Successive Lava Flows Research Articles

Discovery of Stenothyrid Gastropod from Deccan Intertrappean at Barga Village, Madhya Pradesh and its Palaeoenvironmental Significance

Abstract Sedimentary rocks sandwiched between two successive lava flows in Deccan Volcanic Province (DVP) are known as Deccan intertrappean rocks. Fossils of both plants and invertebrates of uppermost Cretaceous to lowermost Palaeocene are present in some of these intertrappean rocks like that in the village Barga. Among the invertebrate fossils, gastropods dominate and because of their benthic nature, they helped a lot to decipher the palaeoenvironment. Gastropod Stenothyra has not yet been reported from any of the Deccan intertrappean. The present work records the presence of the gastropod Stenothyra for the first time from Barga Deccan intertrappean. Its occurrence points to prevalence of brackish water riverine environment at that time in the study area.

A geoheritage valuation to prevent environmental degradation of a new volcanic landscape in the Canary Islands

AbstractOn 19 September 2021, a new monogenetic volcano (Tajogaite) erupted on the Island of La Palma (Canary Islands, Spain). After 85 days of Strombolian style eruption, with emissions of volcanic material, a pyroclastic cone 200 m high and 800 m in its basal diameter was formed. Successive lava flows descended the western slopes and reached the Atlantic Ocean on 29 September. On descending the coastal cliffs and entering the sea, the lava flows formed two lava deltas on the submarine island shelf, backed by fossilized coastal cliffs. This geological event has raised a new challenge: the environmental conservation of new volcanic landscapes in island territories with high anthropogenic pressure on land uses. This work uses comparative and numerical methods in geoheritage to support their conservation from a scientific basis. In a first phase, a cartographic inventory was made of all the volcanic formations similar to the new volcano in the geological framework of the Canary Islands. In a second phase, their representativeness (A), rarity (R), diversity (D), integrity (I), and observability (O) were quantitatively measured by means of dimensional estimators. The results obtained show that the new volcano presents a geological value of the first order in the context of the Canary Islands, which is one of the most prominent oceanic archipelago worldwide. Its value is based above all on its high conservation state with respect to the similar volcanoes in the Canary Islands. The high mismatch found between the intrinsic geological value and the environmental protection of this area, justifies the development and application of urgent basic guidelines for its protection, as well as the promotion of geotourism as opposed to alternative land uses.

Mondaca Volcano lahar of December 3, 1762, Maule Region (35°28’S): one of the largest volcanic disasters in Chilean history

The Mondaca Volcano comprises a thick rhyolitic lava-field and a dome of similar composition, located near the Lontué River Valley headwaters in the northern part of the Southern Andes Volcanic Zone. It reaches a total volume of ~0.85 km3, and it is formed by 4 subunits, named Mondaca 1, 2, 3 and 4, which correspond to successive rhyolitic blocky lava flows, emitted from a rounded dome structure. They present well-preserved flow structures and, in the surroundings, restricted to the south and east of the dome, pyroclastic fall, as well as block and ash deposits are also exhibited. Downstream, along the Lontué River, a laharic deposit is recognized. The lahar was produced after the collapse of an ephemeral ~0.44 km3 lake generated after the river obstruction by viscous lavas, during the 1762 first eruptive phase. Proximal lahar facies are well exposed between 5 and 30 km from their source. The profuse agricultural activity has completely obliterated the lahar’s medium facies deposits along the Central Depression, but are well identified at the mouth of the Mataquito River, 180 km downstream, as a beige-coloured layer, interbedded within dark coastal beach-sands. The identification of overflows and super-elevation deposits formed during the debris flow emplacement along the Lontué River valley, allows to determine a high flow mobility, with estimated velocities that locally reached up to 114 km/h. Petrographic characteristics in addition to chemical composition of lavas from the volcano, pyroclasts and juvenile blocks of the laharic deposit, indicate that all they correspond to high K calcoalkaline rhyolites with subalkaline affinity. These backgrounds, together with the geographical continuity between the lavas and debris deposits along the Lontué and Mataquito rivers, verify facies correlation and common origin as the result of the 1762 Mondaca Volcano eruption complex evolution. Although it was a mainly effusive eruption that could not be observed from Curicó, the collateral consequences would have been catastrophic over a vast area to the south of that city, and evidences one of the largest volcanic disasters in Chilean history. Probably because of the low density polulation at that time, the consequences could have been minor.

