the ad 1362 oraefajokull eruption s e iceland physical volcanology and volatile release abstract the explosive rhyolitic eruption of oraefajokull volcano iceland in ad 1362 is described and interpreted based on the sequence of pyroclastic fall and flow deposits at 10 proximal locations around the south side of the volcano oraefajokull is an ice clad stratovolcano in south central iceland which has an ice filled caldera 4 5 km diameter of uncertain origin the main phase of the eruption took place over a few days in june and proceeded in three main phases that produced widely dispersed fallout deposits and a pyroclastic flow deposit an initial phase of phreatomagmatic eruptive activity produced a volumetrically minor coarse ash fall deposit unit a with a bi lobate dispersal this was followed by a second phreatomagmatic possibly phreatoplinian phase that deposited more fine ash beds unit b dispersed to the sse phases a and b were followed by an intense climactic plinian phase that lasted 8 12 h and produced unit c a coarse lapilli pumice clast dominated fall deposit in the proximal region at the end of plinian activity pyroclastic flows formed a poorly sorted deposit unit d presently of very limited thickness and exposed distribution much of eastern iceland is covered with a very fine distal ash layer dispersed to the ne this was probably deposited from an umbrella cloud and is the distal representation of the plinian fallout a total bulk fall deposit volume of 2 3 km3 is calculated 1 2 km3 dre pyroclastic flow deposit volumes have been crudely estimated to be

tremor and its duration amplitude distribution at popocatepetl volcano mexico the duration amplitude distribution was calculated for the tremor observed at popocatepetl volcano during episodes of activity in 2000 2012 2013 and 2014 an exponential function was used to obtain a good fit for the duration amplitude distribution and the source of volcanic tremor is probably generated by the transportation of magmatic fluids and its coupling with the host rock within the volcanic conduit in particular harmonic tremor has shown large amplitudes durations and mean values of amplitude more than spasmodic or pulsalting tremor this is due to different generation mechanisms in the case of harmonic tremor it is produced during magma ascent and lava dome growth whilst spasmodic and pulsalting tremor are associated with fragmentation of the lava dome and gas emissions this paper presents the duration amplitude distribution as a method to estimate the intensity of the tremor at popocatepetl a volcano with the major risk in all mexico

primary fractures within a tuff cone north menan butte idaho u s a abstract north menan butte is a tuff cone near idaho falls idaho it is a result of the eruption of basaltic magma through shallow water saturated river alluvium of the snake river the cone is characterized by primary fractures that can be classified into four groups on the basis of their physical properties and their orientations relative to the symmetry elements of the cone type i fractures are short closely spaced and usually confined to individual beds they strike approximately at right angles to cone radii and always dip toward the rim of the tuff cone bed segments separated by these fractures have undergone rotation resulting in normal displacements type ii fractures have similar attitudes but are more continuous less frequent and show no shear displacement type iii fractures also strike at right angles to cone radii but they dip away from the cone rim they cut across several beds and reveal inconsistent senses of shear displacement type iv fractures are radial steeply dipping and tend to be the most continuous of all fracture types type i fractures were the earliest to develop age relationships otherwise are uncertain examples of all four types of fractures are exposed on the inner and outer eroded slopes of the cone evidence from the cone indicates that the fractures developed in an unconsolidated aggregate of tuff with low cohesion therefore analysis of fracture genesis should be constrained by principles of soil mechanics type i fractures originated as tension fractures related to early downslope mass movement later movement on type i fractures accompanied the development of type iii shear fractures and possible bedding plane displacements all caused by overloading the crest of the cone by late stage eruptive products the origin of type ii fractures is unknown shrinkage due to desiccation or large scale creep are possible explanations the radial type iv fractures may be a consequence of desiccation shrinkage or possibly of subcone processes such as magma doming or radial hydraulic fracturing

zeolite compositions as proxies for eruptive paleoenvironment we report chemical compositions of authigenic alteration minerals in subglacially erupted hyaloclastite breccias from james ross island antarctica which provide a geochemical proxy for paleoenvironment analyses of zeolites phillipsite and chabazite from several lava fed deltas on the island were compared with published compositions of zeolites from known freshwater and marine environments using values of na k ca 3 0 for phillipsites and 1 0 for chabazites the data successfully distinguish between marine and freshwater alteration however our study also suggests that volcanic breccias may not be sealed to fluids even when the pore spaces are completely filled the james ross island data show that at higher elevations in lava fed deltas the zeolites preserve their original freshwater influenced i e glacial compositions whereas at lower elevations na k ca ratios are generally higher and indicate a later marine influence we suggest that water may penetrate several kilometers into the volcanic pile by movement along fractures and or grain boundaries

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

composite tuff ring cone complexes in jeju island korea possible consequences of substrate collapse and vent migration hydromagmatic volcanoes tuff rings and tuff cones in jeju island korea are divided into holocene volcanoes that were constructed upon rigid plateau forming lavas and early to middle pleistocene volcanoes that underlie the lavas and were constructed upon fragile unconsolidated sediments the older volcanoes are distinguished from the holocene volcanoes in that the former have irregular morphologies the irregular morphologies resulted from superposition or juxtaposition of multiple rim deposits of partial tuff rings and or tuff cones that have contrasting lithofacies characteristics bed attitudes and paleoflow directions the rim deposits are bounded by laterally persistent truncation surfaces which suggest large scale collapse of the substrate together with the overlying tephra piles and or erosion during a break in eruptive activity collapse was probably caused by the instability of the friable sedimentary substrate removal of lateral support because of downward quarrying of volcanic conduits and liquefaction of the water saturated substrate by volcanic seismicity the path of magma supply was diverted in some cases after collapse giving rise to migration of the active vent in other cases the eruption style changed abruptly from cone forming fallout dominated to ring forming surge dominated eruptions after collapse the resultant volcanic edifices thus consist of multiple rim beds of partial tuff rings and or tuff cones commonly with non circular morphologies substrate collapse and lateral vent migration as observed in jeju island appears to be an important phenomenon in hydromagmatic volcanoes because they are commonly constructed upon unstable sedimentary substrates substrate collapse and vent migration needs to be taken into account as an important control on volcano morphology and structures and as a means of lateral enlargement of terrestrial submarine or extraterrestrial volcanoes

