Explosion Craters Research Articles

Chapter 7 The Neogene-Recent volcanic rocks

The late Tertiary–Recent volcanoes are largely confined to the area of the late Tertiary depression, the exceptions being the most southerly volcanoes of Hanang and Kwaraha, and the Basotu explosion craters of the Mbulu Plateau (Dawson 2008, fig. 3.1). The first systematic attempt to classify the volcanoes was that of Guest (1953) who, mainly on the basis of rock type, divided them into the Older Extrusives (volcanoes characterized by extrusion of basalt) and the Younger Extrusives (formed by more alkaline rock types). Guest's classification is broadly correct, but modifications to the scheme have resulted from later regional mapping by the Tanzania Geological Survey, together with radiometric dating of rocks from individual centres. This has resulted in a more refined stratigraphy, particularly with respect to the timing of the volcanicity relative to the regional tectonics. For example, some peralkaline volcanoes, formerly included with the Younger Extrusives, are older than, or contemporaneous with, some Older Extrusive basaltic centres. The term ‘peralkaline volcano’ is used here to indicate that the structure is formed dominantly of peralkaline rocks i.e. those for which whole-rock chemical analyses show (Na + K)/Al>1. Following the faulting that formed the late Tertiary tectonic depression, a group of major volcanoes erupted within the depression, the lavas largely infilling the depression and eventually overstepping the boundary fault-scarps at several points. In the SW, volcanoes erupting in the Crater Highlands infilled the northeastern end of the Eyasi half graben, and in the east, lavas from the Kilimanjaro massif filled the depression to the north of the Pangani graben. The earliest date of eruption is poorly constrained since, even in the major fault escarpments, the first extrusive rocks of most of the volcanoes are not exposed. The only exposure of volcanic rocks in contact with the underlying basement rocks within the depression …

Th-, U- AND LREE-BEARING GROSSULAR, CHROMIAN FERRIALLANITE-(Ce) AND CHROMIAN CERITE-(Ce) IN SKARN XENOLITHS EJECTED FROM THE GOLCUK MAAR CRATER, ISPARTA, ANATOLIA, TURKEY

Thorium-, U- and LREE-bearing skarn xenoliths have been found in the volcaniclastic products of the Golcuk explosive maar crater, Isparta, southwestern Turkey. The samples predominantly consist of clinopyroxene, garnet, plagioclase, wollastonite, esseneite, olivine, calcite and dolomite, with accessory ferriallanite-(Ce), phlogopite, amphibole, quartz, apatite, spinel, cerite-(Ce) and monazite. We report the whole-rock geochemistry and electron-microprobe data for related Th-, U- and LREE-bearing garnet, ferriallanite-(Ce) and cerite-(Ce). Extreme enrichments in the LREE (1600–9400 ppm La, 2200–13500 ppm Ce, 270–900 ppm Pr, 400–2000 ppm Nd), and in Th (200–300 ppm), U (20–400 ppm), Y (50–150 ppm), Be (13–108 ppm), Cu (14–1600 ppm), Zn (50–600 ppm) and Au (up to 9.5 ppb) are observed. A strong correlation between U + Th abundance and ∑REE (5900–26000 ppm) exists in whole-rock samples. Among the minerals, grossular (Th + U: 0.10–3.97 wt.%, ∑LREE: 4.40–7.80 wt.%), chromian ferriallanite-(Ce) (Th + U: 0.02–0.20 wt.%, ∑LREE: 19.30–24.50 wt.%) and a chromian variety of cerite-(Ce) recorded here for the first time (Th + U: 0.97–1.35 wt.%, ∑LREE: 56.83–60.64 wt.%) are the principal hosts of the Th, U and the LREE. The fluids responsible for the geochemically enriched zones are most likely of contact-metasomatic origin, affecting the interface between a shallow magma-chamber and a carbonate wallrock. High concentrations of gold (up to 4.2 ppb) in the magmatic rocks of the Golcuk district demonstrate an important economic potential.

