Volcaniclastic Processes Research Articles

A Confidence-Based Assessment Method for Distinguishing Pyroclastic Density Current Deposits From Other Volcaniclastic Units

Pyroclastic density currents (PDCs) are a destructive volcanic hazard. Quantifying the types, frequency and magnitudes of PDC events is essential for effective risk management, but since historical records at best extend a few hundred years this usually relies on identifying deposits in the geological record. However, small volume unconsolidated PDC deposits have low preservation potential and can be difficult to distinguish from other volcaniclastic units, especially in proximal locations. Consequently many small or poorly exposed deposits can be overlooked. Here, we introduce a structured field method for assessing volcaniclastic deposits of unknown origin with a particular focus on identifying deposits from concentrated PDCs (pyroclastic flows). The method differs from traditional identification schemes in that it does not uniquely attribute a deposit to a single depositional process, but instead assesses how confidently different volcaniclastic processes could explain the observed deposit features. Therefore, the underlying uncertainties in the assessment are explicitly addressed. The method allows consistent, rapid assessment of candidate pyroclastic flow deposits in the field, and the concept could easily be adapted for assessing other types of volcaniclastic deposit. The introduction of confidence levels in deposit interpretations should be useful for carrying though uncertainties into probabilistic assessments of volcanic hazards.

A Middle Miocene carbonate embankment on an active volcanic slope: Ilhéu de Baixo, Madeira Archipelago, Eastern Atlantic

Carbonate factories on insular oceanic islands in active volcanic settings are poorly explored. This case study illuminates marginal limestone deposits on a steep volcanic flank and their recurring interruption by deposits linked to volcaniclastic processes. Historically known as Ilhéu da Cal (Lime Island), Ilhéu de Baixo was separated from Porto Santo, in the Madeira Archipelago, during the course of the Quaternary. Here, extensive mines were tunnelled in the Miocene carbonate strata for the production of slaked lime. Approximately 10 000 m3 of calcarenite (−1 to 1ø) was removed by hand labour from the Blandy Brothers mine at the south end of the islet. Investigations of two stratigraphic sections at opposite ends of the mine reveal that the quarried material represents an incipient carbonate ramp developed from east to west and embanked against the flank of a volcanic island. A petrographic analysis of limestones from the mine shows that coralline red algae from crushed rhodoliths account for 51% of all identifiable bioclasts. This material was transported shoreward and deposited on the ramp between normal wave base and storm wave base at moderate depths. The mine's roof rocks are formed by Surtseyan deposits from a subsequent volcanic eruption. Volcaniclastic density flows also are a prevalent factor interrupting renewed carbonate deposition. These flows arrived downslope from the north and gradually steepened the debris apron westwards. Slope instability is further shown by a coral rudstone density flow that followed from growth of a coral reef dominated by Pocillopora madreporacea (Lamarck), partial reef collapse, and transport from a more easterly direction into a fore-reef setting. The uppermost facies represents a soft bottom at moderate depths in a quiet, but shore-proximal setting. Application of this study to a broader understanding of the relationship between carbonate and volcaniclastic deposition on oceanic islands emphasizes the susceptibility of carbonates to dilution and complete removal by density flows of various kinds, in contrast to the potential for preservation beneath less-disruptive Surtseyan deposits. Copyright © 2013 John Wiley & Sons, Ltd.

Source to sink: A review of three decades of progress in the understanding of volcaniclastic processes, deposits, and hazards