ice melt collapse pits and associated features in the 1991 lahar deposits of volcan hudson chile criteria to distinguish eruption induced glacier melt in subaerial volcaniclastic sequences structures formed by ice blocks can provide information about a volcano s history of lahar generation by glacier melt at volcan hudson in chile catastrophic lahars were initiated by eruption induced melting of glacier ice in august and october 1991 they transported large ice blocks 50 km down the rio de los huemules valley to the sea large current crescents with lee side lenses were formed where ice blocks were deposited during waning stages of the flood when stranded blocks of ice melted they left cone shaped and ring shaped heaps of ice rafted debris on the sediment surface several hundred ice blocks were completely buried within the aggrading lahar sediment and when these melted circular collapse pits formed in the sediment collapse types included subsided coherent blocks of sediment bounded by an outward dipping ring fracture trapdoor structures with horseshoe shaped fractures downsag pits with centroclinal dips locally up to 60 pits with peripheral graben and crevasses piecemeal highly fragmented collapse structures and funnel shaped pits containing disaggregated sediment a sequence of progressive collapse is inferred in which initial downsag and subsidence on an outward dipping ring fracture produces a small diameter pit this is followed by widening of the pit by progressive development of concentric ring fractures and downsag outside the early formed pit and by collapse of overhanging pit walls to produce vertical to inward dipping walls and aprons of collapse debris on the pit floor the various structures have potential for preservation even in regions prone to high rainfall and flooding and they can be used to indicate that former lahars contained abundant blocks of ice

sedimentary architecture of large scale jokulhlaup generated ice block obstacle marks examples from skeiðararsandur se iceland during glacial outburst floods jokulhlaups large ice blocks are often released from the glacier margin ice blocks ground at points of flow deceleration generating distinctive patterns of scour and deposition scaled to the obstacle size the large scale ice block obstacle marks produced within the proglacial zone are diagnostic of jokulhlaups we use ground penetrating radar gpr to investigate the sedimentary architecture of ice block obstacle marks generated by the november 1996 jokulhlaup at skei ararjokull iceland the gpr data reveal a varied sedimentary architecture interpreted to suggest that ice block obstacle mark sedimentology is controlled by flow conditions sediment supply and whether ice blocks are deposited in isolation or as a cluster following ice block grounding deflection of flow down and away from the obstacle develops horseshoe vortices that maintain a non depositional hollow around the obstacle whilst continued outwash deposition away from the obstacle raises the level of the surrounding sandur where ice blocks ground early during the jokulhlaup hollows are infilled with foreset beds at the up flow end and backset beds antidune distally however where ice blocks ground on the waning stage when sediment flux is reduced antidunes are not preserved and the hollow is infilled with low angle foreset beds in both cases the lee side shadow ridge is composed of pseudo anticlinal beds proximal to the obstacle where shed vortices are strong and plane beds distally where flows converge and the shed vortices weaken where flow parallel ice block clusters develop the grounding of smaller ice blocks in the lee of the initial obstacle results in only limited shadow ridge aggradation these data provide improved insight into the controls on ice block obstacle mark sedimentary architecture and allow identification of high magnitude jokulhlaups in the sedimentary record

mechanical and geometric controls on the structural evolution of pit crater and caldera subsidence 1 pit craters and calderas are volcanic depressions produced by subsidence of a magma reservoir roof to identify how geometric and mechanical factors may influence the structural evolution of this subsidence we used two dimensional distinct element method numerical models the reservoir host rock was represented as an assemblage of bonded circular particles that interact according to elastic frictional laws varying particle and bond properties produced a range of bulk material properties characteristic of natural rock masses fracturing results when bonds break once their shear or tensile strength is exceeded the magma reservoir was represented as a region of nonbonded low friction particles withdrawal of magma was simulated by incrementally reducing the area of the reservoir particles resultant gravity driven failure and subsidence of the reservoir roof were explicitly replicated interaction of the roof s strength young s modulus thickness diameter ratio t d and the reservoir s shape yields a variety of model structures and subsidence styles in conceptual terms four end member subsidence styles developed 1 central sagging favored by low strength and low t d 2 central snapping favored by high strength low t d and a sill like reservoir shape 3 single central block favored by low to intermediate strength high young s modulus and intermediate t d and 4 multiple central blocks favored by high strength low young s modulus and high t d most model realizations incorporated some combination of each style however the models provide a geomechanical framework for understanding natural pit crater or caldera structures as at nindiri nicaragua fernandina galapagos dolomieu la reunion and miyakejima japan

