Lee Siebert

Large volcanic debris avalanches: Characteristics of source areas, deposits, and associated eruptions

Abstract Large volcanic debris avalanches, often exceeding a cubic kilometer in volume, create massive amphitheater-shaped reentrants into the volcanic edifice that differ in morphology and origin from normal collapse calderas. The volume of debris avalanche deposits at the base of these breached craters or calderas often correlates closely with the volumes of the missing sectors of the volcanic edifices, indicating that the dominant process in the formation of these depressions is massive slope failure of a portion of the volcanic cone. Debris avalanche deposits display a hummocky topography with numerous small hills and closed depressions, longitudinal and transverse ridges, and locally homogeneous debris with a jigsaw fit, features that are typical of landslide deposits. The size of the hummocks and the maximum size of breccia blocks within them tends to decrease away from the source. Data on travel distance ( L ) of debris avalanches as a function of vertical drop ( H ) demonstrates the great mobility (median H / L = 0.11) of these avalanches, which are emplaced at calculated velocities often exceeding 100 km/hr. Volcanic debris avalanches and associated formation of “avalanche calderas” have occurred at roughly four per century in historic time, several times the historic rate for the formation of Krakatau-type calderas. These depressions often show a preferred orientation normal to the dominant direction of dike emplacement. The differential stress produced by the emplacement of parallel dike swarms is an important factor among the many factors that contribute to large-scale volcanic slope movements. In addition to hazards from rapid emplacement of the avalanches and possible associated directed blasts, a major secondary volcanic hazard from these events is tsunamis produced by the rapid impact of debris avalanches from coastal volcanoes into the sea.

Volcanic hazards from Bezymianny- and Bandai-type eruptions

Major slope failures are a significant degradational process at volcanoes. Slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years; the historic record provides evidence for at least six of these events in the past century. Several historic debris avalanches exceed 1 km3 in volume. Holocene avalanches an order of magnitude larger have traveled 50–100 km from the source volcano and affected areas of 500–1500 km2. Historic eruptions associated with major slope failures include those with a magmatic component (Bezymianny type) and those solely phreatic (Bandai type). The associated gravitational failures remove major segments of the volcanoes, creating massive horseshoe-shaped depressions commonly of caldera size. The paroxysmal phase of a Bezymianny-type eruption may include powerful lateral explosions and pumiceous pyroclastic flows; it is often followed by construction of lava dome or pyroclastic cone in the new crater. Bandai-type eruptions begin and end with the paroxysmal phase, during which slope failure removes a portion of the edifice. Massive volcanic landslides can also occur without related explosive eruptions, as at the Unzen volcano in 1792.The main potential hazards from these events derive from lateral blasts, the debris avalanche itself, and avalanche-induced tsunamis. Lateral blasts produced by sudden decompression of hydrothermal and/or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s−1. The ratio of area covered to distance traveled for the Mount St. Helens and Bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume. The potential for large-scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice, as at Mount St. Helens and Bezymianny.Debris avalanches can move faster than 100 ms−1 and travel tens of kilometers. When not confined by valley walls, avalanches can affect wide areas beyond the volcanos flanks. Tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions. The probable travel distance (L) of avalanches can be estimated by considering the potential vertical drop (H). Data from a catalog of around 200 debris avalanches indicates that the H/L rations for avalanches with volumes of 0.1–1 km3 average 0.13 and range 0.09–0.18; for avalanches exceeding 1 km3, H/L ratios average 0.09 and range 0.5–0.13.Large-scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts. The nature and duration of precursory activity vary widely, and the timing of slope faliure greatly affects the type of associated eruption. Bandai-type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity.

