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Degassing-induced crystallization in volatile rich intermediate composition magmas results in material stiffening and strengthening that prior to solidification is reflected in non-Newtonian rheology. We explore the effects of a spectrum of such rheological regimes on eruptive style and morphologic evolution of lava domes, using a two-dimensional (2D) particle-dynamics model for a spreading viscoplastic (Bingham) fluid. We assume that the ductile magma core of a 2D synthetic lava dome develops finite yield strength, and that deformable frictional talus evolves from a carapace that caps the magma core. Our new model is calibrated against an existing analytical model for a spreading viscoplastic lava dome and is further compared against observational data of lava dome growth. Results indicate that a degassing-induced increase in strength of the injected magma causes a transition in the lava dome morphology from a dome with low surface relief evolving endogenously (with apparent bulk yield strength - 104 105 – 106 Pa) extruded through the dome carapace. The virtual lava dome with τ0a = 0.6 MPa shows good agreement with the observed dome heights observed at the Soufriere Hills Volcano, Montserrat during a period of endogenous growth. The calculated apparent flow viscosity (1.36 × 1011 Pa·s for τ0a = 0.6 MPa) is in the range of estimated viscosities (109 to 1012 Pa·s) for andesitic-dacitic crystal-rich lavas. Our model results indicate a strong correlation between apparent yield strength and dome morphology, with both controlled by degassing-induced crystallization and extrusion rate.

This study of burns patients from two eruptions of Merapi volcano, Java, in 1994 and 2010, is the first detailed analysis to be reported of thermal injuries in a large series of hospitalised victims of pyroclastic surges, one of the most devastating phenomena in explosive eruptions. Emergency planners in volcanic crises in populated areas have to integrate the health sector into disaster management and be aware of the nature of the surge impacts and the types of burns victims to be expected in a worst scenario, potentially in numbers and in severity that would overwhelm normal treatment facilities. In our series, 106 patients from the two eruptions were treated in the same major hospital in Yogyakarta and a third of these survived. Seventy-eight per cent were admitted with over 40% TBSA (total body surface area) burns and around 80% of patients were suspected of having at least some degree of inhalation injury as well. Thirty five patients suffered over 80% TBSA burns and only one of these survived. Crucially, 45% of patients were in the 40–79% TBSA range, with most suspected of suffering from inhalation injury, for whom survival was most dependent on the hospital treatment they received. After reviewing the evidence from recent major eruptions and outlining the thermal hazards of surges, we relate the type and severity of the injuries of these patients to the temperatures and dynamics of the pyroclastic surges, as derived from the environmental impacts and associated eruption processes evaluated in our field surveys and interviews conducted by our multi-disciplinary team. Effective warnings, adequate evacuation measures, and political will are all essential in volcanic crises in populated areas to prevent future catastrophes on this scale.

We provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active Husavik-Flatey transform fault in the southern part of the Tjornes Fracture Zone of north Iceland. In the Flateyjarskagi peninsula immediately south of the fault, dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace. In map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90°. We present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed. We compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation, prior to the appearance of a transform fault system comprised of discrete shears. Our paleomagnetic results were obtained from one relatively undisturbed reference site, and three sites of deformed rocks in Flateyjarskagi and two in Tjornes peninsula. These results indicate large vertical-axis rotations, and the progressive deflection of dike trends throughout northern Flateyjarskagi also serves as a separate quantitative estimate of vertical-axis rotation. The clockwise vertical-axis rotation estimates of 71°–97° from dikes are less than those deduced from paleomagnetism, but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses. The problem is typical for deformed rocks from high-latitude sites with steeply inclined paleomagnetic vectors. We consider our field measurements of dikes as more accurate indicators of true tectonic rotation. This regional deformation of Miocene rocks occurred early in the evolution of the TFZ transform, probably beginning ca. 7 Ma, when an older rift zone present in western North Iceland jumped to its present NVZ location in northeast Iceland. The early deformations involved shear strain distributed over a region about 20-km broad and produced the curved structures. This phase was followed by the sequential development of more narrowly distributed discrete shears, generating the initial Husavik-Flatey transform fault.

One of the World's premier field geologists, Kristjan Saemundsson led immense geological mapping programs and authored or co-authored nearly all geological maps of Iceland during the past half century, including the first modern bedrock and tectonic maps of the whole country. These monumental achievements collectively yield the most inclusive view of an extensional plate boundary anywhere on Earth. When Kristjan began his work in 1961, the relation of Iceland to sea-floor spreading was not clear, and plate tectonics had not yet been invented. Kristjan resolved key obstacles by demonstrating that the active rifting zones in Iceland had shifted over time and were linked by complex transforms to the mid-ocean spreading ridge, thus making the concept of sea-floor spreading in Iceland acceptable to those previously skeptical. Further, his insights and vast geological and tectonic knowledge on both high- and low-temperature geothermal areas in Iceland yielded a major increase in knowledge of geothermal systems, and probably no one has contributed more than he to Icelandic energy development. Kristjan's legacy is comprised by his numerous superb maps on a variety of scales, the high quality papers he produced, the impactful ideas generated that were internationally diffused, and the generations of colleagues and younger people he inspired, mentored, or otherwise positively influenced with his knowledge and generous attitude.

