassy surfaces that are interpreted to have formed by direct contact with surrounding ice, and are termed ice-contact lava confinement surfaces. These surfaces preserve several distinctive structures, such as lava shelves, pillows that have one flat surface and mini-pillow (< 10 cm across) breakouts, which are interpreted to have formed by the interplay of lava chilling and confinement against ice, ice melting and ice fracture. The ice-contact lava confinement surfaces are also associated with zones of distinctive open cavities in the lavas that range from about 1 m to several metres across. The cavities are interpreted as having arisen by lava engulfing blocks of ice, that had become trapped in a narrow zone of meltwater between the lava and the surrounding ice, and are termed ice-block meltout cavities. The same areas of the lavas also display included and sometimes clearly rotated blocks of massive to planar to cross-stratified hyaloclastite lapilli tuffs and tuff–breccias, termed hyaloclastite inclusions, which are interpreted as engulfed blocks of hyaloclastite/pillow breccia carapace and talus, or their equivalents reworked by meltwater. Some of the stage I lavas are mantled at the southern end of the mountain by up to 35 m thickness of well-bedded vitric lapilli tuffs (stage II), of phreatomagmatic origin, which were erupted from a now dissected cone, preserved in this area. The tephra was deposited dominantly by subaqueous sediment gravity flows (density currents) in an ice-bound lake (or less likely a sub-ice water vault), and was also transported to the south by sub-ice meltwater traction currents. This cone is onlapped by a subaerial pahoehoe lava-fed delta sequence, formed during stage III, and which was most likely fed from a now buried vent(s), located somewhere in the north-central part of the mountain. A 150 m rise in lake level submerged the capping lavas, and was associated with progradation of a new pahoehoe lava-fed delta sequence, produced during stage IV, and which was fed from the present summit cone vent. The water level rise and onset of stage IV eruptions were not associated with any obviously exposed phreatomagmatic deposits, but they are most likely buried beneath stage IV delta deposits. Stage IV lava-fed deltas display steep benches, which do not appear to be due to syn- or post-depositional mass wasting, but were probably generated during later erosion by ice. The possibility that they are due to shorter progradation distances than the underlying stage III deltas, due to ice-confinement or lower volumes of supplied lava is also considered.  相似文献   

5.
Products of subglacial volcanism can illuminate reconstructions of paleo-environmental conditions on both local and regional scales. Competing interpretations of Pleistocene conditions in south Iceland have been proposed based on an extensive sequence of repeating lava-and-hyaloclastite deposits in the Síða district. We propose here a new eruptive model and refine the glacial environment during eruption based on field research and analytical data for the Síða district lava/hyaloclastite units. Field observations from this and previous studies reveal a repeating sequence of cogenetic lava and hyaloclastite deposits extending many kilometers from their presumed eruptive source. Glasses from lava selvages and unaltered hyaloclastites have very low H2O, S, and CO2 concentrations, indicating significant degassing at or close to atmospheric pressure prior to quenching. We also present a scenario that demonstrates virtual co-emplacement of the two eruptive products. Our data and model results suggest repeated eruptions under thin ice or partially subaerial conditions, rather than eruption under a thick ice sheet or subglacial conditions as previously proposed.  相似文献   

6.
Emplacement of a giant submarine slide complex, offshore of South Kona, Hawaii Island, was investigated in 2001 by visual observation and in-situ sampling on the bench scarp and a megablock, during two dives utilizing the Remotely Operated Vehicle (ROV) Kaiko and its mother ship R/V Kairei. Topography of the bench scarp and megablocks were defined in 3-D perspective, using high-resolution digital bathymetric data acquired during the cruise. Compositions of 34 rock samples provide constraints on the landslide source regions and emplacement mechanisms. The bench scarp consists mainly of highly fractured, vesiculated, and oxidized aa lavas that slumped from the subaerial flank of ancestral Mauna Loa. The megablock contains three units: block facies, matrix facies, and draped sediment. The block facies contains hyaloclastite interbedded with massive lava, which slid from the shallow submarine flank of ancestral Mauna Loa, as indicated by glassy groundmass of the hyaloclastite, low oxidation state, and low sulfur content. The matrix facies, which directly overlies the block facies and is similar to a lahar deposit, is thought to have been deposited from the water column immediately after the South Kona slide event. The draped sediment is a thin high-density turbidite layer that may be a distal facies of the Alika-2 debris-avalanche deposit; its composition overlaps with rocks from subaerial Mauna Loa. The deposits generated by the South Kona slide vary from debris avalanche deposit to turbidite. Spatial distribution of the deposits is consistent with deposits related to large landslides adjacent to other Hawaiian volcanoes and the Canary Islands.  相似文献   

