Lava Succession Research Articles

The Morphology of Martian Pyroclastic Ramparts and Their Use in Determining Vent‐Proximal Eruption Dynamics

AbstractHigh‐resolution investigations of Late Amazonian volcanic landforms provide previously unrevealed insights into the dynamics of Martian volcanic eruptions. On Earth, the formation of vent‐proximal accumulations of spatter deposits is attributed to low‐intensity lava fountaining episodes representing eruptions on the very edge of explosive activity. Martian spatter deposits form small‐scale volcanic landforms that are rarely reported, and thus the dynamics of Martian mafic explosive eruptions are still not fully constrained. We conducted high‐resolution Context Camera‐based mapping coupled with a stereo‐pair‐generated digital elevation model to reconstruct the eruptive history of a fissure system and its associated products south of Ascraeus Mons, Mars. The studied volcanic fissure clearly demonstrates both explosive and effusive deposits and, in addition, is spatially associated with a lava channel. For the first time, these observations allowed us to conduct a comparative analysis of vent‐proximal volcanic products and reconstruct the late‐stage eruption dynamics of a fissure system. We found that the spatial distribution of the pyroclastic (spatter) rampart along the fissure vent is heterogeneous and generated using dynamic eruption processes. Moreover, the lava channel fed from the fissure vent shows evidence of successive lava overspills whose emplacement was topographically controlled. These observations suggest that, in contrast to the general inference that Amazonian‐age volcanism mainly involves effusive eruptions, explosive‐origin landforms might have been overlooked. Therefore, we argue that high‐resolution mapping of pyroclastic deposits may provide critical insights into understanding the dynamic nature of Martian fissure eruptions and explosive‐associated volatile release during the last stages of eruptions.

Facies architecture and stratigraphy of a basaltic-trachytic polygenetic volcano, Whakaraupō Volcano, NE Banks Peninsula, New Zealand

Polygenetic volcanic systems typically evolve through multiple igneous and sedimentary events of construction and degradation. Understanding the stratigraphy of volcanic rocks is essential for assessing volcanic hazards and natural resources such as hydrocarbons, geothermal heat, and groundwater. It requires identifying the internal and external structures of the volcanic deposits in the geological record. We present a multiscale analysis of the development of the Whakaraupō Volcano at the NE portion of the Miocene Lyttelton Volcanic Complex (LVC), Banks Peninsula, South Island, New Zealand. We combine different stratigraphic approaches (Facies, Lithostratigraphy, and Unconformity boundaries) to characterize the Volcano in time and space. Thirteen lithofacies and seven facies' associations were described and classified into four Formations (Scott Valley, Heathcote Valley, Britten Craig, and Mount Pleasant). Each lithostratigraphy represents a specific depositional and eruptive environmental setting separated by unconformities. The Whakaraupō Volcano evolved progressively shifting the eruptive locus towards ENE. Hawaiian-style eruptions characterize the initial eruptive stage, forming widespread sequences of hawaiite a'ā lava flows with fan-like geometry overlapped by benmoreitic rubbly and blocky lava flows (Scott Valley Fm). Next, the activity shifted to transitional type mild explosive eruptions with numerous air-fall hawaiite to mugearite scoria cones producing ash plains around active vents, co-genetic lava domes, and a‘ā lava flows fed from a NE-SW fissure system, as well as building an eruption-fed conglomeratic volcanic debris apron (Heathcote Valley Fm). The following Formation (Britten Craig) is characterised by a fan-like a‘ā hawaiite to benmoreite flows bounded by paleosoil layers at the top and bottom, indicating a time gap between the emplacement of Formation. The final stage comprises a hawaiite-trachytic lava succession interbedded with epiclastic deposits (Mount Pleasant Fm). Multiple different ages and compositions of dikes crosscut the volcanic succession, mainly feeding trachytic lava domes with a dominant NE-SW trending orientation. Our multiscale analysis provides insights into the three-dimensional architecture of diverse rock bodies within the Whakarupō Volcano. This approach will aid the construction of prospective models that can be applied to understand the hazards and geo-energy resources of large basaltic-trachytic volcanic complexes.