subglacial and ice contact volcanism at the oraefajokull stratovolcano iceland eruptions of oraefajokull have produced mafic and silicic magmas and have taken place in both glacial and interglacial periods the geology of the volcano records the differing response of magmas of contrasting composition to interaction with ice of variable thickness and gives insight into the development of a long lived ice covered stratovolcano vatnafjall a ridge on the southeast flank of oraefajokull is the first area of the volcano to have been mapped in detail and the geological map is presented here alongside descriptions of each erupted unit the oldest units comprise pillow lavas hyaloclastite and jointed lava flows that were formed during subglacial basaltic eruptions involving abundant meltwater the products of a subsequent explosive initially phreatomagmatic subglacial rhyolite eruption were confined by ice to form a tephra pile over 200 m thick that was intruded by dense rhyolite magma towards the end of the eruption confinement by ice caused a later trachydacite lava flow to form buttresses and a steep pillar whilst some of the meltwater produced infiltrated the lava to generate red and black glassy breccias and cause localised steam explosions it is likely that much of it drained down the steep topography the most recently erupted units are subaerial basaltic lava flows the oldest of which were erupted during an interglacial period and have subsequently been partially eroded and scoured by advancing ice ice has been important in shaping the edifice by confining eruptive products to form constructional features and by later eroding parts of them to form deep valleys reconstructions of volcano ice interaction allowed the local thickness of the glacier at the time of each eruption to be estimated and demonstrates that the upper surface of the ice has varied in elevation by over 700 m

candidate volcanic and impact induced ice depressions on mars abstract we present an analysis of two concentrically fractured depressions on mars one in northern hellas and the second in galaxias fossae volumetric measurements indicate that 2 4 km3 and 0 2 km3 of material was removed in order to form the north hellas and galaxias depressions the removed material is inferred to be predominantly water ice calorimetric estimates suggest that up to 103 105 m3 of magma would have been required to melt sublimate such a volume of ice under an ice magma interaction scenario this process would lead to subsidence and cracking of the surface which could produce the observed concentric fracture crevasse like morphology while the galaxias fossae landform morphology is consistent with an impact origin the large volume of removed material in north hellas is less consistent with an impact origin and is interpreted to have resulted from volcanic melting of ice the possibility of liquid water formation during or subsequent to volcanism or an impact could generate locally enhanced habitable conditions making these features tantalizing geological and astrobiological exploration targets

constraining the boundary between the sunda and andaman subduction systems evidence from the 2002 mw 7 3 northern sumatra earthquake and aftershock relocations of the 2004 and 2005 great earthquakes the 2004 m w 9 0 sumatra andaman earthquake initiated along the andaman subduction zone north of the last great sumatra earthquake along the sunda trench in 1861 during the 2005 m w 8 7 banyak islands earthquake a portion of the 1861 rupture subsequently failed the boundary between the 2004 and 2005 ruptures broadly coincides with local trench rotation and the southern edge of the andaman microplate which suggests structural control on fault segmentation aftershock relocations of the 2004 and 2005 earthquakes show little overlap and the sharp boundary between the series locates near the 2002 m w 7 3 northern sumatra earthquake we posit that these features represent the southern extent of the stable andaman microplate 50 100 km northwest of what was previously reported broadband analyses of the 2002 earthquake yield a bilateral rupture pattern that is used to model coulomb stress changes near the 2004 hypocenter to assess stress interactions along adjacent fault segments

ilchulbong tuff cone jeju island korea revisited a compound monogenetic volcano involving multiple magma pulses shifting vents and discrete eruptive phases ilchulbong sunrise peak tuff cone is a united nations educational scientific and cultural organization unesco world heritage site that owes its scientific importance to the outstanding coastal exposures that surround it it is a type locality for the sedimentary evidence of pyroclastic transport and depositional processes that occur during phreatomagmatic basaltic eruptions its classically shaped cone morphology was long considered to result from a single short lived eruption reanalysis of the sedimentary sequence has now revealed two subtle paraconformities in the deposits indicating that the eruption occurred in three separate phases the break between phases i to ii was long i e days to weeks during which erosion and compaction of the lower cone occurred leading to a volcanowide discontinuity surface and reworked deposits in the lower flanks possibly following cooling and solidification of the initial shallow conduit system the phase ii eruption resumed from a new vent location 600 m away before being terminated by a very brief interval and final eruption of phase iii from the same general site the second pause led to a more subtle discontinuity marked by differences in deposit character above and below it and it was associated with a period of seismicity induced slumping and slipping of deposits detailed geochemical study of the juvenile clasts through the sequence reveals that three separate alkali basaltic magma pulses were erupted corresponding directly to the three eruption phases magmas 1 and 3 may be genetically related with the former showing evidence for longer periods of shallow level fractionation whereas magma 2 originated from a slightly different mantle domain although at a similar depth these results demonstrate that breaks in monogenetic eruption sequences may represent deep magma system processes and they raise fundamental questions concerning the mechanisms of triggering and eruption of multiple magma pulses in small volume basaltic volcanoes this study shows the importance of recognizing sedimentary evidence of time breaks in such eruption sequences as potential indications of shifts in eruption chemistry and vent location these have important implications for the controls of vent migration at other monogenetic volcanoes and for hazard planning during future similar eruptions

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

coseismic event of may 15 1992 huon peninsula papua new guinea comparison with quaternary tectonic history the may 15 1992 earthquake on the huon peninsula papua new guinea resulted in the uplift and large scale mortality of intertidal fringing coral reefs reduction in the highest level of survival of intertidal massive corals was used as a proxy for assessing the amount of coseismic uplift over 45 km of the huon coastline measurements were gathered from three sites one and three months after the earthquake sea level pressure gauges in place in the northern end of sialum lagoon showed not uplift but subsidence from the earthquake uplift ranged from 7 to 13 cm and subsidence ranged from 8 to 14 cm the may 15 1992 earthquake corroborates the coseismic origin of the raised reef terraces of the huon peninsula greater uplift of the land in the southeast relative to the northwest is consistent with the regional quaternary uplift pattern the close proximity of subsidence shown by sea level gauges to uplift shown by coral mortality is a manifestation of the abundant fault blocks in the area uplift rates of 3 0 5 2 m ka calculated from the earthquake are only marginally higher than previous estimates based on radiometric age dates and terrace geomorphology the first directly observed earthquake uplift event on the huon peninsula has yielded only centimetre scale coseismic uplift thus individual holocene and pleistocene terraces thought to have been the result of metre scale displacement from single earthquakes may rather have been due to successive episodes of centimetre scale uplift on constructional reef platforms the clustered history of earthquakes on the huon peninsula throughout the past 100 years indicates the complexity involved in assessment of seismic risk throughout the world

