Angular Blocks Research Articles

Geological Studies to Support the Tourism Site: A Case Study in the <i>Rafflesia</i> Trail, Near Kampung Jedip, Lojing Highlands, Kelantan, Malaysia

Some geological studies were performed in the Rafflesia Trail, near Kampung Jedip, in Lojing Highlands, Kelantan. This area is famous for its Rafflesia and has become one of the tourism attractions in Kelantan. However, the recent development of agricultural activities has significant effects on the area. The objective of these studies is to give substantial geological information including geomorphology, lithology, structural features, geohazard potentials, and water quality analysis in the study area. Desk study was conducted by reviewing some literatures related to the topic and the study area. Field work was organized during a scientific expedition in January 2014, to collect data, samples, and photographs. Geomorphologically, the study area is a mountainous area which mostly consists of mountain ridges and mountain valleys. Some fluvial features occur in the study area, such as waterfalls, cascades, rapids, runs, pools, potholes, lateral bars, and point bars. This area is composed of granitic rocks, mostly granite porphyry, and covered by superficial deposits of Quaternary age. The geological structure that is commonly found in the study area is joints, where they divide the rock body into large, roughly angular blocks, which is called as brecciation. The potential geohazard in the study area is landslides, where some of them are composed of soil only and others are mixtures of rock and soil. The water quality analysis has managed to identify that the stream water in this area generally has the quality of Class III (according to INWQS for Malaysia). These studies recommend that this area should be supported as a sustainable tourism site in Lojing Highlands.

Holocene Pedogenesis in Fluvial Deposits of the Conejos River Valley, Southern Colorado

Relatively few geomorphic studies have examined Holocene-aged soils developed in alluvial deposits in the Rocky Mountains. Here, we present a soil morphological investigation from a suite of fluvial terraces in the glaciated portion of the Conejos River Valley, southern Colorado. The surficial geology of 25 km of the glacial valley was mapped in detail. Within three separate sub-reaches (Platoro, Lake Fork and South Fork) a total of thirteen soil pits and exposures were excavated and described on alluvial deposits. Soil samples were analyzed for particle size and extractable iron. Soil horizonation (A/C to A/B/2C), structure (fine sub angular to medium angular blocks), clay content of the B horizon (8.0% to 22.8%) and Feo/Fed (0.39 to 0.80) illustrate trends with relative terrace deposit age in individual sub-reaches. However, only Feo/Fed ratios displayed similar rates of development between all sub-reaches highlighting the usefulness of this metric for determining accurate rates of pedogenesis and relative age for Holocene-aged deposits in sub-alpine environments. Results indicate that clay content and structure developed in alluvial deposits of similar ages vary between sub-reaches. Clay contents were found to be lower in the Platoro sub-reach (e.g. 13.1% at Platoro and 20.0% at South Fork on Qt1 deposits). This variation is attributed to heterogeneity in the nature of the inherited parent material and potential variability in aeolian dust contributions throughout the Holocene.

Unusual lapilli tuff ejecta erupted at Stromboli during the 15 March 2007 explosion shed light on the nature and thermal state of rocks forming the crater system of the volcano

