Dense Rock Equivalent Research Articles

Deposits of the most recent eruption in the Southern Mono Craters, California: Description, interpretation and implications for regional marker tephras

The penultimate eruption in the Mono Craters, Mono County, CA, USA, occurred in the southern section of the volcanic chain, and is herein named the South Mono eruption. The South Mono eruption occurred in 594–648calA.D., and its products consist of widespread Plinian and phreatomagmatic fall, surge and pyroclastic flow deposits. The explosive deposits can be broken into Basal, Orange–Brown (surge dominated) and Upper subunits. The eruptive phase represented by the Upper beds was the most intense and voluminous, dispersing tephra over a wide region of eastern CA and western NV. South Coulee was the only effusive product of the eruption, and comprises the vast majority of the c. 0.4cukm dense-rock equivalent (DRE) volume. The tephra overlies the deposits of Wilson Butte to the south, and is correlated herein with Wood's Tephra 2, and Walker Lake and Turupah Flats regional marker tephra layers. Other dates for these regional tephras may be the result of dating ash redeposited in debris flow events following fire.

A model for calculating eruptive volumes for monogenetic volcanoes — Implication for the Quaternary Auckland Volcanic Field, New Zealand

Monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency. Investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano. Eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios, especially transitions between explosive and effusive behaviour and the length of eruptions. Erosion, superposition and lack of exposure limit the accuracy of volume determination, even for very young volcanoes. In this study, a systematic volume estimation model is developed and applied to the Auckland Volcanic Field in New Zealand. In this model, a basaltic monogenetic volcano is categorised in six parts. Subsurface portions of volcanoes, such as diatremes beneath phreatomagmatic volcanoes, or crater infills, are approximated by geometrical considerations, based on exposed analogue volcanoes. Positive volcanic landforms, such as scoria/spatter cones, tephras rings and lava flow, were defined by using a Light Detection and Ranging (LiDAR) survey-based Digital Surface Model (DSM). Finally, the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes. Considering only those parts with high reliability, the overall magma output (converted to Dense Rock Equivalent) for the post-250 ka active Auckland Volcanic Field in New Zealand is a minimum of 1.704km3. This is made up of 1.329km3 in lava flows, 0.067km3 in phreatomagmatic crater lava infills, 0.090km3 within tephra/tuff rings, 0.112km3 inside crater lava infills, and 0.104km3 within scoria cones. Using the minimum eruptive volumes, the spatial and temporal magma fluxes are estimated at 0.005km3/km2 and 0.007km3/ka. The temporal–volumetric evolution of Auckland is characterised by an increasing magma flux in the last 40ky, which is inferred to be triggered by plate tectonics processes (e.g. increased asthenospheric shearing and backarc spreading of underneath the Auckland region).

Overview of the 2006 eruption of Mt. Merapi

In the last part of the 20th century and the beginning of the 21st century, Mt. Merapi in Central-Java Indonesia erupted about every 2–5years. Most of the eruptions were low in explosivity, with VEI-3 or less. Eruptions usually involve the formation of a lava dome, either in the beginning or in the end of the eruptive episode.For the 2006 eruption, the precursory signal was first observed in the middle of the year 2005 with a decrease in EDM slope distances to points on the rim, an increase of seismicity and a possible increase of SO2 emissions. Those early events marked the beginning of a more continuous period of inflation, which led to the eruption.In total, the pre-eruption displacement of the southern rim reached at least 2.4m toward the measuring station in Kaliurang (KAL). From late April until June 2006, a lava dome grew on the summit with a volume that gradually increased until it reached about 4.1millionm3 in 38days. The total of erupted magma was about 5.3millionm3 dense-rock-equivalent (DRE). The dome subsequently collapsed in three steps from June 4 to June 14, leaving an open scar on its southeast side. In this paper we detail the changes of dome morphology that were monitored by taking successive photographs from similar positions. The eruption in 2006 marked a significant change in summit morphology, from west-southwestward opening during the 20th century to the currently southeast orientation. Also, an Mw 6.4 earthquake occurred on 26 May, midway through the eruption, which adds interesting questions about the relationship of the eruption and the earthquake.EDM data from 2006 and previous eruptions show that the summit remains inflated after each eruption, i.e., no significant deflation occurs following eruptions. The lack of post-eruption deflation suggests that magma remains in the shallow parts of the edifice after the eruption. As a result, the complex of summit lava domes and their intrusive roots grow with time and Merapi's rim and summit become progressively more unstable and prone to collapse.