Global database on large magnitude explosive volcanic eruptions (LaMEVE)

To facilitate the assessment of hazards and risk from volcanoes, we have created a comprehensive global database of Quaternary Large Magnitude Explosive Volcanic Eruptions (LaMEVE). This forms part of the larger Volcanic Global Risk Identification and Analysis Project (VOGRIPA), and also forms part of the Global Volcano Model (GVM) initiative (http://www.globalvolcanomodel.org). A flexible search tool allows users to select data on a global, regional or local scale; the selected data can be downloaded into a spreadsheet. The database is publically available online at http://www.bgs.ac.uk/vogripa and currently contains information on nearly 3,000 volcanoes and over 1,800 Quaternary eruption records. Not all volcanoes currently have eruptions associated with them but have been included to allow for easy expansion of the database as more data are found. Data fields include: magnitude, Volcanic Explosivity Index (VEI), deposit volumes, eruption dates, and rock type. The scientific community is invited to contribute new data and also alert the database manager to potentially incorrect data. Whilst the database currently focuses only on large magnitude eruptions, it will be expanded to include data specifically relating to the principal volcanic hazards (e.g. pyroclastic flows, tephra fall, lahars, debris avalanches, ballistics), as well as vulnerability (e.g. population figures, building type) to facilitate risk assessments of future eruptions.

A statistical analysis of the global historical volcanic fatalities record

A new database of volcanic fatalities is presented and analysed, covering the period 1600 to 2010 AD. Data are from four sources: the Smithsonian Institution, Witham (2005), CRED EM-DAT and Munich RE. The data were combined and formatted, with a weighted average fatality figure used where more than one source reports an event; the former two databases were weighted twice as strongly as the latter two. More fatal incidents are contained within our database than similar previous works; approximately 46% of the fatal incidents are listed in only one of the four sources, and fewer than 10% are in all four. 278,880 fatalities are recorded in the database, resultant from 533 fatal incidents. The fatality count is dominated by a handful of disasters, though the majority of fatal incidents have caused fewer than ten fatalities. Number and empirical probability of fatalities are broadly correlated with VEI, but are more strongly influenced by population density around volcanoes and the occurrence and extent of lahars (mudflows) and pyroclastic density currents, which have caused 50% of fatalities. Indonesia, the Philippines, and the West Indies dominate the spatial distribution of fatalities, and there is some negative correlation between regional development and number of fatalities. With the largest disasters removed, over 90% of fatalities occurred between 5 km and 30 km from volcanoes, though the most devastating eruptions impacted far beyond these distances. A new measure, the Volcano Fatality Index, is defined to explore temporal changes in societal vulnerability to volcanic hazards. The measure incorporates population growth and recording improvements with the fatality data, and shows prima facie evidence that vulnerability to volcanic hazards has fallen during the last two centuries. Results and interpretations are limited in scope by the underlying fatalities data, which are affected by under-recording, uncertainty, and bias. Attempts have been made to estimate the extent of these issues, and to remove their effects where possible.The data analysed here are provided as supplementary material. An updated version of the Smithsonian fatality database fully integrated with this database will be publicly available in the near future and subsequently incorporate new data.

Edifice collapse and related hazards in Guatemala

Abstract Guatemalan volcanoes have at least seven debris-avalanche deposits, associated with Cerro Quemado, Fuego, Pacaya, Tecuamburro and an unidentified volcano. The deposits range in size from less than 0.1 to in excess of 9 km 3 and from 2.5 to in excess of 300 km 2 . The avalanches traveled 3 to 50 km from their sources in the Guatemalan highlands. Three of the avalanches occurred in Late Pleistocene time and four in Holocene time—two of them within the last 2000 years. The avalanches occurred at both andesitic and basaltic stratovolcanoes and at dacitic dome complexes. Laterally directed phreatic or magmatic pyroclastic explosions were associated with two of the debris avalanches. An evaluation of factors that might lead to an edifice collapse in Guatemala is based on the case studies presented in this report and a survey of the literature. Edifice collapses are more apt to occur if zones of weakness exist within the volcanic edifices, such as unwelded pyroclastic rocks and pervasively altered rocks. Further, the trench-ward side of volcano pairs like Fuego and Atitlan may be more likely to fail because it may have weak zones along the contact with the older back-arc volcano. The direction of failure may be influenced by regional slopes, which in Guatemala generally trend southward toward the oceanic trench, and by such structural factors as multiple vents or overly steep slopes reflecting previous activity or erosion. Debris avalanches are more likely to occur in drainages which have headwaters at two or more volcanoes. Domes are especially apt to produce small- to moderate-sized debris avalanches, and, further, if the domes form a coalescing chain, are most likely to fail in a direction normal to the chain. These factors are used at seventeen major volcanic centers in Guatemala to assess their potential for edifice collapse and most probable direction of failure.

