Search

Destruction of human property by lava flow invasion is a significant volcanic hazard at Mount Etna (Italy), where reliable risk maps are important for risk mitigatinn. We present new high-resolution quantitative risk maps of Mount Etna that are based on lava flow simulations starting from more than 70,000 different potential vents, a probability distribution of vent location, an empirical relationship for the maximum length of lava flows, and a database of buildings. In addition to standard risk maps, which classify areas according to the expected damage at each point, we classify each point of the volcano with respect to the damage that would occur if a vent opened at that point. The resulting maps should help local authorities in making the necessary decisions to deal with ongoing eruptions and to plan long-term land use. doi: 10.1130/G30187A.1

Kilauea may be one of the world's most intensively monitored volcanoes, but its eruptive history over the past several thousand years remains rather poorly known. Our study has revealed the vestiges of thin basaltic tephra deposits, overlooked by previous workers, that originally blanketed wide, near-summit areas and extended more than 17 km to the south coast of Hawai'i. These deposits, correlative with parts of tephra units at the summit and at sites farther north and northwest, show that Kilauea, commonly regarded as a gentle volcano, was the site of energetic pyroclastic eruptions and indicate the volcano is significantly more hazardous than previously realized. Seventeen new calibrated accelerator mass spectrometry (AMS) radiocarbon ages suggest these deposits, here named the Kulanaokuaiki Tephra, were emplaced ca. A.D. 400-1000, a time of no previously known pyroclastic activity at the volcano. Tephra correlations are based chiefly on a marker unit that contains unusually high values of Ti[O.sub.2] and [K.sub.2]O and on paleomagnetic signatures of associated lava flows, which show that the Kulanaokuaiki deposits are the time-stratigraphic equivalent of the upper part of a newly exhumed section of the Uwekahuna Ash in the volcano's northwest caldera wall. This section, thought to have been permanently buried by rockfalls in 1983, is thicker and more complete than the previously accepted type Uwekahuna at the base of the caldera wall. Collectively, these findings justify the elevation of the Uwekahuna Ash to formation status; the newly recognized Kulanaokuaiki Tephra to the south, the chief focus of this study, is defined as a member of the Uwekahuna Ash. The Kulanaokuaiki Tephra is the product of energetic pyroclastic falls; no surge- or pyroclastic-flow deposits were identified with certainty, despite recent interpretations that Uwekahuna surges extended 10-20 km from Kilauea's summit. Keywords: Kilauea volcano, pyroclastic eruptions, high Ti[O.sub.2] and [K.sub.2]O marker unit, pyroclastic surges, stratigraphy, radiocarbon ages, paleomagnetism.

The northern Antarctic Peninsula region has undergone ~10 m.y. of eruptive activity by basaltic volcanoes, mainly in subglacial settings. Spectacular exposures of lava-fed deltas, capped by basalt flows and commonly underlain by glacigenic sediments on top of a Cretaceous sedimentary 'basement,' characterize James Ross, Vega, and other islands and promontories in the region. Neogene strata are collectively known as the James Ross Island Volcanic Group and record a cryptic history of glaciation, with the timing of events determinable by argon-isotope dating. Focusing especially on the glacigenic sediments themselves, and their relationships with overlying or bounding volcanic rocks, we define facies associations related to (1) eruptions beneath thick ice (>200 m) that produced lava-fed deltas resting on, and intermingling with, diamictite; and (2) eruptions under marine conditions that typically culminated in the development of several tuff-cone successions, some on top of presumably relict glacially striated surfaces. A combination of provenance studies on clasts in the glacigenic sediments, some of which are derived from the Antarctic Peninsula, and geochronology, leads to the conclusion that an Antarctic Peninsula Ice Sheet extended over James Ross and Vega Islands at about the time that the main volcanic edifices began to grow, i.e., prior to ca. 6.2 Ma at least. Much of the subsequent development of the succession is attributed to the interaction between the growing volcanoes and local ice caps. Full resolution of glacial-interglacial events in this region promises to inform the debate about the stability of the most climatically sensitive part of the Antarctic Ice Sheet during the Neogene Period. Keywords: Neogene glacial fluctuations, James Ross Island, Antarctic Peninsula Ice Sheet, glacier-volcano interaction, glacigenic sediments, lava-fed deltas, argon dating, strontium dating.