a two level model of a thermally forced ocean basin abstract some simple solutions mostly analytic are presented for the large scale baroclinic response to thermal forcing on a mid latitude beta plane surface heat flux is parameterized as ta tt tau with atmospheric temperature ta prescribed as a function of latitude varying ocean surface temperature tt and equilibration time for long times decades benthic forcing is included using a similar representation the model allows horizontal density variations at each level when there are no meridional boundaries there is only a local response to the forcing a geostrophic baroclinic zonal flow is driven by the north south temperature gradient but it has no associated advection or divergence effects this picture is greatly changed when east and or west coasts are added kelvin waves pass information rapidly about 200 km day 1 along coasts and rossby waves travel slowly offshore most effectively from the cut with speed c 1 km day 1 for spin up problems e g the response to a change in for

nloadf a program for computing ocean tide loading the loading of the earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique in order to compute these most accurately a combination of global and local models of the ocean tides may be needed the program nloadf convolves the green functions for loading with ocean tide models using a station centered grid with fixed dimensions making it easy to combine different ocean models without overlap in the convolution the program computes all the quantities of interest gravity displacement tilt and strain and includes the case where measurements are made beneath the surface of the ocean

sinkholes pit craters and small calderas analog models of depletion induced collapse analyzed by computed x ray microtomography volumetric depletion of a subsurface body commonly results in the collapse of overburden and the formation of enclosed topographic depressions such depressions are termed sinkholes in karst terrains and pit craters or collapse calderas in volcanic terrains this paper reports the first use of computed x ray microtomography mu ct to image analog models of small scale similar to 2 km diameter high cohesion overburden collapse induced by depletion of a near cylindrical stock like body time lapse radiography enabled quantitative monitoring of the evolution of collapse structure velocity and volume moreover mu ct scanning enabled nondestructive visualization of the final collapse volumes and fault geometries in three dimensions the results illustrate two end member scenarios 1 near continuous collapse into the depleting body and 2 near instantaneous collapse into a subsurface cavity formed above the depleting body even within near continuously collapsing columns subsidence rates vary spatially and temporally with incremental accelerations the highest subsidence rates occur before and immediately after a surface depression is formed in both scenarios the collapsing overburden column undergoes a marked volumetric expansion such that the volume of subsurface depletion substantially exceeds that of the resulting topographic depression in the karst context this effect is termed bulking and our results indicate that it may occur not only at the onset of collapse but also during progressive subsidence in the volcanic context bulking of magma reservoir overburden rock may at least partially explain why the volume of magma erupted commonly exceeds that of the surface depression

growth and evolution of an emergent tuff cone considerations from structural geology geomorphology and facies analysis of sao roque volcano sao miguel azores abstract the syn eruptive and post eruptive history of sao roque tuff cone its geological setting and volcanological features were studied in detail to understand the role played by the different factors that contributed to the morphological evolution of this relatively simple and small volcanic edifice in addition attention was also focused on the series of natural changes that affected the tuff cone during the course of the years and that finally led to its structural disassembly a novel model is proposed to explain this process the sao roque volcanic centre located on the island of sao miguel azores consists of two well consolidated bodies and numerous small islets that formed more than 4700 years ago during the hydromagmatic activity that took place along an intruding dyke whose nnw sse trend is in agreement with the regional tectonic pattern the eruptive vents probably migrated progressively from sse to nnw forming small edifices through the rapid accumulation of sediments during alternating phases of dry and wet magmatic emissions syn eruptive partial collapses greatly modified the original morphological structure of these edifices probably allowing sea water to continuously flow into the vents the complex interaction of these factors controlled the depth of magma fragmentation producing different types of deposits in which the ash lapilli ratio varies considerably the high water saturation degree of these deposits caused syn eruptive and post eruptive remobilization which resulted in collapses and some small scale landslides post eruptive wnw ese trending transtensional and extensional tectonic activities operated during the initial dissection of the cone generating instability furthermore the rapid accumulation of wet tephra and its following consolidation caused selective collapses that favoured the fragmentation of the deposit and caused the formation of numerous islets separated by radially arranged channels collapses also involved the lava units emplaced in more recent times around the tuff cone which show that brittle deformation has been significant in the area for a prolonged period

deposit temperature of pyroclastic density currents emplaced during the el chichon 1982 and colima 1913 eruptions new data on the pyroclastic density current pdc deposit temperature t dep are provided for two prominent eruptions of mexican volcanoes of the twentieth century the 1982 eruption of el chichon and the 1913 eruption of colima in spite of similar lithofacies magma composition and pre eruptive conditions the t dep of the pdcs from the 1982 el chichon and 1913 colima eruptions differ significantly with intervals of t dep of 360 420 c and 250 330 c respectively these new data emphasize that a full understanding of the physical mechanisms responsible for equilibrium temperature attainment within a pyroclastic deposit has not yet been realized the t dep measured for el chichon pdc deposits confirm the preliminary data published elsewhere while colima magnetic temperatures provide different values to those published previously