Lava domes are structures that grow by the extrusion of viscous silicic or intermediate composition magma from a central volcanic conduit. Repeated cycles of growth are punctuated by collapse, as the structure becomes oversized for the strength of the composite magma that rheologically stiffens and strengthens at its surface. Here we explore lava dome growth and failure mechanics using a two-dimensional particle-dynamics model. The model follows the evolution of fractured lava, with solidification driven by degassing induced crystallization of magma. The particle-dynamics model emulates the natural development of dome growth and rearrangement of the lava dome which is difficult in mesh-based analyses due to mesh entanglement effects. The deformable talus evolves naturally as a frictional carapace that caps a ductile magma core. Extrusion rate and magma rheology together with crystallization temperature and volatile content govern the distribution of strength in the composite structure. This new model is calibrated against existing observational models of lava dome growth. Results show that the shape and extent of the ductile core and the overall structure of the lava dome are strongly controlled by the infusion rate. The effects of extrusion rate on magma rheology are sensitive to material stiffness, which in turn is a function of volatile content and crystallinity. Material stiffness and material strength are key model parameters which govern magma rheology and subsequently the morphological character of the lava dome and in turn stability. Degassing induced crystallization causes material stiffening and enhances material strength reflected in non-Newtonian magma behavior. The increase in stiffness and strength of the injected magma causes a transition in the style of dome growth, from endogenous expansion of a ductile core, to stiffer and stronger intruding material capable of punching through the overlying material and resulting in the development of a spine or possibly inducing dome collapse. Simulation results mimic development of a megaspine upon the influx of fresh magma which leads to the re-direction of magma flow, creating a new shear zone and the switching of dome growth from one side to the other. Our model shows similar dome growth dynamics as observed at Soufriere Hills Volcano, Montserrat, indicating a strong correlation between extrusion rate and its subsequent effect on mechanical properties and variations in magma rheology.

We analyze the textures and mineralogy of Merapi tephra generated during explosive VEI 3–4 eruptions over the past 2000 years, and compare these data with those observed for Merapi dome and flow lavas. We find that the Merapi pumiceous tephra and lava textures differ significantly with respect to small-size crystal populations, but that phenocryst textures are generally similar. A similar initial phase of crystallization is indicated for tephras and lavas in mid-crustal (> 10 km depth) reservoirs. Subsequent textural differences are mainly affected by ascent rate and degassing during ascent, and, for dome lavas, with temporary storage in shallower reservoirs. These differences also correspond to different eruptive styles. Our analyses include study of pumices, lava, and breadcrust-bomb samples, including samples from some of the most recent explosive episodes prior to the 2010 eruption. Textural analyses of youthful breadcrust bomb samples yield insights on one type of transition between effusive and explosive eruptive styles, involving pressure build-up under a degassed crystalline shallow-conduit plug. In general, comparison of the crystal size distributions and calculated residence times among the effusive and explosive eruptive styles suggests that the two main magma-product types resided for similar lengths of time in a mid-crustal reservoir, before ascending toward the surface and either erupting explosively (tephra), or stagnating in a shallow magma chamber prior to extrusion (lava). The interpretation is supported by the occurrences of amphibole in pristine condition in tephra and in altered state in lava. Finally the 1872 and 2010 explosive eruptions are examined and compared with others over the past three millennia. The 2010 bulk-rock compositions overlap with products of other major explosive Merapi eruptions, such as the Ngrangkah, Tegalsruni, Temusari, and Kepuharjo tephras. The 2010 products show a gradational late-stage mafic enrichment, dissimilar to 1872 which showed little (

We analyze and compare the textures of Merapi lavas (basalts and basaltic andesites) ranging in age from Proto-Merapi through modern activity, with the goal of gaining insights on the temporal evolution of Merapi's magmatic system. Analysis of textural parameters, such as phenocryst and microphenocryst crystallinity, coupled with crystal size distribution theory, provides information about the storage and transport of magmas. We combine textural analyses with geochemical investigations for a comprehensive comparison of erupted lavas over time. The chemical analyses identify crystal growth processes in magma chambers and underline differences between sample groups. Our work suggests the occurrence of two distinct histories, presumably associated with (at least) two generally distinct types of rheological behaviors and storage/transport systems. These behaviors are associated with different plagioclase growth patterns, with both groups influenced by late-stage shallow decompression degassing-induced microlite crystallization. Both groups contain amphibole crystals that indicate an early period of mid-crustal to deep-crustal storage of water-rich magmas. Dome lavas from the 20th century eruptive activity indicate quasi-steady-state nucleation-and-growth evolution interspersed with episodes of reheating and textural coarsening, suggesting residence in magma storage at multiple depths, both > 10 km, and