Scoria-cone eruptions are typically low in volume and explosivity compared with eruptions from stratovolcanoes, but they can affect local populations profoundly. Scoriacone eruption effects vary dramatically due to eruption style, tephra blanket extent, climate, types of land use, the culture and complexity of the affected group, and resulting governmental action. A comparison of a historic eruption (Paricutin, Mexico) with prehistoric eruptions (herein we primarily focus on Sunset Crater in northern Arizona, USA) elucidates the controls on and effects of these variables. Long-term effects of lava flows extend little beyond the flow edges. These flows, however, can be used for defensive purposes, providing refuges from invasion for those who know them well. In arid lands, tephra blankets serve as mulches, decreasing runoff and evaporation, increasing infiltration, and regulating soil temperature. Management and retention of these scoria mulches, which can open new areas for agriculture, become a priority for farming communities. In humid areas, though, the tephra blanket may impede plant growth and increase erosion. Cultural responses to eruptions vary, from cultural collapse, through fragmentation of society, dramatic changes, and development of new technologies, to little apparent change. Eruptions may also be viewed as retribution for poor behavior, and attempts are made to mollify angry gods. Keywords: Scoria cones, geoarchaeology, volcanic risk, agriculture, Sunset Crater, Paricutin.

The volcanic hazard potential of Mount Etna volcano is currently nourished by longlasting, powerful eruptions of basaltic magmas coupled with increased seismicity and ground deformation, and the world's largest discharge of volcanic gases. The current evolutionary cycle of Mount Etna activity is consistent with subduction-related chemical modifications of the mantle source. Arrival of a new mantle-derived magma batch beneath the volcano has been hypothesized, but is still elusive among the erupted products. Here we demonstrate petrological and geochemical affinities between the magmas supplying modern eruptions and high-Mg, fallstratified (FS) basalts ejected violently ~4 k.y. ago. The FS primitive magmas (~13 wt% MgO) are characteristically volatile enriched (at least 3.8 wt% [H.sub.2]O and 3300 ppm C[O.sub.2]), and bear a trace element signature of a garuet-bearing, metasomatized source (high Gd/Yb, K/La, U/Nb, Pb/Ce, Ca/Al). They started crystallizing olivine ([Fo.sub.91]), clinopyroxene (Mg# 92.5), and Cr spinel deep in the plumbing system (>5 kbar), contributing to the cumulate piles at depth and to differentiated alkaline basalt and trachybasalt magmas in the shallow conduit. Continuous influx of mantle-derived, volatile-rich magmas, such as those that supplied the FS fallout, provides a good explanation for major compositional and eruptive features of Mount Etna. Keywords: Mount Etna, volcano, picrite, melt inclusions, volatiles, degassing, mantle, metasomatism.

A common association of thin, late Holocene Mono and Inyo crater tephra beds buried by debris flows within a 250-km-long corridor of western Nevada suggests that there has been at times a genetic link between these two otherwise independent processes. The unambiguous depositional relations and the undisturbed character of the tephra beds at more than 20 sites indicate that tephra deposition was often followed by burial under debris flows produced by intense precipitation. Considering that average return periods are hundreds of years for such storms, it is highly improbable that this stratigraphic association is random coincidence. We propose that the most plausible explanation for this association is that some Plinian ash columns produced intense thunderstorms, resulting in large debris flows that buried the tephra beds. Keywords: Plinian eruptions, Mono and Inyo Craters, tephra, debris flows.