A revision of the structure and stratigraphy of pre-Green Tuff ignimbrites at Pantelleria (Strait of Sicily)

At Pantelleria, peralkaline silicic magmas were erupted across a range of eruptive typologies and magnitudes: pyroclastic flows, Plinian to strombolian pumice fallout and lava flows. In this paper we focus on the intermediate cycle of eruptive activity which is bracketed by ignimbrite units slightly older than the two caldera collapses which marked the volcanological activity of the island. This age interval (180–85ka) was punctuated by six ignimbrite-forming eruptions (silicic and variably peralkaline) for a cumulative erupted magma volume of approximately 6km3 dense rock equivalent. Based on new 40Ar/39Ar (Na,K)-feldspar ages and petrographic data, we propose an updated volcanostratigraphic scheme for these welded and rheomorphic ignimbrites that can be summarised as follows: (i) the age of the old (‘La Vecchia’) caldera collapse is now tightly constrained between 140 and 146ka and the caldera-forming eruption can be traced to a lithic-rich welded tuff breccia that outcrops in two opposite sectors of the island (south-west and north-east); (ii) four ignimbrite units previously considered unrelated are now merged in two distinct eruptive paroxysmal events at 107 and 85ka. In particular, the 85ka eruptive event is comparable in magnitude to the younger (caldera forming) Green Tuff Plinian eruption; (iii) the recurrence patterns of the 107 and 85ka eruptions, compared to the Green Tuff, allow us to qualitatively assess that the climax in production of low-temperature silicic and peralkaline melt was focused in the age interval 85–45ka.

An overview of the 2009 eruption of Redoubt Volcano, Alaska

In March 2009, Redoubt Volcano, Alaska erupted for the first time since 1990. Explosions ejected plumes that disrupted international and domestic airspace, sent lahars more than 35km down the Drift River to the coast, and resulted in tephra fall on communities over 100km away. Geodetic data suggest that magma began to ascend slowly from deep in the crust and reached mid- to shallow-crustal levels as early as May, 2008. Heat flux at the volcano during the precursory phase melted ~4% of the Drift glacier atop Redoubt's summit. Petrologic data indicate the deeply sourced magma, low-silica andesite, temporarily arrested at 9–11km and/or at 4–6km depth, where it encountered and mixed with segregated stored high-silica andesite bodies. The two magma compositions mixed to form intermediate-silica andesite, and all three magma types erupted during the earliest 2009 events. Only intermediate- and high-silica andesites were produced throughout the explosive and effusive phases of the eruption. The explosive phase began with a phreatic explosion followed by a seismic swarm, which signaled the start of lava effusion on March 22, shortly prior to the first magmatic explosion early on March 23, 2009 (UTC). More than 19 explosions (or “Events”) were produced over 13days from a single vent immediately south of the 1989–90 lava domes. During that period multiple small pyroclastic density currents flowed primarily to the north and into glacial ravines, three major lahars flooded the Drift River Terminal over 35km down-river on the coast, tephra fall deposited on all aspects of the edifice and on several communities north and east of the volcano, and at least two, and possibly three lava domes were emplaced. Lightning accompanied almost all the explosions.A shift in the eruptive character took place following Event 9 on March 27 in terms of infrasound signal onsets, the character of repeating earthquakes, and the nature of tephra ejecta. More than nine additional explosions occurred in the next two days, followed by a hiatus in explosive activity between March 29 and April 4. During this hiatus effusion of a lava dome occurred, whose growth slowed on or around April 2. The final explosion pulverized the very poorly vesicular dome on April 4, and was immediately followed by the extrusion of the final dome that ceased growing by July 1, 2009, and reached 72Mm3 in bulk volume. The dome remains as of this writing. Effusion of the final dome in the first month produced blocky intermediate- to high-silica andesite lava, which then expanded by means of lava injection beneath a fracturing and annealing, cooling surface crust. In the first week of May, a seismic swarm accompanied extrusion of an intermediate- to high-silica andesite from the apex of the dome that was highly vesicular and characterized by lower P2O5 content. The dome remained stable throughout its growth period likely due to combined factors that include an emptied conduit system, steady degassing through coalesced vesicles in the effusing lava, and a large crater-pit created by the previous explosions. We estimate the total volume of erupted material from the 2009 eruption to be between ~80M and 120Mm3 dense-rock equivalent (DRE).The aim of this report is to synthesize the results from various datasets gathered both during the eruption and retrospectively, and which are represented by the papers in this publication. We therefore provide an overall view of the 2009 eruption and an introduction to this special issue publication.

Identifying potentially active volcanoes in the Andes: Radiometric evidence for late Pleistocene-early Holocene eruptions at Volcán Imbabura, Ecuador