We identify and quantify controls on the timescales and magnitudes of cyclic (periodic) volcanic eruptions using the numerical model DOMEFLOW ( de’ Michieli Vitturi et al., 2010 ) which was developed by the authors for magma systems of intermediate composition. DOMEFLOW treats the magma mixture as a liquid continuum with dispersed gas bubbles and crystals in thermodynamic equilibrium with the melt and assumes a modified Poiseuille form of the viscous term for fully developed laminar flow in a conduit of cylindrical cross-section. During ascent, magma pressure decreases and water vapor exsolves and partially degasses from the melt as the melt simultaneously crystallizes, causing changes in mixture density and viscosity. Two mechanisms previously proposed to cause periodic eruption behavior have been implemented in the model and their corresponding timescales explored. The first applies a stick–slip model in which motion of a shallow solid plug is resisted by static/dynamic friction, as described in Iverson et al. (2006) . For a constant magma supply rate at depth, this mechanism yields cyclic extrusion with timescales of seconds to tens of seconds with values generally depending on assumed friction coefficients. The second mechanism does not consider friction but treats the plug as a high-viscosity Newtonian fluid. During viscous resistance, pressure beneath the degassed plug can increase sufficiently to overcome dome overburden, plug weight, and viscous forces, and ultimately drive the plug from the conduit. In this second model cycle periods are on the order of hours, and decrease with increasing magma supply rate until a threshold is reached, at which point periodicity disappears and extrusion rate becomes steady (vanishingly short periods). Magma volatile content for fixed chamber pressure has little effect on cycle timescales, but increasing volatile content increases mass flow rate and cycle magnitude as defined by the difference between maximum and minimum extrusion rates. Increasing mass flow rate also increases the magnitude of predicted deformation cycles. Both models have been applied to dome-building eruptions using conditions appropriate for the Soufriere Hills volcano, Montserrat. Results suggest that the modeled stick–slip mechanism cannot explain the cycles of extrusion and explosion well-documented at Montserrat (timescales of hours). However, a subset of the simulations for the viscous plug model are consistent with the Montserrat data and therefore demonstrate the feasibility of this second formulation.

Strikingly similar examples of edifice collapse and directed blast are the 18 May 1980 eruption of Mount St. Helens (MSH), Washington, USA, and the 30 March 1956 eruption of Bezymianny Volcano (BZ), Kamchatka, Russia. In these cases, flank failures led to near-instantaneous decompression and fragmentation of intra-edifice cryptodome magma, which produced catastrophic, laterally directed blasts. In both instances, the blast products consisted of juvenile material with bimodal density/vesicularity distributions: low- and high-density modes at 1900 and 2400 kg m− 3 for BZ, 1600 and 2300 kg m− 3 for MSH, although the proportion of high-density material is greater at BZ. Blast materials also exhibit striking variety in groundmass crystallinity ( 90 vol.%) despite having fairly uniform pheno-crystallinities, suggesting that degassing-driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes. New bulk-rock H2O and ∂D measurements confirm that progressive open-system outgassing occurred prior to both blasts. The correlations between crystallinity, clast density, and bulk H2O contents suggest that syn-blast magma expansion was modulated both by non-uniform volatile distribution within the cryptodome and rheological controls associated with non-uniform crystal content. Spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin, which probably correlates with timing of magma injection. The greater proportion of high-density material at BZ is speculatively related to lower blast energy compared with MSH.

The transient dynamics of magma ascent during dome-forming eruptions were investigated and the effects of magma chamber pressure perturbations on eruption rate are illustrated. The numerical model DOMEFLOW, developed by the authors for this work, is applied to the problem. DOMEFLOW is a transient 1.5D isothermal two-phase flow model of magma ascent through an axisymmetric conduit of variable radius, which accounts for gas exsolution, bubble growth, crystallization induced by degassing, permeable gas loss through overlying magma and through conduit walls, as well as viscosity changes due to crystallization and degassing. For runs in which chamber pressure increases, the time required to reach the new steady state (transition time) is a complex function of the pressure perturbation, while for decreasing chamber pressure, transition time is a monotonic function of the magnitude of the pressure perturbation. The transition to the new steady state is mainly controlled by magma compressibility, travel time (time required for one parcel of magma to travel from chamber to surface), and the time over which the pressure perturbation occurs. Results of many runs (> 300) were analyzed using dimensional analysis to reveal a general relationship which predicts the temporal evolution of magma effusion rate for a given sudden increase in chamber pressure; the product of the change in steady-state extrusion rate and the time required to reach the new steady state is linearly proportional to the normalized change in chamber pressure, the volume of the conduit, and the ratio of top and bottom conduit radii, and inversely proportional to the cubic root of volatile fraction. This relationship is used to interpret observed variations in two ongoing dome-building eruptions, the Soufriere Hills volcano, Montserrat, and Merapi volcano, Indonesia.

A dyke conduit has been shown to be a realistic model for the shallow magmatic feeder system for the dome forming eruption at Soufriere Hills Volcano, Montserrat. Here we use a three-dimensional Finite Element model to examine the ground deformation that can be expected due to the pressurization of a dyke conduit. We find that the generated deformation has a bilateral symmetry with nearly no displacement in the direction of dyke strike, and a maximal ground deformation about 1 km away from the vent in the perpendicular direction. Resultant surface deformation is mainly triggered by the upper part of the feeder system, where the dyke opens into a cylindrical conduit. We apply our deformation model to investigate tilt data collected in 1997 in order to infer the orientation of the dyke. We obtain a best-fit for a NNW–SSE trending dyke, which matches observations of the ground displacement field obtained by differential GPS and the alignment of main structural geologic features of southern Montserrat.