The transition from backarc extension to Andean growth: Insights from geochronologic, sedimentologic, and structural studies of Mesozoic and Cenozoic sedimentary and volcanic rocks in south-central Chile (36°S)

Many studies propose a significant shift in the tectonic and paleogeographic evolution of the Andes in south-central Chile and Argentina during the Late Cretaceous. It has been proposed that the preceding Jurassic-Early Cretaceous extensional regime that resulted in a low-relief volcanic arc and the backarc Neuquén basin came to an end, giving way to shortening and Andean growth from the Late Cretaceous onward. Nevertheless, there are disagreements regarding the timing and nature of this transition to Andean orogenesis. To address these issues, we conducted geochronologic (U–Pb and 40Ar/39Ar), sedimentologic, and structural studies on Mesozoic-Cenozoic sedimentary and volcanic rocks in the Río Maule area (Principal Cordillera, Chile, 36°S). From our findings and prior analyses, we propose the following tectono-stratigraphic evolution of the region. (1) Marine deposition of the Tithonian-Hauterivian Baños del Flaco Formation took place in an extensional backarc basin. (2) After a ∼ 40 Myr hiatus, fluvial deposits of the Colimapu Formation and volcanic rocks of the Plan de los Yeuques Formation accumulated during the Cenomanian-Danian. Whereas the Colimapu Formation displays evidence of syndepositional shortening, the Plan de los Yeuques Formation exhibits synextensional growth strata. Contrary to other studies, our results suggest that the Chilean part of the Principal Cordillera was largely a zone of active deposition rather than an elevated fold-thrust belt during the Late Cretaceous. We propose that sedimentation occurred within a series of relatively stable intermontane subbasins generated by shortening, followed by extension. (3) After a ∼ 20 Myr hiatus, middle Eocene to early Miocene (Lutetian-Aquitanian) accumulation of a thick succession of andesitic lavas and minor clastic sediments of the Abanico Formation occurred in an intraarc extensional basin. (4) Finally, major shortening and uplift of previously deposited Mesozoic-Cenozoic rocks took place throughout the Neogene. This phase constituted the primary contractional deformation in the Andes of south-central Chile and Argentina. In terms of the transition to early Andean deformation, we propose that structural deformation did not generate a major, regional-scale fold-thrust belt during the late Albian-Santonian. Modest extension, tectonic quiescence, or low-magnitude shortening seem to have dominated during the early to middle Cenozoic.

Magmatic Evolution of the Fossil Melanesian Island Arc: Evidence From Lower Miocene Lavas of Malekula Island (Vanuatu)

AbstractThe subduction zones in the SW Pacific Ocean are some of the most dynamic plate boundaries on Earth with changes in subduction polarity and subduction initiation processes. The New Hebrides island arc formed some 10 million years ago after the Melanesian island arc was abandoned due to the cessation of subduction of the Pacific Plate after collision of oceanic plateaus with the island arc. Parts of the Melanesian island arc occur within the New Hebrides island arc and we show that Miocene volcanic rocks exposed on Malekula island in the New Hebrides island arc consist of island arc tholeiites to alkaline basalts with variable slab contribution. The lava erupted between 22.5 and 16.3 Ma, similar to the Wainimala Group lavas on Fiji representing the eastern portion of the Melanesian island arc. Partial melts from subducted sediments affected a Pacific MORB‐type mantle, but we do not find evidence for a slab component derived from the Ontong Java Plateau or for assimilation of continental crust. Thus, no Miocene subduction of the Ontong Java Plateau occurred beneath Malekula and the basement of the island probably does not consist of continental crust rifted from Australia. The Malekula lava succession resembles that of other portions of the Miocene Melanesian island arc between New Britain and Fiji, indicating continuous subduction of the Pacific plate along this arc.

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.