tephra snow and water complex sedimentary responses at an active snow capped stratovolcano ruapehu new zealand a feature of small scale explosive volcanism at stratovolcanoes is the rapid destruction of primary near vent pyroclastic deposits by sedimentary processes a protracted series of explosive eruptions of moderate volume from september 1995 until july 1996 at mount ruapehu in new zealand its largest eruptive episode this century afforded an opportunity to study these remobilisation processes in detail all significant sub plinian eruptions occurred in mid winter forming metre thick tephra accumulations on steep slopes covered with perennial ice and seasonal snow subsequent events demonstrated the variety and complexity of the erosion processes that remobilise primary pyroclasts in such a setting these processes arose from the complex interactions of tephra with snow and ice and liquid water in varying proportions and were very diverse in nature and scale their effectiveness can be gauged from the fact that there is almost no stratigraphic record of any of the 40 eruption episodes recorded in the past 100 years at ruapehu syn eruptive remobilisation processes included the generation of eruption triggered lahars by the ejection of hot water from the crater lake post eruptive interactions mainly remobilised fall deposits from proximal areas and included rain triggered lahars which were among the largest and most hazardous events with the greatest distal impacts

icelandic jokulhlaup impacts implications for ice sheet hydrology sediment transfer and geomorphology abstract glaciers and ice sheets erode entrain and deposit massive quantities of debris fluxes of subglacial meltwater exert a fundamental control on ice dynamics and sediment transport budgets within many glacial systems outburst floods jokulhlaups constitute high magnitude high frequency fluxes of meltwater relative to normal ablation controlled discharge this paper presents a synthesis of research on recent icelandic jokulhlaups and their geomorphological and sedimentary impact we identify jokulhlaup impacts within subglacial englacial and proglacial settings and discuss their wider significance for ice sheet hydrology sediment transfer and geomorphology because jokulhlaups erode deposit and re work sediment simultaneously they usually cause significant glaciological and sedimentological impacts jokulhlaups that propagate as subglacial flood waves often produce widespread hydromechanical disruption at the glacier base recent icelandic jokulhlaups have been recognised as highly efficient agents of reworking subglacial sediment and glacial sediment entrainment models of jokulhlaup impact therefore need to encompass the sub and englacial environment in addition to the proglacial zone where research has traditionally been focussed most jokulhlaups transport sediment to proglacial sandar and often directly to oceans where preservation potential of the impact is greater proglacial jokulhlaup deposits form distinctive sedimentary assemblages coupled with suites of high energy erosional landforms this study of modern jokulhlaup processes and sedimentary products may be useful for the interpretation of meltwater processes associated with quaternary ice sheets

the surface energy balance of an active ice covered volcano villarrica volcano southern chile the energy balance of bare snow and tephra covered ice near the glacier equilibrium line elevation on villarrica volcano southern chile was investigated during 2004 and 2005 combining meteorological surface temperature and ablation measurements with energy balance modelling a tephra thermal conductivity of 0 35 w m 1 k 1 and a critical tephra thickness of 0 1 m tephra windblown tephra impurities lowered snow albedo but increased snowmelt by only an estimated 0 28 m w e over the same period the net mass balance impact of supraglacial tephra at villarrica volcano is therefore positive as thick ash and lapilli mantle most of the glacier ablation zones probably reducing annual ablation by several metres w e in the accumulation seasons frequent melting events were recorded with modelled daily snowmelt rates of up to 50 mm w e

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

fractures in a trachyandesitic lava at oraefajokull iceland used to infer subglacial emplacement in 1727 8 eruption abstract we present detailed field observations of cooling fractures in a small volume trachyandesitic lava informally named the slaga lava on the south west flank of oraefajokull volcano south east iceland columnar joints pseudopillow fracture systems and curved platy jointing occur in the lava whose exposed section is approximately 600 m in length and generally 2 3 m in thickness columnar jointing may occur at the base of flow lobes whereas pseudopillow fracture systems occur throughout the lava in an outer glassy fractured carapace and curved platy fractures occur in the centres of larger flow lobes pseudopillow fracture systems composed of a single master fracture and multiple subsidiary fractures formed normal to the master fracture are of two types g type pseudopillow fracture systems have very narrow striae chisel marks on their master fractures indicative of rapid cooling and brittle fracture sr type pseudopillow fracture systems display alternating smooth and rough master fracture surface textures evidence of alternating brittle and ductile fracture propagation mechanisms subsidiary fractures in both types show curved striae on their fracture surfaces which enable the determination of fracture propagation directions pseudopillow fracture systems are thought only to form in the presence of water including water caused by the melting of ice and snow the curved platy fractures display prominent river lines and may have resulted from cooling contraction post emplacement degassing flow deflation or shearing in the flow against the outer solid crust of the flow during inflation due to recent advances in the understanding of the formation mechanisms of pseudopillow fracture systems they and the other fractures present in the flow can be used to reconstruct the cooling environment the lava is inferred to have been emplaced within subglacial drainage channels incised into or beneath a thin alpine type glacier with coolant infiltrating the lava radially from all sides and ponding or draining of water at or along the lava base we link this flow to the 1727 8 eruption of oraefajokull volcano on the basis of historical records which describe lava effusion in this approximate location and striking chemical compositional similarity between 1727 tephra and the slaga lava

formation of caldera periphery faults an experimental study changing stresses in multi stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism s associated with caldera formation and the corresponding zones of structural weakness we evaluate characteristic structures resulting from doming chamber inflation evacuation collapse chamber deflation and cyclic resurgence inflation and deflation and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation the structural dome so produced is characterised bysteepening in the periphery whereas the broadening apex subsides pure evacuation causes the chamber roof to collapse along adjacent bell shaped reverse faults the distribution of concentric faults is influenced by the initial edifice morphology steep and irregular initial flanks result in a tilted or chaotic caldera floor the third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation following doming caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome the flanks of the edifice relax resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults the latter form locally uplifted and tiltedwedges half grabens that grade into horst and graben structures this superimposed fault pattern also extends inside the caldera we suggest that major pressure deviations in magma chamber s are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first order indication of the processes and mechanisms involved in caldera formation