We report on detailed laboratory experiments designed to illustrate the frictional behavior of granular volcanic debris. The materials include pyroclastic flow debris from Soufriere Hills Volcano, Montserrat, and from Stromboli Volcano, Italy, and lahar deposits from Mount St. Helens. Experiments were conducted in a servo-controlled, double-direct shear apparatus under conditions of displacement-control and with monitored temperature and humidity. The effects of loading velocity, normal stress, grain-size range, and saturation state were examined for normal stresses from 0.75 to 8 MPa. The Soufriere Hills debris was sampled in the field using a 2 mm sieve. Samples from Stromboli and MSH represent the original bulk material. The influence of grain-size and size distribution were examined in detail for the Soufriere Hills material for 1) two narrow size ranges (3–4 /, 0.063–0.125 mm; and 0–1 /, 0.5–1 mm), 2) a wide size range (3–0 /, 0.125–1 mm), and 3) the natural size distribution in the range 0–1 mm. These four data sets show remarkably uniform properties: coefficients of residual internal friction varied from 0.62 to 0.64 and coefficients of peak internal friction varied from 0.66 to 0.69, with zero cohesion in each case. For the natural grain-size distribution, we find a small but clear increase in residual sliding friction with increasing slip velocity in the range 10 to 900 μm/s, i.e. velocity strengthening frictional behavior. For Soufriere Hills pyroclastic material the coefficient of internal friction changes very little when water saturated. The frictional characteristics were remarkably similar for the three volcanoes. For the natural grain-size distributions, the coefficient of residual internal friction ranged from 0.61 to 0.63 and peak internal friction values ranged from 0.65 to 0.66. Pre-loaded and over-compacted materials sheared at reduced normal stress gave peak coefficients as high as 0.80. Our results imply that granular volcanic debris should often fail via stable creep, but may exhibit stick-slip instability under some conditions. © 2008 Elsevier B.V. All rights reserved.

Styles of dome collapse at Soufriere Hills volcano (SHV; November 1995–March 1998) are classified by relations between extrusion rate prior to collapse and collapse volume. Four separate modes of collapse behavior are apparent. Notably, moderate rates of extrusion are shown to result in two disparate modes of collapse: small-to-large collapses on steeply inclined failure planes that switch to collapse volumes an order of magnitude larger that cut deeply into the dome core. For constant effusion rates, this bifurcation in behavior is explained by the monotonic growth of a soft core that ultimately promotes the development of a deep-seated failure over previously favored shallow failure modes. Models are developed to test this hypothesis that first constrain magnitudes of cohesive and frictional strength with observed dome collapse morphologies and volumes. Evaluations of dome strengths confirm the important role of a soft core in promoting deep failure. A nested model representing a cohesive dome core, surrounded by a frictional rind, with constant rate of magma input, confirms the observed bifurcation in behavior, and for invariant effusive activity. Importantly, failure volumes are shown to increase by close to an order of magnitude for a few percent change in the proportion of dome core comprising cohesive material. This model is capable of replicating, a posteriori, the approximate timing of failure for both small (250 m) and large (325 m) domes. The timing and style of the 17 September 1996 and June to November 1997 collapses are honored.

The way in which strands of uncertain volcanological evidence can be used for decision-making, and the weight that should be given them, is a problem requiring formulation in terms of the logical principles of Evidence Science. The basic ideas are outlined using the explosion at Galeras volcano in Colombia in January 1993 as an example. Our retrospective analysis suggests that if a robust precautionary appraisal had been made of the circumstances in which distinctive tornillo signals were detected at Galeras, those events might have been construed as stronger precursory evidence for imminent explosive activity than were the indications for quiescence, given by the absence of other warning traits. However, whilst visits to the crater might have been recognised as involving elevated risk if this form of analysis had been applied to the situation in January 1993, a traditional scientific consideration of the available information was likely to have provided a neutral assessment of short-term risk levels. We use these inferences not to criticise interpretations or decisions made at the time, but to illustrate how a structured, evidence-based analysis procedure might have provided a different perspective to that derived from the conventional scientific standpoint. We advocate a formalism that may aid such decision-making in future: graphical Bayesian Belief Networks are introduced as a tool for performing the necessary numerical procedures. With this approach, Evidence Science concepts can be incorporated rationally, efficiently and reliably into decision support during volcanic crises.

During the sequential dome collapse of Merapi Volcano on 22 November 1994, a broadband seismic station on the western slope was the only operational seismic equipment that provided continuous on-scale recording of the event. According to visual and seismic observations, the collapse activity lasted about 10 h. We divide the activity into two phases: (I) a period with fluctuating but generally increasing seismic activity associated with 24 moderate to large dome-collapse nuees ardentes, lasting about 40 min and culminating with the largest event at 10:54 (all times reported as local time); and (II) activity from 11:42 until 20:00 described by scattered clusters of individual rockfalls and 20 nuees ardentes. The broadband data were evaluated using the assumption that avalanches with the same source areas and descent paths exhibit a linear relation between source volume and recorded seismic-amplitude envelope area. A result of the analysis is the determination of the volume of selected individual events. From the field surveys, the total volume of the collapsed dome lava is 2.6 Mm3. Discounting the volumetric influence of rockfalls, the average size of the 44 nuees ardentes is therefore about 60,000 m3. The largest collapse event at 10:54 is estimated to involve 260,000 m3, based on an analysis of the seismicity. The remaining 23 phase I events averaged 60,000 m3, with the total volume of all phase I events accounting for 63% of the unstable dome. The 20 phase II events comprised 37% of the total volume and averaged 47,000 m3. The methods described here can be put to practical use in real-time monitoring situations. Broadband data were essential in this study primarily because of the wide dynamic range.

