Large Explosive Eruptions Research Articles

Phase petrology reveals shallow magma storage prior to large explosive silicic eruptions at Hekla volcano, Iceland

Understanding the conditions that culminate in explosive eruptions of silicic magma is of great importance for volcanic hazard assessment and crisis mitigation. However, geological records of active volcanoes typically show a wide range of eruptive behavior and magnitude, which can vary dramatically for individual eruptive centers. In order to evaluate possible future scenarios of eruption precursors, magmatic system variables for different eruption types need to be constrained. Here we use petrological experiments and microanalysis of crystals to clarify the P–T–x state under which rhyodacitic melts accumulated prior to the H3 eruption; the largest Holocene Plinian eruption of Hekla volcano in Iceland. Cobalt-buffered, H2O-saturated phase equilibrium experiments reproduce the natural H3 pumice phenocryst assemblage (pl>fa+cpx>ilm+mt>ap+zrc) and glass chemistry, at 850±15°C and PH2O of 130 to 175 MPa, implying shallow crustal magma storage between 5 and 6.6 km. The systematics of FeO and anorthite (CaAl2Si2O8) content in plagioclase reveal that thermal gradients were more important than compositional mixing or mingling within this magma reservoir. As these petrological findings indicate magma storage much shallower than is currently thought of Hekla's mafic system, we use the constrained storage depth in combination with deformation modeling to forecast permissible surface uplift patterns that could stem from pre-eruptive magma intrusion. Using forward modeling of surface deformation above various magma storage architectures, we show that vertical surface displacements caused by silicic magma accumulation at ∼6 km depth would be narrower than those observed in recent mafic events, which are fed from a lower crustal storage zone. Our results show how petrological reconstruction of magmatic system variables can help link signs of pre-eruptive geophysical unrest to magmatic processes occurring in reservoirs at shallow depths. This will enhance our abilities to couple deformation measurements (e.g. InSAR and GPS) to petrological studies to better constrain potential precursors to volcanic eruptions.

Surface morphology of caldera-forming eruption deposits revealed by lidar mapping of Crater Lake National Park, Oregon – Implications for deposition and surface modification

Large explosive eruptions of silicic magma can produce widespread pumice fall, extensive ignimbrite sheets, and collapse calderas. The surfaces of voluminous ignimbrites are rarely preserved or documented because most terrestrial examples are heavily vegetated, or severely modified by post-depositional processes. Much research addresses the internal sedimentary characteristics, flow processes, and depositional mechanisms of ignimbrites, however, surface features of ignimbrites are less well documented and understood, except for comparatively small-volume deposits of historical eruptions. The ~7700 calendar year B.P. climactic eruption of Mount Manama, USA, vented ~50km3 of magma, deposited first as rhyodacite pumice fall and then as a zoned rhyodacite-to-andesite ignimbrite as Crater Lake caldera collapsed. Lidar collected during summer 2010 reveals the remarkably well-preserved surface of the Manama ignimbrite and related deposits surrounding Crater Lake caldera in unprecedented detail despite forest cover. The ±1m lateral and ±4cm vertical resolution lidar allows surface morphologies to be classified. Surface morphologies are created by internal depositional processes and can point to the processes at work when pyroclastic flows come to rest. We describe nine surface features including furrow-ridge sets and wedge-shaped mounds in pumice fall eroded by high-energy pyroclastic surges, flow-parallel ridges that record the passage of multiple pyroclastic flows, perched benches of marginal deposits stranded by more-mobile pyroclastic-flow cores, hummocks of dense clasts interpreted as lag deposit, transverse ridges that mark the compression and imbrication of flows as they came to rest, scarps indicating ignimbrite remobilization, fields of closely spaced pits caused by phreatic explosions, fractures and cracks due to extensional processes resulting from ignimbrite volume loss, and stream channels eroded in the newly formed surface. The nine morphologies presented here illustrate a dynamic depositional environment that varied spatially and with time during the eruption, and show that multiple processes modified the ignimbrite after deposition, both during and after the eruption.