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

propagation linkage and interaction of caldera ring faults comparison between analogue experiments and caldera collapse at miyakejima japan in 2000 the formation of ring faults yields important implications for understanding the structural and dynamic evolution of collapse calderas and potentially associated ash flow eruptions caldera collapse occurred in 2000 at miyakejima island japan in response to a lateral intrusion based on geophysical data it is inferred that a set of caldera ring faults was propagating upward to understand the kinematics of ring fault propagation linkage and interac tion we describe new laboratory sand box experiments that were analyzed through digital image correlation dic and post processed using 2d strain analysis the results help us gain a better understanding of the processes occurring during caldera subsidence at miyakejima we show that magma chamber evacuation induces strain localization at the lateral chamber margin in the form of a set of reverse faults that sequentially develops and propagates upwards then a set of normal faults initiates from tension fractures at the surface propagating downwards to link with the reverse faults at depth with increasing amounts of subsidence interaction between the reverse and normal fault segments results in a deactivation of the reverse faults while displacement becomes focused on the outer normal faults modeling results show that the area of faulting and collapse migrates successively outward as peak displace ment transfers from the inner ring faults to later developed outer ring faults the final structural architecture of the faults bounding the subsiding piston like block is hence a consequence of the amount of subsidence in agreement with other caldera structures observed in nature the experimental simulations provide an analogy to the obser vations and seismic records of caldera collapse at miyake jima volcano but are also applicable to caldera collapse in general

plate boundary deformation associated with the great sumatra andaman earthquake the sumatra andaman earthquake of 26 december 2004 is the first giant earthquake moment magnitude m w 9 0 to have occurred since the advent of modern space based geodesy and broadband seismology it therefore provides an unprecedented opportunity to investigate the characteristics of one of these enormous and rare events here we report estimates of the ground displacement associated with this event using near field global positioning system gps surveys in northwestern sumatra combined with in situ and remote observations of the vertical motion of coral reefs these data show that the earthquake was generated by rupture of the sunda subduction megathrust over a distance of 1 500 kilometres and a width of 150 kilometres megathrust slip exceeded 20 metres offshore northern sumatra mostly at depths shallower than 30 kilometres comparison of the geodetically and seismically inferred slip distribution indicates that 30 per cent additional fault slip accrued in the 1 5 months following the 500 second long seismic rupture both seismic and aseismic slip before our re occupation of gps sites occurred on the shallow portion of the megathrust where the large aceh tsunami originated slip tapers off abruptly along strike beneath simeulue island at the southeastern edge of the rupture where the earthquake nucleated and where an m w 7 2 earthquake occurred in late 2002 this edge also abuts the northern limit of slip in the 28 march 2005 m w 8 7 nias simeulue earthquake

deformation and slip along the sunda megathrust in the great 2005 nias simeulue earthquake seismic rupture produced spectacular tectonic deformation above a 400 kilometer strip of the sunda megathrust offshore northern sumatra in march 2005 measurements from coral microatolls and global positioning system stations reveal trench parallel belts of uplift up to 3 meters high on the outer arc islands above the rupture and a 1 meter deep subsidence trough farther from the trench surface deformation reflects more than 11 meters of fault slip under the islands and a pronounced lessening of slip trenchward a saddle in megathrust slip separates the northwestern edge of the 2005 rupture from the great 2004 sumatra andaman rupture the southeastern edge abuts a predominantly aseismic section of the megathrust near the equator

origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife aggregation of airborne particles is an important way in which the atmosphere is cleansed of fine dust particles such as following explosive eruptions and meteorite impacts we identify successive stages in the growth history of particle aggregates based upon well preserved ash aggregate bearing pyroclastic layers on tenerife the layers are persistently organized into couplets made up of a lower ignimbrite layer and an upper widespread coignimbrite ash fall layer the upper part of each ignimbrite contains whole and fragmented concentric laminated accretionary lapilli whereas the overlying coignimbrite ash fall layer lacks accretionary lapilli and is composed of framework supported smaller and nonlaminated ash pellets sometimes slightly deformed or partly disaggregated the pellets resemble the cores of the larger accretionary lapilli in the underlying ignimbrite layer these field relations are repeated numerous times in several different successions and they indicate that ash pellets not accretionary lapilli form within the coignimbrite ash plumes some pellets fell directly to the ground producing coignimbrite ash fall layers but others settled into pyroclastic density currents where they accreted successive concentric laminations of fine ash as they circulated through the variously turbulent levels of the stratified current and heat of the lower part of the current dried and partly lithified them into brittle accretionary lapilli the fully formed whole and broken accretionary lapilli were then deposited from the current along with ash and pumice lapilli numerous ignimbrite veneers on tenerife have the form of ash layers a few centimeters thick that drape topography and locally contain matrix supported accretionary lapilli most volcanoes lack laterally continuous field exposure and such accretionary lapilli bearing layers might be mistaken for ash fall deposits we highlight the value of careful distinction between different types of ash aggregate facies when interpreting the origin of pyroclastic deposits for example during hazard assessments