Recent eruptions from volcanoes with no previously known historical activity in Chile and Indonesia have raised the importance of the early identification of potentially active centers for the purpose of hazard assessment. Here we bring radiometric evidence ( 14 C, 39Ar– 40Ar) of previously unrecognized but significant magmatic activity at partly eroded Imbabura volcano (Ecuador) in late Pleistocene to early Holocene times, on whose perimeter live more than 300,000 persons. Following an effusive stage from 50 to 30 ka with the emplacement of andesitic lava flows on different flanks of its edifice, the activity became explosive with the generation of andesitic block-and-ash flows on its eastern side, beginning at ~ 35 ka cal BP. Subsequently a flank collapse associated with a volcanic blast occurred on the volcano's SW flank at ~ 30 ka cal BP. The resulting debris avalanche and blast breccias cover an area now heavily populated around San Pablo Lake and its source was later concealed by successive dome building episodes at Huarmi which produced ~ 2.8 km 3 of silicic andesite. Renewed dome activity at the edifice's Taita summit occurred at ~ 17 ka cal BP and continued intermittently into early Holocene times, as indicated by pyroclastic flow deposits overlying a palaeosoil dated at ~ 9 ka cal BP. In summary, this study reveals an eruptive behavior characterized by a low recurrence rate but with quite large eruptions, a pattern which is also observed at other silicic volcanoes of Ecuador's Western Cordillera. It is now imperative to reconsider the origin and source of the many tephra layers catalogued in Holocene lacustrine sediments in the Imbabura area. Tephra and lava volume estimates for Imbabura volcano converted to Dense Rock Equivalent values yield a minimum magmatic output rate of 0.13 km 3/ka in the past 35,000 years, which argues for sustained magma production for this volcano in recent geological times. The Imbabura example thus raises the question of how to improve population preparedness for volcanoes with infrequent eruptions, and how to guide authorities' decisions concerning the development of urban areas and infrastructures near presently inactive but potentially highly dangerous volcanoes.

Tephrochronology and eruptive history of Kirishima volcano in southern Japan

Kirishima volcano has been active for the last 600 ka. From 600 to 330 ka, the activity mainly consisted of caldera-forming eruptions with two large-scale pyroclastic flows of >50 km 3 in dense-rock equivalent (DRE). From 600 ka to present, 83% of the discharged magma has erupted as pyroclastic materials. After 330 ka, volcanic activity changed and stratovolcanoes started to be constructed with fallout tephras from plinian, vulcanian, Strombolian, and phreatomagmatic eruptions. Eruptive centers have roughly migrated from west to east, forming more than 25 small-scale stratovolcanoes and pyroclastic cones. The magma discharge rate has increased from 0.05 to 0.6 km 3/ky. Kirishima is becoming increasingly more active, and a more accurate and detailed warning system for its eruptions is required.

A re-appraisal of the stratigraphy and volcanology of the Cerro Galán volcanic system, NW Argentina

From detailed fieldwork and biotite 40Ar/39Ar dating correlated with paleomagnetic analyses of lithic clasts, we present a revision of the stratigraphy, areal extent and volume estimates of ignimbrites in the Cerro Galan volcanic complex. We find evidence for nine distinct outflow ignimbrites, including two newly identified ignimbrites in the Toconquis Group (the Pitas and Vega Ignimbrites). Toconquis Group Ignimbrites (~5.60–4.51 Ma biotite ages) have been discovered to the southwest and north of the caldera, increasing their spatial extents from previous estimates. Previously thought to be contemporaneous, we distinguish the Real Grande Ignimbrite (4.68 ± 0.07 Ma biotite age) from the Cueva Negra Ignimbrite (3.77 ± 0.08 Ma biotite age). The form and collapse processes of the Cerro Galan caldera are also reassessed. Based on re-interpretation of the margins of the caldera, we find evidence for a fault-bounded trapdoor collapse hinged along a regional N-S fault on the eastern side of the caldera and accommodated on a N-S fault on the western caldera margin. The collapsed area defines a roughly isosceles trapezoid shape elongated E-W and with maximum dimensions 27 × 16 km. The Cerro Galan Ignimbrite (CGI; 2.08 ± 0.02 Ma sanidine age) outflow sheet extends to 40 km in all directions from the inferred structural margins, with a maximum runout distance of ~80 km to the north of the caldera. New deposit volume estimates confirm an increase in eruptive volume through time, wherein the Toconquis Group Ignimbrites increase in volume from the ~10 km3 Lower Merihuaca Ignimbrite to a maximum of ~390 km3 (Dense Rock Equivalent; DRE) with the Real Grande Ignimbrite. The climactic CGI has a revised volume of ~630 km3 (DRE), approximately two thirds of the commonly quoted value.

Physical volcanology of the post-twelfth-century activity at Cotopaxi volcano, Ecuador: Behavior of an andesitic central volcano