Five Pleistocene basaltic volcanoes in Crater Flat (southern Nevada) demonstrate the complexity of eruption processes associated with small-volume basalts, and the effects of initial emplacement characteristics on posteruptive geomorphic evolution of the volcanic surfaces. The volcanoes record explosive processes ranging from 'classical' Strombolian mechanisms to, potentially, violent Strombolian mechanisms. Cone growth was accompanied and sometimes disrupted by effusion of lavas from the bases of cones. Pyroclastic cones were built upon a gently southward-sloping surface and were prone to failure of their downslope (southern) flanks. Early lavas flowed primarily southward and, at Red and Black Cone volcanoes, carried abundant rafts of cone material on the tops of the flows. These resulting early lava fields eventually built platforms such that later flows erupted from the eastern (at Red Cone) and northern (at Black Cone) bases of the cones. Three major surface features--scoria cones, lava fields with abundant rafts of pyroclastic material, and lava fields with little or no pyroclastic material--experienced different posteruptive surficial processes. Failure to account for the different initial surface features can lead to erroneous conclusions about the relative ages of the surfaces based upon modern morphology. Conversely, the recognition of different volcanic surfaces provides the opportunity to study the effects of different initial properties upon posteruptive geomorphic processes. Contrary to previous interpretations that the individual volcanoes were emplaced by polycyclic eruptions (separated by thousands or tens of thousands of years), we argue that the Pleistocene Crater Flat volcanoes are monogenetic, each having formed in a single eruptive episode lasting up to a few years. We show that all eruptive products emanated from the areas of the volcanoes' main cones rather than from scattered vents, as inferred by previous workers. These conclusions provide insight into the processes associated with continental basaltic magmatism and information related to the geometry and timing of volcanic events, which is useful in risk assessments. Keywords: scoria cone, basalt, lava flow, polycyclic, monogenetic, Strombolian, violent Strombolian.

Evaluating volcanic hazards requires knowledge of the processes that trigger eruptions and the nature and timing of geophysical signals related to these processes. One approach to addressing this need is to link geophysically observable signals (e.g., those related to seismic, aeromagnetic, inflationary, or degassing processes) to pre-eruptive magmatic events deduced (in hindsight) from studies of erupted magmas. Here we present data on sanidine crystals from the A.D. 79 eruption of Vesuvius that show abrupt changes in Ba concentration caused by magma chamber recharge events prior to eruption. These changes have been degraded by diffusion during the time interval between recharge and eruption, and we have determined the length of this time interval by modeling the measured Ba concentration gradients. The results identify three distinct recharge events in the century before the eruption, the most recent occurring ~20 yr beforehand. Keywords: magma, Vesuvius, recharge, diffusion, strontium isotopes.

Strombolian and fire fountain activities represent a common expression of explosive basaltic eruptions. However, the transition between these two eruptive styles and their source mechanisms are still debated. We use textural and compositional studies to characterize pyroclastic material from both the Strombolian and Hawaiian-style fire fountain phases of the January-June 2000 Etna activity. We find that basaltic scoria presents distinctive textural and compositional features that reflect different modes of magma vesiculation and crystallization in the two eruptive regimes. Overall, magma that forms Strombolian scoria is far more crystallized, less vesicular, and more evolved, indicating strong volatile depletion and longer residence time before being erupted. Fire fountain scoria indicates a fast-rising magma with evidence of moderate syneruptive volatile exsolution. The new textural and compositional data set is integrated with previous volcanological and geophysical investigations to provide further insights into the dynamics of fire fountains, and to frame the transition from Strombolian explosions to fire fountain activity into a model that may apply to future eruptions at Mount Etna as well as other active basaltic volcanoes. Keywords: Mt. Etna, explosive activity, scoria, textures, glass compositions, eruption dynamics.