Geology and geochemistry characteristics of the Chiapanecan Volcanic Arc (Central Area), Chiapas Mexico

The Chiapanecan Volcanic Arc (CVA), located in the central portion of the State of Chiapas, is a 150 km stretch of volcanoes irregularly aligned in the northwest direction between two great volcanic features: the Trans-Mexican Volcanic Belt to the northwest and the Central American Volcanic Arc to the southeast. The CVA is located in a complex zone marking the interaction of the North American, Caribbean and Cocos plates, near the Motagua–Polochic fault system, the boundary between North American and Caribbean plates. The central part of the CVA is composed of an irregular northwest alignment of at least 10 volcanic structures generally lying along NNW–SSE-trending faults splayed from the Motagua–Polochic system. Among the structures there are seven volcanic domes (Huitepec, Amahuitz, La Iglesia, Mispía, La Lanza, Venustiano Carranza and Santotón), one explosion crater (Navenchauc), one collapse structure (Apas), and one dome complex (Tzontehuitz). In the majority of the structures there is a clear resurgence with the formation of several domes in the same structure, with the destruction of previous domes (Navenchauc) or with the formation of new explosion craters or collapse structures (Apas). The volcanic activity in the CVA was mainly effusive accompanied by explosive and phreatomagmatic events and is characterized by volcanic domes accompanied by block-and-ash-flows, ash flows with accretionary lapilli, falls, and pumice flows. The volcanic structures and deposits are calcalkaline in composition with a medium to high content of potassium. CVA volcanic rocks vary from andesite to dacite with SiO 2 between 57 and 66 wt.%, show low concentrations of Ti, P, Nb and Ta, are enriched in Light Rare Earths, depleted in Heavy Rare Earths, and show a small Eu anomaly; all indicative of arc-related volcanism associated with subduction of the Cocos plate under the North American plate, but complicated by the geometry of the plate boundary fault system.

Timing of Glacial Advances and Volcanic Activities on Mt. Tateyama, Hida Range, Central Japan during the Late Stade of the Last Glacial Period

The timing of the glacial advances and volcanic activities on Mt. Tateyama (3,015m) and Tateyama Volcano in the Hida Range in central Japan during the late stade of the last glacial period were clarified based on three moraines and their stratigraphic relationships with volcanic ejecta. The late stade was subdivided into Tateyama Stades I (just before 29cal ka), II (18-20 10Be ka) and III (10-11 10Be ka). The glaciers during the late stade attained their maximum advances in Tateyama Stade I and retreated with time. The maximum glacial advances during the late stade were caused just before 29cal ka in MIS 3 on the high mountains in the Hida Range simultaneously. Between Tateyama Stades I and II, the Tateyama Volcano gave rise to phreatic explosions that fragmented a lava layer overlying explosion crater. The fall of the lava fragments, however, did not trigger off the start of the glacial retreat and advance on Mt. Tateyama.

Gravity and magnetic investigation of maar volcanoes, Auckland volcanic field, New Zealand

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).

Deep impact craters in the Isidis and southwestern Utopia Planitia regions of Mars: High target material strength as a possible cause

Using THEMIS, MOC and MOLA data, we have found 51 craters in the diameter range 6–11.8 km within southwestern Utopia Planitia and Isidis Planitia that are significantly deeper than typical fresh craters in the northern lowlands of Mars. The restricted geographic distribution of these craters, their simple morphology, and data from impact and explosion crater studies suggest that unusually strong target materials (as much as a factor of 2 greater than average materials in the Martian lowlands) are the cause of the excessive crater depth. We propose that the greater target material strength acts to delay gravity‐dominated collapse to larger crater sizes. Furthermore, we suggest that a regional, olivine‐rich mafic to untramafic rock unit identified by TES and THEMIS is a reasonable candidate for these strong materials. The unit is exposed on the southern edge of Isidis basin and in crater ejecta within the basin, and forms layers that dip toward the Isidis Basin center.