Cotopaxi volcano, situated in the Eastern Cordillera of the Ecuadorian Andes, is one of the most active volcanoes on Earth. The volcano is well known for the magnificence of its almost perfectly symmetrical cone topped by ice and snow and for the destructive power of its large-scale, syneruptive lahars. This paper presents a stratigraphic study of the post–twelfth-century eruptive products that reveals the existence of 21 continuous tephra beds. Most of them were characterized from both a physical (dispersal areas, deposit volumes, peak Mass Discharge Rate [MDR] of the eruptions) and compositional point of view. New 14 C dates, linked with a new examination of historical chronicles, allow us to create a new chronostratigraphic scheme for this period of activity, which is bracketed by the emplacement of a regional tephra marker (A.D. 1140 ash bed from Quilotoa volcano) and the present day. The first period (A.D. 1150–1742) included only two moderate-intensity explosive eruptions, the oldest being possibly related to a dome disruption. In contrast, the period A.D. 1742–1880 started with two high-intensity, Plinian eruptions (maximum column heights of 25 and 29 km), followed by several short-lived but sustained, convective episodes. Deposits of pyroclastic surges and scoria flows were emplaced during some of these short-lived events and may have been related to column collapse and boiling over activity, respectively. Post-1880 activity, reported in 1904, 1906, and 1912, likely consisted of minor explosions that affected only the crater area. Our study of recent activity at Cotopaxi shows that high dispersive power (peak mass discharge rates from 1.1 to 9.3 × 10 7 kg/s) is associated with the eruption of only moderate amounts of magma (1.1 × 10 10 –6.0 × 10 11 kg, or ∼0.005–0.2 km 3 , Dense Rock Equivalent [DRE]). Additionally, al-though the past 2000 yr of activity at Cotopaxi have been interpreted to reflect a fairly uniform magma supply rate, detailed analysis of the past centuries, and a reanalysis of data from the past 2000 yr show that Cotopaxi9s eruptive activity is characterized by clusters of eruptive events that are separated by periods of long quiescence punctuated by isolated eruptions, often of slightly more evolved magma. No systematic variations in composition emerge in the time sequence. Although new magmatic phases commonly start with the eruption of mafic magma, this is not always observed. Additionally, eruption clusters may show either compositional trends of increasing SiO 2 content or abrupt compositional changes within a cluster. We interpret the temporal and compositional variations in eruptive activity to reflect the complex interplay of deep versus shallow magmatic processes. An important result from the perspective of volcanic hazards is our conclusion that, over the studied period, no clear relation exists among repose time, eruption magnitude, and magma composition. This conclusion contrasts with the periodic eruptive behavior that has been postulated at many central volcanoes worldwide, thus inviting a reexamination of other intermediate-composition volcanic systems and a reassessment of the assumption of periodic activity.

The formation and evolution of Hule and Río Cuarto maars, Costa Rica

The Hule and Río Cuarto maars are respectively located 11 and 18 km northward of the active crater of Poás volcano, on the Caribbean side of the Central Volcanic Range of Costa Rica. They lie on the northern part of Poás volcano massif, along a N–S trending, ~ 27 km-long volcanic fracture crossing the Poás volcano. The volcanic products from Hule maar (2.3 km × 1.8 km, area ~ 3.5 km 2) are mainly pyroclastic surges (poorly vesiculated andesites with very small plagioclases), silica-rich andesitic pumice flows, air-fall deposits, ballistic blocks, and reworked deposits that overlie the regional Pleistocene volcanic basement. They were produced during three main explosive phases. Two overlapping pyroclastic cones have developed within the Hule maar, and at least three lava fields are related to them (high-Al basalt to basaltic andesite). Another maar, Pata de Gallo (400 m across), is located less than 1 km off the SE rim of Hule. Río Cuarto is a nearly circular maar (700–850 m across) with a surface area of 0.33 km 2. Río Cuarto products include surges, ballistics and air-fall tephra, produced during three main explosive phases. These deposits show a narrow fan oriented westward, according to westerly wind direction. They indicate a westerly-directed surge (first 2 km), followed by air-fall deposits (up to 5 km away). Radiocarbon dating has shown that Hule was formed ~ 6.2 ka ago and Pata de Gallo probably formed ~ 2.8 ka ago, while the intra-maar products could have ages of ~ 1.7 ka or ~ 0.7 ka, indicating that Hule is a polygenetic maar. There are no radiocarbon ages yet for dating the formation of Río Cuarto maar, but archaeological data suggest that it erupted between 3–4 ka ago. The volume of pyroclastic deposits associated to Hule maar is estimated to be 0.51–0.53 km 3, from which ~ 20% is juvenile material, therefore 0.07–0.08 km 3 of new dense rock equivalent (DRE) magma, after subtracting 20–30% of porosity. The tephra from Río Cuarto is estimated to be 4.4 × 10 7 m 3, of which 0.008 m 3 correspond to DRE magma. The Hule and Río Cuarto maars are occupied by lakes and, in the last decades, several lake-overturn events have taken place, with a repeat cycle of six to seven years. The main outcome of these events has been the mass death of fish accompanied by changes in the lake color. In these systems, the hazard related to the possible occurrence of Nyos-type gas eruptions can be considered negligible or very local, but significant for tourists who camp by the lakes.