Weathering Controlled Landslide in Deccan Traps: Insight from Mahabaleshwar, Maharashtra

Landslide is one of the devastating natural phenomenon that threatens human life and property. Every year a number of persons lost their lives due to the landslides. Therefore, a better understanding and characterization of landslide is very essential for adopting mitigation strategies to contain the adversities of this natural hazard. Information on landslides from different climatic setup are very essential for better understanding of the influence of weathering, rainfall, or topography on landslide generation. Weathering is one of the important causative factor for landslide generation in the moderate topography or inactive mountainous terrain. The Western Ghats including the Deccan Traps, an inactive mountain range, receives torrential rainfall. Intense rainfall in these areas enhances the weathering processes and fabricates thick soil covers. Mahabaleshwar area, Maharashtra was chosen as a case study, where high elevated part is covered by lateritic layer and each lava flow unit is separated by a thin weathered bed of red bole. The area experiences series of landslides during the summer monsoon months. Mainly two types of landslides have been identified in the area confined with the red bole bed and powdery lateritic soil. The first type of landslides occur at higher elevations (≥1200m) where horizontal beds of permeable laterites underlined by impermeable thick basalt beds. The rain water infiltrates down and spread laterally within the permeable lateritic beds. It finally spouts at lower plateau elevations and triggers mainly debris flows. The other category of landslides occurs where the weathered red bole bed separates two successive lava flows. The percolating water from the secondary porosities (joints and inter connected vugs) comes out from the contact zones of basalt and red bole bed in the form of seepages. It erodes the red bole bed and as a result the overlying masses hang and consequently lead to rock fall. The Chemical Index of Alteration (CIA) of the representative samples from landslide locations indicates significant weathering. The CIA values for the fine lateritic soil are up to 98% whereas for the red bole bed it varies from 77 to 85%. This suggests a high chemical weathering and higher erodibility. The association of active landslide locations with the red bole bed and fine lateritic soil suggests a close relation between weathering and landslide occurrences in the area.

Morphometry of terrestrial shield volcanoes

Shield volcanoes are described as low-angle edifices built primarily by the accumulation of successive lava flows. This generic view of shield volcano morphology is based on a limited number of monogenetic shields from Iceland and Mexico, and a small set of large oceanic islands (Hawaii, Galápagos). Here, the morphometry of 158 monogenetic and polygenetic shield volcanoes is analyzed quantitatively from 90-meter resolution SRTM DEMs using the MORVOLC algorithm. An additional set of 24 lava-dominated ‘shield-like’ volcanoes, considered so far as stratovolcanoes, are documented for comparison. Results show that there is a large variation in shield size (volumes from 0.1 to >1000km3), profile shape (height/basal width (H/WB) ratios mostly from 0.01 to 0.1), flank slope gradients (average slopes mostly from 1° to 15°), elongation and summit truncation. Although there is no clear-cut morphometric difference between shield volcanoes and stratovolcanoes, an approximate threshold can be drawn at 12° average slope and 0.10 H/WB ratio. Principal component analysis of the obtained database enables to identify four key morphometric descriptors: size, steepness, plan shape and truncation. Hierarchical cluster analysis of these descriptors results in 12 end-member shield types, with intermediate cases defining a continuum of morphologies. The shield types can be linked in terms of growth stages and shape evolution, related to (1) magma composition and rheology, effusion rate and lava/pyroclast ratio, which will condition edifice steepness; (2) spatial distribution of vents, in turn related to the magmatic feeding system and the tectonic framework, which will control edifice plan shape; and (3) caldera formation, which will condition edifice truncation.