7.
In exceptional cases also acid hyaloclastites can be originated by submarine extrusion of acid lavas. Acid hyaloclastites — which hitherto were not known — could be found for the first time on the western Ponza-Islands/Tyrrhenian Sea, Italy. The formation of acid hyaloclastites is closely connected with the process of auto-brecciation proceeding during the extrusion of acid lavas; both processes work together and cause the disintegration of parts of extruding lavabodies (domes etc.) into a mass of volcano-clastic materials. Hyaloclastization that is both the chilling of fluid lava in contact with water, ice or water bearing country rocks and the shattering of the congealed glassy skin by the pressure of the inflowing lava. Auto-brecciation that is destruction of parts of the extruding mass, not only by movements proceeding during the slow cruption but also by explosive phenomena depending on the release of the vapors. Hyaloclastization and auto-brecciation of acid lavas are an example of convergency: through both processes more or less similar deposits are formed. Thus brecciated acid hyaloclastites are not to be distinguished from auto-breccias. Two occurrences of brecciated lavas in California are interpreted as acid and intermediate hyaloclastites. Both are originated by the intrusion of rhyolitic respectively of andesitic lavas into water-bearing country rocks. In particularity for Iceland the possibility is discussed, that the formation of acid hyaloclastites also may occur in case of subglacial eruptions of acid lavas.  相似文献   

8.
An exceptionally well-exposed, ancient, intra-arc basin in the Permian Takitimu Group of New Zealand contains 14 km of interbedded primary volcanic and marine volcaniclastic rocks of basaltic to rhyodacitic composition. These are the products of subaerial and submarine arc volcanism and closely associated turbidite sedimentation. The Takitimu oceanic arc/basin setting formed a dynamic closed sedimentary system in which large volumes of volcaniclastic material generated at the arc was rapidly redeposited in marine basins flanking the eruptive centres. Volcanism probably included (1) moderate- to deep-water extrusion of lava and deposition of hyaloclastite, (2) extrusive and explosive eruptions from shallow marine to marginally emergent volcanoes in or on the margin of the basin, and (3) Plinian and phreato-Plinian eruptions from more distant subaerial vents along the arc. Much of the newly erupted material was rapidly transported to the adjacent marine basin by debris flows, slumping and sliding. Hemipelagic sedimentation predominated on the outer margin of the basin, infrequently interrupted by deposition of ash from the most explosive arc volcanism and the arrival of extremely dilute turbidites. Turbidite sedimentation prevailed in the remainder of the basin, producing a thick prograding volcaniclastic apron adjacent to the arc. The volcaniclastic strata closely resemble classic turbidite deposits, and show similar lateral facies variations to submarine fan deposits. Study of such sequences provides insight into poorly understood processes in modern arc-related basins.  相似文献   

9.
 Pliocene–Recent volcanic outcrops at Seal Nunataks and Beethoven Peninsula (Antarctic Peninsula) are remnants of several monogenetic volcanoes formed by eruption of vesiculating basaltic magma into shallow water, in an englacial environment. The diversity of sedimentary and volcanic lithofacies present in the Antarctic Peninsula outcrops provides a clear illustration of the wide range of eruptive, transportational and depositional processes which are associated with englacial Surtseyan volcanism. Early-formed pillow lava and glassy breccia, representing a pillow volcano stage of construction, are draped by tephra erupted explosively during a tuff cone stage. The tephra was resedimented around the volcano flanks, mainly by coarse-grained sediment gravity flows. Fine-grained lithofacies are rare, and fine material probably bypassed the main volcanic edifice, accumulating in the surrounding englacial basin. The pattern of sedimentation records variations in eruption dynamics. Products of continuous-uprush eruptions are thought to be represented by stacks of poorly bedded gravelly sandstone, whereas better bedded, lithologically more diverse sequences accumulated during periods of quiescence or effusive activity. Evidence for volcano flank failure is common. In Seal Nunataks, subaerial lithofacies (mainly lavas and cinder cone deposits) are volumetrically minor and occur at a similar stratigraphical position to pillow lava, suggesting that glacial lake drainage may have occurred prior to or during deposition of the subaerial lithofacies. By contrast, voluminous subaerial effusion in Beethoven Peninsula led to the development of laterally extensive stratified glassy breccias representing progradation of hyaloclastite deltas. Received: 5 February 1996 / Accepted: 17 January 1997  相似文献   