A study of emplacement temperatures was carried out for the largest of the 22 November 1994 nuee ardente deposits at Merapi Volcano, based mainly on the response of plastic and woody materials subjected to the hot pyroclastic current and the deposits, and to some extent on eyewitness observations. The study emphasizes the Turgo–Kaliurang area in the distal part of the area affected by the nuee ardente, where nearly 100 casualties occurred. The term nuee ardente as used here includes channeled block-and-ash flows, and associated ash-clouds of surge and fallout origins. The emplacement temperature of the 8 m thick channeled block-and-ash deposit was relatively high, ∼550°C, based mainly on eyewitness reports of visual thermal radiance. Emplacement temperatures for ash-cloud deposits a few cm thick were deduced from polymer objects collected at Turgo and Kaliurang. Most polymers do not display a sharp melting range, but polyethylene terephthalate used in water bottles melts between 245 and 265°C, and parts of the bottles that had been deformed during fabrication molding turn a milky color at 200°C. The experimental evidence suggests that deposits in the Turgo area briefly achieved a maximum temperature near 300°C, whereas those near Kaliurang were

The common occurrence of tree and pole blow-down from pyroclastic currents provides an opportunity to estimate properties of the currents. Blow-down may occur by uprooting (root zone rupture), or flexure or shear at some point on the object. If trees are delimbed before blow-down, each tree or pole can be simulated by a cylinder perpendicular to the current. The force acting on a cylinder is a function of flow dynamic pressure, cylinder geometry, and drag coefficient. Treated as a cantilever of circular cross-section, the strength for the appropriate failure mode (rupture, uprooting or flexure) can then be used to estimate the minimum necessary current dynamic pressure. In some cases, larger or stronger standing objects can provide upper bounds on the dynamic pressure. This analysis was treated in two ways: (1) assuming that the current properties are vertically constant; and (2) allowing current velocity and density to vary vertically according to established models for turbulent boundary layers and stratified flow. The two methods produced similar results for dynamic pressure. The second, along with a method to approximate average whole-current density, offers a means to estimate average velocity and density over the height of the failed objects. The method is applied to several example cases, including Unzen, Mount St. Helens, Lamington, and Merapi volcanoes. Our results compare reasonably well with independent estimates. For several cases, we found that it is possible to use the dynamic pressure equations developed for vertically uniform flow, along with the average cloud density multiplied by a factor of 2–5, to determine average velocity over the height of the failed object.

Multiphase (MP) and low frequency (LF) earthquakes with spectral peak amplitudes at 3–4 and 1 Hz, respectively, are two common types of shallow volcanic earthquakes previously recognized at Merapi Volcano. Their mechanisms are poorly understood but MPs have been temporally associated with lava dome growth. We conducted a seismic experiment in January–February 1998, using four broadband seismographs to investigate the nature of seismic activity associated with dome growth. During our experiment, Merapi experienced mild dome growth with low-level seismic activity. We compare our data to that recorded on a local short-period (SP) network, with the following preliminary results. MP and LF events as recorded and classified on the short-period network instruments were recognized on the broadband network. Frequency spectrograms revealed similar patterns in the near summit region at widely separated broadband stations. Higher frequency spectra than previously recognized were identified for both MP and LF events, and were strongly attenuated as a function of radial distance from the source. Thus the spectral characteristics of these events as recorded on far-field stations are not fully indicative of the source processes. In particular, many events classified as LF-type appear to have much high frequency energy near the source. This aspect of these so-called LF earthquakes, and their association with very-long-period (VLP) pulses, suggests that many events identified in the far-field as LF events are in actuality a variety of the MP event and involve similar source processes. Broadband records indicated that simple large-amplitude VLP pulses were embedded in MP and LF wavetrains. From event to event these pulses were similar in their waveforms and had periods of 4 s. VLP events embedded in LF and MP earthquakes were located using particle motions. The epicenters were clustered in a central region of the dome complex, and preliminary source depths were within about 100 m of the dome surface, suggesting a source region deep within the dome or the uppermost conduit. A similar source location was established by study of MP high-frequency onsets. Our broadband data suggests that we have recorded both elastic seismic waves and a simple embedded pulse that is interpreted to represent a surface tilt at the seismometer site. The inferred tilt indicates an inflation and subsequent deflation, possibly caused by a gas pressure pulse or episodic shallow magma transport with stick-slip movement of the conduit wall.

Information on Merapi eruptive activity is scattered and much is remotely located. A concise and well-documented summary of this activity has been long needed to assist researchers and hazard-mitigation efforts, and the aim of this paper is to synthesize information from the mid-1700s to the present. A descriptive chronology is given, with an abbreviated chronology in a table that summarizes events by year, assigns preliminary Volcanic Explosivity Index (VEI) ratings and Hartmann classifications, and provides key references. The history of volcano monitoring is also outlined. The study reveals that a major difference in eruption style exists between the twentieth and nineteenth centuries, although the periodicity between larger events seems about the same. During the twentieth century, activity has comprised mainly the effusive growth of viscous lava domes and lava tongues, with occasional gravitational collapses of parts of oversteepened domes to produce the nuees ardentes—commonly defined as “Merapi-type”. In the 1800s, however, explosive eruptions of relatively large size occurred (to VEI 4), and some associated “fountain-collapse” nuees ardentes were larger and farther reaching than any produced in the twentieth century. These events may also be regarded as typical eruptions for Merapi. The nineteenth century activity is consistent with the long-term pattern of one relatively large event every one or two centuries, based on the long-term eruptive record deduced by others from volcanic stratigraphy. It is uncertain whether or not a “recurrence-time” model continues to apply to Merapi, but if so, Merapi could soon be due for another large event and its occurrence with only modest (or inadequately appreciated) precursors could lead to a disaster unprecedented in Merapi's history because the area around the volcano is now much more densely populated.

Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases. We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986‐88, 1992‐93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts 1 microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab63An29Or8) surrounded by alkali feldspar rims (Ab53An5Or42), compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock. As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions (PH2 O and temperature) were achieved throughout this extrusive period. However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995. Crystal textures and compositions in the 1992‐93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic. q 2000 Elsevier Science B.V. All rights reserved.

Following the eruption of January 1992, episodes of lava dome growth accompanied by generation of dome-collapse nuees ardentes occurred in 1994–1998. In addition, nuees ardentes were generated by fountain-collapse in January 1997, and the 1998 events also suggest an explosive component. Significant tilt and seismic precursors on varying time scales preceded these events. Deformation about the summit has been detected by electronic tiltmeters since November 1992, with inflation corresponding generally to lava dome growth, and deflation (or decreased inflation) corresponding to loss of dome mass. Strong short-term (days to weeks) accelerations in tilt rate and seismicity occurred prior to the major nuees ardentes episodes, apart from those of 22 November 1994 which were preceded by steadily increasing tilt for over 200 days but lacked short-term precursors. Because of the combination of populated hazardous areas and the lack of an issued warning, about 100 casualties occurred in 1994. In contrast, the strong precursors in 1997 and 1998 provided advance warning to observatory scientists, enabled the stepped raising of alert levels, and aided hazard management. As a result of these factors, but also the fortunate fact that the large nuees ardentes did not quite descend into populated areas, no casualties occurred. The nuee ardente episode of 1994 is interpreted as purely due to gravitational collapse, whereas those of 1997 and 1998 were influenced by gas-pressurization of the lava dome.

Edifice deformations are reported here for the period 1988–1995 at Merapi volcano, one of the most active and dangerous volcanoes in Indonesia. The study period includes a major resumption in lava effusion in January 1992 and a major dome collapse in November 1994. The data comprise electronic distance measurements (EDM) on a summit trilateration network, slope distance changes measured to the upper flanks, and other data collected from 1988 to 1995. A major consequence of this study is the documentation of a significant 4-year period of deformation precursory to the 1992 eruption. Cross-crater strain rates accelerated from less than 3×10 −6 /day between 1988 and 1990 to more than 11×10 −6 /day just prior to the January 1992 activity, representing a general, asymmetric extension of the summit during high-level conduit pressurization. After the vent opened and effusion of lava resumed, strain occurred at a much-reduced rate of less than 2×10 −6 /day. EDM measurements between lower flank benchmarks and the upper edifice indicate displacements as great as 1 m per year over the four years before the 1992 eruption. The Gendol breach, a pronounced depression formed by the juxtaposition of old lava coulees on the southeast flank, functioned as a major displacement discontinuity. Since 1993, movements have generally not exceeded the 95% confidence limits of the summit network. Exceptions to this include 12 cm outward movement for the northwest crater rim in 1992–1993, probably from loading by newly erupted dome lava, and movements as much as 7 cm on the south flank between November 1994 and September 1995. No short-term precursors were noted before the November 1994 lava dome collapse, but long-term adjustments of crater geometry accompanied lava dome growth in 1994. Short-term 2-cm deflation of the edifice occurred following the November 1994 dome collapse.

The complex 1980 Mount St. Helens debris avalanche is modeled numerically as a transient biviscous fluid flow. Several approaches are considered, including two-step rheologically-distinct models for avalanche I and combined avalanches II/III, and a composite flow model consisting of retrogressive slides of identical rheology successively accreted to the main avalanche flow. For the two-step situation, flow rheologies are evaluated separately for the initial avalanche, comprising the debris avalanche block facies, and an ensuing explosive-influenced flow. Strengths (normalized by density) as high as 250 m2/s2 and apparent Newtonian viscosities as much as 275 m2/s were established for the block facies. These parameters for the explosively-influenced flow are an order of magnitude lower. The distribution of stratigraphic units within flowing model debris, compared with field distributions, suggests that the higher-strength emplacement models are appropriate for debris deposited on Johnston Ridge and in the upper parts and flanks of the North Fork Toutle River valley. In general, models for which constant rheology is assumed throughout the flow process provide lower-bound emplacement times, and excessive early velocities, as compared to the prototype event. Because model calibration is based on matching runout by trial and error, it is therefore biased toward the rheologic parameters essential to achieving that runout. These values characterize the flow in its latter stages, whereas the actual strength and viscosity may have substantially decreased as a function of displacement. Two-dimensional models predict debris accumulation about twice as thick as that observed at the foot of Mount St. Helens, where flow divergence was significant. This discrepancy is lessened with a quasi-three-dimensional modification of the flow model. Accretionary composite flow models with homogeneous rheology simulate the overriding of early avalanche debris by later debris pulses. The accretionary composite model also predicts a thick flow snout down-valley. Since this feature is not indicated in the debris deposit, the model lends support to the concept of flow separation by rheology. The two-step and accretionary composite flow models complement one another and provide model-based support for the Harry Glicken hypothesis for complex emplacement at Mount St. Helens.