Northwest Africa 5790: Revisiting nakhlite petrogenesis

Northwest Africa 5790, the latest nakhlite find, is composed of 58vol.% augite, 6% olivine and 36% vitrophyric intercumulus material. Its petrology is comparable to previously discovered nakhlites but with key differences: (1) Augite cores display an unusual zoning between Mg# 54 and 60; (2) Olivine macrocrysts have a primary Fe-rich core composition (Mg#=35); (3) The modal proportion of mesostasis is the highest ever described in a nakhlite; (4) It is the most magnetite-rich nakhlite, together with MIL 03346, and exhibits the least anisotropic fabric. Complex primary zoning in cumulus augite indicates resorption due to complex processes such as remobilization of former cumulates in a new magma batch. Textural relationships indicate unambiguously that olivine was growing around resorbed augite, and that olivine growth was continuous while pyroxene growth resumed at a final stage. Olivine core compositions (Mg#=35) are out of equilibrium with the augite core compositions (Mg# 60–63) and with the previously inferred nakhlite parental magma (Mg#=29). The presence of oscillatory zoning in olivine and augite precludes subsolidus diffusion that could have modified olivine compositions. NWA 5790 evidences at least two magma batches before eruption, with the implication that melt in equilibrium with augite cores was never in contact with olivine. Iddingsite is absent.Accordingly, the previous scenarios for nakhlite petrogenesis must be revised. The first primary parent magmas of nakhlites generated varied augite cumulates at depth (Mg# 66–60) as they differentiated to different extents. A subsequent more evolved magma batch entrained accumulated augite crystals to the surface where they were partly resorbed while olivine crystallized. Trace element variations indicate unambiguously that they represent consanguineous but different magma batches. The compositional differences among the various nakhlites suggest a number of successive lava flows. To account for all observations we propose a petrogenetic model for nakhlites based on several (at least three) thick flows. Although NWA 5790 belongs to the very top of one flow, it should come from the lowest flow sampled, based on the lack of iddingsite.

Weathering characteristics of interflow volcanic boles from Mandla lobe, Eastern Deccan volcanic province

The Deccan volcanism straddling the Cretaceous–Paleogene boundary resulted into one of the most extensive continental flood basalts in the Western and Central Peninsular India. The interflow sedimentary units (boles) often cited as paleosols are the rare archives of paleoenvironmental conditions existed during successive lava flows. A debate on the origin of boles as true paleosols exists. Therefore, in order to trace out regional extent of such conditions (pedogenic/non-pedogenic) in the context of their paleoenvironmental significance during the Deccan volcanism, we studied here mineral magnetic characteristics of 36 color differentiated bole beds from the Mandla lobe, in the eastern Deccan volcanic province (EDVP) classified into four types (gray, yellow, brown and red) based on their Munsell notations. The gray, yellow and brown boles show dominantly ferrimagnetic composition (chiefly magnetite and titanomagnetite), while red boles characterize mixed ferri- and antiferromagnetic (mainly hematite) mineralogy. The granulometric (domain) parameters suggest larger single domain (SD) and the multidomain (MD) ferrimagnets in the majority of cases, whereas superparamagnetic (SP) particles (with frequency dependent susceptibility χfd~2.6 to 20.3×10−8m3kg−1) are limited to the few red and brown boles only. The significant presence of primary parent magnetic minerals (i.e. coarse SD–MD titanomagnetite and magnetite) in the majority of boles depict incipient to moderate weathering owing to limited time intervals during successive eruptions. The combination of hematite and SP particles present only in the few red and brown boles depicts baking effect rather than pedogenesis. The comparison of magnetic properties of boles from EDVP and western Deccan volcanic province (WDVP) shows significant differences in the concentration of SP ferrimagnetic particles and hematite attributed to variable weathering intensity, duration and heating related alterations from the upper flows. Both EDVP and WDVP boles indicate similar first order paleoenvironmental conditions without any major pedogenesis that can be regionally correlated.