Extrusive carbonatites: A brief review

49 known extrusive carbonatite occurrences are listed with brief details of their tectonic setting, structure, lithologies, associated silicate rocks, chemistry and presence or absence of included mantle materials. Half the occurrences appear to be related to tephra cones, tuff rings, diatremes and maars and the rest occur within strato-volcanoes. Pyroclastic carbonatitic rocks are present at all the localities, with carbonatite lava flows occurring at only 14 of them. The pyroclastic rocks, which include fallout tephra and deposits from pyroclastic surges and flows and products of phreatomagmatic eruptions, vary from rocks composed principally of carbonate to varieties with as little as 20% igneous carbonate. The most abundant silicate rocks associated with extrusive carbonatites are melilite-bearing rocks, nephelinite and/or ijolite, and phonolite and/or nepheline syenite; seven occurrences have no associated silicate rocks. 16 occurrences, most of them associated with small extrusive centres, contain mantle xenoliths or megacrysts, details of which are tabulated, with spinel lherzolite the most abundant rock type, but amphibole, phlogopite and garnet are also recorded. The lack of such materials in intrusive carbonatites may reflect their less energetic environment of emplacement. It is proposed that carbonatites are essentially of two types: (a) those rising energetically and rapidly from the mantle, which form small explosion craters, ash or tuff cones, or diatremes, have only low-volume associated silicate rocks, and entrain mantle debris, and (b) those which occur in strato-volcanoes, are associated with large volumes of silicate rocks and follow a more complex genesis, probably involving ponding and differentiation (separation from carbonate-bearing silicate magma) at higher levels in the mantle and/or crust. Most of the classic intrusive carbonatite complexes probably fall into the second category.

Limnological Aspects of Two Tropical Crater Lakes (Lago Biao and Lago Loreto) on the Island of Bioko (Equatorial Guinea)

The limnology of Lago Biao and Lago Loreto, two tropical crater lakes on the island of Bioko, was investigated for the first time in January of 2002. Lago Biao fills a caldera at 1750 m altitude and is 18.5 m deep. This lake was characterized by low conductivity (11.6-13.3 μS cm−1), low pH (5.9-6.2), an oxycline between 6 and 10 m depth (90-27% saturation), and a Secchi depth of 5.5 m. Dinoflagellates, desmids, and green algae dominated the phytoplankton community. The rotifers Anuraeopsis fissa, Polyarthra indica, Trichocerca simoneae, and T. similis were most abundant above or in the thermocline. During the day Tropocyclops prasinus tjeefi and Chaoborus cf. ceratopogones concentrated in the oxycline, whereas Daphnia laevis was most abundant between 0 and 4 m depth. In contrast, L. Loreto is 82 m deep and probably fills an explosion crater (‘maar’) at 1050 m altitude. The lake had a turbid, yellowish appearance and a Secchi depth of only 0.6 m. Oxygen content dropped from 121% at the surface to 6% in 10 m depth. The pH declined from 9.15 at the surface to 7.4 in only 2.5 m depth and became acidic at approximately 15 m depth. One species of cyanobacterium, one dinoflagellate and one green alga dominated the phytoplankton and together reached densities of 12 100 cells ml−1. Only one rotifer (Brachionus angularis) and one copepod (Tropocyclops prasinus tjeefi) species colonized the open water. Intact algal cells and zooplankton specimens were encountered near the bottom.

Explosive craters and soil liquefaction.

Curious dry craters formed in the aftermath of a disastrous earthquake are explained. The Bhuj earthquake in January 2001, with a moment magnitude of 7.7, was one of the most disastrous in India's recorded history. Although its fault plane did not rupture the surface, the earthquake caused ground failure, including the formation of sand-blow craters and sand fissures, indicative of liquefaction of subsurface sediment, over an area of about 15,000 square kilometres1. Here we investigate the two large, dry craters that formed during the earthquake, and estimate the likely subsurface pressure at the time from a trajectory analysis of the ejected clasts. We conclude that the craters were created by explosive deformation of the ground that probably resulted from the delayed effects of soil liquefaction.