Growth of the lava dome and extrusion rates at Soufrière Hills Volcano, Montserrat, West Indies: 2005–2008

The third episode of lava dome growth at Soufrière Hills Volcano began 1 August 2005 and ended 20 April 2007. Volumes of the dome and talus produced were measured using a photo‐based method with a calibrated camera for increased accuracy. The total dense rock equivalent (DRE) volume of extruded andesite magma (306 ± 51 Mm3) was similar within error to that produced in the earlier episodes but the average extrusion rate was 5.6 ± 0.9 m3s−1 (DRE), higher than the previous episodes. Extrusion rates varied in a pulsatory manner from <0.5 m3s−1 to ∼20 m3s−1. On 18 May 2006, the lava dome had reached a volume of 85 Mm3 DRE and it was removed in its entirety during a massive dome collapse on 20 May 2006. Extrusion began again almost immediately and built a dome of 170 Mm3 DRE with a summit height 1047 m above sea level by 4 April 2007. There were few moderate‐sized dome collapses (1–10 Mm3) during this extrusive episode in contrast to the first episode of dome growth in 1995–8 when they were numerous. The first and third episodes of dome growth showed a similar pattern of low (<0.5 m3s−1) but increasing magma flux during the early stages, with steady high flux after extrusion of ∼25 Mm3.

Dynamics of the ca. 4965 yr 14C BP “Ochre Pumice” Plinian eruption of Popocatépetl volcano, México

The Ochre Pumice (OP) Plinian fallout was produced by Popocatépetl volcano in central Mexico, during a major Plinian eruption that occurred 4965 ± 65 14C yr BP (3700 BC). The OP is part of the Ochre Pumice Sequence (OPS), that consists of surge, fall, and pyroclastic flow deposits. The OP Plinian fallout shows a bimodal grain size distribution with poor to moderate sorting that improves progressively towards the upper beds. The juvenile component is mainly pumice (>83 wt.%), whereas the accidental components consist of igneous, sedimentary, and metamorphic clasts from the walls of the magma chamber and/or conduit. The vesicularity of the pumice decreases from bottom to top of the stratigraphic sequence while the crystal and glass contents increase. This suggests increasing magma degassing prior and during the eruption. The chemical composition of the pumice varies insignificantly; at the base it is less evolved (SiO 2 = 61 wt.%) and it becomes slightly more silicic towards the top (SiO 2 = 63 wt.%). Pumice clasts have a crystallinity index that ranges between 10 and 25 vol.% and display a seriate texture with phenocrysts of euhedral plagioclase (Pl) + clinopyroxene (Cpx) + orthopyroxene (Opx) + olivine (Ol) ± oxide (Ox) ± apatite (Ap). The size of the maximum lithic (2.5 cm) at a horizontal distance of 19 km indicates an eruption column height of 37–41 km. The isopach map allows the recognition of a dispersal axis that points towards the NNE, where an area of ~ 300 km 2 was covered by >6 cm of pumice and ash. A minimum volume for the OP Plinian fallout deposit was calculated using a minimum thickness of 6 cm found at distal outcrops, and was estimated at ~ 4.9 km 3 which corresponds to 2 km 3 of dense rock equivalent (DRE). Stratigraphic relations indicate that the entire OPS was emplaced during a short time interval of a few days to weeks and consisted of three main different phases: phreato-magmatic, Plinian, and vulcanian. It started violently with the emplacement of surges and then culminated with the eruption of the OP fallout deposit. As the eruption advanced, discharge rates became more intermittent and the height of the column fluctuated and finally collapsed, generating pumice-and-ash flows that were emplaced around the volcano. This short but intense activity was followed during subsequent years by the growth of domes in the crater and the emplacement of block-and-ash-flows generated by vulcanian explosions.

Eyewitness, stratigraphy, chemistry, and eruptive dynamics of the 1913 Plinian eruption of Volcán de Colima, México

Based on the stratigraphic record of the deposits, analysis of previous works, historic archives, and eyewitness accounts the 1913 eruption of Volcán de Colima was reconstructed. The eruption started in 17 January and peaked in 20 January, 1913. It occurred in three main phases: 1) An opening phase with the generation of Merapi-type pyroclastic flows (units F 1, F 2, F 3) and a pyroclastic surge (S 1), 2) A vent-clearing phase with strong explosions that produced Vulcanian-Soufriere-type pyroclastic flows (F 4) and a pyroclastic surge (S 2) which destroyed the summit dome decompressing the magma system, and 3) a Plinian phase with the establishment of a ∼ 23 km high column dispersing a fallout (C 1) to the NE followed by the collapse of the column that generated a pyroclastic surge (S 3) and 15-km long pumice-rich pyroclastic flows (F 5). After the eruption, remobilization of the pyroclastic material generated lahars in main gullies around the crater. Fallout C 1 blanketed an area of ∼ 191,318 km 2 covering the cities of Guzman, Guadalajara, and Saltillo (Coahuila) located at 725 km from the source. It had a volume of 1.4 km 3 (0.57 km 3 DRE = Dense Rock Equivalent). The total volume of pyroclastic flow and surge deposits was 0.26 km 3 (0.07 km 3 DRE) giving a total volume of the eruption of 1.66 km 3 (0.64 km 3 DRE). The Plinian column lasted 4.6 h with a total mass of 1.5 × 10 12 kg and a mass eruption rate of 9.02 × 10 7 kg/s. The column height and the ejected magma volume indicate that the 1913 eruption had a VEI = 5 being the largest event in the historical record of Colima Volcano. Juvenile scoria and pumice consisted of Pl > Opx > Cpx > Hbl + accessory titanomagnetite + Ap and Ol with reaction rims. Chemistry of juvenile scoria and pumice samples is andesitic very homogeneous (58.3 ± 0.5 wt.% SiO 2) and similar to the 1818 juvenile products (58.9 ± 0.2 wt.% SiO 2). The presence of banded scoria, olivine phenocrysts with reaction rims, and trace element variations strongly suggest that the 1913 eruption was caused by a magma mixing event.