Methane and hydrogen sulfide vent from a cold seep above a shallowly buried methane hydrate in a mud volcano located 24 km offshore southern California in 800 m of water. Bivalves, authigenic calcite, and methane hydrate were recovered in a 2.1 m piston core. Aragonite shells of two bivalve species are unusually depleted in [sup.13C] (to -19 [per thousand] [[delta].sup.13]C), the most [sup.13]C-depleted shells of marine macrofauna yet discovered. Carbon isotopes for both living and dead specimens indicate that they used, in part, carbon derived from anaerobically oxidized methane to construct their shells. The [[delta].sup.13]C values are highly variable, but most are within the range -12 [per thousand to -19 [per thousand] This variability may be diagnostic for identifying cold-seep-hydrate systems in the geologic record. Authigenic calcite is abundant in the cores down to ~1.5 m subbottom, the top of the methane hydrate. The calcite is depleted in [sup.13C] [[delta].sup.13]C = -46 [per thousand] to -58 [per thousand]), indicating that carbon produced by anaerobically oxidized methane is the main source of the calcite. Methane sources include a geologic hydrocarbon reservoir from Miocene source rocks, and biogenic and thermogenic degradation of organic matter in basin sediments. Oxygen isotopes indicate that most calcite formed out of isotopic equilibrium with ambient bottom water, under the influence of gas hydrate dissociation and strong methane flux. High metal content in the mud volcano sediment indicates leaching of basement rocks by fluid circulating along an underlying fault, which also allows for a high flux of fossil methane. Keywords: [sup.13C]-depleted bivalve shells, cold seep, methane hydrate, mud volcano, California margin basin.

The 10-km-wide caldera of the historically active Aniakchak volcano, Alaska, subsides ~13 mm/yr, based on data from 19 European Remote Sensing Satellite (ERS-1 and ERS-2) interferometric synthetic aperture radar (InSAR) images from 1992 through 2002. The pattern of subsidence does not reflect the distribution of pyroclastic deposits from the last eruption in 1931 and therefore is not related to compaction of fragmental debris. Weighted least-squares inversion of the deformation maps indicates a relatively constant subsidence rate. Modeling the deformation with a Mogi point source locates the source of subsidence at ~4 km below the central caldera floor, which is consistent with the inferred depth of magma storage before the 1931 eruption. Magmatic C[O.sub.2] and He have been measured at a warm soda spring within the caldera, and several sub-boiling fumaroles persist elsewhere in the caldera. These observations suggest that recent subsidence can be explained by the cooling or degassing of a shallow magma body (~4 km deep), and/or the reduction of the pore-fluid pressure of a cooling hydrothermal system. Ongoing deformation of the volcano detected by InSAR, in combination with magmatic gas output from at least one warm spring, and infrequent low-level bursts of seismicity below the caldera, indicate that the volcanic system is still active and requires close attention for the timely detection of possible hazards. Keywords: volcanic processes, deformation, SAR, interferometry.

The geologic map pattern of the Qiangtang terrane in central Tibet defines a >600-km-long and up to 270-kin-wide east-plunging structural culmination. It is characterized by early Mesozoic blueschist-bearing melange and upper Paleozoic strata in the core and mainly Triassic-Jurassic strata along the limbs. In the western Qiangtang terrane (~84[degrees]E), the culmination is unconformably overlain by weakly deformed mid-Cretaceous volcanic flows and tuffs. Along the Bangong suture to the south (32[degrees]N, 84[degrees]E), mid-Cretaceous nonmarine red beds and volcanic rocks lie unconformably on Jurassic suture zone rocks. These relationships demonstrate that west-central Tibet was above sea level during the mid-Cretaceous and experienced significant denudation prior to mid-Cretaceous time. Growth of the Qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early Mesozoic melange and upper Paleozoic strata during the Early Cretaceous Lhasa-Qiangtang collision. The north-south width of the inferred thrust sheet provides a minimum slip estimate of ~150 km at 84[degrees]E, decreasing eastward to -70 km at 87[degrees]E. Paleogene deformation in the Qiangtang terrane is characterized by widely distributed, mainly north-dipping thrust faults that cut Eocene-Oligocene red beds and volcanic rocks in their footwall. Along the Bangong suture, the north-dipping Shiquanhe-Gaize-Amdo thrust system cuts 64 and 43 m.y. old volcanic tuffs in its footwall and accommodated >40 km of post-mid-Cretaceous shortening. The Tertiary south-dipping Gaize-Siling Co backthrust bounds the southern margin of the Bangong suture and marks the northernmost limit of mid-Cretaceous marine strata in central Tibet. Cretaceous deformation and denudation in central Tibet is attributed to northward underthrusting of the Lhasa terrane beneath the Qiangtang terrane along the Bangong suture. This model implies that (1) Cretaceous strata along the Bangong suture and in the northern Lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the Qiangtang terrane, and (2) the central Tibetan crust was thickened substantially prior to the Indo-Asian collision. Although its magnitude is poorly known, Tertiary shortening in the Qiangtang terrane is more prevalent than in the Lhasa terrane; this difference may be attributed to the presence of underthrust melange in the deeper central Tibetan crust, which would have made it weaker than the Lhasa terrane during the Indo-Asian collision. Keywords: Tibet, plateau formation, continental collision, underthrusting, suture, shortening.