A 28 000–7600 cal yr BP pollen record of vegetation and climate change from Pukaki Crater, northern New Zealand

A pollen record of vegetation and climate change representing the interval ca. 28 000–7600 cal yr BP was obtained from a 52.5-m sediment core taken from a volcanic explosion crater at present sea level in Auckland, New Zealand (36°59′S). This provides one of the few continuous terrestrial records covering Marine Isotope Stage 2 and part of Stage 1 from the mid-latitudes of the Southern Hemisphere. Chronology of the record is underpinned by eight distal Taupo Volcanic Zone rhyolitic tephra of known age augmented by thirteen AMS radiocarbon dates. The Last Glacial Maximum cooling extended from at least c. 28 000 cal yr BP to ca. 18 000 cal yr BP when vegetation apparently consisted of patches of beech-dominated forest embedded within shrubland and grassland. Forest patches were probably larger or more widespread than those in central North Island to the south but less extensive than further to the north, in Northland. Climate warming commenced ca. 18 000 cal yr BP, marked by the replacement of beech by conifers and angiosperms, and by the expansion of forest at the expense of shrubland and grassland. No clear Late Glacial climatic reversal is evident at this site. Early Holocene forest is characterised by the replacement of the dominant forest tree Prumnopitys taxifolia by Dacrydium cupressinum. Following breach of the crater rim ca. 7600 cal yr BP by rising sea level the maar was rapidly infilled with marine sediments over a period of ca. 1000 years.

Exploration and discovery in Yellowstone Lake: results from high-resolution sonar imaging, seismic reflection profiling, and submersible studies

‘No portion of the American continent is perhaps so rich in wonders as the Yellow Stone’ (F.V. Hayden, September 2, 1874) Discoveries from multi-beam sonar mapping and seismic reflection surveys of the northern, central, and West Thumb basins of Yellowstone Lake provide new insight into the extent of post-collapse volcanism and active hydrothermal processes occurring in a large lake environment above a large magma chamber. Yellowstone Lake has an irregular bottom covered with dozens of features directly related to hydrothermal, tectonic, volcanic, and sedimentary processes. Detailed bathymetric, seismic reflection, and magnetic evidence reveals that rhyolitic lava flows underlie much of Yellowstone Lake and exert fundamental control on lake bathymetry and localization of hydrothermal activity. Many previously unknown features have been identified and include over 250 hydrothermal vents, several very large (>500 m diameter) hydrothermal explosion craters, many small hydrothermal vent craters (∼1–200 m diameter), domed lacustrine sediments related to hydrothermal activity, elongate fissures cutting post-glacial sediments, siliceous hydrothermal spire structures, sublacustrine landslide deposits, submerged former shorelines, and a recently active graben. Sampling and observations with a submersible remotely operated vehicle confirm and extend our understanding of the identified features. Faults, fissures, hydrothermally inflated domal structures, hydrothermal explosion craters, and sublacustrine landslides constitute potentially significant geologic hazards. Toxic elements derived from hydrothermal processes also may significantly affect the Yellowstone ecosystem.

Hydrothermal and tectonic activity in northern Yellowstone Lake, Wyoming

Yellowstone National Park is the site of one of the world9s largest calderas. The abundance of geothermal and tectonic activity in and around the caldera, including historic uplift and subsidence, makes it necessary to understand active geologic processes and their associated hazards. To that end, we here use an extensive grid of high-resolution seismic reflection profiles (∼450 km) to document hydrothermal and tectonic features and deposits in northern Yellowstone Lake. Sublacustrine geothermal features in northern Yellowstone Lake include two of the largest known hydrothermal explosion craters, Mary Bay and Elliott9s. Mary Bay explosion breccia is distributed uniformly around the crater, whereas Elliott9s crater breccia has an asymmetric distribution and forms a distinctive, ∼2-km-long, hummocky lobe on the lake floor. Hydrothermal vents and low-relief domes are abundant on the lake floor; their greatest abundance is in and near explosion craters and along linear fissures. Domed areas on the lake floor that are relatively unbreached (by vents) are considered the most likely sites of future large hydrothermal explosions. Four submerged shoreline terraces along the margins of northern Yellowstone Lake add to the Holocene record of postglacial lake-level fluctuations attributed to “heavy breathing” of the Yellowstone magma reservoir and associated geothermal system. The Lake Hotel fault cuts through northwestern Yellowstone Lake and represents part of a 25-km-long distributed extensional deformation zone. Three postglacial ruptures indicate a slip rate of ∼0.27 to 0.34 mm/yr. The largest (3.0 m slip) and most recent event occurred in the past ∼2100 yr. Although high heat flow in the crust limits the rupture area of this fault zone, future earthquakes of magnitude ∼5.3 to 6.5 are possible. Earthquakes and hydrothermal explosions have probably triggered landslides, common features around the lake margins. Few high-resolution seismic reflection surveys have been conducted in lakes in active volcanic areas. Our data reveal active geothermal features with unprecedented resolution and provide important analogues for recognition of comparable features and potential hazards in other subaqueous geothermal environments.

