Hyaloclastite Deposits Research Articles

Submarine Basaltic Magmatism in the Subbetic Basin (Southern Spain): Insights into Melt-Weakening Processes during Mesozoic Continental Rifting

Abstract Mantle-derived volcanic rocks from the Subbetic hyperextended basin in SE Spain provide new insights into the composition and mechanical behavior of the mantle during continental rifting. The present study describes a sequential restored cross-section along with geochemical characteristics of the basaltic rocks interbedded within the Mesozoic succession of the basin. Sedimentary stacking patterns of minibasins above the mobilized salt reflect the relationships with coeval basaltic volcanism. We recognize two type localities on the basis of volcanic facies, the presence of shallow intrusive bodies, and age of the associated sedimentary formations. The first type corresponds to subaqueous pillow-lava flows and subvolcanic sills and dikes associated with Lower Jurassic marly limestones and Middle Jurassic oolitic limestones. The Jurassic basalts present enriched MORB compositions with moderate La/Sm and low Sm/Yb ratios. Interestingly, a significant group of this Jurassic basaltic magmatism departs from the typical MORB-OIB array, showing deep Nb-Ta negative anomalies and high Th/Nb ratios. The second type comprises subaqueous lava flows, also including pillow-shaped basalts interlayered with hyaloclastite deposits and Upper Cretaceous clays, radiolarites, and marly limestones. The Cretaceous magmatism is characterized by highly enriched MORB compositions. Furthermore, the moderate Sm/Yb values and the positive correlation between LREE/HREE and Zr point to the involvement of deep (Grt-present) mantle sources in the origin of the Cretaceous basaltic melts. We interpret the Lower-Middle Jurassic calc-alkaline signal as due to the partial melting of recycled crustal rocks within the upper mantle, i.e., associated with remnants of pre-Mesozoic subducted slabs. These characteristics are similar to those described in Triassic basaltic rocks widespread throughout the External Zone of the Betic Cordillera. Mantle-derived basalts interlayered within the Lower Jurassic syn-rift deposits indicate that melting and deformation within the lithospheric mantle was initiated early during continental rifting. Accordingly, we suggest that Early to Middle Jurassic mantle melts promoted failure within the upper mantle, thus contributing to the inception of lithospheric-scale shear zones, which, in turn, controlled the evolution of this magma-poor hyperextended margin. Subsequently, rift evolution gave way to the activation of deeper melt sources in the mantle and an increase of the alkaline signature at the Cretaceous time.

Volcanic Structures and Magmatic Evolution of the Vesteris Seamount, Greenland Basin

The formation of isolated seamounts distant from active plate boundaries and mantle plumes remains unsolved. The solitary intraplate volcano Vesteris Seamount is located in the Central Greenland Basin and rises ∼3,000 m above the seafloor with a total eruptive volume of ∼800 km3. Here, we present a new high-resolution bathymetry of Vesteris Seamount and a detailed raster terrain analysis, distinguishing cones, irregular volcanic ridges, volcanic debris fans, U-shaped channels and lava flows. The slope angles, ruggedness index and slope direction were combined with backscatter images to aid geologic interpretation. The new data show that the entire structure is a northeast to southwest elongated stellar-shaped seamount with an elongated, narrow summit surrounded by irregular volcanic ridges, separated by volcanic debris fans. Whole-rock geochemical data of 78 lava samples form tight liquid lines of descent with MgO concentrations ranging from 12.6 to 0.1 wt%, implying that all lavas evolved from a similar parental magma composition. Video footage from Remotely Operated Vehicle (ROV) dives shows abundant pyroclastic and hyaloclastite deposits on the summit and on the upper flanks, whereas lavas are restricted to flank cones. The seamount likely formed above a weak zone of the lithosphere possibly related to initial rifting parallel to the nearby Mohns Ridge, but the local stress field increasingly affected the structure of the volcano as it grew larger. Thus, we conclude that the evolution of Vesteris Seamount reflects the transition from deep, regional lithospheric stresses in the older structures to shallower, local stresses within the younger volcanic structures similar to other oceanic intraplate volcanoes. Our study shows how the combination of bathymetric, visual and geochemical data can be used to decipher the geological evolution of oceanic intraplate volcanoes.