Textural and mineralogical study of high-temperature, angular blocks erupted during the Stromboli explosion of 15 March 2007 was used to make inferences on the nature and thermal state of rocks forming the subsurface of the volcano' summit crater terrace. The studied ejecta consist of lapilli tuff that formed as a result of the transformation and high temperature induration (sintering) of the basaltic scoriae, lapilli and ash originally accumulated as loose tephra during the current activity of the volcano. The main processes leading to the tephra transformation were investigated through microstructural observations, mineral and glass analyses (SEM-EDS and EMP analyses). Investigations revealed that subsolidus reactions and partial melting of the tephra occurred, at temperatures higher than 600°C and under variable fO2 conditions from QFM to HM buffering curves. In some blocks, evidence of high-T reheating and partial melting at the expense of secondary hydrothermal minerals was also observed. In order to track the subsolidus reheating history of the basaltic pyroclasts, a detailed study of the pseudomorphic phases and reactions after olivine, driven by iron oxidation under high-T conditions, was performed. The observed mineralogical transformation suggests that the lapilli tuff material, originating from the burial of tephra routinely accumulated by persistent Strombolian explosions within the crater terrace, were in some cases altered by the circulation of acidic fluids and were in any case reheated due to isotherm rise forced by high heat flux and gas streaming delivered by the underlying magma system. It is worth noting that the ejection of these unusual volcanic lithotypes was possible because a few days before the 15 March 2007 event, the craters were clogged with lapilli tuff material that slid into the crater bottom between 7 and 9 March. Findings of this study suggest that the scattered permanently active vents and shallow conduits of Stromboli are surrounded sideways and underneath the crater terrace, by a fairly large volume of high temperature rocks with variable degree of compaction, sintering up to partially melted. Such a spectrum of rock types is in good agreement with the conceptual model of prominent thermal zoning all around (sideway and upwards) the active magmatic system. We speculate that continuous migration upwards of isotherms led to transformation and partial melting of the normal Strombolian tephra.

In vitro evaluation of the imaging accuracy of C‐arm conebeam CT in cerebral perfusion imaging

The authors have developed a method to enable cerebral perfusion CT imaging using C-arm based conebeam CT (CBCT). This allows intraprocedural monitoring of brain perfusion during treatment of stroke. Briefly, the technique consists of acquiring multiple scans (each scan comprised of six sweeps) acquired at different time delays with respect to the start of the x-ray contrast agent injection. The projections are then reconstructed into angular blocks and interpolated at desired time points. The authors have previously demonstrated its feasibility in vivo using an animal model. In this paper, the authors describe an in vitro technique to evaluate the accuracy of their method for measuring the relevant temporal signals. The authors' evaluation method is based on the concept that any temporal signal can be represented by a Fourier series of weighted sinusoids. A sinusoidal phantom was developed by varying the concentration of iodine as successive steps of a sine wave. Each step corresponding to a different dilution of iodine contrast solution contained in partitions along a cylinder. By translating the phantom along the axis at different velocities, sinusoidal signals at different frequencies were generated. Using their image acquisition and reconstruction algorithm, these sinusoidal signals were imaged with a C-arm system and the 3D volumes were reconstructed. The average value in a slice was plotted as a function of time. The phantom was also imaged using a clinical CT system with 0.5 s rotation. C-arm CBCT results using 6, 3, 2, and 1 scan sequences were compared to those obtained using CT. Data were compared for linear velocities of the phantom ranging from 0.6 to 1 cm∕s. This covers the temporal frequencies up to 0.16 Hz corresponding to a frequency range within which 99% of the spectral energy for all temporal signals in cerebral perfusion imaging is contained. The errors in measurement of temporal frequencies are mostly below 2% for all multiscan sequences. For single scan sequences, the errors increase sharply beyond 0.10 Hz. The amplitude errors increase with frequency and with decrease in the number of scans used. Our multiscan perfusion CT approach allows low errors in signal frequency measurement. Increasing the number of scans reduces the amplitude errors. A two-scan sequence appears to offer the best compromise between accuracy and the associated total x-ray and iodine dose.

Morphology-controlled synthesis of tungsten oxide hydrates crystallites via a facile, additive-free hydrothermal process

Tungsten oxide hydrates crystallites with a diversity of morphologies were successfully synthesized by employing peroxo-polytungstic acid as the precursor via a facile and additive-free hydrothermal process. The crystal structure, morphology and orientation relationship were investigated by using X-ray diffractometer, scanning electron microscopy and transmission electron microscopy. Results show that with the increase of the hydrothermal reaction time, the product morphology shows an obvious evolution from hexagonal plates to round angular blocks and to hexagonal gears, through irregular nanosheets and to cuboid rods finally. Furthermore, the driving force of this morphology evolution was found to be correspondent with the interesting change of the orientation growth and the phase transition of tungsten oxide hydrates under the hydrothermal process.