Waterbirds of Alkaline Lakes in Western Uganda

ABSTRACT Uganda's only alkaline lakes are found in the Queen Elizabeth Conservation Area and the adjoining Kyambura Wildlife Reserve. Both are Important Bird Areas, a status to which the birds of the lakes contribute. A total of 179 waterbird counts were made between 1984 and 2000, covering eight of the nine alkaline lakes, all of which are small explosion craters. Of the 75 species counted, all but three were non-specialists. Maxima are given for all species, together with seasonal data for five crater lakes. Four lakes regularly supported more than 1,000 Lesser Flamingos each, the maximum being 60,000. They were Maseche, Bagusa, Nshenyi and Munyanyange. Breeding has been attempted, but has been unsuccessful so far. The more important flamingo lakes had high values for conductivity, above 15,000 μS cm−2, whilst species richness is associated with muddy shores and, probably, intermediate levels of alkalinity, between 10 and 50 Meq−1. The lakes are important scenically, for ecotourism, and for the conserva...

The case for rainfall on a warm, wet early Mars

Valley networks provide compelling evidence that past geologic processes on Mars were different than those seen today. The generally accepted paradigm is that these features formed from groundwater circulation, which may have been driven by differential heating induced by magmatic intrusions, impact melt, or a higher primordial heat flux. Although such mechanisms may not require climatic conditions any different than today's, they fail to explain the large amount of recharge necessary for maintaining valley network systems, the spatial patterns of erosion, or how water became initially situated in the Martian regolith. In addition, there are no clear surface manifestations of any geothermal systems (e.g., mineral deposits or phreatic explosion craters). Finally, these models do not explain the style and amount of crater degradation. To the contrary, analyses of degraded crater morphometry indicate modification occurred from creep induced by rain splash combined with surface runoff and erosion; the former process appears to have continued late into Martian history. A critical analysis of the morphology and drainage density of valley networks based on Mars Global Surveyor data shows that these features are, in fact, entirely consistent with rainfall and surface runoff. The necessity for a cold, dry early Mars has been predicated on debatable astronomical and climatic arguments. A warm, wet early climate capable of supporting rainfall and surface runoff is the most plausible scenario for explaining the entire suite of geologic features in the Martian cratered highlands.

Distal record of multi-sourced tephra in Onepoto Basin, Auckland, New Zealand: implications for volcanic chronology, frequency and hazards

We have documented 80 tephra beds dating from ca. 9.5 to >50 ka, contained within continuously deposited palaeolake sediments from Onepoto Basin, a volcanic explosion crater in Auckland, New Zealand. The known sources for distal (>190 km from vent) tephra include the rhyolitic Taupo Volcanic Centre (4) and Okataina Volcanic Centre (14), and the andesitic Taranaki volcano (40) and Tongariro Volcanic Centre (3). The record provides evidence for four new events between ca. 50 and 28 ka (Mangaone Subgroup) suggesting Okataina was more active than previously known. The tephra record also greatly extends the known northern dispersal of other Mangaone Subgroup tephra. Ten rhyolitic tephra pre-date the Rotoehu eruption (>ca. 50 ka), and some are chemically dissimilar to post-50 ka rhyolites. Some of these older tephra were produced by large-magnitude events; however, their source remains uncertain. Eight tephra from the local basaltic Auckland Volcanic Field (AVF) are also identified. Interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from Taranaki volcano in the 27.5–9.5-ka period. In addition, 28 older events are recognised. The tephra are trachytic to rhyolitic in composition. All have high K2O contents (>3 wt%), and there are no temporal trends. This contrasts with the proximal lava record that shows a trend of increasing K2O with time. By combining the Onepoto tephra record with that of the previously documented Pukaki crater, 15 AVF basaltic fall events are constrained at: 34.6, 30.9, 29.6, 29.6, 25.7, 25.2, 24.2, 23.8, 19.4, 19.4, 15.8 and 14.5 ka, and three pre-50 ka events. This provides some of the best age constraints for the AVF, and the only reliable data for hazard recurrence calculations. The minimum event frequency of both distal and local fall events can be estimated, and demonstrates the Auckland City region is frequently impacted by ash fall from many volcanoes.