10.
Hyaloclastites are volcaniclastic rocks generated by non-explosive granulation of volcanic glass which takes place when basaltic magmas are quenched by contact with water; hyaloclastites are common products of deep submarine basaltic central volcanoes (seamounts and guyots). We suggested (Honnorlz, 1966) calling hyalotuffs the actual pyroclastic rocks which are generated by phreatomagmatic and phreatic explosions taking place when basaltic volcanoes crupt in shallow waters; hyalotuffs are restricted to shallow subaqueous conditions since no volcanic explosion can occur in deep seas. The distinction between hyaloclastites and hyalotuffs is therefore a useful tool when reconstructing the paleogeography of ancient submarine volcanic edifices and the mechanism by which their lavas were emplaced. We propose using two sets of morphometric parameters to discriminate the hyaloclastites from the hyalotuffs. The granules making up these two volcaniclastic rock types plot in different areas of either 1) a ternary diagram the apexes of which represent the grain planarity (P), convexity (V) and concavity (C) % («roundness» according to Szadeczky-Kardoss); or 2) a binary diagram relating the number of grain corners (N) to their planarity (P) %.  相似文献   

11.
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   

12.
In this paper we present a model for the growth of a maar-diatreme complex in a shallow marine environment. The Miocene-age Costa Giardini diatreme near Sortino, in the region of the Iblei Mountains of southern Sicily, has an outer tuff ring formed by the accumulation of debris flows and surge deposits during hydromagmatic eruptions. Vesicular lava clasts, accretionary lapilli and bombs in the older ejecta indicate that initial eruptions were of gas-rich magma. Abundant xenoliths in the upper, late-deposited beds of the ring suggest rapid magma ascent, and deepening of the eruptive vent is shown by the change in slope of the country rock. The interior of the diatreme contains nonbedded breccia composed of both volcanic and country rock clasts of variable size and amount. The occurrence of bedded hyaloclastite breccia in an isolated outcrop in the middle-lower part of the diatreme suggests subaqueous effusion at a low rate following the end of explosive activity. Intrusions of nonvesicular magma, forming plugs and dikes, occur on the western side of the diatreme, and at the margins, close to the contact between breccia deposits and country rock; they indicate involvement of volatile-poor magma, possibly during late stages of activity. We propose that initial hydromagmatic explosive activity occurred in a shallow marine environment and the ejecta created a rampart that isolated for a short time the inner crater from the surrounding marine environment. This allowed explosive activity to draw down the water table in the vicinity of the vent and caused deepening of the explosive center. A subsequent decrease in the effusion rate and cessation of explosive eruptions allowed the crater to refill with water, at which time the hyaloclastite was deposited. Emplacement of dikes and plugs occurred nonexplosively while the breccia sediment was mostly still soft and unconsolidated, locally forming peperites. The sheltered, low-energy lagoon filled with marine limestones mixed with volcaniclastic material eroded from the surrounding ramparts. Ultimately, lagoonal sediments accumulated in the crater until subsidence or erosion of the tuff ring caused a return to normal shallow marine conditions.  相似文献   

13.
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   

14.
We reconstructed the evolution of the volcanic activity within the central Garrotxa monogenetic Volcanic Field, the youngest volcanic area of the Iberian Peninsula, by investigating the stratigraphy of the volcanic successions and the morphology of the monogenetic eruptive centres. Analysis of this volcanic succession has been conducted following the Unconformity Bounded Stratigraphic Units criteria. The detailed stratigraphy of the volcanic successions shows that the central Garrotxa Volcanic Field (GVF) evolved through four main periods of volcanic activity (Synthems) represented by the eruptive products of the mafic monogenetic volcanoes and associated syn-eruptive reworked deposits (Eruptive Units) and by the inter-eruptive deposits (Epiclastic Units). The distribution and the morphologies of the monogenetic volcanoes suggest that feeder dykes were emplaced under influence of the present stress filed and along pre-existing fractures of the basement. Our facies analysis of the deposits and their distribution shows that migration of volcanism toward the centre of the basin was accompanied by a trend of increasing explosivity. Episodic hydromagmatism in central Garrotxa occurred without a specific geographic locus or obvious temporal correlation. Finally, by integrating field data with the stratigraphy extracted from water wells, we determined the volume of the volcanic deposits. The small average volume of products emitted during each eruptive period, and the long quiescence separating them, allow us to classify the GVF as a low-output rate volcanic field.  相似文献   