Integrated photogrammetry, lava geochemistry and palynological re-evaluation of the early evolution of the topographically constrained Mull Lava Field, Scotland

ABSTRACTPhotogrammetry was used to elucidate complex strata relationships between isolated outcrops of the Palaeocene lava fields of SW Mull, part of the North Atlantic Igneous Province. Subsequent sampling for lava geochemistry and palynology was undertaken based on interpretation of these photogrammetry models. Coherent units of Plateau- and Staffa-type lavas were identified using lava geochemistry, in particular using rare earth elements (REEs), divisions supported by multivariate statistics. Lavas with three different REE compositional clusters were identified within the Staffa magma type and four within the Plateau type of SW Mull. Understanding the distribution of these lava types was achieved using the interpreted photogrammetry models and analysis of prominent interbedded sedimentary rock units and their correlative unconformities. Probably the most renowned rocks of SW Mull are the thick columnar jointed lavas, including those of the Isle of Staffa. REE geochemistry reveals that lavas of both Staffa- and Plateau-type geochemistry occur as columnar jointed facies associated with what has previously been attributed to the Staffa Lava Formation. Instead, the SW Mull Lava Field was initiated by eruption of Plateau-type lava into a fault-controlled valley. Subsequent eruptions of Staffa-type lavas partially infilled this structure, which was finally overfilled by a thick succession of younger Plateau-type lavas. The geochemical characteristics of this Plateau–Staffa–Plateau lava succession indicate that magma reservoirs deep in the crust were succeeded by shallow melts during a period of crustal extension. This phase of shallow melting induced topographical instability and formation of sedimentary interbeds and correlative unconformities that characterise the Staffa-type lava succession. Subsequent compressive tectonics forced a return to deep crustal melts. Interbed palynofloral compositional data indicate that eruption of the later Plateau-type lavas likely took place within a period of ~2.5 million years during the Selandian to early Thanetian period.

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.

Age of the Valle del Bove formation and chronology of the post-collapse flank eruptions, Etna volcano (Italy)

The Valle del Bove is a profound and wide scar on the east Etna flank witnessing the Holocene main volcano-tectonic event of the volcano, frequently invaded by lava flows during the last centuries. The Valle del Bove slope failure produced the Milo debris avalanche deposit on the lower east flank that is partially covered by the Mongibello lavas and a pyroclastic succession. In this paper, we constrain for the first time the age of the Milo debris avalanche deposit and the overlying lava succession exposed at three quarries recently caved at the valley mouth through a multidisciplinary approach integrating stratigraphic and petrographic analyses, 14C, and paleomagnetic dating. In particular, 14C age determinations of the Milo debris avalanche deposit indicate that the initial stage of the catastrophic flank collapse of the Valle del Bove occurred at 7478–7134 BCE during the Mesolithic age. Conversely, the main portion of the lava succession filling the valley floor emplaced after the sub-Plinian picritic eruption occurred at 2579–2278 BCE (FS tephra layer) consistently with the increasing occurrence frequency of flank eruptions documented in the geological record of Etna during the past 4000 yrs. Paleomagnetic dating highlighted that in the study area the sub-Plinian eruption was followed by two quasi-contemporaneous flank eruptions during the Late Copper age (2600–2400 BCE), whereas other two flank eruptions occurred during Greek-Roman and Medieval ages. These results have relevant implications on the stratigraphy and evolution of Etna, particularly on the Valle del Bove initial collapse and the relative emplacement of the Chiancone detritic-alluvial sequence.

Further evidence of unstable reverse polarity geomagnetic field across the Olduvai subchron: paleomagnetic and multispecimen paleointensity study on Khertvisi lava flows (Lesser Caucasus)