The Materials Failure Forecasting Method for volcanic eruptions (FFM) analyses the rate of precursory phenomena. Time of eruption onset is derived from the time of “failure” implied by accelerating rate of deformation. The approach attempts to fit data, Ω, to the differential relationship Ω ¨ = A Ω ˙ , where the dot superscript represents the time derivative, and the data Ω may be any of several parameters describing the accelerating deformation or energy release of the volcanic system. Rate coefficients,A and α, may be derived from appropriate data sets to provide an estimate of time to “failure”. As the method is still an experimental technique, it should be used with appropriate judgment during times of volcanic crisis. Limitations of the approach are identified and discussed. Several kinds of eruption precursory phenomena, all simulating accelerating creep during the mechanical deformation of the system, can be used with FFM. Among these are tilt data, slope-distance measurements, crater fault movements and seismicity. The use of seismic coda, seismic amplitude-derived energy release and time-integrated amplitudes or coda lengths are examined. Usage of cumulative coda length directly has some practical advantages to more rigorously derived parameters, and RSAM and SSAM technologies appear to be well suited to real-time applications. One graphical and four numerical techniques of applying FFM are discussed. The graphical technique is based on an inverse representation of rate versus time. For α = 2, the inverse rate plot is linear; it is concave upward for α 2. The eruption time is found by simple extrapolation of the data set toward the time axis. Three numerical techniques are based on linear least-squares fits to linearized data sets. The “linearized least-squares technique” is most robust and is expected to be the most practical numerical technique. This technique is based on an iterative linearization of the given rate-time series. The hindsight technique is disadvantaged by a bias favouring a too early eruption time in foresight applications. The “log rate versus log acceleration technique”, utilizing a logarithmic representation of the fundamental differential equation, is disadvantaged by large data scatter after interpolation of accelerations. One further numerical technique, a nonlinear least-squares fit to rate data, requires special and more complex software. PC-oriented computer codes were developed for data manipulation, application of the three linearizing numerical methods, and curve fitting. Separate software is required for graphing purposes. All three linearizing techniques facilitate an eruption window based on a data envelope according to the linear least-squares fit, at a specific level of confidence, and an estimated rate at time of failure.

Benchmarks installed on the upper eastern flank of Mt. Etna in 1982 have subsided continually since then, with the rate of subsidence twice accelerating prior to eruptions. The first of these eruptions was in December 1985, and the second in September 1989. This pattern of accelerating downslope movement has also been observed prior to landslides, and recent work applying knowledge of the failure of materials has shown that analysis of the inverse rate of these movements can be used to predict the time of failure. Post-eruption analyses of geodetic or seismic data from volcanoes has shown that in several cases, this approach could have been used to forecast eruptions weeks in advance. Applying the same principles to the accelerating subsidence on Mt. Etna's eastern flank prior to the eruptions of 1985 and 1989 shows that rough estimates of eruption dates could have been obtained several months in advance. These observations also suggest a speculative eruption-triggering mechanism involving an interplay between slope creep deformation and extensional weakening over the zone of intrusion.

Seismic activity during the December 1989 to April 1990 eruption of Redoubt Volcano, Alaska, has been tracked by the Alaska Volcano Observatory using Real-time Seismic Amplitude Measurement (RSAM) and Seismic Spectral Amplitude Measurement (SSAM) systems with up to five stations. Data consist of 10-minute averages of the absolute amplitudes of seismometer output. These data have been used to test in hindsight the Materials Failure Forecast Method (FFM), which attempts to define the time of eruption with a time series for precursory phenomena whose rate of change accelerates measurably before events. Practical application of the method emphasizes inverse-rate plots, both for early detection of signal emergent from background, and for event forecasting. Eruption windows are determined by graphical or numerical extrapolation of the inverse-rate trend and by intersection of a data envelope, reflecting data scatter and consistency, with an empirical critical rate near the time of eruption. Prior to the dome-destroying eruption of January 2, rate changes were of sufficient consistency, duration, and intensity for a qualitative or quantitative FFM predictive analysis, using either RSAM or SSAM. Signal-to-noise ratio was high for both RSAM or SSAM data sets, and FFM analyses could have provided useful support for decision making. Following January 2, in association with a rapid succession of dome-collapse eruptions, signal strength diminished and RSAM signal-to-noise ratio declined. The most distinct patterns on RSAM reflected noise rather than signal, and forecasting exclusively based for RSAM would have been misleading. By eliminating the noise in excluded frequencies, an enhanced signal-to-noise ratio was generally produced by SSAM for banded frequencies near 2 Hz. After January 2, only SSAM exhibited signal-to-noise ratios suitable for FFM analysis. Inverse-SSAM plots could have been informative for early detection of precursory long-period seismicity. The prospect of combining the FFM approach with spectral amplitudes in selected frequencies offers considerable promise.