15.
The Highway–Reward massive sulphide deposit is hosted by a silicic volcanic succession in the Cambro-Ordovician Seventy Mile Range Group, northeastern Australia. Three principal lithofacies associations have been identified in the host succession: the volcanogenic sedimentary facies association, the primary volcanic facies association and the resedimented syn-eruptive facies association. The volcanogenic sedimentary facies association comprises volcanic and non-volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base. Lithofacies of the primary volcanic facies association include coherent rhyolite, rhyodacite and dacite, and associated non-stratified breccia facies (autoclastic breccia and peperite). The resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes, but then redeposited by mass-flow processes. Resedimentation was more or less syn-eruptive so that the deposits are essentially monomictic and clast shapes are unmodified. This facies association includes monomictic rhyolitic to dacitic breccia (resedimented autoclastic facies), siltstone-matrix rhyolitic to dacitic breccia (resedimented intrusive hyaloclastite or resedimented peperite) and graded lithic-crystal-pumice breccia and sandstone (pumiceous and crystal-rich turbidites). The graded lithic-crystal-pumice breccia and sandstone facies is the submarine record of a volcanic centre(s) that is not preserved or is located outside the study area. Pumice, shards, and crystals are pyroclasts that reflect the importance of explosive magmatic and/or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings.The lithofacies associations at Highway–Reward collectively define a submarine, shallow-intrusion-dominated volcanic centre. Contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0.5 km3. Single porphyritic units vary from <10 to 350 m in thickness and some are less than 200 m in diameter. Ten of the porphyritic units studied in the immediate host sequence to the Highway–Reward deposit are entirely intrusive. Two of the units lack features diagnostic of their emplacement mechanism and could be either lavas and intrusions. Direct evidence for eruption at the seafloor is limited to a single partly extrusive cryptodome. However, distinctive units of resedimented autoclastic breccia indicate the presence nearby of additional lavas and domes.The size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption/intrusion, the style of emplacement, and the subaqueous emplacement environment. Due to rapid quenching and mixing with unconsolidated clastic facies, the sills and cryptodomes did not spread far from their conduits. The shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units. Magma preferentially invaded the sediment, avoiding the older units or conforming to their margins. Large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma.  相似文献   

16.
We examined the distribution of submerged and emergent macrophyte species and the entire macrophyte community within and between five lake types (highland reservoirs, alkali lakes, large shallow lakes, small to medium sized shallow lakes, marshes) in the Pannon Ecoregion, Hungary. The lowest submerged, emergent and total species richness was found in alkali lakes. The highest submerged macrophyte richness was in small to medium sized lakes, while the highest emergent macrophyte species richness was in reservoirs, small to medium sized lakes, and marshes. The values of within-lake type beta diversity were generally lower than the values of alpha diversity, especially for submerged macrophytes, indicating between site homogeneity in species composition within the lake types. Emergent macrophyte communities contributed the most to within and between lake type diversity and total (gamma) diversity. Canonical correspondence analyses showed that the main environmental variables which influenced the distribution of submerged macrophytes were conductivity, Secchi transparency and water nitrogen contents. For emergent macrophytes conductivity, lake width, altitude and water depth proved to be the most influential variables. Our results contribute to the knowledge of large-scale distribution of macrophytes in the Pannon Ecoregion and to the identification of conservation value of lakes using macrophytes. The results support the importance of small lakes and artificial lakes in the conservation of macrophyte diversity compared to large and natural lakes in the Pannon Ecoregion.  相似文献   

17.
Detailed mapping of Tok Island, located in the middle of the East Sea (Sea of Japan), along with lithofacies analysis and K-Ar age determinations reveal that the island is of early to late Pliocene age and comprises eight rock units: Trachyte I, Unit P-I, Unit P-II, Trachyandesite (2.7±0.1 Ma), Unit P-III, Trachyte II (2.7±0.1 Ma), Trachyte III (2.5±0.1 Ma) and dikes in ascending stratigraphic order. Trachyte I is a mixture of coherent trachytic lavas and breccias that are interpreted to be subaqueous lavas and related hyaloclastites. Unit P-I comprises massive and inversely graded basaltic breccias which resulted from subaerial gain flows and subaqueous debris flows. A basalt clast from the unit, derived from below Trachyte I, has an age of 4.6±0.4 Ma. Unit P-II is composed of graded and stratified lapilli tuffs with the characteristics of proximal pyroclastic surge deposits. The Trachyandesite is a massive subaerial lava ponded in a volcano-tectonic depression, probably a summit crater. A pyroclastic sequence containing flattened scoria clasts (Unit P-III) and a small volume subaerial lava (Trachyte II) occur above the Trachyandesite, suggesting resumption of pyroclastic activity and lava effusion. Afterwards, shallow intrusion of magma occurred, producing Trachyte III and trachyte dikes.The eight rock units provide an example of the changing eruptive and depositional processes and resultant succession of lithofacies as a seamount builds up above sea level to form an island volcano: Trachyte I represents a wholly subaqueous and effusive stage; Units P-I and P-II represent Surtseyan and Taalian eruptive phases during an explosive transitional (subaqueous to emergent) stage; and the other rock units represent later subaerial effusive and explosive stages. Reconstruction of volcano morphology suggests that the island is a remnant of the south-western crater rim of a volcano the vent of which lies several hundred meters to the north-east.  相似文献   