Reservoir architectures of interlava systems: a 3D photogrammetrical study of Eocene cliff sections, Faroe Islands

Abstract Following the intra-volcanic Rosebank discovery in the Faroe–Shetland Basin, NE Atlantic, there has been a need to find suitable analogues to characterize reservoir architectures, connectivities and compartmentalization of interlava clastic beds. The Faroe Islands, situated c. 160–190 km to the NW of this discovery, are an exposed remnant of the Palaeogene lava field host and the near-vertical cliff sections afford the opportunity to map lateral variations over many kilometres. This was achieved through 3D photogrammetry based on high-resolution digital photographs taken from a helicopter. The study focused on the Eocene Enni Formation, which is dominated by a mixture of simple and compound lava flows commonly separated by minor volcaniclastic beds, including the widespread Argir Beds. In general, the interlava beds are tabular shaped and <1 to c. 6 m thick. Locally they thicken in depressions formed by the wedging out or differential erosion of underlying lava flow lobes. Connectivity may be caused by the wedging out of successive lava flows leading to the lateral merging of interlava beds or more rarely be hydraulically connected via conglomeratic fills of major channels. Lateral compartmentalization may be caused by the intersection of interlava beds by dykes, lava tubes, lava-filled channels or reverse faults.

A combined paleomagnetic/dating investigation of the upper Jaramillo transition from a volcanic section at Tenerife (Canary Islands)

A coupled paleomagnetic/dating investigation has been conducted on a sequence of 25 successive lava flows, emplaced during the upper transition of the Jaramillo subchron in Tenerife, Canary Islands. This sequence is located along the western wall of the Güímar collapse scar, in the south central part of the island. Nine flows distributed throughout this sequence were dated using unspiked K/Ar and 40Ar/39Ar methods. They bracket the section between 1009±22 ka and 971±21 ka (2σ). A first group of 8 flows at the bottom of the sequence is characterized by normal polarity with paleointensity values of the order of present-day field intensity in the Canary Islands. The virtual geomagnetic poles (VGP) of these 8 flows describe a short loop at high latitudes. Seven overlying flows are transitional in directions and dated between 991±14 ka and 1002±11 ka consistently with published ages of the upper Jaramillo reversal. This second group of flows is characterized by low paleointensity values (around 8–12 μT) that are less than 30% of the present dipole value in Tenerife. The VGPs of the first two transitional flows lie over northeastern Pacific whereas the five following transitional flows have all negative inclinations and their VGPs lie initially over East Antarctica, then describe a northward loop almost reaching New Zealand. The final group of ten flows yield intensities varying between 20 and 35 μT and VGPs close to the southern pole with two of them describing a small amplitude second loop to southeastern Pacific. Assuming a constant extrusion rate as a very first approximation, the distribution of the obtained ages suggests a duration of 7.6±5.6 ka for the transitional interval. The obtained transitional positions of VGPs are consistent with the path reported for the same reversal from North Atlantic sediments but are different from the only other volcanic record from Tahiti. The intensity low characterizing the transitional interval remains the best tie point, centered at 996±7 ka (2σ) relative to 28.02 Ma FC sanidine.

Palaeomagnetism and 40Ar/39Ar age of a Pliocene lava flow sequence in the Lesser Caucasus: record of a clockwise rotation and analysis of palaeosecular variation