A10 近畿地方の第四紀テフラ層序に基づく大規模爆発的噴火の層位と噴出体積(火山の地質・形成史(2),日本火山学会2006年秋季大会)

A10 近畿地方の第四紀テフラ層序に基づく大規模爆発的噴火の層位と噴出体積(火山の地質・形成史(2),日本火山学会2006年秋季大会)

Long-term changes in explosive and effusive behaviour at andesitic arc volcanoes: Chronostratigraphy of the Centre Hills Volcano, Montserrat

Volcanism on Montserrat (Lesser Antilles arc) has migrated southwards since the formation of the Silver Hills ~2.5Ma, and has formed three successively active volcanic centres. The Centre Hills volcano was the focus of volcanism from ~1–0.4Ma, before activity commenced at the currently active Soufrière Hills volcano. The history of activity at these two volcanoes provides an opportunity to investigate the pattern of volcano behaviour on an andesitic arc island over the lifetime of individual volcanoes. Here, we describe the pyroclastic stratigraphy of subaerial exposures around central Montserrat; identifying 11 thick (>1m) pumiceous units derived from sustained explosive eruptions of Centre Hills from ~0.8–0.4Ma. Over 10 other, less well- exposed pumiceous units have also been identified. The pumice-rich units are interbedded with andesite lava breccias derived from effusive, dome-forming eruptions of Centre Hills. The stratigraphy indicates that large (up to magnitude 5) explosive eruptions occurred throughout the history of Centre Hills, alongside effusive activity. This behaviour at Centre Hills contrasts with Soufrière Hills, where deposits from sustained explosive eruptions are much less common and restricted to early stages of activity at the volcano, from ~175–130ka. Subsequent eruptions at Soufriere Hills have been dominated by andesitic effusive eruptions. The bulk composition, petrography and mineral chemistry of volcanic rocks from Centre Hills and Soufrière Hills are similar throughout the history of both volcanoes, except for occasional, transient departures to different magma compositions, which mark shifts in vent location or dominant eruption style. For example, the final recorded eruption of Centre Hills, before the initiation of activity at Soufrière Hills, was more silicic than any other identified eruption on Montserrat; and the basaltic South Soufrière Hills episode marked the transition to the current stage of predominantly effusive Soufrière Hills activity. The compositional stability observed throughout the history of Centre Hills and Soufrière Hills suggests that a predominance towards effusive or explosive eruption styles is not driven by major compositional shifts of magma, but may reflect local changes in long-term magma storage conditions that characterise individual episodes (on 105year timescales) of volcanism on Montserrat.

Ancient Turks and Climate Disasters in Central Asia (534–679 AD)

This paper is the first to analyse the effect climate disasters (AD 536, 581, 626, 679) had on the historical processes in the east of Central Asia in the Early Middle Ages. Climate disasters are abrupt, severe, and sometimes protracted periods of cooling and drought that produce a negative effect on the economy of a nomadic society, the reason for this being large explosive volcanic eruptions. Cooling and drought brought famine to the nomads and caused illnesses. In addition to that, all the natural catastrophes were accompanied by plague pandemics among the eastern Turks. The Turkic Empire can be split into several chronological periods characterised by significant events that changed the course of history of the nomadic state, namely, AD 534–545 — the rise of the Turkic Empire; AD 581–583 — the division of the Turkic Empire into the Western and the Eastern Empires; AD 627–630 — the Eastern Turks are conquered by China; AD 679–687 — the second rise of the Eastern Turkic Empire. The research carried out shows that there is a certain connection between the important historical events and climate disasters in the history of the Turkic Empire (AD 534–745), this factor making it possible for the author to come forth with the proposition that the climatic factor did have an effect on the historical processes in the east of Central Asia, especially on the territories that had a nomadic economy.