Goguitchaichvili et al. (Physics of the Earth and Planetary Interiors, doi.org/10.1016/j.pepi.2020.106641, 2021) reported a detailed rock-magnetic and paleointensity study of a Lesser Caucasus lava sequence comprised between 1.93 ± 0.09 and 1.78 ± 0.11 Ma. Reverse polarity magnetization determined for 20 consecutive lava flows permitted to suggest that the Olduvai subchron is probably disrupted by a short reverse-polarity episode. No paleointensity was obtained because of thermally unstable samples. Here, we report a detailed rock-magnetic, paleomagnetic, and multispecimen parallel differential pTRM absolute paleointensities performed on the nearby parallel Khertvisi lava succession. New isotopic age determinations indicate that the lavas of the Khertvisi section erupted in the Early Pleistocene, at the boundary of Gelasian and Calabrian ages in a time span of 1.88 ± 0.10 to 1.71 ± 0.12 Ma. All lava flows yielded well-defined reverse polarity magnetization with site mean directions Inc. = −56.1°, Dec = 189.5°, N = 20 (lava flows), α95 = 2.3°, k = 187. These directions are close to the Toloshi mean paleodirections, and both slightly deviate clockwise from the geocentric axial dipole and expected Plio-Quaternary directions. The values of the virtual geomagnetic pole (VGP) scatter parameters indicate that the studied sequence was formed during a very short time, most probably insufficient to average paleosecular variation. The multispecimen methodology provided a relatively low paleointensity for most analyzed lava flows, comparable to the transitional field intensity in Georgia during the Bruhnes and Matuyama chrons. These results definitively confirm a rather unstable, reverse polarity geomagnetic field regime across the Olduvai normal superchron. Thus, we propose to consider the Khertvisi section as the firm, volcanic evidence of Olduvai's short-lived geomagnetic event.

Petrogenetic implications of Fe–Mg substitution in clinopyroxene lattices of basaltic suites of Khandwa area, eastern Deccan volcanic province, Central India

Field investigations and lava flow mapping around Khandwa (21°49′ N:76°21′ E), Central India (within the eastern Deccan volcanic province) record three distinct lava flows which are in general, characterized by several structural zones, for example, Lower Vesicular Zone, Lower Colonnade Zone, Entablature Zone (EZ), Upper Colonnade Zone (UCZ), and Upper Vesicular Zone (UVZ). Different lava flows (often with chilled zones) in the field have been marked by discontinuous thin intertrappean beds. The occasional presence of chilled dyke and feeder dyke (both associated with Flow II) has also been noticed. The entire lava succession rests over coarse‐grained (cumulate) gabbroic (apparent) basement. Deduced clinopyroxene ordering clearly indicates that 100% initial ordering had been achieved for Flow II chilled zone, Flow II UCZ, Flow III UCZ, Flow III EZ, and cumulate gabbro. On the contrary, Flow II UCZ (97.35% initial ordering), feeder dyke (98.76% initial ordering), chilled dyke (99.82% initial ordering), and Flow I chilled zone (99.07% initial ordering) show initial clinopyroxene disordering (<100% ordering), which were later subjected to a gradual process to achieve ordering. Second‐order regression analyses clearly indicate that the progressive clinopyroxene ordering‐process is controlled by several differentiation parameters namely An% of coexisting plagioclase, lower thermometric values, and FeOt/MgO. In addition, roles of AlVI, Ti4+, and Fe3 were also suggested to influence the clinopyroxene ordering‐style. Based on Ti/6ox versus Al/6ox relation of clinopyroxene, it was deduced that cooling rate appreciably controls the clinopyroxene ordering‐behaviour. We contemplate that variation in clinopyroxene ordering‐pattern in basaltic rocks of Khandwa represents interplay of differentiation parameters and cooling rate of the ambient magma.

Ti-Rich Tholeiitic Picrites from the Prinsen af Wales Bjerge, East Greenland, and Evidence for a Pyroxene-Rich Mantle Source at Breakup in the North Atlantic

Abstract Highly magnesian, olivine-phyric tholeiitic basaltic and picritic lavas with >5 wt% TiO2 from the Prinsen af Wales Bjerge (PAWB) region are chemically distinct from all other Paleogene East Greenland flood basalts and from basalts in the North Atlantic Igneous Province. The ~100-m-thick lava succession rests on the 61 Ma Urbjerget Formation, is intercalated with volcaniclastic sediments, and has 57 Ma 40Ar/39Ar stepwise degassing ages. It is part of the Milne Land Formation, the first of the major flood basalt formations in East Greenland, and the result of plume impingement of the Kangerlussuaq area in East Greenland during the initial stages of continental breakup. The Ti-rich picrites have relatively primitive compositions and contain Mn- and Ni-rich olivine up to Fo88. Intermediate to high 87Sr/86Sri (0.7034–0.7044) and low Pb isotopic compositions reflect 4–11% crustal contamination, whereas the initial εNd (+4 − +5) and 187Os/188Os ratios (0.121–0.129) overlap with recent Icelandic basalts and appear little affected by contamination processes. The mantle source of the Ti-rich picrites contained garnet and was pyroxene-rich and similar to that of later low-Si alkaline basalts. The Ti-rich picrites of the PAWB, similar to other Ti-rich melts of the Kangerlussuaq region, represent analogies of MgO-rich and variably TiO2-enriched melts from pyroxene rich sources of traditionally accepted mantle plumes like Hawaii.