SUMMARY A palaeomagnetic and rock-magnetic investigation has been carried out on a Pliocene lava flow sequence in the Djavakheti Highland in the central Lesser Caucasus in the Republic of Georgia. In addition, a 40 Ar/ 39 Ar dating and electronic microscopic studies were performed on samples of this sequence, named the Saro section, which consists of 39 successive lava flows of doleritic basalts. A characteristic magnetization could be isolated in all studied 39 flows,yielding reverse-polarity directions inall cases,amean directionD =202.2 ◦ ,I =–60.6 ◦ (N = 39, α95 = 2.0 ◦ , k = 138) being obtained. Thermomagnetic experiments (strong-field versus temperature curves) suggested low-Ti titanomagnetites and low Curie-temperature titanomagnetites with a rather high titanium content (x ≈ 0.5–0.7) as the main carriers of remanence. Their domain structure is characterized by a mixture of single- and multidomain grains. 40 Ar/ 39 Ar dating yielded an age of 1.73 ± 0.03 Ma, interpreted as the eruption age of the uppermost lava flow of the sequence. Analysis of palaeomagnetic results and radiometric data from the present and a previous study allows two different explanations about the time of emplacement of the section: (i) The lower 36 flows of the sequence might have been emitted between the normal-polarity Reunion and Olduvai chrons, and the upper three flows after the Olduvai chron, with a long hiatus in volcanic activity of more than 150 kyr or (ii) The whole sequence has been emitted between 1.778 and 1.73 ± 0.03 Ma, after the Olduvai chron. Comparison of the palaeomagnetic results obtained in this study with the expected direction shows that while inclination values agree well, declination shows an eastward deviation of 19.2 ◦ ± 5.8 ◦ . This discrepancy can be explained with a clockwise vertical-axis rotation of the sequence, which might have been produced by extensional structures with strike-slip component, which can be found in the study area. Virtual geomagnetic pole (VGP) scatter recorded in the sequence is somewhat lower than expected. To analyse this behaviour, VGP scatter of the sequence was first calculated with respect to the palaeomagnetic pole obtained from the Fisher mean of all VGPs. Then it was calculated with respect to the appropriate Eurasian palaeomagnetic pole from VGPs obtained after applying a 19.2 ◦ counter-clockwise rotation of the sequence (i.e. undoing the observed vertical axis rotation). The observed scatter with both independent methods appeared to be very similar, allowing the deduction that

U–Pb systematics of the unique achondrite Ibitira: Precise age determination and petrogenetic implications

Ibitira is an unbrecciated, equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites. We present the first combined high-precision U and Pb isotopic data for this unique meteorite. The 238U/235U value of 137.777±0.013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites. This value results in corrections of −1.1Ma for Pb–Pb dates calculated using the previously assumed invariant 238U/235U value of 137.88. Using the determined 238U/235U value, the 7 most radiogenic Pb isotopic analyses for acid-leached pyroxene-rich and whole rock fractions yield an isochron Pb–Pb age of 4556.75±0.57Ma, in excellent agreement with the results of Mn–Cr chronology which give the ages of 4557.4±2.5Ma and 4555.9±3.2Ma using the U-corrected Pb–Pb age of D’Orbigny as a time anchor. Along with the previously proposed thermal history of Ibitira and our closure temperature estimates for Pb diffusion, the Pb–Pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism. The metamorphism may have been caused by burial of Ibitira lava under successive lava flows and, if so, the Pb–Pb age should post-date the crystallization by a short time interval. The Pb isotopic data for acid leachates suggest partial re-equilibration of Pb between plagioclase and phosphate, perhaps during an impact event at 4.49Ga, as recorded by K–Ar systematics. The whole rock 238U/204Pb indicates that compared to CI chondrites, Ibitira is less depleted in Pb than in some alkali elements despite a lower condensation temperature of Pb than the alkali elements. The restricted Pb depletion may reflect preferential concentration of metals with high fluid/melt partition coefficients including Pb and Zn as a result of fluid exsolution and migration within the parent magma. We discuss the implications of the U–Pb systematics for the origin and differentiation of the parent body.

Magnetic interactions at the origin of abnormal magnetic fabrics in lava flows: a case study from Kerguelen flood basalts