Volcaniclastic sediments, broadly defined as clastic deposits derived from the transport, deposition and/or redeposition of the products of volcanic activity, have long been a Cinderella of the geosciences. This status is a function of the inherent complexity of the fragmentation, transport and depositional processes that operate in volcanically-impacted environments and the comparatively recent development of the discipline as a specialist area. Volcaniclastic studies are truly interdisciplinary, drawing on many elements of physical volcanology, fluid dynamics, classical clastic sedimentology, hydrology and geomorphology. In the past 30 years volcaniclastic studies have blossomed, partly in response to a number of catastrophic and high-profile volcanic eruptions, including Mount St. Helens in 1981, Nevado del Ruiz in 1985 and Pinatubo in 1991, and partly due to integration with the maturing science of fluid dynamics and an increased understanding of the behaviour of particulate dispersions and two-phase granular flows. These historical events have demonstrated that the sedimentary repercussions of volcanic eruptions can have more severe, far-reaching, and prolonged impacts than the initial volcanism. In parallel, studies of well-preserved examples from the geological record have extended our understanding of landscape and environmental responses to styles and scales of volcanism that have not been recorded historically, such as the impacts of caldera-forming eruptions from silicic calderas. As a consequence, studies have expanded beyond classical stratigraphic and sedimentological studies of ancient successions in a variety of plate tectonic settings to a more dynamic focus on process. Ultimately, volcaniclastic successions are the product of the interplay between the volcano, in terms of the style, magnitude and explosivity of the eruption, and the environment, as expressed by physiography, hydrology, energy, and accommodation space.

Sedimentology of cold-climate aeolian sandsheet deposits in the Askja region of northeast Iceland

Active aeolian sandsheets cover an area of 270 km 2 in the Askja region of NE Iceland. The sandsheet is divided into (i) a deflationary upwind part that is characterised by deflation lags, ventifacted boulder fields and restricted aeolian sand patches in the lee of obstacles, (ii) an accumulating central part that is characterised by continuous aeolian sand cover up to 10 m thick, where three scales of aeolian ripples are evident along with larger scale forms including longitudinal zibars, and (iii) a downwind part that is currently subject to aeolian bypass with only localised accumulation occurring in topographic hollows between basaltic lava fields. Trains of aeolian granule megaripples are a characteristic landform of the downwind part of the sandsheet. These sinuous-crested, oblique bedforms attain heights of 0.35 m and downwind spacings of 6 m. They are composed principally of low-density, pebble-grade pumice clasts and other reworked volcaniclastic detritus derived from the nearby Askja volcanic centre. The distribution of landforms across the sandsheet reflects a regional aeolian sediment budget that is controlled by dynamic interactions between glacial, ice-margin fluvial, aeolian and volcaniclastic processes under the influence of a distinctive climatic regime. Sediment supply at the upwind sandsheet margin, adjacent to the Dyngjujökull glacier, increases during summer periods when rates of glacial ablation are at a maximum. Longer term controls on sediment supply rate are determined by episodes of glacier surging and retreat, and by periodic eruptions of volcaniclastic debris from the Askja volcanic centre. The availability of sediment for aeolian transport is controlled by the level of the groundwater table, the depth and continuity of the permafrost layer in the proglacial area, the degree of development of armoured pebble lags and freezing of the sandsheet surface during winter months. The transport capacity of the wind varies on a seasonal basis with mean wind velocities and wind gustiness reaching a maximum during winter periods. Both winter and summer mean wind velocities have been steadily increasing since formal records began in 1969. The suite of aeolian landforms developed within the Askja region, as defined by sediment texture and composition, and bedform scale, morphology and spatial distribution, reflects a special set of controlling circumstances, which mean that the Askja Sandsheet deposits differ significantly from those described from elsewhere.

The 472 AD Pollena eruption of Somma-Vesuvius (Italy) and its environmental impact at the end of the Roman Empire