Regional variability in the frequency and magnitude of large explosive volcanic eruptions

Quantifying the frequency at which volcanic eruptions of different size occurs is impor- tant for hazard assessment. Volcanic records can be used to estimate the recurrence rate of large-magnitude eruptions (magnitude ≥4), but recording biases that impact data complete- ness complicate analysis. To overcome these biases, we conceptualize the volcanic record as a series of individual and unique time series associated by a common behavior. Thus, we approach issues of completeness on a volcano-by-volcano basis and use a hierarchical Bayes- ian approach to characterize the common frequency-magnitude (f-M) behavior for different groups of volcanoes. We identify variations in the f-M relationship between different volcano types and between different volcanic arcs. By accounting for systematic under-recording in the volcanic record, we also calculate the global recurrence rates for large-magnitude erup- tions during the Holocene, which are similar to previous estimates. However, higher recur- rence rates for smaller-magnitude events are observed, which is a result of our adjustments for data completeness. Quantifying how the f-M relationship varies between different groups of volcanoes provides an opportunity to understand how the tectonic setting in uences f-M behavior, which is important to quantify long-term regional volcanic hazard.

Paleoproterozoic andesitic volcanism in the southern Amazonian craton, the Sobreiro Formation: New insights from lithofacies analysis of the volcaniclastic sequences

The Sobreiro Formation (SF) records one excellent and well-preserved example of subaerial Precambrian (ca. 1.88Ga) volcanism on earth. It is located in the São Felix do Xingu region (SFX), in the eastern part of Pará State, southern Amazonian craton, northern of Brazil. The high-K calc-alkaline composition of the Sobreiro rocks indicates that this formation likely generated in an ocean-continent convergent margin. This paper documents the architecture of a series of basaltic-andesite to andesite and minor dacite lava flows and associated volcaniclastic rocks. These rocks are divided into primary and secondary lithotypes, depending if they resulted from a direct volcanic activity (pyroclastic) or reworked processes and massive to stratified, depending on the different transport and emplacement mechanisms. Primary lithofacies is formed of pyroclastic flow and surge deposits, volcanic breccias and welded ignimbrites; secondary volcaniclastic lithofacies consist of reworked debris. Mass-flows, hyperconcentrated flows and stream floods are interpreted as fluvial/alluvial deposits representing periods of stream and river reworking and re-establishment after an eruptive phase or an edifice failure event. These different lithofacies record the volcanic history of the Sobreiro Formation. A complex volcanic environment is thought to have existed with emission of large lava flows and explosive eruptions. The modern volcanological approach used here can serve as a model for the evolution of Precambrian volcano-sedimentary basins. Our approach sheds new light on the different processes operating on volcanic edifices and to constrain the depositional environment and thus geodynamic setting of Precambrian continental volcanic belts.

Sequential development of tidal ravinement surfaces in macro- to hypertidal estuaries with high volcaniclastic input: the Miocene Puerto Madryn Formation (Patagonia, Argentina)

The late Miocene beds of the Puerto Madryn Formation (Provincia del Chubut, Argentina) are formed by shallow marine and estuarine sediments. The latter include several tidal-channel infills well exposed on the cliffy coast of the Peninsula Valdes. The Bahia Punta Fosil and Cerro Olazabal paleochannels are end members of these tidal channels and show a fining-upward infilling starting with intraformational channel lag conglomerates above deeply erosional surfaces interpreted as fluvial ravinement surfaces (the erosion surface formed in the purely fluvial or the fluvially dominated part of the estuary, where erosion is driven by fluvial processes). These are overlain and eventually truncated (and suppressed) by the tidal ravinement surface (TRS), in turn covered with high-energy, bioclastic conglomerates mostly formed in the “tidally dominated/fluvially influenced” part of an estuary. Above, large straight or arcuate point bars with alternatively sandy/muddy seasonal beds and varying trace and body fossil contents were deposited from the freshwater fluvially dominated to saline-water tidally dominated part of the estuary. The upper channel infill is formed by cross-bedded sands with mud drapes and seaward-directed paleocurrents, together with barren, volcaniclastic sandy to muddy heterolithic seasonal rhythmites, both deposited in the fluvially dominated part of the estuary. Volcanic ash driven by the rivers after large explosive volcanic eruptions on land resulted in sedimentation rates as high as 0.9 m per year, preserving (through burial) the morphology of tidal channels and TRSs. The channel deposits were formed in a tide-dominated, macrotidal to hypertidal open estuary with well-developed TRSs resulting from strong tidal currents deeply scouring into the transgressive filling of the channels and eventually cutting the fluvial ravinement surface. The TRSs extended upstream to the inner part of the estuary during long periods of low sedimentation rates, extended channel migration and sediment bypass, interrupted by transient, high volcaniclastic input. The tidal channels of the Puerto Madryn Formation constitute a unique example of estuary sedimentation with pulsed sediment supply in a macrotidal to hypertidal estuary.