Volcanism and rapid sedimentation affect the benthic communities of Deception Island, Antarctica

Deception Island is amongst the most active volcanoes in the Southern Ocean, with over 20 explosive eruptions in the last ca. 200 years. The eruption that formed the caldera at Deception Island occurred 3980 ± 125 calendar years Before Present, and it is the largest eruptive event documented in Antarctica during Holocene. Since then, post-caldera volcanic activity has comprised many scattered eruptive vents across the island. Mortality of benthic organisms has been reported during the most recent eruptions occurred on the island, in 1967, 1969, and 1970 Common Era (CE), with very low abundances of organisms during the 1967–1973 CE period. Within the sea-flooded part of the caldera depression, named Port Foster, a submarine volcanic axis with several volcanic cones is observed. An interdisciplinary team sampled the best morphologically preserved volcanic edifice within Port Foster, the so-called Stanley Patch. Geophysical data traced the volcano and characterized its morphology and inner structure. Underwater scuba sampling allowed to acquire sediment and rock samples, photographs and video images of the benthic organisms and seascape. Morphology of Stanley Patch cone and textural characteristics of the collected pyroclastic rocks indicate that the volcanic edifice was originated during an explosive eruption. Furthermore, the lack of palagonitization, quenched pyroclast margins, and hyaloclastite deposits indicate that this cone has formed on-land, before the caldera floor became inundated by the seawater, highlighting the complex intra-caldera evolution of Deception Island. A sediment core from the crater was collected for sedimentological, and geochemical analysis. Antarctic climate and seasonal sea ice, together with organic degradation due to high sedimentation rates, explain the low total organic carbon data measured. The volcanic history of the island has probably avoided the development of a stable benthic community over time, similar to other Antarctic shallow communities. Moreover, the current geomorphological conditions still shape different benthic communities than in the surrounding coastal ecosystems. Stanley Patch, and the whole Port Foster, provide a natural laboratory for benchmarking the reestablishment of benthic communities on a volcanic-influenced shallow marine environment, offering relevant data for future studies evaluating global climate change effects on the Antarctic seabed.

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.

Facies architecture and palaeoenvironmental constraints of subaqueous felsic volcanism in the Iberian Pyrite Belt: The Paymogo Volcano-Sedimentary Alignment

The Paymogo Volcano-Sedimentary Alignment (PVSA) is an essentially igneous sequence associated with VHMS deposits of the Iberian Pyrite Belt. Unlike other areas of this mining district, where mafic igneous rocks are abundant, in the PVSA these are essentially felsic. Two facies associations have been distinguished based on petrographic and chemical features: a lower dacite facies association and an upper rhyolite facies association. The main ore horizons appear related to volcanic breccias of the rhyolite facies association. A detailed petrographic study of these breccias suggests they are resedimented hyaloclastite deposits. The analysis of volcanic facies indicates that the volcanism developed at water depths deeper than 500 m, consistent with the minimum depth at which VHMS deposits can form. We conclude that detailed volcanological studies provide effective constraints to palaeoenvironmental interpretations, which are relevant in assessing regional stratigraphy and area selection for VHMS exploration.

The 3D facies architecture and petrophysical properties of hyaloclastite delta deposits: An integrated photogrammetry and petrophysical study from southern Iceland

AbstractHyaloclastites develop where lava interacts with water resulting in deposits that have a unique and often complex range of petrophysical properties. A combination of eruptive style and emplacement environment dictates the size, geometry and distribution of different hyaloclastite facies and their associated primary physical properties such as porosity, permeability and velocity. To date, links between the 3D facies variability within these systems and their petrophysical properties remain poorly understood. Hjörleifshöfði in southern Iceland presents an exceptional outcrop exposure of an emergent hyaloclastite sequence >1 km wide by >200 m high and enables an investigation of the distribution of the hyaloclastite deposits at seismic scale. Within this study we present a photogrammetry‐based 3D model from part of this recent hyaloclastite delta and incorporate previous work by Watton et al. (Journal of Volcanology and Geothermal Research, 2013, 250, 19) to undertake detailed facies interpretation and quantification. Laboratory petrophysical analyses were performed on 34 core plugs cut from key field facies samples, including P‐ and S‐wave velocity, density, porosity and permeability at both ambient and confining pressure. Integration of the 3D model with the petrophysical data has enabled the production of pseudo‐wireline logs and property distribution maps which demonstrate the variability of physical properties within hyaloclastite sequences at outcrop to seismic scale. Through comparison of our data with examples of older buried hyaloclastite sequences we demonstrate that the wide‐ranging properties of young hyaloclastites become highly uniform in older sequences making their identification by remote geophysical methods for similar facies variations more challenging. Our study provides an improved understanding of the petrophysical property distribution within hyaloclastite sequences and forms a valuable step towards improving the understanding of similar subsurface sequences and their implications for imaging and fluid flow.