The 1944 eruption of Vesuvius, Italy: combining contemporary accounts and field studies for a new volcanological reconstruction

AbstractWe integrate the different contemporary sources together with new field data on the pyroclastic deposits to make a new volcanological reconstruction of the explosive phases of the 1944 Vesuvius eruption. We adopt the four successive phases of the eruption first defined by Imbò (1945), who made the most detailed contemporary description of the eruption: Phase 1 – effusive, Phase 2 – lava fountains, Phase 3 – mixed explosions and Phase 4 – seismic-explosive. Phase 1 consisted of four days of effusive activity. Phase 2 generated eight successive lava fountains which formed agglutinated spatter in a restricted area around the crater. At distances of > 1 km from the crater, reverse graded, well-sorted, scoria lapilli with up to 94 wt % juvenile material and calculations indicate a volume of 8.2 × 106 m3 DRE (Dense Rock Equivalent) for Phase 2. A short pause in scoria fallout was observed that coincides with the transition between Phases 2 and 3 of the eruption. On the crater rim there is clear evidence for the different phases, owing to the stratification of the deposits; however, away from the crater, stratigraphic breaks suggesting any discontinuity in the eruptive activity are absent. The beginning of Phase 3 is marked by the appearance of abundant dense scoria fragments, coincident with the coarsest part of the lapilli. High-density scoria forms 10 wt % of juvenile material in Phase 2, increasing to 45% in the upper part of Phase 3. Isopach maps derived from field measurements indicate a mean volume of 40.2 × 106 m3 DRE for Phase 3. Distal ash, mainly formed during Phase 3, was dispersed to the SE as far as Albania, and calculations yield a volume of 102 × 106 m3 DRE. Intermittent activity associated with Phase 4 generated ash-rich plumes dispersed towards the SW and contemporary thickness descriptions yield a bulk volume of 4.2 × 106 m3 (2.5 × 106 m3 DRE). Small pyroclastic density currents (PDCs) were observed during Phases 3 and 4. The deposits (200 m from the crater rim) of these currents have been identified on the flanks of the cone. Thin, massive and poorly sorted ash layers, that occur up to 2.5 km from the crater rim, are interpreted to represent the distal facies of these PDCs. Mass discharge rate (MDR) estimates for the paroxysmal phase (end of Phase 2 and start of Phase 3) of this event are around 3.5 × 106 kg/s, however, this increases to > 107 kg/s if the mass of distal ash is taken into account. Column height estimates from fallout isopleths associated with the eruption's paroxysmal phase are > 10 km. Based on the contemporaneous chronicles, we were able to define the type and extent of damage associated with the different styles (or temporal phases) of the eruption. Our calculations demonstrate that the present-day population at risk has doubled compared to 1944. The contemporaneous (and also subsequent) scientific literature underestimated the magnitude and intensity of this eruption and very little attention has been dedicated to the damage that occurred. We suggest that this is at least partly related to the extensive destruction of Neapolitan area and the deaths of tens of thousands of civilians related to the Second World War.

A 90,000-year tephrostratigraphic framework of Aso Volcano, Japan

A detailed 90,000-year tephrostratigraphic framework of Aso Volcano, southwestern Japan, has been constructed to understand the post-caldera eruptive history of the volcano. Post-caldera central cones were initiated soon after the last caldera-forming pyroclastic-flow eruption (90 ka), and have produced voluminous tephra and lava flows. The tephrostratigraphic sequence preserved above the caldera-forming stage deposits reaches a total thickness of 100 m near the eastern caldera rim. The sequence is composed mainly of mafic scoria-fall and ash-fall deposits but 36 silicic pumice-fall deposits are very useful key beds for correlation of the stratigraphic sequence. Explosive, silicic pumice-fall deposits that fell far beyond the caldera have occurred at intervals of about 2500 years in the post-caldera activity. Three pumice-fall deposits could be correlated with lava flows or an edifice in the western part of the central cones, although the other silicic tephra beds were erupted at unknown vents, which are probably buried by the younger products from the present central cones. Most of silicic eruptions produced deposits smaller than 0.1 km 3, but bulk volumes of two silicic eruptions producing the Nojiri pumice (84 ka) and Kusasenrigahama pumice (Kpfa; 30 ka) were on the order of 1 km 3 (VEI 5). The largest pyroclastic eruption occurred at the Kusasenrigahama crater about 30 ka. This catastrophic eruption began with a dacitic lava flow and thereafter produced Kpfa (2.2 km 3). Total tephra volume in the past 90,000 years is estimated at about 18.1 km 3 (dense rock equivalent: DRE), whereas total volume for edifices of the post-caldera central cones is calculated at about 112 km 3, which is six times greater than the former. Therefore, the average magma discharge rate during the post-caldera stage of Aso Volcano is estimated at about 1.5 km 3/ky, which is similar to the rates of other Quaternary volcanoes in Japan.