complex circular subsidence structures in tephra deposited on large blocks of ice varða tuff cone oraefajokull iceland several broadly circular structures up to 16 m in diameter into which higher strata have sagged and locally collapsed are present in a tephra outcrop on southwest oraefajokull southern iceland the tephra was sourced in a nearby basaltic tuff cone at var a the structures have not previously been described in tuff cones and they probably formed by the melting out of large buried blocks of ice emplaced during a preceding jokulhlaup that may have been triggered by a subglacial eruption within the oraefajokull ice cap they are named ice melt subsidence structures and they are analogous to kettle holes that are commonly found in proglacial sandurs and some lahars sourced in ice clad volcanoes the internal structure is better exposed in the var a examples because of an absence of fluvial infilling and reworking and erosion of the outcrop to reveal the deeper geometry the ice melt subsidence structures at var a are a proxy for buried ice they are the only known evidence for a subglacial eruption and associated jokulhlaup that created the ice blocks the recognition of such structures elsewhere will be useful in reconstructing more complete regional volcanic histories as well as for identifying ice proximal settings during palaeoenvironmental investigations

capelinhos 1957 1958 faial azores deposits formed by an emergent surtseyan eruption the 1957 1958 eruption of capelinhos faial island azores involved three periods of surtseyan hydromagmatic activity two in 1957 and one in 1958 deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally 1 km from the vent five lithofacies are identified lithofacies i is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout lithofacies ii consists of beds with internally discontinuous lenses and has sand wave structures that increase in abundance toward the outer margins of the tuff cone this lithofacies is interpreted as having been deposited from pyroclastic surges lithofacies iii is composed of mantle bedded deposits with laterally discontinuous internal stratigraphy this lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges in the outer flanks of the tuff cone lithofacies iii grades laterally into fallout beds of lithofacies i lithofacies iv consists of alternating beds of coarse ash aggregates and non aggregated fine ash and is particularly well developed in distal regions some of this facies was formed by fallout alternating beds also occur plastered against obstacles up to 2 km from the vent indicating an origin from wet pyroclastic surges the orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated the distinction between surge and fallout in distal regions is uncertain because wind drifted fallout and decelerating surge clouds can generate similar deposits lithofacies v consists of scoria lapilli beds interpreted to be fallout from hawaiian style fire fountaining in the later stages of the eruption juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular 25 60 of clasts 30 vesicles however on both micro and macroscopic scales there is a wide range in clast vesicularity up to 70 vesicles indicating that although fragmentation was predominantly hydromagmatic vesiculation and magmatic volatile driven fragmentation operated simultaneously

relationship between caldera collapse and magma chamber withdrawal an experimental approach collapse calderas have received considerable attention due to their link to earth s ore deposits and geothermal energy resources but also because of their tremendous destructive potential although calderas have been investigated through fieldwork numerical models and experimental studies some important aspects on their formation still remain poorly understood one key issue concerns the volume of magmas involved in caldera forming eruptions we perform analogue experiments to correlate the structural evolution of a collapse with the erupted magma chamber volume fraction the experimental device consists of a transparent box 60 60 40 cm filled with dry quartz sand and a water filled latex balloon as a magma chamber analogue evacuation of water through a pipe causes a progressive deflation of the balloon that leads to a collapse of the overlying structure the experimental design allows to record the temporal evolution of the collapse and to track the evolution of fractures and faults we study the appearance and development of specific brittle structures such as surface fractures or internal reverse faults and correlate each different structure with the corresponding removed magma chamber volume fraction we also determine the critical conditions for caldera onset experimental results show that at any stage of caldera developments the experimental relationship between volume fraction and chamber roof aspect ratio fits a logarithmic curve it implies that volume fractions required to trigger caldera collapse are lower for chambers with low aspect ratios shallow and wide than for chambers with high aspect ratios deep and small these results are in agreement with natural examples and previous theoretical studies

the validation and sensitivity of a model of the icelandic ice sheet abstract the derivation and implementation of a three dimensional model used to investigate the last glacial maximum ice sheet across iceland by hubbard et al 2006 a modelling insight into the icelandic last glacial maximum ice sheet quaternary science reviews is described it is applied at 2 km resolution and requires boundary distributions of topography geothermal heat flux surface air temperature and mass balance calculated using a temperature index approach based on reference distributions of annual temperature and precipitation the model enables the variables of ice thickness stress strain and temperature to evolve freely through time and caters for the coupling of thermally triggered basal sliding with non local dynamics through the computation of longitudinal stresses it is driven through perturbations in sea level and annual precipitation and temperature a series of contemporary experiments are initiated to validate the model against the present ice cover across iceland forcing the model from ice free conditions with the 1961 1990 reference climatology yields a good simulation of all the ice masses except for vatnajokull where the model falls well short of its present margins however an experiment forced from ice free conditions for 1000 years with a 2 c cooling perturbation followed by 100 years of reference climatology yields a good simulation of vatnajokull in addition to other ice masses implying that it is a remnant icecap inherited from the little ice age and perpetuated through strong ice elevation mass balance coupling an ensemble of experiments are initiated to investigate the sensitivity of the optimum lgm model isolated in hubbard et al 2006 ice sheet volume and aspect ratio but not area are found to be sensitive to basal boundary conditions in particular to the choice of sliding parameter and the applied geothermal conditions due to strong topographic control in particular the configuration of offshore bathymetry and shelf break ice sheet volume and area is sensitive to the calving parameter and sea level change however an asymmetrical response indicates that the ice sheet is effectively decoupled from further climatic deterioration once it advances to the continental shelf break these experiments imply that there is little latitude in the selection of model parameters which yields an ice sheet compatible with the available evidence and that the optimum lgm experiment represents a sound result by inference at least 63 of the optimum lgm ice sheet was grounded below sea level implying potential instability with the onset of deglaciation