Towards a unified genetic model for the Au-Ag-Cu Pueblo Viejo district, central Dominican Republic

The origin of the world-class Pueblo Viejo deposit, central Dominican Republic, remains controversial. In this article we scale up the area of study to the Cabirma del Cerro mining concession, which abuts the Pueblo Viejo mining area, and integrate new data on the stratigraphy, lithogeochemistry and U-Pb geochronology of volcanogenic units with previous descriptions of the geology of the Pueblo Viejo deposit. Volcanic, volcaniclastic and sedimentary lithofacies of the ore-hosting Los Ranchos Formation reveal a progression from deposition in submarine to subaerial environments. Volcanic and hyaloclastite deposits are of exclusive basaltic composition, and were deposited at ca. 122–112Ma. Lithogeochemistry of basaltic rocks indicates tholeiitic and boninitic affinities. Chemostratigraphic relationships denote a progressive shift from LREE-depleted low-Ti island-arc tholeiitic and boninitic lavas to LREE-richer island-arc tholeiitic lavas. Basaltic deposits were intruded by tonalite batholiths and plagiorhyolite stocks at ca. 113–109Ma. These acid intrusives have tholeiitic, M-type affinities. Subsequently, diorites with transitional tholeiitic-calc-alkaline, and monzodiorites and andesitic domes with calc-alkaline affinities were emplaced at 109–106Ma. The progressive lithogeochemical changes throughout the magmatic sequence of the Los Ranchos Formation mirror those in so-called subduction-initiation ophiolites. Hydrothermal alteration and sulfide mineralization are ubiquitous in the area of study, and include volcanogenic massive sulfide (VMS) and epithermal deposit types. By assessing the location of these mineralization types in the stratigraphic sequence, an integrated metallogenic model with two distinct mineralization events is presented: (1) VMS mineralization took place during the subduction-initiation stage of the island-arc, and was genetically associated to LREE-depleted tholeiitic and boninitic basalts; subsequently (2) porphyry Cu(-Mo)-high sulfidation epithermal mineralization at ca. 112Ma was most likely connected to a widespread episode of M-type acid magmatism of tholeiitic affinity during the steady-state or true-subduction regime.

Formation and evolution of the end-Permian Oktyabrsk volcanic crater in the Tunguska Basin, Eastern Siberia

The Siberian Traps large igneous province comprises lava flows, volcaniclastic and hyaloclastite deposits, and a network of sills and dikes that were emplaced into the sediments beneath the volcanics. Spectacular examples of these intrusions are found in the Tunguska Basin sediments in Eastern Siberia. Hundreds of diatremes have been identified, rooted above sill intrusions, linking the contact metamorphic aureoles around the intrusive complexes in the Tunguska Basin with the paleo-surface. The diatremes in the southern part of the basin have been proposed as the main conduits for metamorphic gas release to the atmosphere, and thus a trigger leading to the end-Permian terrestrial mass extinction. The content of these diatremes has yet to be presented in detail in English-language literature, and fundamental questions remain regarding the content and formation history of these structures. This study examines the geochemistry and petrography of crater sediments within the uppermost portion of the Oktyabrsk diatreme complex in the southern Tunguska Basin, Eastern Siberia. We focus primarily on 505m of lacustrine sediments within the western crater. The sediments are tuffaceous silt- and sandstones intercalated with conglomerates and cemented by calcite. The major element composition of the bulk crater sediments indicates a mixture of basalt and carbonate. The sedimentary mineral assemblages do not reflect primary basaltic rock, but rather altered volcaniclastic material comprising clays, zeolites, calcite, and pyrite with minor detrital magnetite, fluorite, apatite, and garnets originating from hydrothermal diatreme deposits. The western crater sediments contain no evidence of allochthonous sediment sources and were subjected to only low-temperature diagenesis following burial, making this deposit a prime locale for further investigations of in situ processes and biologic activity in Siberia at the end-Permian. The Oktyabrsk complex formed via multiple episodes of mineral deposition and eruption events and the diatremes represent a likely source of the thick volcaniclastic deposits in the southern Tunguska Basin. This system enables further investigation of the connection between the Siberian Traps magma-sediment interaction and the end-Permian terrestrial environment.