When Might the Campi Flegrei Caldera Erupt Again?

The clock may be ticking for Italy's Campi Flegrei caldera, a region with a pattern of numerous and sometimes large explosive eruptions. The next explosion could be less than 100 years away.

Alkalic marine tephra layers at ODP Site 1241 - Major explosive eruptions from an oceanic volcano in a pre-shield stage?

We report a series of fourteen marine tephra layers that are the products of large explosive eruptions of Subplinian to Plinian intensities and magnitudes (VEI >4) from Cocos Island, Costa Rica. Cocos Island is a volcanic island in the eastern Central Pacific Ocean ~500km offshore Costa Rica, and is situated on the northwestern flank of the aseismic Cocos Ridge. Geochemical fingerprinting of Pleistocene (~2.4–1.4Ma) marine tephra layers from Ocean Drilling Project (ODP) Leg 202 Site 1241 using major and trace element compositions of volcanic glass shards demonstrates unequivocally their origin from Cocos Island rather than the Galápagos Archipelago or the Central American Volcanic Arc (CAVA). Cocos Island and the adjacent seamounts of the Cocos Island Province have alkalic compositions and formed on young (≤3Ma) oceanic crust from an extinct spreading ridge bounded by a transform fault against the older and thicker crust of the aseismic Cocos Ridge. Cocos Island has six times the average volume of the adjacent seamounts although all appear to have formed during the 3–1.4Ma time period. Cocos Island lies closest to the transform fault and we explain its excessive growth by melts rising from garnet-bearing mantle being deflected from the thick Cocos Ridge lithosphere toward the thinner lithosphere on the other side of the transform, thus enlarging the melt catchment area for Cocos Island compared to the seamounts farther away from the transform. This special setting favored growth above sea level and subaerial explosive eruptions even though the absence of appropriate compositions suggests that the entirely alkalic Cocos Island (and seamounts) never evolved through the productive tholeiitic shield stage typical of other Pacific Ocean islands, possibly because melt production rates remained too small. Conditions of magma generation and ascent resembled Hawaiian pre-shield volcanoes but persisted for much longer (<1m.y.) and formed evolved, trachytic magmas. Therefore Cocos Island may be a unique example for a volcanic ocean island that did not pass through the typical growth stages.

A pulse of mid-Pleistocene rift volcanism in Ethiopia at the dawn of modern humans.

The Ethiopian Rift Valley hosts the longest record of human co-existence with volcanoes on Earth, however, current understanding of the magnitude and timing of large explosive eruptions in this region is poor. Detailed records of volcanism are essential for interpreting the palaeoenvironments occupied by our hominin ancestors; and also for evaluating the volcanic hazards posed to the 10 million people currently living within this active rift zone. Here we use new geochronological evidence to suggest that a 200 km-long segment of rift experienced a major pulse of explosive volcanic activity between 320 and 170 ka. During this period, at least four distinct volcanic centres underwent large-volume (>10 km3) caldera-forming eruptions, and eruptive fluxes were elevated five times above the average eruption rate for the past 700 ka. We propose that such pulses of episodic silicic volcanism would have drastically remodelled landscapes and ecosystems occupied by early hominin populations.

The 1257 Samalas eruption (Lombok, Indonesia): the single greatest stratospheric gas release of the Common Era.