Quantifying recent pyroclastic and lava flows at Arenal Volcano, Costa Rica, using medium‐footprint lidar

Arenal volcano is a small, active stratovolcano in Costa Rica. In 1998 and 2005, NASA's Laser Vegetation Imaging Sensor (LVIS) was used to collect wide‐swath 3‐dimensional topographic images of the volcano. The LVIS is a full‐waveform, scanning, medium‐sized footprint, airborne laser altimeter system. By digitally recording the shape of the returning laser pulse (waveform), the LVIS provides a precise and accurate view of both the sub‐canopy and canopy‐top topographies as well as the vertical and horizontal structure of vegetation at 15–25 m horizontal resolution. By comparing georeferenced waveform data collected in 1998 and 2005, we mapped lava and pyroclastic flows deposited during this period. The active crater grew by 3.82 m yr−1. A flow volume estimate of 2.19 × 107 m3 (Dense Rock Equivalent of 1.89 × 107 m3 or 0.085 m3s−1) was obtained for the period 1998 to 2005. Precise elevation and elevation change data such as those provided by the LVIS are essential to calculate eruption volume and to study magma‐supply dynamics, as well as assess the danger posed by the volcano to the local population from hazards such as pyroclastic flows.

Magma–tectonic interactions in Nicaragua: the 1999 seismic swarm and eruption of Cerro Negro volcano

A low-energy (Volcanic Explosivity Index [VEI] 1), small-volume (0.001 km 3 Dense Rock Equivalent [DRE]) eruption of highly crystalline basalt occurred at Cerro Negro volcano, Nicaragua, August 5–7, 1999. This eruption followed three earthquakes (each ∼ M w 5.2) with strike-slip and oblique-slip focal mechanisms, the first of which occurred approximately 11 h before eruptive activity and within 1 km of Cerro Negro. Surface ruptures formed during these events extend up to 4 km from Cerro Negro, but concentrate ∼1 km south of Cerro Negro. Surface ruptures did not occur within 300 m of the cone, however, three new vents formed on the south flank and base of Cerro Negro and on trend with the Cerro La Mula–Cerro Negro volcanic alignment. Earthquake swarms were located northwest and southeast of Cerro Negro and seismicity was elevated for up to 11 days after the initial event. The temporal and spatial patterns of earthquake swarms, surface ruptures, and the eruption location can be explained using the Hill [J. Geophys. Res. 82 (1977) 1347] model for earthquake swarms in volcanic regions, where an eruption is triggered by tectonically induced changes in the regional stress field. In this model, tectonic strain, rather than magmatic overpressure causes dilation of the conduit for magma ascent. Numerical simulations for the 1999 eruption illustrate that the observed velocities (up to 75 m s −1) and fountain heights (50–300 m) can be achieved by eruption of magma with little excess magmatic pressure, in response to changes in Coulomb stress along the Cerro La Mula–Cerro Negro alignment. These observations and models show that 1999 Cerro Negro activity was a tectonically induced small-volume eruption in an arc setting, with the accommodation of extensional strain by dike injection.

A 550‐year‐old Plinian eruption at El Chichón Volcano, Chiapas, Mexico: Explosive volcanism linked to reheating of the magma reservoir

Some 550 years ago (1320–1433 A.D.), a powerful Plinian eruption at El Chichón Volcano in southern Mexico produced a widespread pumice fall deposit. We subdivided the deposit into three parts on the basis of structural and textural characteristics, pumice lithology and density, granulometry, and petrologic‐geochemical attributes. The deposit covers an area of 1500 km2 within the 1‐cm isopach and has a minimum estimated bulk volume of 2.8 km3 (1.1 km3 dense rock equivalent (DRE)); its eruptive column reached an altitude of ∼31 km. Consideration of field evidence, the presence and nature of mafic enclaves, and chemical data strongly suggest that the 550 year B.P. eruption is linked with the intrusion of a high‐temperature basaltic magma into preexisting but stagnated trachyandesitic magma beneath El Chichón. Thorough mixing of the two magmas produced a compositionally uniform hybrid trachyandesite magma (average SiO2 55.3 wt %), which subsequently underwent crystal growth and gas exsolution, ultimately overpressurizing the zoned magmatic system to erupt explosively. On the basis of El Chichón's known eruptive history, the intrusion‐mixing event occurred sometime after the 900 year B.P. eruption. The hybrid magma had a preeruption temperature of 820–830°C and was water undersaturated (5–6 wt % H2O) at pressures of ∼2–2.5 kbar.