Magma flow within dykes in submarine hyaloclastite environments: an AMS study of the Miocene Cabo de Gata volcanic units

Abstract The Miocene Cabo de Gata volcanic arc in SE Spain comprises a wide variety of volcanic facies and eruptive styles in subaqueous to subaerial environments. In the SW sector of the area, 5–100 m-thick, NNW–SSE-orientated dykes feed and intrude submarine hyaloclastite deposits. We analysed the anisotropy of magnetic susceptibility (AMS) of six dykes and five hyaloclastite sites from three volcanic units: the Cerro Cañadillas, Los Frailes, and El Barronal formations. The main magnetic minerals are primary low-Ti titanomagnetite and magnetite. The AMS ellipsoids in the dykes are generally oblate-triaxial in shape, with magnetic foliations subparallel to the dyke walls. Kinematic field evidence supports the inferred flow directions deduced from magnetic lineation and imbrication of magnetic foliation. The geometric relationships between dyke margins and AMS axes indicate that dykes at El Barronal were emplaced via prevalent subvertical upward magma flow. The inferred flow directions are reproduced well by analogue models of AMS simulating magma migration in dykes with a diapiric geometry. The other dykes were emplaced by lateral magma propagation. Conversely, hyaloclastite shows a large scatter of the AMS axes reflecting different degrees of fragmentation. We observe a gradual increase in scatter in the AMS from confined dykes to fragmented hyaloclastite.

The petrophysical and petrographical properties of hyaloclastite deposits: Implications for petroleum exploration

Offshore sequences of volcaniclastic rocks (such as hyaloclastite deposits) are poorly understood in terms of their rock properties and their response to compaction and burial. As petroleum exploration targets offshore volcanic rifted margins worldwide, understanding of volcanic rock properties becomes important both in terms of drilling and how the rocks may behave as seals, reservoirs, or permeability pathways. The Hawaiian Scientific Drilling Project phase II in 2001 obtained a 3 km-(2-mi)-long core of volcanic and volcaniclastic rocks that records the emergence of the largest of the Hawaiian islands. Core recovery of 2945 m (9662 ft) resulted in an unparalleled data set of volcanic and volcaniclastic rocks. Detailed logging, optical petrology, and major element analysis of two sections at depths 1831–1870 and 2530–2597 m (6007–6135 and 8300–8520 ft) are compared to recovered petrophysical logs (gamma ray, resistivity, and P-wave velocity). This study concludes deviation in petrophysical properties does not seem to correlate to changes in grain size or clast sorting, but instead correlates with alteration type (zeolite component) and bulk mineralogy (total olivine phenocryst percentage component). These data sets are important in helping to calibrate well-log responses through hyaloclastite intervals in areas of active petroleum exploration such as the North Atlantic (e.g., Faroe-Shetland Basin, United Kingdom, and Faroe Islands, the Norwegian margin and South Atlantic margins bordering Brazil and Angola).

Magma–ice–sediment interactions and the origin of lava/hyaloclastite sequences in the Síða formation, South Iceland

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

Three-dimensional lithofacies variations in hyaloclastite deposits

Analysis of the spatial lithofacies variability within lava-fed delta formations in southern Iceland has revealed complex three-dimensional volcanic architectures in hyaloclastite deposits in non-glacial settings. Two depositional environments are studied, (a) lava entering a marine embayment (Stóri-Nupur) and; (b) lava advancing into a body of water of the flanks of a Surtseyan cone (Hjörleifshöfði). Interaction between environmental factors such as shoreline geomorphology, water depth, wave energy levels, the nature of the lava transport system, lava supply rate all affect the resulting lava deltas creating complex lithofacies arrangements and stacking patterns. Recognised here are two types of hyaloclastite deltas. One of syn-sedimentary origin (Hjörleifshöfði) and one derived from primary fragmentation processes (Stóri-Nupur). Syn-sedimentary systems are dominated by destabilisation of the hyaloclastite pile leading to reworking downslope and share similarities to alluvial delta systems. Conversely, primary fragmented systems are controlled by waxing and waning cycles in volcanic effusivity whereby the hyaloclastite unit recorded is not the product of one lava flow rather than one eruptive event.