Large explosive eruptions inject volcanic gases and fine ash to stratospheric altitudes, contributing to global cooling at the Earth’s surface and occasionally to ozone depletion. The modelling of the climate response to these strong injections of volatiles commonly relies on ice-core records of volcanic sulphate aerosols. Here we use an independent geochemical approach which demonstrates that the great 1257 eruption of Samalas (Lombok, Indonesia) released enough sulphur and halogen gases into the stratosphere to produce the reported global cooling during the second half of the 13th century, as well as potential substantial ozone destruction. Major, trace and volatile element compositions of eruptive products recording the magmatic differentiation processes leading to the 1257 eruption indicate that Mt Samalas released 158 ± 12 Tg of sulphur dioxide, 227 ± 18 Tg of chlorine and a maximum of 1.3 ± 0.3 Tg of bromine. These emissions stand as the greatest volcanogenic gas injection of the Common Era. Our findings not only provide robust constraints for the modelling of the combined impact of sulphur and halogens on stratosphere chemistry of the largest eruption of the last millennium, but also develop a methodology to better quantify the degassing budgets of explosive eruptions of all magnitudes.

The feeder system of the Toba supervolcano from the slab to the shallow reservoir.

The Toba Caldera has been the site of several large explosive eruptions in the recent geological past, including the world's largest Pleistocene eruption 74,000 years ago. The major cause of this particular behaviour may be the subduction of the fluid-rich Investigator Fracture Zone directly beneath the continental crust of Sumatra and possible tear of the slab. Here we show a new seismic tomography model, which clearly reveals a complex multilevel plumbing system beneath Toba. Large amounts of volatiles originate in the subducting slab at a depth of ∼150 km, migrate upward and cause active melting in the mantle wedge. The volatile-rich basic magmas accumulate at the base of the crust in a ∼50,000 km3 reservoir. The overheated volatiles continue ascending through the crust and cause melting of the upper crust rocks. This leads to the formation of a shallow crustal reservoir that is directly responsible for the supereruptions.

Increased rates of large-magnitude explosive eruptions in Japan in the late Neogene and Quaternary.

Tephra layers in marine sediment cores from scientific ocean drilling largely record high‐magnitude silicic explosive eruptions in the Japan arc for up to the last 20 million years. Analysis of the thickness variation with distance of 180 tephra layers from a global data set suggests that the majority of the visible tephra layers used in this study are the products of caldera‐forming eruptions with magnitude (M) > 6, considering their distances at the respective drilling sites to their likely volcanic sources. Frequency of visible tephra layers in cores indicates a marked increase in rates of large magnitude explosive eruptions at ∼8 Ma, 6–4 Ma, and further increase after ∼2 Ma. These changes are attributed to major changes in tectonic plate interactions. Lower rates of large magnitude explosive volcanism in the Miocene are related to a strike‐slip‐dominated boundary (and temporary cessation or deceleration of subduction) between the Philippine Sea Plate and southwest Japan, combined with the possibility that much of the arc in northern Japan was submerged beneath sea level partly due to previous tectonic extension of northern Honshu related to formation of the Sea of Japan. Changes in plate motions and subduction dynamics during the ∼8 Ma to present period led to (1) increased arc‐normal subduction in southwest Japan (and resumption of arc volcanism) and (2) shift from extension to compression of the upper plate in northeast Japan, leading to uplift, crustal thickening and favorable conditions for accumulation of the large volumes of silicic magma needed for explosive caldera‐forming eruptions.

Stratospheric Aerosols from Major Volcanic Eruptions: A Composition-Climate Model Study of the Aerosol Cloud Dispersal and e-folding Time

Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulfur gases above the tropopause, causing large increases in stratospheric aerosols. Five major volcanic eruptions after 1960 (i.e., Agung, St. Helens, El Chichón, Nevado del Ruiz and Pinatubo) have been considered in a numerical study conducted with a composition-climate coupled model including an aerosol microphysics code for aerosol formation and growth. Model results are compared between an ensemble of numerical simulations including volcanic aerosols and their radiative effects (VE) and a reference simulations ensemble (REF) with no radiative impact of the volcanic aerosols. Differences of VE-REF show enhanced diabatic heating rates; increased stratospheric temperatures and mean zonal westerly winds; increased planetary wave amplitude; and tropical upwelling. The impact on stratospheric upwelling is found to be larger when the volcanically perturbed stratospheric aerosol is confined to the tropics, as tends to be the case for eruptions which were followed by several months with easterly shear of the quasi-biennial oscillation (QBO), e.g., the Pinatubo case. Compared to an eruption followed by a period of westerly QBO, such easterly QBO eruptions are quite different, with meridional transport to mid- and high-latitudes occurring later, and at higher altitude, with a consequent decrease in cross-tropopause removal from the stratosphere, and therefore longer decay timescale. Comparing the model-calculated e-folding time of the volcanic aerosol mass during the first year after the eruptions, an increase is found from 8.1 and 10.3 months for El Chichón and Agung (QBO westerly shear), to 14.6 and 30.7 months for Pinatubo and Ruiz (QBO easterly shear). The corresponding e-folding time of the global-mean radiative flux changes goes from 9.1 and 8.0 months for El Chichón and Agung, to 28.7 and 24.5 months for Pinatubo and Ruiz.