Mineralogy and geochemistry of lavas from the submarine lower caldera walls of Santorini Volcano (Greece)

Santorini Volcano, located in the central sector of the South Aegean volcanic arc, is one of the most active and potentially dangerous volcanoes in Europe. It has hosted Plinian eruptions over the last 350 ka of which at least four eruptions were accompanied by caldera collapses. Even though Santorini Volcano is considered a major threat, the main focus of research, thus far, has been on the comparatively young and subaerial deposits, whereas older stages of volcanism have been poorly studied. Our study focusses on samples from the submarine inner caldera walls and gives new insights into the early evolutionary stages of Santorini Volcano, contributing to a better understanding of its eruptive history and thus potential risks. The submarine lava successions were sampled along the inner caldera wall by a remotely operated vehicle (ROV) during R/V POSEIDON cruise 511 in 2017.The recovered lavas comprise two magmatic series, a low-K basaltic series overlain by a medium- to high-K series, which includes basaltic andesites, andesites and occasional dacites to trachytes. Major and trace element compositions and mineral zonation patterns suggest that fractional crystallisation and periodic magma replenishment were the dominant processes controlling magma evolution of both magmatic series. In addition, repeated magma mixing events played an important role, as indicated by zonation patterns in plagioclase and clinopyroxene ante- and phenocrysts. The thickness of the submarine lava successions (≥100 m) and the occurrence of similarly zoned plagioclase throughout indicate long-lived magma plumbing systems characterised by repeated differentiation and magma mixing. Furthermore, the incompatible element and radiogenic isotope ratios indicate a heterogeneous mantle source for Santorini magmas, which reflects the relative contribution of subduction (sedimentary) input and mantle wedge influx. A lava sample from the northern submarine caldera wall probably represents a deep level of the original Peristeria volcano. 40Ar/39Ar dating of andesite lavas from the southern submarine caldera produced ages of ~255 ka, whereas one basalt lava produced an age of 309 ± 30 ka. Interestingly, the new ages of both groups fall within an age gap in the volcanic history of Santorini Volcano. Even though it was not possible to unequivocally correlate the sampled submarine lava series to known subaerial units, the major and trace element compositions, and Sr-Nd-Pb isotopic compositions of our intermediate lavas show a strong similarity to subaerial lavas of Peristeria volcano, the second oldest major stage of Santorini volcanism. It however seems more likely that we have sampled a thus far unknown stage of volcanism at Santorini Volcano.

Geochronological evolution of the potentially active Iliniza Volcano (Ecuador) based on new K-Ar ages

The youngest volcanism of the Ecuadorian Volcanic Front (Western Cordillera) is mainly dominated by highly explosive events, including the growth and violent destruction of lava domes, and the formation of thick pyroclastic sequences. Deposits associated with such eruptive dynamics have been identified at Iliniza, a compound volcano located in the Western Cordillera with a poorly defined evolutionary history. We present the first K-Ar ages of Iliniza volcano combined with stratigraphic data, numerical reconstructions, and geochemical analyses, providing a new perspective on its evolution. Our results show that Iliniza volcano is much younger than previously proposed. The Iliniza twin-peaked shape is the result of the superposition of two andesitic to dacitic stratovolcanoes. (1) The North Iliniza (NI) edifice was constructed by two lava successions and an intermediate satellite vent, showing a short range of mainly effusive activity between 123 ± 6 and 116 ± 2 ka. (2) The South Iliniza (SI) edifice began its construction through the Lower SI stage when massive lavas dated at ~45 ka formed a basal cone. During the Upper SI stage, the uppermost part of this edifice was destroyed by the highly explosive Jatuncama phase (VEI 5) leaving a 30–40 m-thick ignimbrite sequence. The subsequent extrusion of several dacitic lava domes reconstructed the South Iliniza summit at around 35 ka. The Terminal SI stage corresponds to the emission of several andesitic lavas between 31 ± 4 and 25 ± 3 ka. The Iliniza eruptive activity extended into the Late Pleistocene and Holocene with the extrusion of the Tishigcuchi lava dome, and the emission of the Pongo lava flow dated at 6 ± 4 ka. Based on the proposed eruptive history, we suggest that a revised volcanic hazard assessment of the potentially active Iliniza volcano is required.