This paper seeks to analyze in an objective way the circumstances and events that contributed to the 1985 Nevado del Ruiz catastrophe, in order to provide useful guidelines for future emergencies. The paper is organized into two principal parts. In the first part, an Anatomy of the catastrophe is developed as a step-by-step chronicle of events and actions taken by individuals and organizations during the period November 1984 through November 1985. This chronicle provides the essential background for the crucial events of November 13. This year-long period is broken down further to emphasize important chapters: the gradual awareness of the awakening of the volcano; a long period of institutional skepticism reflecting an absence of credibility; the closure of the credibility gap with the September 11 phreatic eruption, followed by an intensive effort to gird for the worst; and a detailed account of the day of reckoning. The second part of the paper, Retrospection , examines the numerous complicated factors that influenced the catastrophic outcome, and attempts to cull a few “lessons from Armero” in order to avoid similar occurrences in the future. In a nutshell, the government on the whole acted responsibly but was not willing to bear the economic or political costs of early evacuation or a false alarm. Science accurately foresaw the hazards but was insufficiently precise to render reliable warning of the crucial event at the last possible minute. Catastrophe was therefore a calculated risk, and this combination - the limitations of prediction/detection, the refusal to bear a false alarm and the lack of will to act on the uncertain information available - provided its immediate and most obvious causes. But because the crucial event occurred just two days before the Armero emergency management plan was to be critically examined and improved, the numerous circumstances which delayed progress of emergency management over the previous year also may be said to have contributed to the outcome. Thus the catastrophe was not caused by technological ineffectiveness or defectiveness, nor by an overwhelming eruption, or by an improbable run of bad luck, but rather by cumulative human error - by misjudgment, indecision and bureaucratic shortsightedness. Armero could have produced no victims, and therein dwells its immense tragedy.

The decrease in potential energy of gravity-driven overthrusts due to change in position can be equated to the sum of strain energy required for deformation, the work done against friction, and the kinetic energy. An analytical model based on this relationship allows the strain energy in pure bending to be expressed as a function of displacement. The model applies to an elastic material, and assumes frictionless slip for the multilayered case. Minimization of potential energy leads to an estimate of the minimum initial length required for gravitational sliding, and to the apparent coefficient of friction, displacements, and velocities for given slide geometries and properties. An example problem is worked out for the case of the Heart Mountain detachment of Wyoming, U.S.A. Minimum initial length for frictionless sliding is about 5.9 km, ignoring the effect of bending, and 8.5 km for a case of multilayered bending. Blocks of this size would move only a short distance as the increase in potential energy experienced in the riser prevents further sliding. But multilayered bending seems to have been necessary for sliding, compared to bending of the overthrust sheet as a single layer. Kinetic energy relationships suggest that the apparent coefficient of sliding friction for the 45 km long block was less than 0.031; e.g., for a friction coefficient of 0.025, a velocity of 19 m/sec is predicted at a characteristic displacement of 7 km. The model thereby illustrates the need for drastic friction reduction, but does not attempt to address the question of mechanism by which friction was reduced. The energy required to deform the slide mass is thought to be small compared to the decrease in potential energy due to changing position.

Some aspects of Iceland geodynamics are investigated from Landsat multispectral scanner (MSS) digital data in conjunction with field studies. Lineaments at the 1: 500,000 scale were mapped from computer processed imagery designed to enhance subtle structural fabric elements within the data. The analyses indicate pervasive north and west-northwest-trending lineaments. These trends are interpreted in terms of alternative complex models involving Riedel shear deformation or thermoelastic contraction adjacent to a spreading center. West-northwest-trending lineaments are concentrated along the Husavik-Flatey fault system. East of Axarfjordur, a 20-km wide band of west-northwest lineaments could mark the onshore continuation of the Grimsey submarine fault proposed on the basis of earthquake distributions.

The geometry and thermal history of fractures have been determined at 59 stations from Reykjavik to Hvalfjordur in southwestern Iceland. The data provide information on crustal stress regimes in the vicinity of mid-ocean ridges. Two major, generalized fracture orientations are present 1. (1) a northeast system, trend 010°–030°, except on Akranes where the orientation is 040°–060° 2. (2) a broad east—west system containing one or more sets with strike between 070°–130°. Thermal history of the host rock and fractures was determined from secondary minerals in vugs and fractures. The thermal history indicates that the northeast fracture set opened while the area was within the relatively hot axial zone of active volcanism and rifting. Some of the east—west trending fractures also opened at this time but many formed later, after the area had begun to cool and drift from the active zone. The northeast fracture set is essentially parallel to the trend of dikes and normal faults in southwestern Iceland. They have been interpreted as extension fractures (resulting in about 0.4% maximum extension) forming generally from the same stress field associated with normal faulting and dike injection in the active zone. Fracturing in an east-west direction (estimated 0.1% maximum extension), mainly near the edge and outside the active zone, indicates a reorientation of this stress field. The dominant mechanism related to the origin of the east—west fractures may be thermoelastic stresses arising from axial and basal accretion and cooling of lithospheric plates. Both fracture systems are inferred to have formed, in the Griffiths idealization, under nearly biaxial effective compressive loading on the order of 200 bar. The discrepancy between this value and the kilobar-order strengths of short-time laboratory tests reflects such factors as high temperature stress corrosion and fatigue. Fracture propagation is assumed to have been stable, but governed primarily by lateral load-diminishing mechanisms rather than by progressive loading. These relaxation mechanisms may have been episodic (northeast-system fissure swarm activity) or steady-state (thermoelastic contraction) in time.

Thermal stresses associated with a temperature change of only 10°C are on the order of 10–100 bars. This illustrates the important influence thermal stresses can impose on the results of rock stress measurements by the hydraulic fracturing method. In order to examine the problem, expressions are derived to describe the stress field produced by non-steady state heat conduction in an infinite elastic medium surrounding a cylindrical borehole, with the borehole wall kept at a constant temperature different from the original temperature of the medium. These expressions are then used to modify stress equations for hydraulic fracturing as derived from elasticity theory. The main thermal influence is on the determination of the maximum far-field compression stress; lowering of temperature leads to high estimate, whereas raising of temperature leads to a low estimate. The determination of principal stress directions is not generally affected.