Facies architecture of the Upper Oligocene submarine Smrekovec stratovolcano, Northern Slovenia

The Upper Oligocene Smrekovec Volcanic Complex (SVC) represents the remains of a submarine stratovolcano which underwent Early Miocene tectonic dissection and large-scale displacement along the Periadriatic Line. According to the lithofacies architecture, abundance and spatial distribution, central, medial and distal zones of the former volcano edifice have been recognized. The zones extend at a distance of 0–2km, 2–5km and 5–20km south of the Periadriatic Line, respectively.Lavas, shallow intrusive bodies, autobreccia, peperite and hyaloclastite deposits occur mainly in the central zone, which is extensively altered under hydrothermal conditions, characterised by the development of zeolites (mainly laumontite), prehnite, chlorite, albite and epidote. Explosive volcanic activity intensified with the evolution of magma from basaltic andesitic to dacitic. Gas- and water-supported eruption-fed density flows are assumed. Their deposits show lithofacies organisation in pyroclastic depositional units (PDUs) similar to either volcaniclastic turbidites (Type 1) or thin subaqueous ignimbrites (Type 2).Volcaniclastic debris flow deposits are developed as volcaniclastic breccia and sandy debris flow deposits; the former occurs in the central and medial zones, and the latter occurs in the medial zone only.The most widespread lithofacies in the SVC are volcaniclastic turbidite deposits. Three types of sedimentary unit (TSU) have been distinguished. Type 1 is 2–5m thick and dominated by the massive division. It mainly occurs in the central zone. Type 2 is 0.7–3.5m thick and occurs in the central and medial zones. It comprises the massive division, bedded fining- and thinning-upward division, and a horizontally, wavy and cross-laminated division, topped by massive fine-grained tuff. Type 3 is 0.1–1.5m thick and mainly occurs in the distal zone. The massive division is absent, and thinly-bedded and laminated divisions are topped by massive fine-grained tuff. Volcaniclastic turbidites were settled from low-density turbidity currents and related suspension clouds.Mixed volcaniclastic–siliciclastic deposits are subordinate in occurrence and restricted to the distal zone. They are commonly reworked by bottom currents.

Permian rhyolitic volcanism, changing from subaqueous to subaerial in post-Variscan intra-continental Sirinia Basin (SW Romania–Eastern Europe)

The Sirinia Basin is a well-exposed, Upper Carboniferous–Lower Permian intra-continental extensional basin, containing 1–2 km of Permian interbedded terrigeneous sediments and primary volcanic and volcaniclastic rocks of rhyolitic composition. These are mostly the products of subaqueous volcanism passing into a subaerial volcanism. The intra-continental basin lies on the Danubian metamorphic units in the south westernmost part of the Carpathians (SW Romania) and is N-S oriented (18 × 8 km), as result of Alpine tectonic evolution and deformation. The Sirinia Basin setting formed a dynamic closed intra-continental sedimentary system that experienced a successive subsidence; it started with lacustrine sediments and then with large volumes of rhyolitic volcanic and volcaniclastic material dominantly sited in the central part of the basin. Volcanism included (1) subaqueous extrusion of domes surrounded by hyaloclastite deposits closely associated with secondary sedimentation, (2) extrusive and explosive Surtseyan-type and subaerial eruptions from shallow water to emergent volcanoes on dome-cap and associated reworked deposits, towards the southern side of the basin, and (3) subaerial extrusive domes. Fluvial erosion and deposition completed the evolution of the emerged marginal part of the basin. Most of the erupted material of the first and second phase filling the central part of the basin along extensional normal faults was rapidly transported on sides of the basin by turbidite sedimentation, debris flows, slumping and sliding. While turbidites prevail toward the central part of the basin, the debris flow sedimentation predominated at the margin of the basin, infrequently interrupted by deposition of fallout ash from hydromagmatic explosive eruptions related to the dome-cap tuff and pumice cones.