Anisotropy of low-field magnetic susceptibility (AMS) of basaltic lava flows can give some clues about post-emplacement tilting occurring in volcanic sequences. Such a study has been carried out on a sequence of 19 successive lava flows from Kerguelen Archipelago. Surprisingly, two different patterns were observed. The first one—herein called normal fabric—is consistent with the lava-flow direction inferred from geological observations, whereas the second one observed for about 70 per cent of the analysed samples—herein called abnormal fabric—appears to be unrelated to the shear history of the lava flows during their emplacements. These abnormal fabrics are not strictly inverse fabric but seem to have a more complex origin. The unusual nature of these abnormal fabrics has been confirmed by lattice preferred orientation of plagioclases obtained from high-resolution electron backscattered diffraction measurements. The aim of this study is to propose a physical interpretation of these fabrics. A first step in our understanding was to clearly identify the nature and the magnetic properties of the Fe-Ti oxides assemblage. Thus, we present a comprehensive rock magnetic analysis relying on low- and high-temperature thermomagnetic curves (K–T), crossover diagrams, first-order reversal curves, ore-microscopy, Mössbauer measurements and electron backscattered images. We found that in the present case the abnormal fabrics are probably linked to a mixture of prevailing single domain (SD) strongly interacting grains population and a subsidiary multidomain grains population. SD grains are identified as small magnetite crystallized within the interstitial glass in globular aggregates along the silicate framework. Then a question arises: to what extent such SD interacting grains in globular aggregates can affect the magnetic susceptibility and its anisotropy? To answer this question, we developed a physical model in which the total energy density of an SD grain with an uniaxial anisotropy is minimized. In this model, the distribution function of the direction of the magnetic interaction field is comprised between two boundary states. It is either isotropic, which formally corresponds to the thermal demagnetized state, or it is ordered, which formally corresponds to the Alternative Field demagnetized state. We show that in both cases the degree of anisotropy decreases as the interaction field increases. Thus, we conclude that the abnormal fabric encountered in this study can be simply explained by strong magnetostatic interactions that would explain the large scattering of AMS often observed in basaltic lava flows.

Paleointensity variation across the Matuyama-Brunhes polarity transition: Observations from lavas at Punaruu Valley, Tahiti

[1] We have conducted a paleointensity study of the Matuyama-Brunhes (M-B) polarity transition recorded in 34 successive lava flows of Punaruu Valley on Tahiti. A reversed polarity is obtained from the lower part of the record, major directional changes are derived from the middle part of the record, and a normal polarity is recorded in the upper part of the record. These paleomagnetic directions and five 40Ar/39Ar ages yielding a weighted mean of 771 ± 8 (1σ) ka indicate that 30 lava flows recorded the geomagnetic field across the M-B transition. The 215 specimens from 32 flows were subjected to the double-heating technique of the Shaw method combined with low-temperature demagnetization (LTD-DHT Shaw method), yielding 73 successful results from 18 flows. For the reversed polarity period just prior to the major directional changes, paleointensity shows an oscillation-like variation between 3 and 38 μT corresponding to virtual dipole moments (VDMs) between 0.9 × 1022 and 9.6 × 1022 Am2. For the major directional changes, a weak paleointensity of 5 μT is obtained, which gives a VDM of 1.0 × 1022 Am2. For the normal polarity period, paleointensities are 14–21 μT, giving VDMs of 3.5–5.2 × 1022 Am2. For the reversed polarity period just prior to the major directional changes, a linear relationship with a correlation coefficient of 0.96 is recognized on the diagram of VDM versus virtual geomagnetic pole latitude. This linear relationship may be a precursory feature of the geodynamo at the onset of the M-B transition.

A paleomagnetic and rock-magnetic study of a neogene lava flow sequence in La Gomera (Canary Islands, Spain)

We present rock-magnetic and paleomagnetic results obtained on samples belonging to a Neogene sequence of 11 successive lava flows and a dyke from La Gomera (Canary Islands, Spain). Analysis of thermomagnetic curves allows to distinguish three types of samples: (i) Type H samples with low-Ti titanomagnetite as the only carrier of remanence; (ii) type M samples with a main intermediate Curie-temperature phase (TC = 450°C) and low-Ti titanomagnetite; (iii) type L curves with a low Curie-temperature phase (TC = 120 to 200°C) and an intermediate Curie-temperature phase (TC = 400°C). Analysis of hysteresis parameters suggests that the grain size of most studied samples corresponds to pseudo single-domain particles, which can be also interpreted as a mixture of single-domain and multi-domain particles. Paleomagnetic experiments reveal only a single paleomagnetic component. Characteristic remanence of all studied lava flows and the dyke shows reverse polarity. The mean direction of the whole sequence is D = 188.2°, I =−35.4° (k = 46.9; α95 = 6.4°) and the calculated paleomagnetic pole yields a longitude λ= 150.7° and a latitude ϕ= 78.8° (k = 59.4; A95 = 5.7°). Secular variation is analysed through the scatter of virtual geomagnetic poles (VGP). A VGP angular scatter SB = 5.9 with an upper confidence limit Sup = 8.0 and a lower confidence limit Slow = 4.6 are obtained. This scatter is clearly smaller than the average for this latitude obtained for the last 5 Ma. The studied lava flows were probably emitted in a relatively short time interval.