Geology of Stephens Island and the age of Stephens formation

On Stephens Island, northernmost Marlborough Sounds, New Zealand, 1100 m of southeast-dipping sedimentary rocks, ranging from mudstone with limestone lenses to volcanogenic sandstone and conglomerate, comprise Stephens Formation, the youngest formation within the Maitai Group. The rocks form the most northerly exposure of the Nelson Regional Syncline; younging directions show that they are overturned and therefore are on the eastern limb of the syncline. They are well indurated with widespread metamorphic prehnite, pumpel1yite and lawsonite. No top or bottom to the formation is exposed. The formation is subdivided into Queens Beach, Incline, Takapourewa, and Titapua Members. The oldest rocks (Queens Beach Member) comprise 150 m of rounded cobble conglomerate which encloses angular blocks of coarse volcanogenic sandstone. One such block, 4 m long and up to 1.5 m thick, contains a diverse but indifferently preserved marine fossil assemblage dominated by brachiopods and pectinid bivalves of Late Permian age....

Spatial arrangement of soil mantle in Glacis de Buenavista, Mexico as a product and record of landscape evolution

Factors controlling spatial soil organization have recently attracted the attention of soil scientists because they can contribute to regional management of soil resources. We present here a case study in the Glacis de Buenavista (Mexico), an extended volcanic piedmont, in order to develop an integral understanding of its soil cover as a product and, at the same time, as a record of the main stages of landscape evolution. Three phenomena were considered: Luvisol and Vertisol type soils, and pedosediments, all of them analyzed in terms of their development degree (clay content, macro and micromorphology, selective extractions of Fe (Fe d, Fe o), silicon (Si o) and aluminum (Al o), weathering index and clay mineralogy). Luvisols, represented in the Ahuatenco, Mexicapa and Buenavista sections, are located in the northern and central part of the area. They are red-clayey, polygenetic soils that show strong weathering (with high kaolinitic clay content and high weathering index values), clay illuviation, and reductomorphic processes, combined with vertic features in some parts of the profiles. Vertisols dominate the central and southern portion of the Glacis. They exhibit cracking, angular blocks with wedge-like shapes, slickensides and stress-cutans. The pedogenesis of the Vertisol type is clearly associated with the presence of smectitic minerals in the clay fraction as a product of neoformation. Factors controlling this pedodiversity are: relief, which affects bioclimatic differentiation within the studied landsurface and defines the lateral redistribution of moisture and dissolved substances; and time. Luvisols represent a longer pedogenetic phase that started in the Late Pleistocene, according to the age of buried paleosols, while Vertisols mainly originated in the Late Holocene (according to the age of buried organic horizons). Pedosediments are located in the central area of the Glacis. Here, past and present geomorphic processes interact to produce the greatest soil diversity. The recent human-induced erosion partly destroyed polycyclic Luvisols and exposed ancient subsurface pedosedimentary strata. At the same time it produced patches of unconsolidated pedosediments consisting mostly of redeposited Luvisol materials.

Hominin occupations at the Dmanisi site, Georgia, Southern Caucasus: Raw materials and technical behaviours of Europe’s first hominins

Dmanisi is the oldest site outside of Africa that records unquestioned hominin occupations as well as the dispersal of hominins in Europe and Asia. The site has yielded large numbers of artefacts from several periods of hominin occupation. This analysis of Dmanisi stone tool technology includes a review of all the pieces recovered during the last 15 years of excavations. This lithic assemblage gives insights into the hominin behaviour at 1.7–1.8 Ma in Eurasia. Dmanisi hominins exploited local rocks derived from either nearby riverbeds or outcrops, and petrographic study provides data on patterns of stone procurement. Recent geological surveys and technological studies of the artefacts illustrate the roles of hominins in composing the assemblage. Dmanisi hominins selected two types of blanks, including cobbles and angular blocks, of basalt, andesite, and tuffs. Many complete cobbles, pebbles, and rolled blocks in basalt were unmodified, and geological analyses and surveys indicate that hominins brought manuports back to the site, suggesting a complex procurement strategy. Cores, flakes and debris show that all stages of flaking activity took place at the site. Numerous unifacial cores suggest that knapping was not very elaborate. Centripetal knapping is observed on some flake-cores. Knapping was influenced by the blank shape and natural angles. Most flaked objects were either cores or chopper-cores. Flakes predominate while flake tools are rare. The Dmanisi lithic assemblage is comparable to Oldowan sites in Africa in terms of reduction sequence, organisation of the removals, platform types, and the lack of retouched flakes. Dmanisi artefacts and may have been produced by the original hominins in Europe and Asia.