The 1883 and late-prehistoric eruptions of Augustine volcano, Alaska

Abstract The eruptive history of Augustine volcano has been characterized by cycles of growth and destruction of the volcano. Repeated failure of 5 – 10% of the edifice has produced mobile debris avalanches that reached the sea on all sides. High lava extrusion rates rapidly restore the volcano to its pre-failure configuration. This equilibrium between constructive and destructive processes has resulted in a relatively low lava-dome complex surrounded by an apron of volcaniclastic debris three times the volume of the dome complex. The most recent edifice collapse occurred in 1883, producing the 0.3 km3 Burr Point debris-avalanche deposit. Three major slide blocks extended the shoreline up to 2 km and produced a tsunami that swept across Cook Inlet. Hummock morphologies change from a proximal radial orientation to a dominantly transverse alignment reflecting deceleration and compression of the avalanches as they enter the sea. The breached crater formed by collapse was then largely filled by a 0.09 km3 lava dome and a 0.04 km3 lava flow travelled down the north flank. Lithic block-and-ash flows and pyroclastic surges reached the coast. The Burr Point avalanche deposit partially overlaps the Rocky Point debris-avalanche deposit to the west that was probably emplaced 200–400 years B.P. A major collapse event at ca. 1540 ± 110 A.D. produced the West Island debris avalanche, ending a period of expansion of the western side of the island. An accompanying lateral blast overtopped the avalanche, covering a 40 ° sector of the west flank. The plinian tephra layer B may also have been erupted at the time of the West Island eruption. Today Augustine volcano has rebuilt itself to a size similar to that which preceded the last edifice failure in 1883. The frequency of past collapses (three in the last 500 years) suggests that summit collapse is a possibility during any future eruption. The next major collapse is expected to involve 0.1 – 0.5 km3 of the summit; the ensuing debris avalanche would likely reach the coast, producing a tsunami that could impact populated areas of the Kenai Peninsula. The largest tsunami would result from collapse in directions other than to the north or west. Tsunami magnitude is contingent on failure volume, direction and timing with respect to tides.

Late-Pleistocene to precolumbian behind-the-arc mafic volcanism in the eastern Mexican Volcanic Belt; implications for future hazards

An area of widespread alkaline-to-subalkaline volcanism lies at the northern end of the Cofre de Perote^ Citlaltepetl (Pico de Orizaba) volcanic chain in the eastern Mexican Volcanic Belt (MVB). Two principal areas were active. About a dozen latest-Pleistocene to precolumbian vents form the 11-km-wide, E^W-trending Cofre de Perote vent cluster (CPVC) at 2300^2800 m elevation on the flank of the largely Pleistocene Cofre de Perote shield volcano and produced an extensive lava field that covers s 100 km 2 . More widely dispersed vents form the Naolinco volcanic field (NVF) in the Sierra de Chiconquiaco north of the city of Jalapa (Xalapa). Three generations of flows are delineated by cone and lava-flow morphology, degree of vegetation and cultivation, and radiocarbon dating. The flows lie in the behind-the-arc portion of the northeastern part of the MVB and show major- and trace-element chemical patterns transitional between intraplate and subduction zone environments. Flows of the oldest group originated from La Joya cinder cone (radiocarbon ages V42 000 yr BP) at the eastern end of the CPVC. This cone fed an olivine-basaltic flow field of V20 km 2 that extends about 14 km southeast to underlie the heavily populated northern outskirts of Jalapa, the capital city of the state of Veracruz. The Central Cone Group (CCG), of intermediate age, consists of four morphologically youthful cinder cones and associated vents that were the source of a lava fields 27 km 2 of late-Pleistocene or Holocene age. The youngest group includes the westernmost flow, from Cerro Colorado, and a lava flow V2980 BP from the Rincon de Chapultepec scoria cone of the NVF. The latest eruption, from the compound El Volcancillo scoria cone, occurred about 870 radiocarbon years ago and produced two chemically and rheologically diverse lava flows that are among the youngest precolumbian flows in Mexico and resemble paired aa^pahoehoe flows from Mauna Loa volcano. The El Volcancillo eruption initially produced the high effusion rate, short-duration Toxtlacuaya alkaline aa lava flow from the southeastern crater. This 12-km-long hawaiite (average 50.5% SiO2) flow was followed by extrusion of the calc-alkaline R| ¤o Naolinco lava flow from the northwestern crater. This large-volume (V1.3 km 3 ) tube-fed basaltic pahoehoe flow (average SiO2 49%) traveled 50 km. Inferred effusion rates suggest emplacement over a decade-long period. Flows of all three age groups are transected by Highway 140 and the railway that form major transportation arteries between Jalapa and Puebla. This area has not previously been considered to be at volcanic risk, but volcanism here has continued into precolumbian time. Future eruptions of similar magnitude and location to those documented here could pose significant hazards to transportation corridors and to densely populated areas in and to the north of Jalapa. Slight