Volcano-tectonic activity along structures of the unstable NE flank of Mt. Etna (Italy) and their possible origin

We describe the prehistoric–historic activity of the main and minor volcano-tectonic structures of the northeastern flank of Mt. Etna, giving new data which provide a clearer view of how destabilized volcanoes react to magma pressure and gravity forces. Data are mostly based on new lithostratigraphic surveys and on offset measurements of late Pleistocene–Holocene deposits and historic man-made features. The North-East Rift (NE Rift) is a volcanic rift that has guided mostly explosive, but also secondary effusive, activity since the end of the Pleistocene. Monogenetic cones and explosive craters are strongly elongated and aligned along 66 main surface fractures and several others, hidden, but inferred, from sinkhole alignments. The opening direction is locally oblique, with a left-lateral component in the north-eastern segment of the rift. At the NE tip of the rift, the movements are relieved by the normal left-lateral Pernicana Fault (PF), an 11-km-long structure comprising E–W fault segments connected by right-stepping en échelon faults. Our study demonstrates that the very high slip rates at a centennial scale of these structures are well constrained and similar (NE Rift=0.6–1.8 cm/a, PF=0.5–2.2 cm/a) and have been also recognized in the Latest Pleistocene–Holocene (2.7 cm/a). The E–W Fiumefreddo Fault (FF) has normal left-lateral kinematics and a historic slip rate of 0.1 cm/a. Ripa della Naca Faults strike NE and are covered by unfaulted <15-ka-old lavas. Relative sinking to the SE gives a late Pleistocene slip rate of 0.2 cm/a. Newly recognized NNE- to NE-striking active normal faults interfere at the PF–FF junction zone. The active East-North-East Rift, which is described in detail for the first time, is another volcanic rift affecting the eastern flank of Mt. Etna. This rift shows left-lateral normal motions and has guided the location of several pyroclastic cones since at least the end of the Pleistocene. Stratigraphic and structural data prove the presence of a North–South Rift which has existed across the Mt. Etna summit zone during at least the last 14 ka. Our data coupled with published seismological data indicate that these structures are the surface expressions of the seaward sliding of the eastern flank of Etna along two slip surfaces in a two-component principal stress field, produced by gravity loading and magma intrusion. A shallower slide surface can be activated following dominance of gravity instability triggered by the mass of the volcano flank, or movements can occur along a deeper surface triggered by injection phases, or the two surfaces can move at the same time by a combination of effects. The existence of this double system of deformation can explain several apparently contradictory phenomena, such as discontinuous correlation of deformation, earthquakes and eruptions, and different earthquake foci, focal mechanisms and fault slip rates.

Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano

Mesoscopic structures in four large cavities on the Big Obsidian Flow, OR, USA, provide evidence of links between large-scale surface folding, cavity formation, and hazardous explosive activity originating from within the flow. Stereographic analysis of flow banding, stretched bubble lineation, striations, and mesoscopic fold axes indicates that three of the cavities are near-cylindrical folds. Cavity fold axes are oriented parallel to large-scale compressional flow ridge axes, suggesting that cavities form in response to shortening during flow advance. Large surface folds develop as the upper 10 m of the cooler flow surface buckles when the flow front cools, slows, and stalls against topographic barriers. Void space is created in fold hinges as obsidian layers shear past each other and pull apart. The resultant cavities may serve as reservoirs for exsolved volatiles or surface water which may later vent explosively to form explosion craters on the flow surface.