A recent landslide on the east Devon coast, UK

The largest landslides on the south coast of England are those associated with failure surfaces in montmorillonite- rich mudstones close to the base of the Gault Formation (mid Cretaceous). Those at Folkestone, Sussex (Trenter & Warren 1996), Ventnor, Isle of Wight (Hutchinson 2001) and Black Ven, west Dorset (Brunsden 2002) are among the largest active landslide systems in the UK. Westwards from Dorset, the Gault facies of the Albian Stage is replaced on the east Devon coast by sandstones and sandy calcarenites of the Upper Greensand Formation. These are mostly moderately strong rocks that crop out in almost continuous, precipitous cliffs between Sidmouth and the county boundary at Lyme Regis. They are a major component of the landslide debris, but the formation itself has not previously been shown to have initiated a failure. For example, the extensive Bindon landslide [SY 277 895] near Axmouth, which incorporates large masses of Upper Greensand, was attributed to failure surfaces in the Gault (Conybeare et al . 1840). However, that formation has not been recorded in a coastal section west of Lyme Regis. Joint/bedding-bounded toppling failures have been observed from time to time in the highest, more massive part of the Upper Greensand along the east Devon coast. The most recent of these occurred after a prolonged period of wet weather in 2001 when a single block of about 3000 tonnes in the highest part of the Upper Greensand fell over 120 m from the western end of Higher Dunscombe Cliff, Salcombe Regis [SY 150 878] and broke up on the beach. A similar mechanism has until now seemed to be the most likely explanation to account for the accumulations of angular blocks of Upper Greensand up to several hundred tonnes in weight that are common in the intertidal area between Beer Head and Sidmouth …

Deformations in Transform Faults with Rotating Crustal Blocks

A continuum model of deformation of a transform fault is considered. The fault interior is modelled as a part of a plate fragmented into a large number of angular blocks. The blocks are not joined together by any binder, but are rather held together due to external compression such that they can, in principle, rotate independently. Significant numbers of blocks involved in the deformation process permit a continuum description of the deformation, which in view of the possibility of independent rotations, necessitates the application of the Cosserat theory. The crucial point in the described model is the interconnection between the rotations and the normal stresses associated with the angular shape of the blocks: the rotating blocks ‘elbow’ one another. Furthermore, elbowing produces compressive stresses independent of the direction of rotations. Consequently, the constitutive equations become non-linear involving absolute values of the components of the curvature tensor. The paper analyses a simple shear of a fault under constraining compression acting in the direction normal to the fault. An infinite layer subjected to opposite displacements and zero rotations at the edges is considered. It is shown that block rotations can lead to complex deformation patterns. There exists a displacement threshold proportional to the pressure: for imposed displacements below the threshold, the deformation pattern coincides with the conventional one as predicted for a classical elastic isotropic layer with uniform displacement gradient in the absence of block rotations. When the imposed displacement exceeds the threshold value, boundary zones of non-uniform rotations, displacement gradients and dilatation emerge. It is interesting to note that these features, which could be mistaken for indicators of non-elastic or localisation processes, occur in a situation where only elastic processes are acting at the scale of blocks and no friction or other energy dissipation processes take place.