Catastrophic sedimentary processes associated with explosive eruptions represent a significant geologic hazard in volcanic areas. Here we report a striking historic example of an intermediate-scale explosive event whose environmental effects were strongly amplified by secondary rapid mass flows and hydrogeologic disasters. The 472 AD Pollena eruption of Somma-Vesuvius (Campania, Italy) took place in the critical period of the fall of the Western Roman Empire. On the basis of an integrated geologic–archaeologic study we point out evidence of human habitation at the time of the eruption, effects induced and recovery time in a wide territory of Campania, and how the eruption significantly accelerated the deterioration of the local society during the Late Ancient age. The eruption began with a pulsating, sustained eruption column, followed by pyroclastic surges and scoria flows. Hydromagmatism acted early in the event, different from the typical Plinian eruptions of Somma-Vesuvius. Specific facies associations of primary and secondary volcaniclastic deposits characterize three depositional domains, including the volcano slopes, the surrounding alluvial plains and the distal mountains of the Apennine Range. Both volcano slopes and distal mountain slopes supplied loose pyroclastic material to the hyperconcentrated floods and debris flows that spread across the alluvial plains. The great impact of secondary volcaniclastic processes arose from: (1) the high vulnerability of the territory due to its geomorphic context; (2) the humid climatic conditions; (3) the hydromagmatic character of the eruption; (4) the decline of land management at the end of the Roman Empire.

Turbidites associated with bentonites in the Upper Cretaceous of the Campos Basin, offshore Brazil

More than fifteen bentonite beds have been identified in the Santonian/Campanian section of the Campos Basin, offshore Brazil. These range in thickness up to 1 m and are mainly composed of interstratified illite/smectite. Relict textural features can still be observed, identifying them as former volcanic ash layers. They also display gradational or sharp contacts with the associated lithologies. In the Carapeba Field, `pure' bentonite beds, formed mainly by expandable clay minerals, occur intercalated with pelitic rocks. Bentonite beds with up to 20% terrigenous silty material (micas, quartz and feldspars), here classified as `impure', are found directly overlying siliciclastic turbidite sandstones. These bentonite beds incorporate constituents of turbiditic dust and are interpreted as equivalent to the Bouma E division of the underlying turbidite layers. This fact suggests a contemporaneity of the turbidite and the volcaniclastic processes. Therefore, the volcanism and associated tectonic seismicity are considered a potential mechanism for the onset of turbidite sedimentation.

Volcaniclastic processes on modern submarine arc stratovolcanoes: sidescan and photographic evidence from the Rumble IV and V volcanoes, southern Kermadec Arc (SW Pacific)

Studies of volcaniclastic processes on modern, submarine, arc stratovolcanoes are rare. Facies models of submarine volcanism and associated volcaniclastic deposits have to date emphasized general and/or regional processes. Two andesite-alkali basaltic volcanoes (Rumble IV and V volcanoes) from the southern Kermadec arc are “mapped” with swath bathymetry and sidescan imagery, and photographic data, to characterise volcaniclastic processes on these modern, submarine, arc stratovolcanoes, and their interaction with ambient marine processes. The principal observation from the Rumble IV and V data is the fine-scale spatial variability in the distribution of effusive lavas, various volcaniclastic deposits, and hemipelagic-pelagic sediments. Mass-movement of volcaniclastic deposits is common, and includes (1) epiclastic debris flows, (2) pillow breccia talus deposition penecontemporaneous with lava eruption, (3) traction particulate gravity flows, and possibly (4) sector collapse. Epiclastic redeposition is inferred to be mobilised by episodic, but frequent, localised, small volume flows, which largely redeposit debris on the edifice. Winnowed and rippled volcaniclastic sediment indicates bottom-current activity is common, but localised, over the entire height of both volcanoes, down to a water depth of ~ 2200 m. These currents also possibly entrain detritus for subsequent mobilisation by particulate gravity flows. Irregular and ragged spatter agglutinates on Rumble IV, interpreted as evidence of lava fountaining, and wellsorted, fines poor, scoriaceous material on Rumble V, interpreted as evidence of explosive eruption, indicate the transition between effusive and explosive volcanism is between 650 and 700 m water depth. Other southern Kermadec volcanoes (e.g. Tangaroa) have scoriaceous detritus at 960–970 m water depth. Positions of these effusive-explosive transition zones are likely to oscillate up and down the volcano during glacioeustatic fluctuations.