Volcanological evolution of Mount Erciyes stratovolcano and origin of the Valibaba Tepe ignimbrite (Central Anatolia, Turkey)

Mount Erciyes (3917 m) is the largest stratovolcano of Central Anatolia (Turkey). The volcanological evolution of Mount Erciyes from Pliocene–Quaternary to historical times exhibits two distinct stages: (1) Koç Dağ and (2) Erciyes. During the Koç Dağ stage, basaltic and andesitic lava flows were emitted from the cinder cones of Kızıl Tepe and Topakkaya Tepe. Then, Koç Dağ pyroclastics were emplaced, leading to a caldera collapse of 14×18 km in diameter with a volume estimated at 110 km 3. Two eruption phases separated by scoria fall and mud flow deposits are recognised associated with the caldera forming event. During Phase 1, plinian fall and pumice flows were emplaced on the eastern part of the volcano. Plinian fall deposits occur more than 50 km away from the source area. The volume of tephra is estimated at 63.3 km 3 (16 km 3 dense rock equivalent (DRE)), including 62 km 3 of plinian fall and 1.3 km 3 pumice flow deposits. Phase 2 mainly consists of pyroclastic flow deposits (two pumiceous flow units and Valibaba Tepe Ignimbrite (VTI)). The pumiceous flow deposits (4.2 km 3 DRE) extend 30 km from the inferred source area towards the north and northeast. The final pyroclastic flows of Phase 2 occurred 2.8 Ma ago and produced the VTI. The VTI is a low aspect ratio welded ignimbrite and its volume is estimated at 40 km 3 DRE. Plinian fall deposits (0.8 km 3 DRE) preceded the VTI and are only observed on the eastern part of the volcano, covering an area of 1500 km 2. During the Erciyes stage, two eruptive cycles are defined. The first cycle (2.6–0.17 Ma) is characterised by effusive, extrusive and weakly explosive activity with emplacement of andesitic lava flows, dacitic dome flows, basaltic andesite lava flows, and andesitic cones. The second cycle involved dacitic extrusive and explosive activity, located at the summit area, and produced block-and-ash flows derived from rhyodacitic domes. Plinian fall, surge and pumice flow deposits were emplaced prior to rhyodacitic dome extrusions. The last event was the emplacement of debris avalanche deposits related to sector collapse caldera.

Volumetric characteristics of lava flows from interferometric radar and multispectral satellite data: the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Gal�pagos

We have used a suite of remotely sensed data, numerical lava flow modeling, and field observations to determine quantitative characteristics of the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galapagos Islands. Flank lava flow areas, volumes, instantaneous effusion rates, and average effusion rates were all determined for these two eruptions, for which only limited syn-eruptive field observations are available. Using data from SPOT, TOPSAR, ERS-1, and ERS-2, we determined that the 1995 Fernandina flow covers a subaerial area of 6.5×106 m2 and has a subaerial dense rock equivalent (DRE) volume of 42×106 m3. Field observations, ATSR satellite data, and the FLOWGO numerical model allow us to determine that the effusion rate declined exponentially from a high of ~60–200 m3 s-1 during the first few hours to <5 m3 s-1 prior to ceasing after 73 days, with a mean effusion rate of 4–16 m3 s-1. Integrating the ATSR-derived, exponentially declining effusion rate over the eruption duration produces a total (subaerial + submarine) DRE volume of between 27 and 100×106 m3, the range in values being due to differing assumptions about heat loss characteristics; only values in the higher part of this range are consistent with the independently derived subaerial volume. Using SPOT, TOPSAR, ERS-1, and ERS-2 data, we determine that the 1998 Cerro Azul flow is 16 km long, covers 16 km2, and has a DRE volume of 54×106 m3. FLOWGO produces at-vent velocity and effusion rate values of 11 m s-1 and ~600 m3 s-1, respectively. The velocity value agrees well with the 12 m s-1 estimated in the field. The mean effusion rate (total DRE volume/duration) was 7–47 m3 s-1. Dike dimensions, fissure lengths, and pressure gradients along the conduit based on magma chamber depth estimates of 3–5 km produce mean effusion rates for the two eruptions that range over nearly four orders of magnitude, the range being due to uncertainty in the magma viscosity, dike dimensions, and pressure gradient between magma chamber and vent. Although somewhat consistent with mean effusion rates from other techniques, their wide range makes them less useful. The exponentially declining effusion rates during both eruptions are consistent with release of elastic strain being the driving mechanism of the eruptions. Our results provide independent input parameters for previously published theoretical relationships between magma chamber pressurization and eruption rates that constrain chamber volumes and increases in volume prior to eruption, as well as time constants of exponential decay during the eruption. The results and theoretical relationships combine to indicate that at both volcanoes probably 25–30% of the volumetric increase in the magma chamber erupted as lava onto the surface. In both eruptions the lava flow volumes are less than 1% of the magma chamber volume.