Arenal-type pyroclastic flows: A probabilistic event tree risk analysis

A quantitative hazard-specific scenario-modelling risk analysis is performed at Arenal volcano, Costa Rica for the newly recognised Arenal-type pyroclastic flow (ATPF) phenomenon using an event tree framework. These flows are generated by the sudden depressurisation and fragmentation of an active basaltic andesite lava pool as a result of a partial collapse of the crater wall. The deposits of this type of flow include angular blocks and juvenile clasts, which are rarely found in other types of pyroclastic flow. An event tree analysis (ETA) is a useful tool and framework in which to analyse and graphically present the probabilities of the occurrence of many possible events in a complex system. Four event trees are created in the analysis, three of which are extended to investigate the varying individual risk faced by three generic representatives of the surrounding community: a resident, a worker, and a tourist. The raw numerical risk estimates determined by the ETA are converted into a set of linguistic expressions (i.e. VERY HIGH, HIGH, MODERATE etc.) using an established risk classification scale. Three individually tailored semi-quantitative risk maps are then created from a set of risk conversion tables to show how the risk varies for each individual in different areas around the volcano. In some cases, by relocating from the north to the south, the level of risk can be reduced by up to three classes. While the individual risk maps may be broadly applicable, and therefore of interest to the general community, the risk maps and associated probability values generated in the ETA are intended to be used by trained professionals and government agencies to evaluate the risk and effectively manage the long-term development of infrastructure and habitation. With the addition of fresh monitoring data, the combination of both long- and short-term event trees would provide a comprehensive and consistent method of risk analysis (both during and pre-crisis), and as such, an ETA is considered to be a valuable quantitative decision support tool.

Molecular assembly of 1,3,5-tris(cyanomethyl) and 1,4-bis(cyanomethyl)arenes with silver triflate

Dicyano- and tricyano-substituted aromatic angular building blocks were systematically complexed with silver triflate, and their structures were determined by means of single-crystal X-ray diffraction. The molecular assembly of 1,3,5-tris(cyanomethyl)-2,4,6-triethylbenzene with silver triflate from benzene resulted in a layered structure with distorted square pyramidal silver sites. The structure resulting from the complexation of 1,3,5-tris(cyanomethyl)-2,4,6-trimethylbenzene with silver triflate is dependent on the solvent of crystallization. From benzene or toluene, reaction of 1,3,5-tris(cyanomethyl)-2,4,6-trimethylbenzene with silver triflate yielded a porous, channel-containing, solvated structure, but from acetone the resulting material was a network solid containing no solvent. Complexation of 1,4-bis(cyanomethyl)-2,3,5,6-tetraethylbenzene and 1,4-bis(cyanomethyl)-2,3,5,6-tetramethylbenzene with silver triflate resulted in network solids where the triflate anions were strongly coordinated to the silver.

Relationships in the alpine Ranunculus haastii (Ranunculaceae) complex and recognition of R. piliferus and R. acraeus from southern New Zealand

Growth habit, vegetative, floral characters, and molecular data support the recognition of Ranunculus haastii and R. piliferus at species rank, rather than as intraspecific taxa at the rank of subspecies. Therefore, R. piliferus is a new combination at species rank for plants previously known as R. haastii subsp. piliferus. R. acraeus is a new species distinguished from R. piliferus by its finely crenate leaf and bract margins, glabrous peduncle, and in having 6–7 sepals that are hairy on the abaxial surface and glabrous on the adaxial surface. R. piliferus occurs in northern Southland and South Otago and R. acraeus occurs in North Otago and South Canterbury. R. piliferus and R. acraeus grow in relatively stable alpine rock‐fields that comprise rock fractured into large, coarse, and angular blocks of differing sizes and shapes. R. haastii occurs in Canterbury and Marlborough where it grows on mobile fine‐grained scree in the alpine zone. Phylogenetic analyses of nrDNA ITS and cpDNA JSA sequence data provide support for the segregation of R. piliferus from R. haastii, and for the recognition of R. acraeus. R. piliferus forms a distinct genotype in both the ITS and JSA analyses. R. acraeus and one sample of R. haastii have a distinct JSA genotype, and R. acraeus and R. buchananii share a nine‐base‐pair indel. In the ITS analysis R. acraeus and most samples of R. grahamii share the same genotype, and along with R. haastii form a northern, alpine group of diverse genotypes.