Characterisation of the Quaternary eruption record: analysis of the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database

The Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database contains data on 1,883 Quaternary eruption records of magnitude (M) 4 and above and is publically accessible online via the British Geological Survey. Spatial and temporal analysis of the data indicates that the record is incomplete and is thus biased. The recorded distribution of volcanoes is variable on a global scale, with three-quarters of all volcanoes with M≥4 Quaternary activity located in the northern hemisphere and a quarter within Japan alone. The distribution of recorded eruptions does not strictly follow the spatial distribution of volcanoes and has distinct intra-regional variability, with about 40% of all recorded eruptions having occurred in Japan, reflecting in part the country’s efforts devoted to comprehensive volcanic studies. The number of eruptions in LaMEVE decreases with increasing age, exemplified by the recording of 50% of all known Quaternary eruptions during the last 20,000 years. Historical dating is prevalent from 1450 AD to the present day, substantially improving record completeness. The completeness of the record also improves as magnitude increases. This is demonstrated by the calculation of the median time, T50, for eruptions within given magnitude intervals, where 50% of eruptions are older than T50: T50 ranges from 5,070 years for M4-4.9 eruptions to 935,000 years for M≥8 eruptions. T50 follows a power law fit, suggesting a quantifiable relationship between eruption size and preservation potential of eruptive products. Several geographic regions have T50 ages of <250 years for the smallest (~M4) eruptions reflecting substantial levels of under-recording. There is evidence for latitudinal variation in eruptive activity, possibly due to the effects of glaciation. A peak in recorded activity is identified at 11 to 9 ka in high-latitude glaciated regions. This is absent in non-glaciated regions, supporting the hypothesis of increased volcanism due to ice unloading around this time. Record completeness and consequent interpretation of record limitations are important in understanding volcanism on global to local scales and must be considered during rigorous volcanic hazard and risk assessments. The study also indicates that there need to be improvements in the quality of data, including assessment of uncertainties in volume estimates.

Earth's Volcanoes and Their Eruptions: An Overview

Volcanoes are not random phenomena, but owe their existence, location, morphology, and eruptive styles to tectonic plate motions. Volcanic landforms vary widely in size and morphology from the stereotypical towering glacier-clad stratovolcano. They range from small spatter cones to massive shield volcanoes several thousand cubic kilometers in volume or broad volcanic depressions, cinder cones being the most common volcanic construct. Their eruptions likewise span many orders of magnitude in volume. Evaluation of eruption magnitudes and frequencies contrasts the vast majority of mild-to-moderate eruptions (∼80% with volcanic explosivity index, or VEI ≤2) with the more infrequent larger, higher-impact events (5% with VEI ≥4). Increased risk from population growth in proximity to volcanoes has been mitigated by enhanced monitoring and hazard-reduction efforts.

Tom Simkin (1933–2009)

Tom Simkin, friend of volcanologists and volcanology worldwide, died on 10 June 2009 in Baltimore, Md. Tom was a stalwart of the volcanological community and leaves an unparalleled legacy of information on Earths recent volcanism.