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.

Constraints on duration, age and migration of the feeder systems of the Madagascan Flood Basalt Province from high-precision 40 Ar/ 39 Ar chronology

Abstract The Late Cretaceous magmatism in Madagascar is correlated with the break-up between Madagascar and Greater India, with a presumed track of a hotspot from Madagascar towards the Marion Island and with an anoxic event in the Late Cretaceous. The lava succession and associated dyke swarms and sills of western Madagascar (Mailaka area) represent a volumetrically important area of the igneous province, where dykes with random orientation, several igneous intrusions and a flood basalt to rhyodacite sequence do occur. The magmas have a tholeiitic and weakly alkaline affinity. Using plagioclase separates, we obtained two plateau 40 Ar/ 39 Ar ages, and an inverse isochron age statistically indistinguishable, ranging from 92.9 ± 3.8 to 91.2 ± 1.3 Ma (2 σ ). These ages indicate that tholeiitic and alkaline rocks were erupted in the same age span. In addition, these ages are close to the Cenomanian–Turonian (C–T; 93.9 ± 0.2 Ma) boundary and are indistinguishable from the U–Pb ages available for the capping rhyodacitic unit of the Mailaka lava succession. A filtered compilation of eight ages for northern and central-western Madagascar rocks suggests a duration for the magmatic activity in this part of Madagascar province of the order of c. 3 Ma. If the western Madagascar magmatism is plume related, the plume head would need to have been located near the Mailaka area at c. 93 Ma. The geochemistry of the mafic lavas and dykes of western Madagascar is barely distinguishable from mid-ocean ridge basalt (MORB), with an increasing crustal contamination towards the evolved rocks, and does not constrain input of typical components derived by plume magmatism.

The Thórólfsfell tuya, South Iceland – A new type of basaltic glaciovolcano

Basaltic tuyas are glaciovolcanoes that form when substantial focused eruptions take place beneath thick ice. None have been witnessed, so models reconstructing tuya formation are grounded in detailed fieldwork. A key feature of many basaltic tuyas is the presence of volcanic and volcaniclastic rocks that indicate the sustained presence of an encircling meltwater lake during the eruption.Here we provide the first description of Þórólfsfell (Thórólfsfell), a basaltic tuya from Iceland, which is sufficiently distinct from previously described tuyas to be considered a new type of basaltic glaciovolcano.Thórólfsfell is an asymmetric tuya with an area of c.8 km2, base-to-top height of c.450 m, and volume of c.2.2 ± 0.4 km3 that has been emplaced onto the approximately 12o sloping lower southern flanks of Tindfjallajökull central volcano.Thórólfsfell shares only two major morphological characteristics with other basaltic tuyas: (1) a sub-horizontal top comprising subaerial lavas; (2) a clear vertical topographic expression, which reflects preferential upwards edifice growth due to lateral confinement by encircling ice and/or meltwater. There is no evidence for the presence of a large and long-lived syn-eruptive meltwater lake.The Thórólfsfell eruption is effusion-dominated, and there is a gradual reduction in cooling fractures in lavas with elevation. The eruption is divided into three Stages. Stage I forms a c.110 m thick drape onto an irregular but persistently c.12o dipping basement of older basaltic tuffs; Stage I consists of palaeoslope-parallel lava lobes with abundant cooling fractures, accompanied by abundant breccias. Stage II comprises a c.240 m thick stack of c.12o dipping stacked lava lobes with abundant cooling fractures, and occasional autobreccias. Stage III is c.110 m thick, and whilst early lavas have cooling fractures, the final Stage III lavas are sub-horizontal, subaerial pahoehoe lava flows.Our model for the formation of Thórólfsfell has two key features. The first is that the inclined basement has facilitated the downslope movement of meltwater away from the eruption site into an efficient gravity-assisted subglacial meltwater drainage system. The second is that there is a close connection between the vertical growth of the tuya and the ice above, with each successive lava in the growing stack being close to and/or in contact with the overlying ice. This repeated process provides the regular (but transient) meltwater supply necessary to produce a c.350 m stack of similarly-cooled lava carapaces.From a hazards perspective, a Thórólfsfell-style eruption is of little concern as rapid and steady meltwater drainage away from the eruption site would prevent the high-magnitude glacial outburst floods that require accumulated meltwater. The Thórólfsfell eruption provides a new perspective on effective meltwater dispersal away from tuya-building eruptions on dipping palaeoslopes, and on lava-ice interactions during subglacial eruptions. The products of other subglacial eruptions onto dipping basements, producing Thórólfsfell-type tuyas, await study.This first description of a new type of basaltic glaciovolcano may aid in the identification and interpretation of similar glaciovolcanoes on Earth and Mars that have yet to be discovered.