Frost damage of bricks composing a railway tunnel monument in Central Japan: field monitoring and laboratory simulation

Abstract. Bricks of tunnels and bridges of Usui Pass railway (Japan) exposed to north are subject to frost damage. Average depth of erosion due to detachment of angular blocks is around 1-1.5 cm. In order to assess this weathering and to understand its mechanism, an experimental study was carried out in the field and laboratory. Field monitoring showed the combination of seasonal and diurnal freezing with a maximum of heave when the freezing front reached 5 cm depth. Bricks taken from the site were submitted to unidirectional freezing at capillary and vacuum saturation in the laboratory. Results showed that frost damage of bricks was favoured by high saturation level and repetition of freeze-thaw cycles.

Experimenting in the Kitchen

GEOPHYSICS The surfaces of basaltic lavas commonly exhibit two kinds of textures: Pāhoehoe flows form a ropy and relatively smooth surface, and ‘a'ā flows look like jumbled, sharp, angular blocks. It is generally thought that these types reflect an interaction between the viscosity of the lava, which varies as it cools and crystals form, and the shear rate of the flow. Many flows change their morphology from pa−hoehoe to ‘a'ā, and a few change back. To investigate this transition, Soule and Cashman carried out a series of laboratory experiments using corn syrup (diluted to the viscosity of hot basaltic magma) and rice (which has the same density as the diluted syrup and represents the lava crystals). They observed four different regimes: With increasing amounts of rice (corresponding to increasing viscosity), flow is laminar; the rice grains aggregate into clumps; shear zones form between the clumps; and finally, a thin film of rice-free syrup appears along the flow boundary, perhaps by cavitation, and the main flow is thus detached. This evolution and the abrupt transitions between these regimes are consistent with field measurements of the pā transition. — BH Geology 33 , 361 (2005).

Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits

Submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur. Consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows, however, that significant pyroclastic activity can occur even at depths in excess of 3000 m. In order to document and illustrate the full range of submarine eruption styles, we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions: (1) no gas exsolution; (2) gas exsolution but no magma disruption; (3) gas exsolution, magma disruption, and hawaiian-style fountaining; (4) volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse; (5) magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity; and (6) volatile content builds up in the top of a dike leading to vulcanian eruptions. We also examine the role of bulk-interaction steam explosivity and contact-surface steam explosivity as processes contributing to volcaniclastic formation in these environments. We concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities, the most likely circumstance is the quiet effusion of magma with minor gas exsolution, and the production of somewhat vesicular pillow lavas or sheet flows, depending on effusion rate. The amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption. The most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity, due to the localized concentration of volatiles in magma that has a low rise rate; magmatic gas collects by bubble coalescence, and ascends in large isolated bubbles which disrupt the magma surface in the vent, producing localized blocks, bombs, and pyroclastic deposits. Another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions; these deposits, being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater, should be easily distinguishable from strombolian and hawaiian eruptions. However, we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles. In this case, a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian-style activity; the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile-induced hawaiian eruption. Specifically, typical pyroclast sizes might be smaller; fountain heights may exceed those expected for the purely magmatic hawaiian case; cooling of descending pyroclasts would be more efficient, leading to different types of proximal deposits; and runout distances for density flows would be greater, potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer. In addition, flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles, and thus extremely poor in vesicles relative to typical flows associated with hawaiian-style eruptions in the primary magmatic gas case. We examine several cases of reported submarine volcaniclastic deposits found at depths as great as ∼3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid-ocean ridges. Furthermore, the latter stages of development of volcanic edifices (seamounts) formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth, but also to: (1) the presence of a summit magma reservoir, which favors the buildup of magmatic foams (enhancing hawaiian-style activity) and episodic dike emplacement (which favors strombolian-style eruptions); and (2) the common occurrence of alkalic basalts, the CO 2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts. These models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles.