facies analysis of proximal subglacial and proglacial volcaniclastic successions at the eyjafjallajokull central volcano southern iceland

magnitude and frequency in proglacial rivers a geomorphological and sedimentological perspective abstract proglacial fluvial sedimentary systems receive water from a variety of sources and have variable discharges with a range of magnitudes and frequencies little attention has been paid to how these various magnitude and frequency regimes interact to produce a distinctive sedimentary record in modern and ancient proglacial environments this paper reviews the concept of magnitude and frequency in relation to proglacial fluvial systems from a geomorphic and sedimentary perspective rather than a hydrological or statistical perspective the nature of the meltwater inputs can be characterised as low magnitude high frequency primarily controlled by ablation inputs from the source glacier or high magnitude low frequency primarily controlled by exceptional inputs the most important high magnitude low frequency inputs are catastrophic outburst floods often referred to by the term jokulhlaup icelandic for glacier burst glacier surges are an additional form of cyclical variation impacting the proglacial environment which briefly alter the volumes and patterns of meltwater input the sedimentary consequences of low magnitude high frequency discharges are related to frequent variations in stage the greater directional variability that sediment will record and the increased significance of channel confluence sedimentation in contrast the most significant characteristics of high magnitude low frequency flooding include the presence of large flood bars and mid channel jokulhlaup bars hyperconcentrated flows large gravel dunes and the formation of ice block kettle hole structures and rip up clasts glacier surges result in a redistribution of low magnitude high frequency processes and products across the glacier margin and small floods may occur at the surge termination criteria for distinguishing magnitude and frequency regimes in the proglacial environment are identified based on these major characteristics studies of quaternary proglacial fluvial sediments are used to determine how the interaction of the various magnitude and frequency regimes might produce a distinctive sedimentary record consideration of sandur architecture and stratigraphy shows that the main controls on the sedimentary record of proglacial regions are the discharge magnitude and frequency regime sediment supply the pattern of glacier advance or retreat and proglacial topography a model of sandur development is suggested which shows how discharge magnitude and frequency in combination with sandur incision and aggradation controlled by glacier advance and retreat can control sandur stratigraphy

analogue models of collapse calderas and resurgent domes collapse calderas and resurgent domes are a common association related to inflation deflation processes in volcanic systems the structure of calderas and domes depends upon the permitted relative movements of crustal volumes at depth the so called space problem in order to study the structures of collapse calderas and resurgent domes and to take the space problem into account several analogue models were made dry quartz sand was used to simulate the rheology of the brittle crust while newtonian silicone putty located at the base of the sand pack simulated the ductile behaviour of the magma a piston moved the silicone putty downward or upward inducing collapse and doming within the sand three separate sets of experiments simulated 1 caldera collapse 2 resurgence and 3 superimposition of resurgence on collapse and vice versa collapse experiments are characterized by the development of two concentric depressions the first formed depression is bordered by outward dipping reverse ring faults the subsequent outer concentric depression is bordered by inward dipping normal ring faults the deformation pattern during resurgence is a function of the overburden thickness t and the dome diameter d for higher t d ratios a dome forms bordered by inward dipping high angle reverse ring faults outward dipping normal ring faults develop at late stages for lower t d ratios the dome shows at late stages a crestal depression accompanied by radial fractures subsequently an apical extrusion of silicone occurs the superimposition of resurgence over collapse and vice versa is characterized by the complete reactivation with opposite kinematics of all the pre existing ring faults during inversion both in caldera and resurgence reverse ring faults form in the early stages due to differential uplift extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults the experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences

sub surface structures and collapse mechanisms of summit pit craters abstract summit pit craters are found in many types of volcanoes and are generally thought to be the product of collapse into an underpressured reservoir caused by magma withdrawal we investigate the mechanisms and structures associated with summit pit crater formation by scaled analogue experiments and make comparisons with natural examples models use a sand plaster mixture as analogue rock over a cylinder of silicone simulating an underpressured magma reservoir experiments are carried out using different roof aspect ratios roof thickness roof width of 0 2 2 they reveal two basic collapse mechanisms dependant on the roof aspect ratio one occurs at low aspect ratios 1 as illustrated by aspect ratios of 0 2 and 1 outward dipping reverse faults initiated at the silicone margins propagates through the entire roof thickness and cause subsidence of a coherent block collapse along the reverse faults is accommodated by marginal flexure of the block and tension fractures at the surface aspect ratio of 0 2 or by the creation of inward dipping normal faults delimiting a terrace aspect ratio of 1 at an aspect ratio of 1 overhanging pit walls are the surface expressions of the reverse faults experiments at high aspect ratio 1 2 reveal a second mechanism in this case collapse occurs by stopping which propagates upwards by a complex pattern of both reverse faults and tension fractures the initial underground collapse is restricted to a zone above the reservoir and creates a cavity with a stable roof above it an intermediate mechanism occurs at aspect ratios of 1 1 1 2 in this case stopping leads to the formation of a cavity with a thin and unstable roof which collapses suddenly the newly formed depression then exhibits overhanging walls surface morphology and structure of natural examples such as the summit pit craters at masaya volcano nicaragua have many of the features created in the models indicating that the internal structural geometry of experiments can be applied to real examples in particular the surface area and depth of the underpressured reservoir can be roughly estimated we present a morphological analysis of summit pit craters at volcanoes such as kilimanjaro tanzania san cristobal telica and masaya nicaragua and ubinas peru and indicate a likely type of subsidence and possible position of the former magma reservoir responsible for collapse in each case