Hyaloclastites and the slope stability of Hawaiian volcanoes: Insights from the Hawaiian Scientific Drilling Project's 3-km drill core

Core samples recovered during the Hawaiian Scientific Drilling Project (HSDP) drilling project reveal that the upper 1 km of the submarine flank of Mauna Kea is comprised mainly of hyaloclastites. Progressive, very low-temperature alteration of these hyaloclastites has been accompanied by systematic transformations in physical properties of these deposits. Hyaloclastite deposits which directly underlie ca. 1 km of subaerially-emplaced lavas are very poorly consolidated. But over a depth interval of ca. 500 m, compaction and, especially, precipitation of zeolitic, pore-filling cements associated with palagonitization of sideromelane, have eliminated porosity as well as promoted the consolidation of these hyaloclastites. The latter is reflected in unconfined compressive strengths which increase from mean values, respectively, of 2.5 and 4.6 MPa in weakly consolidated, smectite-rich hyaloclastites from the incipient (1080 to 1335 mbsl) and smectitic (1405–1573 mbsl) alteration zones, to a mean value of 10.0 MPa in the more highly consolidated hyaloclastites of the palagonitic zone of alteration (from 1573 mbsl to the bottom of the drill hole). Conversely, overlying, intercalated, and underlying lava flows are generally much less altered, and have mean compressive strengths which are 1 to 2 orders of magnitude greater then hyaloclastites at equivalent depths. The shear strengths of the hyaloclastites also increase with depth and grade of alteration, but are uniformly and substantially lower in the lavas. Those hyaloclastites exhibiting the highest grade of alteration (i.e., palagonitic) also exhibit the highest measured strengths, and thus the alteration of hyaloclastites appears to strengthen as opposed to weaken the flanks of the edifice. However, the contrast in strength between hyaloclastites and lavas may be a primary factor in localizing destabilization, and the zones of weak and poorly consolidated hyaloclastites may facilitate slumping by servings as hosts for shallow detachment surfaces on the unsupported flanks of these volcanoes.

Geotectonic setting of hydrothermal activity on the summit of Lucky Strike Seamount (37°17′N, Mid‐Atlantic Ridge)

We have investigated the relations between volcanic, tectonic, and hydrothermal activity on Lucky Strike Seamount (37°17′N, Mid‐Atlantic Ridge) using a nested survey strategy involving collection of data from different deep‐sea mapping systems. The highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center. Hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake. Isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic (pillow lava) and hydrothermal (sulfide rubble and hydrothermally cemented breccias) substrates. The large volume of sulfide rubble, together with the nature of the sulfide structures, indicates that hydrothermal activity has been episodic but ongoing for a long period of time (hundreds to thousands of years). On the basis of the distribution of hydrothermal deposits, we propose a model of alternation between tectonic and volcanic control on hydrothermalism at Lucky Strike Seamount. Midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high‐temperature hydrothermal activity over long time periods. During periods of amagmatic extension, active faulting within the summit depression provides multiple, near‐surface fluid flow pathways for discharge of high‐temperature fluids and widespread deposition of massive sulfides. During eruptive events, rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits. The new sheet flows form a cap on the hydrothermal system, and fluid upflow is reorganized. Discharge of high‐temperature fluids is restricted to isolated sites with relatively high permeability, for example, the edges of the lava lake. Much of the upwelling hydrothermal fluid pools in the subsurface, conductively cools, and mixes with entrained seawater before discharging as widespread low‐temperature diffuse flow. Hyaloclastites become cemented, further augmenting the sealing of the system. Present‐day activity at Lucky Strike Seamount represents this locally volcanically controlled phase of activity, despite the segment as a whole being dominantly amagmatic.