Windowing petrogenesis of continental flood basalts through mineralogical investigations: a case study from the Eastern Deccan Volcanic Province

Evaluation of the petrogenetic history of continental flood basalt using mineral–chemical data has been sparingly used for several flood basalt provinces of the world. For the Deccan flood basalts, such attempts are still rudimentary. The present contribution highlights several facets of petrogenesis of the Eastern Deccan Volcanic Province using details of constituent phases including glass. The present-study area around Khandwa (21°49′N, 76°21′E) indicates the presence of three distinct lava flows, which were identified on the basis of physical volcanological features based on a three-tier classification. Moreover, the occasional presence of chilled dykes and feeder dykes has been documented. The lava succession rests above (apparent) cumulate gabbro basement. The lava flows, in general, are characterized by dominant pyroxene (mostly augite, locally pigeonite) and plagioclase, opaque (magnetite and ilmenite) and glass, whereas olivine is rare. Both of these principal mineral phases (namely, plagioclase and pyroxene) occur both as phenocrysts and groundmass, while opaque phases occur only as groundmass. Pyroxene and plagioclase are generally zoned due to the changing milieu of crystallization conditions. Pyroxene thermometry data show the equilibration temperature range of 1050–1300 °C. Except for two samples of Flow II and one sample of chilled zone, the pyroxene thermometry data are broadly similar to that obtained from magnetite–ilmenite thermometry. As expected, deduced glass thermometric data reveal a relatively higher temperature. Crystallization history of parental magma was quantitatively modeled using Petrolog 3 software. The results indicate that the parental magma underwent both equilibrium and fractional crystallization (for example initially crystallized olivine reacted with ambient liquid to give rise to pyroxene). Phenocrystal and groundmass plagioclase and pyroxene show distinct patterns of compositional zoning corresponding to fluctuating physical parameters within the magma. Some of the glass [especially from the chilled dyke, feeder dyke, and Upper Colonnade Zone (UCZ) of flow II and flow III] represents ‘higher magnitude pressure ambience’ (fossilized glass) which is distinct from ‘surficial glass’ caused by equilibrium quenching. The glass and pyroxene compositions (using tectonic discrimination diagrams) suggest a non-orogenic continental setting of the parent magma. It is concluded that the studied Deccan volcanism shows a complex interplay of repeated pulses of magma ascent, fluctuation of crystallization condition, effervescence, and local magma mixing.