Bubble accumulation and its role in the evolution of magma reservoirs in the upper crust

Volcanic eruptions transfer huge amounts of gas to the atmosphere. In particular, the sulfur released during large silicic explosive eruptions can induce global cooling. A fundamental goal in volcanology, therefore, is to assess the potential for eruption of the large volumes of crystal-poor, silicic magma that are stored at shallow depths in the crust, and to obtain theoretical bounds for the amount of volatiles that can be released during these eruptions. It is puzzling that highly evolved, crystal-poor silicic magmas are more likely to generate volcanic rocks than plutonic rocks. This observation suggests that such magmas are more prone to erupting than are their crystal-rich counterparts. Moreover, well studied examples of largely crystal-poor eruptions (for example, Katmai, Taupo and Minoan) often exhibit a release of sulfur that is 10 to 20 times higher than the amount of sulfur estimated to be stored in the melt. Here we argue that these two observations rest on how the magmatic volatile phase (MVP) behaves as it rises buoyantly in zoned magma reservoirs. By investigating the fluid dynamics that controls the transport of the MVP in crystal-rich and crystal-poor magmas, we show how the interplay between capillary stresses and the viscosity contrast between the MVP and the host melt results in a counterintuitive dynamics, whereby the MVP tends to migrate efficiently in crystal-rich parts of a magma reservoir and accumulate in crystal-poor regions. The accumulation of low-density bubbles of MVP in crystal-poor magmas has implications for the eruptive potential of such magmas, and is the likely source of the excess sulfur released during explosive eruptions.

Rheomorphic ignimbrites of the Rogerson Formation, central Snake River plain, USA: record of mid-Miocene rhyolitic explosive eruptions and associated crustal subsidence along the Yellowstone hotspot track

Rogerson Graben, USA, is critically placed at the intersection between the Yellowstone hotspot track and the southern projection of the west Snake River rift. Eleven rhyolitic members of the re-defined, ≥420-m-thick, Rogerson Formation record voluminous high-temperature explosive eruptions, emplacing extensive ashfall and rheomorphic ignimbrite sheets. Yet, each member has subtly distinct field, chemical and palaeomagnetic characteristics. New regional correlations reveal that the Brown’s View ignimbrite covers ≥3300 km2, and the Wooden Shoe ignimbrite covers ≥4400 km2 and extends into Nevada. Between 11.9 and ∼8 Ma, the average frequency of large explosive eruptions in this region was 1 per 354 ky, about twice that at Yellowstone. The chemistry and mineralogy of the early rhyolites show increasing maturity with time possibly by progressive fractional crystallisation. This was followed by a trend towards less-evolved rhyolites that may record melting and hybridisation of a mid-crustal source region. Contemporaneous magmatism-induced crustal subsidence of the central Snake River Basin is recorded by successive ignimbrites offlapping and thinning up the N-facing limb of a regional basin-margin monocline, which developed between 10.59 and 8 Ma. The syn-volcanic basin topography contrasted significantly with the present-day elevated Yellowstone hotspot plateau. Concurrent basin-and-range extension produced the N-trending Rogerson Graben: early uplift of the Shoshone Hills (≥10.34 Ma) was followed by initiation of the Shoshone Fault and an E-sloping half-graben (∼10.3–10.1 Ma). The graben asymmetry then reversed with initiation of the Brown’s Bench Fault (≥8 Ma), which remained intermittently active until the Pliocene.