A new map of the lava flow field of Nyamulagira (D.R. Congo) from satellite imagery

Nyamulagira (3058 m a.s.l.), a volcano of the Virunga volcanic province in the western branch of the East African Rift, is Africa’s most active volcano with one eruption every 2–4 years. It represents a hazard for the Virunga National Park and its vicinity. Despite such a frequent activity, Nyamulagira remains poorly studied. The only existing volcanological map was produced in the sixties by Thonnard et al. (1965). The occurrence of 19 eruptions since its publication makes it obsolete. In the present study we mapped the Nyamulagira lava flows from 1938 up to the last eruption to date in 2010 using optical (Landsat, ASTER) and radar (ENVISAT-ASAR, ERS, JERS) imagery. The results are integrated into a Geographical Information System (GIS) and coupled with additional data sources. GIS use makes the new database a flexible – and easy-to-update – tool for scientific purposes as well as for risk, environmental and humanitarian management. Here a new lava flow map was produced. Volumes of the successive lava flows and affected areas of the Virunga National Park were estimated.

Review: Groundwater development and management in the Deccan Traps (basalts) of western India

The Deccan Traps or the basalts of western India are the largest exposure of basic lava flows covering about 500,000 km2. Groundwater occurrence in the Deccan Traps is in phreatic condition in the weathered zone above the hard rock and in semi-confined condition in the fissures, fractures, joints, cooling cracks, lava flow junctions and in the inter-trappean beds between successive lava flows, within the hard rock. Dug wells, dug-cum-bored wells and boreholes or bore wells are commonly used for obtaining groundwater. The yield is small, usually in the range of 1–100 m3/day. The average land holding per farming family is only around 2 ha. Recently, due to the ever increasing number of dug wells and deep bore wells, the water table has been falling in several watersheds, especially in those lying in the semi-arid region of the traps, so that now the emphasis has shifted from development to sustainable management. Issues like climatic change, poverty mitigation in villages, sustainable development, rapid urbanization of the population, and resource pollution have invited the attention of politicians, policy makers, government agencies and non-governmental organizations towards watershed management, forestation, soil and water conservation, recharge augmentation and, above all, the voluntary control of groundwater abstraction in the Deccan Traps terrain.

Recent volcanic events and the distribution of hydrothermal venting at the Lucky Strike hydrothermal field, Mid‐Atlantic Ridge

We present new high‐resolution bathymetry and backscatter data acquired in 2006 with the ROV Victor 6000 over the Lucky Strike hydrothermal field, Mid‐Atlantic Ridge. As long‐term monitoring of the Lucky Strike area (MoMAR project) is being implemented, these new high‐resolution data offer an unprecedented view of the distribution of hydrothermal edifices, eruptive facies, and small‐scale tectonic features in the Lucky Strike vent field. We show that vents located in the NW and NE correspond with wide expanses of lumpy seafloor which we interpret as primarily made of broken chimneys and sulfide edifices. They are found above scarps with relief >50 m or on associated mass wasting deposits. By contrast, the SE and SW vents correspond with small expanses of lumpy seafloor and are located near smaller scarps which we interpret as more recent faults. Hydrothermal edifices in the SW venting area appear very recent, postdating the emplacement and faulting of the most recent lava. We propose that this difference in the age of hydrothermal edifices does not mean that hydrothermal venting itself is more recent in the southern part of the Lucky Strike field because preexisting sulfide deposits there may have been buried by recent volcanic deposits. Instead, the older edifices in the northern part of the hydrothermal field may have been allowed more time to grow because they are set above the level of recent lava flows. The formation of a lava lake is the most recent eruptive event detected at Lucky Strike. Lava drainback is evidenced by benches and lava pillars, suggesting a close connection with an underlying magma reservoir, which probably corresponds to the melt body imaged by Singh et al. (2006). We have found no evidence that this lake was active for months to decades, as lava lakes at terrestrial volcanoes. It may instead have formed as a lava pond, with successive lava flows covering the eruptive vents, as proposed for similar features at the EPR. The horizontal surface of the lake is deformed only near its southwestern shore, along a NNE‐trending set of faults and fissures, which appear to control the distribution of hydrothermal chimneys.