Chapter 2.1a Ferrar Large Igneous Province: volcanology

Abstract Preserved rocks in the Jurassic Ferrar Large Igneous Province consist mainly of intrusions, and extrusive rocks, the topic of this chapter, comprise the remaining small component. They crop out in a limited number of areas in the Transantarctic Mountains and southeastern Australia. They consist of thick sequences of lavas and sporadic occurrences of volcaniclastic rocks. The latter occur mainly beneath the lavas and represent the initial eruptive activity, but also are present within the lava sequence. The majority are basaltic phreatomagmatic deposits and in at least two locations form immense phreatocauldrons filled with structureless tuff breccias and lapilli tuffs with thicknesses of as much as 400 m. Stratified sequences of tuff breccias, lapilli tuffs and tuffs are up to 200 m thick. Thin tuff beds are sparsely distributed in the lava sequences. Lava successions are mainly 400–500 m thick, and comprise individual lavas ranging from 1 to 230 m thick, although most are in the range of 10–100 m. Well-defined colonnade and entablature are seldom displayed. Lava sequences were confined topographically and locally ponded. Water played a prominent role in eruptive activity, as exhibited by phreatomagmatism, hyaloclastites, pillow lava and quenching of lavas. Vents for lavas have yet to be identified.

Noise across Olduvai Subchron: Paleomagnetic study of a Pliocene lava succession from Javakheti Highland (Georgia, Lesser Caucasus)

Well-exposed lava sequences with available geochronological information may provide invaluable information on the fine characteristics of the Earth's magnetic field variation. A paleomagnetic study has been performed on twenty Pleistocene age, sub-horizontal lava flows in the Javakheti Highland (Lesser Caucasus). The sequence is formed by calc-alkaline basaltic lavas and is divided by a thin sedimentary layer in its upper part. We obtained two new radiometric ages: an age of 1.93±0.09 Ma for the flows below the sedimentary layer and 1.78±0.11 Ma for the upper part of the sequence. Thermomagnetic susceptibility vs. temperature curves indicate that remanence is carried by ferrimagnetic minerals like magnetite, titanomagnetite or titanomaghemite. Reliable paleomagnetic results could be obtained from all 20 analyzed flows, yielding the following reverse-polarity mean direction for the whole sequence: inclination I =-61.5°and declination D =200.1° (N = 20; α95 =3.1°; k=114). This direction does not agree with the expected one. The discrepancy can be explained by an apparent clockwise 17.1 ± 6.1° vertical-axis rotation. Alternatively, such moderate deviation may be due to some unstable geomagnetic regime or undersampled paleosecular variation. Isotope ages obtained in the present study indicate that the Toloshi sequence was emplaced during the normal-polarity Olduvai subchron, but palaeomagentic results indicate a reverse-polarity magnetization. At the same time, the analysis of virtual geomagnetic pole (VGP) scatter indicates that the Toloshi sequence was probably formed during a short time, insufficient to average paleosecular variation. These results suggest that the Olduvai subchron is probably disrupted by a short reverse-polarity episode.

Reconstruction of Late Pleistocene-Holocene Deformation through Massive Data Collection at Krafla Rift (NE Iceland) Owing to Drone-Based Structure-from-Motion Photogrammetry

In the present work, we demonstrate how drone surveys coupled with structure-from-motion (SfM) photogrammetry can help to collect huge amounts of very detailed data even in rough terrains where logistics can affect classical field surveys. The area of study is located in the NW part of the Krafla Fissure Swarm (NE Iceland), a volcanotectonic rift composed of eruptive centres, extension fractures, and normal faults. The surveyed sector is characterized by the presence of a hyaloclastite ridge composed of deposits dated, on a stratigraphic basis, to the Weichselian High Glacial (29.1–12.1 ka BP), and a series of lava flows mostly dating back to 11–12 ka BP. The integration of remotely sensed surveys and field inspections enabled us to recognize that this segment of the Krafla rift is made of grabens arranged en-échelon with a left-stepping geometry. A major graben increases in width in correspondence of the hyaloclastite cone; we interpret this geometry as resulting from the mechanical contrast between the stiffer lava succession and the softer hyaloclastites, which favours the development of concentric faults. We also measured a total extension of 16.6 m and 11.2 m along the fractures affecting the lava units, and a total extension in the hyaloclastites of 29.3 m. This produces an extension rate of 1.4 mm/yr in the Holocene lavas and 1.7 ± 0.7 mm/yr in the Weichselian hyaloclastite deposits. The spreading direction we obtained for this area is N97.7° E, resulting from the av. of 568 opening direction values.