Impact of explosive volcanic eruptions around Vesuvius: a story of resilience in Roman time

Large explosive eruptions have reshaped the landscape around Vesuvius many times in prehistoric and historical times. Previous stratigraphic surveys suggested that people living in this area have probably abandoned their settlements (in the Bronze Age) or towns and villas (in the Roman period) for centuries after each major plinian eruption. New archaeological excavations on the northern slope of Vesuvius suggest a much more intriguing scenario. At Pollena Trocchia, an ongoing excavation has shown the superimposition of three different Roman structures, sandwiched between the deposits of the AD 79, AD 472, and AD 512 Vesuvius eruptions. Each of these eruptions more or less completely destroyed and buried the buildings under meters of volcanic products. Surprisingly, after a few years or decades, a new settlement was established exactly on the top of the buried one, indicating the immediate recovery of part of the devastated area. Our research documents the destruction of Roman buildings by volcanic eruptions over a period of five centuries (first to sixth century AD) and provides new insight into human behavior after major explosive eruptions.

Synchronisation of sedimentary records using tephra: A postglacial tephrochronological model for the Chilean Lake District

Well-characterised tephra horizons deposited in various sedimentary environments provide a means of synchronising sedimentary archives. The use of tephra as a chronological tool is however still widely underutilised in southern Chile and Argentina. In this study we develop a postglacial tephrochronological model for the Chilean Lake District (ca. 38 to 42°S) by integrating terrestrial and lacustrine records. Tephra deposits preserved in lake sediments record discrete events even if they do not correspond to primary fallout. By combining terrestrial with lacustrine records we obtain the most complete tephrostratigraphic record for the area to date. We present glass geochemical and chronological data for key marker horizons that may be used to synchronise sedimentary archives used for palaeoenvironmental, palaeoclimatological and palaeoseismological purposes. Most volcanoes in the studied segment of the Southern Volcanic Zone, between Llaima and Calbuco, have produced at least one regional marker deposit resulting from a large explosive eruption (magnitude ≥ 4), some of which now have a significantly improved age estimate (e.g., the 10.5 ka Llaima Pumice eruption from Llaima volcano). Others, including several units from Puyehue-Cordón Caulle, are newly described here. We also find tephra related to the Cha1 eruption from Chaitén volcano in lake sediments up to 400 km north from source. Several clear marker horizons are now identified that should help refine age model reconstructions for various sedimentary archives. Our chronological model suggests three distinct phases of eruptive activity impacting the area, with an early-to-mid-Holocene period of relative quiescence. Extending our tephrochronological framework further south into Patagonia will allow a more detailed evaluation of the controls on the occurrence and magnitude of explosive eruptions throughout the postglacial.

Dynamic implications of ridges on a debris avalanche deposit at Tutupaca volcano (southern Peru)

Catastrophic volcanic landslides can involve different parts of a volcano that can be incorporated into any resulting debris avalanche. The different material properties may influence the mechanical behaviour and, hence, the emplacement mechanisms of the different avalanche units. We present data from a coupled hydrothermal- and magmatic-related volcanic landslide at Tutupaca volcano (Peru). Around ad 1802, the hydrothermal system under Tutupaca’s growing dacite dome failed, creating a debris avalanche that triggered a large explosive eruption. A typical debris avalanche hummocky unit is found, formed out of rock from the dome foot and the underlying hydrothermally altered lavas. It is covered by a more widespread and remarkable deposit that contains remnants of the hot dome core and the inner hydrothermal material. This deposit has ridges 20–500-m long, 10–30-m wide and 1–5-m high, regularly spaced and that fan slightly outward. Cross sections exposed within the ridges reveal coarser cores and finer troughs, suggesting grain size segregation during emplacement. Ridge morphology and granulometry are consistent with fingering known to occur in granular flows. The ridges are also associated with large blocks that have evidence of differential movement compared with the rest of the flowing mass. The presence of both ridged and hummocky deposits in the same event shows that, as different lithologies combine and collapse sequentially, materials with different mechanical properties can coexist in one landslide, leading to contrasting emplacement dynamics. The different structures thus highlight the complexity of such hazardous volcanic events and show the difficulty we face with modelling them.