Existence of karsts into silicated non-carbonated crystalline rocks in Sahelian and Equatorial Africa, hydrogeological implications

Various cavities studied in western Niger and South Cameroon show the existence of important karstic phenomena into metagabbros and gneisses. These large-sized caves resulted from generalized dissolution of silicate formations in spite of their low solubility. Karstification is produced by deep hydrous transfer along lithological discontinuities and fracture net works. The existence of such caves has major implications in geomorphology, under either Sahelian and Equatorial climate, and in hydrogeology and water supply, particularly in the Sahel area

Seismogenic faulting in an area of low seismic activity: Paleoseismicity of the El Camp fault (Northeast Spain)

AbstractGiven that earthquakes do not occur only along high slip-rate faults, slow moving seismogenic faults should be characterized in order to minimize seismic hazard uncertainties. Although no historical earthquakes related to the El Camp fault have been documented, earlier regional geological analysis and the presence of a fault scarp provide evidence of its activity. A paleoseismological study on the southern part of the fault was performed in accordance with the following steps: 1) geological and géomorphologie analysis focussing on the detection of evidence for the seismogenic behavior of the fault, 2) near fault analysis to select the best trenching sites, and 3) trenching to establish and characterize the paleoseismic events. Different dating procedures were used in regional and trenching analyses (Thermoluminiscence, U/Th, Radiocarbon, Pollen analysis). The seismogenic nature of the fault was established by the presence of liquefaction features related to the fault, and by the presence of colluvial wedges composed of large angular blocks. We identified a segment boundary to the north of the Porquerola creek and we focussed our attention on the southern segment, which was active after 125000 yr. The slip rate in this southern segment is 0.02 mm/yr. A minimum of three seismic events were detected, from young to old: the last event Z took place some time prior to 1195 yr AD, the penultimate event Y between event X and the Holocene, and, finally, event X occurred after 125000 yr and prior to 60000 yr. The recurrence period is between 25000 and 35000 yr, the elapsed time is estimated to be no longer than 3000 yr; and the maximum estimated earthquake considering both the onshore and the offshore part of the fault is Mw 6.7 +/-0,5.

Electrical conductivity and pore‐space topology of Merapi Lavas: Implications for the degassing of porphyritic andesite magmas

Pore‐space topology is studied in recent compact angular blocks and scoria fragments from block‐and‐ash flow deposits of Merapi volcano, Indonesia. Connected porosity and electrical conductivity of cored clay‐free samples are measured in the laboratory at varying fluid salinity. The electrical formation factor, tortuosity and cementation index are derived and the pore‐space topology is found to be porosity‐dependent. The electrical flow pattern is controlled by crack‐like paths in compact angular block, and by vesicles geometry in more porous scoria fragments. Because Merapi lavas are highly viscous and crystal‐rich, we infer that the development of pore connectedness and vesicle coalescence is promoted by shear strain. Along with ascent rate considerations we conclude that degassing of Merapi magma occurs mostly in the conduit during ascent.

Pyroclastic flow generated by crater-wall collapse and outpouring of the lava pool of Arenal Volcano, Costa Rica

The pyroclastic flow that issued from the Arenal summit crater on 28 August 1993 came from the collapse of the crater wall of the cone and the drainage of a lava pool. The 3-km-long pyroclastic flow, 2.2±0.8·10 6 m 3 in volume, was confined to narrow val- leys (30-100 m wide). The thickness of the pyroclastic deposit ranged from 1 to 10 m, and its temperature was about 400 °C, although single bombs were up to 1,000 °C. The deposit is clast-supported, has a bimodal grain size distribution, and consists of an intimate mix- ture of finely pulverized rock ash, lapilli, small blocks, and cauliflower bread-crusted bombs, in which are set meter-size lava fragments and juvenile and non-juvenile angular blocks, and bombs up to 7 m in diameter. Large faceted blocks make up 50% of the total volume of the deposit. The cauliflower bombs have deep and intricate bread-crust texture and post-depositional vesiculation. It is proposed that the juvenile material was produced en- tirely from a lava pool, whereas faceted non-juvenile blocks come from the crater-wall collapse. The concen- tration and maximum diameter of cauliflower bread- crusted bombs increases significantly from the base (rockslide + pyroclastic flow) to the top (the pyroclastic flow) of the deposit. An ash cloud deposited accretionary lapilli in the proximal region (outside of the pyroclastic flow deposit), and very fine ash fell in the distal region (between 5 and 30 km). The accretionary lapilli deposit is derived from the fine, elutriated products of the flow as it moved. A turbulent overriding surge blew down the surrounding shrubbery in the flow direction. The pyro-