Pre-eruptive physical conditions of El Reventador volcano (Ecuador) inferred from the petrology of the 2002 and 2004–05 eruptions

A petrological study of the eruptive products of El Reventador allowed us to infer the magmatic processes related to the 2002 and 2004–05 eruptions of this andesitic stratovolcano. On November 3, 2002, El Reventador experienced a highly explosive event, which was followed by emplacement of two lava flows in November–December 2002. Silica contents range from 62 to 58 wt.% SiO 2 for the November 3 pyroclastic deposits to 58–56 and 54–53 wt.% SiO 2 for the successive lava flows. In November 2004 eruptive activity resumed supplying four new lava flows (56–54 wt.% SiO 2) between November 2004 and August 2005. Volatile contents in matrix glasses and glass inclusions from the November 3 pyroclastic deposits allow us to estimate the total amount of SO 2 and HCl released into the atmosphere during the paroxysmal phase (i.e. 80 kT of SO 2 and 280 kT of HCl). Pre-eruptive pressure-temperature conditions of the magmas range from 300 to 150 MPa and ~ 1000 °C with high water contents (~ 5 wt.%). We propose the existence of an andesitic magma body located at ~ 7–12 km depth that is frequently intruded by more primitive, hydrous magmas from a deeper source. The initial crystallization of amphibole from the hydrous primitive magma seems typical of El Reventador, as well as the historically recurrent and regular periods of eruptive activity lasting several years. This eruptive behaviour coupled with the fractionation and mixing processes inferred from the 2002 and 2004–05 petrologic data suggest that deep magmatic recharge at El Reventador is frequent, and is probably responsible for the high frequency of eruptions.

Preliminary comparison of ancient bole beds and modern soils developed upon the Deccan volcanic basalts around Pune (India): Potential for palaeoenvironmental reconstruction

Relict weathering mantles are frequently preserved in the form of paleosols, which generally document the long-term evolution of earth-surface processes. Bole beds occur as prominent horizons composed of fine-grained clayey or earthy material having colors in shades of red to chocolate brown, green, purple or gray, often marking the flow contacts of Deccan basalts, which date to approximately 65 Ma. Bole beds preserved between two successive lava flows can be studied relative to soils developed into the same basalts during the Holocene. The bole beds have slightly lower pH but significantly lower electrical conductivity values and organic matter contents compared to modern soils. They also have higher SiO 2 and lower Al 2O 3 and H 2O + (crystal lattice water) content than modern soils. Results indicate that the bole beds may not have been subjected to intense weathering and long-term pedogenesis, but nonetheless formed under uniform rainfall with surface runoff exceeding evaporation. Weathering chemistry suggests different types of protolith for red and green boles. The high stratigraphic frequency of bole beds throughout the Deccan Volcanics makes them an important source of information about the frequency and environment of inter-eruption intervals.

Post-emplacement tilting of lava flows inferred from magnetic fabric study: the example of Oligocene lavas in the Jeanne d’Arc Peninsula (Kerguelen Islands)

Statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results. A difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement. For 2 successive series of flows on the Jeanne d’Arc Peninsula presently NNW dipping, this method shows lava emplacement along a SSW–NNE direction. This indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago. Additionally, the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series.