Glacial-interglacial history of the Skaftafell region, southeast Iceland, 0–5 Ma

Volcanic strata in the Skaftafell region, southeast Iceland, record a sequence of at least 16 glacial and interglacial intervals since 5 Ma. Two composite sections of 2 to 2.8 km thickness have been constructed from multiple, overlapping, cliff profiles. The timing of alternating sequences of subaerial lava flows, pillow basalts, and hyaloclastite deposits is provided by magnetostratigraphic mapping and K-Ar radiometric dating. We find that the frequency and intensity of glaciations increased significantly at ca. 2.6 Ma, and particularly since 0.8 Ma, amplifying topographic relief in this area from

Englacial vs lacustrine origin of volcanic table mountains: evidence from iceland

Detailed facies analysis of hyaloclastites and associated lavas from eight table mountains and similar "hyaloclastite volcanoes" in the Icelandic rift zone contradict a rapid and continuous, "monogenetic", entirely subglacial evolution of most volcanoes studied. The majority of the exposed hyaloclastite deposits formed in large, stable lakes as indicated by widespread, up to 300-m-thick, continuous sections of deep water, shallow water and emergent facies. Salient features include extensively layered or bedded successions comprising mainly debris flow deposits, turbidites, base surge and fallout deposits consisting of texturally and compositionally variable, slightly altered hyaloclastites, as well as sheet and pillow lavas. In contrast, chaotic assemblages of coarser-grained, more poorly sorted and more strongly palagonitized hyaloclastite tuffs and breccias, as well as scoria and lava are interpreted to have formed under sub- or englacial conditions in small, chimney-like ice cavities or ice-bound lakes. Irregularly shaped and erratically arranged hyaloclastite bodies produced at variable water levels appear to have resulted mainly from rapid changes of the eruptive environment due to repeated build-up and drainage of ice-bound lakes as well as the restricted space between the ice walls. We distinguish a "deep water" facies formed during high water levels of the lake, a hydroclastic shallow water and emergent facies (leakage of the lake or growth of the volcano above the water surface). Our model implies the temporary existence of large, stable lakes in Iceland probably formed by climatically induced ice melting. The highly complex edifices of many table mountains and similar volcanoes were constructed during several eruptive periods in changing environments characterized by contrasting volcanic and sedimentary processes.

A record of Early Pleistocene glaciation on the Mount Edziza Plateau, northwestern British Columbia

Mount Edziza is a Plio-Pleistocene volcanic complex that is located in the Stikine Terrane in northwestern British Columbia. A sequence of diamictites preserved between Ice Peak Formation basalts on the northwestern blank of Mount Edziza records an Early Pleistocene regional glaciation. The lowest Ice Peak Formation basalt flow (IP1; about 1 Ma) was probably extruded onto glacial ice because it is deformed and brecciated, it is pillowed at the base, it lies directly on hyaloclastite deposits, and there is a lack of fluvial and lacustrine sediments at the base. Fabric measurements from the underlying diamictites are consistent with lodgement processes and indicate northwest and southwest transport directions. These data, and an abundance of striated exotic cobbles, indicate that the sediment was deposited by Coast Mountain ice. Radiometric, paleomagnetic, and stratigraphic data all support the interpretation that diamictites at the section are the sedimentary record of an Early Pleistocene (about 1.1 Ma, isotope stage 32–34) regional glaciation(s). The normal paleomagnetic polarity of one of the Ice Peak Formation basalts (IP2) records extrusion during the Jaramillo normal polarity subchron (1.07–0.99 Ma) and further constrains the age of the underlying diamictites.

A core through the Monaro basalt: Bega (BMR) no. 7

A vertical borehole has been drilled through 215.8 m of the Monaro volcanic sequence at a location 20 km south of Cooma. The site is on basalt outcrops near the crest of the Main Divide. The core comprises 198.2 m of mafic volcanic and weathered volcanic rocks overlying 8.5 m of lacustrine sedimentary and hyaloclastite deposits and 9.0 m of weathered basement composed of schist and meta‐sandstone. About 55% of the volcanic sequence is weathered; and some of the weathered material may have been originally volcaniclastic. Seven weathering profiles, up to 12.5 m thick have bauxite zones up to 3.5 m thick at the top. Bauxite comprises ∼9% of the sequence. The fresher rocks are mainly alkali basalt flows, up to 27 m thick, but also include a basanite flow at the top of the sequence and two intervals of alkali dolerite between 65.0 and 75.0 m. The deeply weathered nature of many of the flows indicates prolonged periods of intense chemical weathering under wet climatic conditions between lava eruptions.