18.
上新世盐类沉积仅分布于柴达木盆地两部、昆仑山和唐古拉山北坡;晚更新世-全新世盐类沉积广布于羌塘和柴达木盆地。盐湖沉积只有蒸发岩和碎屑沉积物互层特点, 系干湿交替的内陆气候和动荡浅水环境下的湖相沉积。迄今已发现62种盐类矿物, 其析出具有阶段性特征, 青藏更新世-全新世盐湖自析阶段的起始时间分别为14C 24000和9000a B.P., 钾盐和硼酸盐各有两种典型的矿物共生组合。丰富的盐湖矿物资源是该区走向繁荣的物质基础之一。  相似文献   

19.
Hundreds of small (diameter 2–50 m) hydrothermal explosion craters are dispersed across the top plateau of a hyaloclastite ridge in central Iceland. The craters are undisturbed by erosion and must be of Recent age. The ridge, Dyngjufjöll Ytri, is a tectonic horst, separated from the Dyngjufjöll volcanic center and the Askja caldera by a narrow graben. The ridge is 20 km by 6 km with a flat top partly covered with glacial sediments and air-fall tephra. It is composed of two major volcanic units, a lower hyaloclastite flow and an upper complex series containing pillow lavas, air-fall and surge tephra deposits and water lain sediments. Large (max. 9 m diameter) cylindrical gas pipes, coated with calcite, are locally exposed in the hyaloclastite flow. Evidence suggests that volcanism contributing to the formation of the ridge was long extinct when its surface was broken by the hydrothermal explosions. The elevated position of the ridge and its narrow form make unlikely the existence of a long-lived hydrothermal system. This is also borne out by the absence of intense thermal alteration of the material blown out in the explosions. The ridge is surrounded by postglacial lavas. One of these lavas issued from a fissure with the same strike as the ridge that terminates in a small crater at the base of its southern slope. It is suggested that the fissure continues as an intrusion into the basal hyaloclastite flow where water in gas pipes of the hyaloclastite was converted into high-pressure steam that exploded through the overlying unconsolidated formations.  相似文献   

20.
Thermokarst lakes cover > 20% of the landscape throughout much of the Alaskan Arctic Coastal Plain (ACP) with shallow lakes freezing solid (grounded ice) and deeper lakes maintaining perennial liquid water (floating ice). Thus, lake depth relative to maximum ice thickness (1·5–2·0 m) represents an important threshold that impacts permafrost, aquatic habitat, and potentially geomorphic and hydrologic behaviour. We studied coupled hydrogeomorphic processes of 13 lakes representing a depth gradient across this threshold of maximum ice thickness by analysing remotely sensed, water quality, and climatic data over a 35‐year period. Shoreline erosion rates due to permafrost degradation ranged from < 0·2 m/year in very shallow lakes (0·4 m) up to 1·8 m/year in the deepest lakes (2·6 m). This pattern of thermokarst expansion masked detection of lake hydrologic change using remotely sensed imagery except for the shallowest lakes with stable shorelines. Changes in the surface area of these shallow lakes tracked interannual variation in precipitation minus evaporation (P ? EL) with periods of full and nearly dry basins. Shorter‐term (2004–2008) specific conductance data indicated a drying pattern across lakes of all depths consistent with the long‐term record for only shallow lakes. Our analysis suggests that grounded‐ice lakes are ice‐free on average 37 days longer than floating‐ice lakes resulting in a longer period of evaporative loss and more frequent negative P ? EL. These results suggest divergent hydrogeomorphic responses to a changing Arctic climate depending on the threshold created by water depth relative to maximum ice thickness in ACP lakes. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

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