The Quetzalapa Pumice: a voluminous late Pleistocene rhyolite deposit in the eastern Trans-Mexican Volcanic Belt

The study area is located in the east part of the Trans-Mexican Volcanic Belt, in the Las Cumbres Volcanic Complex (LCVC) which lies between two large stratovolcanoes: Pico de Orizaba (5700 m a.s.l.) to the south, and Cofre de Perote (4200 m a.s.l.) to the NNE. The most conspicuous structure of the LCVC is a 4-km-diameter circular crater with a dacitic dome in the center, which constitutes the remains of a destroyed stratovolcano. The Quetzalapa Pumice (QP) was produced by a plinian eruption that was dated by the 14C method at 20 000 yr. BP. The eruptive sequence consists predominantly of pumice fall deposits and scarce intra-plinian pyroclastic flow deposits, which crop out on the west flank of the LCVC. The absence of post-plinian ignimbrite deposits is striking. The deposits are well sorted, clast-supported with reverse grading at the base, with a medium to high accessory lithics content. The maximum average thickness of the deposit in the proximal areas is about 15 m and has been divided into three members: the Basal Member (BM), 2 m thick with four submembers (BMf 1, BMf 2, BMf 3, and BMafl), the Intermediate Member (IM), 10 m thick with two submembers (IMpf and IMaf), and the Upper Member (UM), 3 m thick with four submembers (UMpl, UMsdf, UMwaf, and UMpls). The predominant component of the fall deposits is a white, highly vesiculated pumice with 71% SiO 2 content. Plagioclase is the most abundant mineral followed by 1–3-mm-long biotite phenocrysts. The accessory lithics are lavas mostly of andesitic composition. Their abundance increases toward the uppermost levels of the sequence. We calculate a minimum volume of 8.4 km 3 (2.22 km 3 dense rock equivalent), for the entire QP deposit. Isopach and isopleth maps show that the IM deposit has an elongated distribution with a NNE–SSW direction, whereas the UM deposit has a circular distribution. We estimate a maximum eruptive column height for the IM of 20 km. Field studies and isopach and isopleth maps indicate that the eruptive column was affected by a strong wind. Previous studies located the QP source in the Las Cumbres crater. However, based on the isopach and isopleth distribution, and the lack of pumice fall deposits inside the Las Cumbres crater, we consider that the QP emission center is located on the west flank of the LCVC, and was buried by its own pumice fall deposits. It coincides with an explosion crater called La Capilla formed during the closing phase of the QP eruption. A ‘pumice fountain’ model is proposed to explain the observed sequence of deposits. According to this model, the material was emitted through a ‘hose-type’ conduit during a monogenetic eruption of rhyolitic composition. This kind of volcanic activity is not extensively reported in the literature.

Pyroclastic flow and explosive activity at Soufrière Hills Volcano, Montserrat, during a period of virtually no magma extrusion (March 1998 to November 1999)

Abstract Dome growth at Soufrière Hills Volcano halted in early March 1998. After dome growth ceased, seismicity reduced significantly, but activity related to dome disintegration and degassing of magma at depth continued. A sustained episode of pyroclastic flows on 3 July 1998 marked the single largest collapse from March 1998 to November 1999. This led to a remarkable episode of dome collapses, low-energy explosions and ash-venting that resulted in the regular production of ash plumes, commonly reaching 1.5-6 km above sea level (a.s.l), but sometimes up to 11 km a.s.l., and the development of a small block-and-ash cone around the explosion crater. During the period of this residual activity, higher levels of activity occurred approximately every five to six weeks. This periodicity was similar to the cycles observed during active dome growth during 1995 to 1998, and probably had a similar cause. The relatively high level of observed activity caused continued concern regarding volcanic hazards and their potential to impact upon the resident population. Vigorous magma extrusion resumed in November 1999. The activity of the intervening period is attributed to the continued cooling and degassing of the dome, conduit and deep magma body, the impact of rising volcanic gases in the volcanic edifice, and limited magma flow in the conduit.

Взрывные кратеры на поверхности космических аппаратов, образованные метеороидами и частицами космического мусора

Взрывные кратеры на поверхности космических аппаратов, образованные метеороидами и частицами космического мусора