At long-lived arc volcanic centers, open-system interactions between new magma and the plutonic roots of the arc will increase with time relative to interactions with older crust. Assimilative recycling of mafic plutonic arc roots will have different isotopic consequences than incorporation of aged continental crust. In the Tatara-San Pedro complex, 30 comagmatic basaltic lavas are characterized by xenocrysts and microxenoliths of olivine, augite, and plagioclase derived from solidified cumulates, wide but poorly correlated ranges of incompatible and compatible major and trace elements, and limited isotopic variability. This mineralogical-chemical signature reflects assimilation wherein grain-boundary melting and disaggregation of xenoliths led to blending of incompatible element-enriched melts derived mainly from hornblende, phlogopite, and plagioclase in combination with variable retention of xenocrysts, hence variable but high compatible element concentrations. Keywords: crustal assimilation, arc volcanism, xenocrysts, Tatara-San Pedro, Andean Southern Volcanic Zone.

The origin of coastal and high-elevation marine gravels on the Hawaiian islands of Lanai and Molokai is controversial, because the vertical tectonics of these islands is poorly constrained. The gravels are either from eustatic highstands or were left by massive tsunamis from offshore giant landslides. In contrast, at Kohala on the island of Hawaii, where continuous subsidence is well established, lithofacies analysis and dating of a fossiliferous marine conglomerate 1.5-61 m above present sea level support a tsunami origin and indicate a runup of >400 m >6 km inland. The conglomerate age, 110 [+ or -] 10 ka, suggests a tsunami caused by the ca. 120 ka giant Alika 2 landslide from nearby Mauna Loa volcano. Keywords: landslide, tsunami, deposits, reef, subsidence, Hawaii.

Satellite remote sensing offers a convenient way to monitor changes in the thermal budgets of Earth's subaerially active volcanoes. By using data acquired by the National Aeronautics and Space Administration's Moderate Resolution Imaging Spectro-radiometer, we have calculated the amount of heat released into the atmosphere by 45 volcanoes active during 2001 and 2002, in order to quantify the contribution active volcanism makes to Earth's energy budget. We report that the amount of heat radiated into the troposphere by these volcanoes, as detected from space, was ~5.34 x [10.sup.16] and 5.30 x [10.sup.16] J/yr during 2001 and 2002, respectively. This energy flux is three orders of magnitude less than the amount of energy consumed by the United States of America for residential, manufacturing, and transportation purposes during 1999. Keywords: volcanoes, remote sensing, thermal emission, mass balance.

Volcanic-ash clouds can be fed by an upward-directed eruption column (Plinian column) or by elutriation from extensive pyroclastic flows (coignimbrite cloud). There is considerable uncertainty about which mechanism is dominant in large-scale eruptions. Here we analyze in a novel way a comprehensive grain-size database for pyroclastic deposits. We demonstrate that the Mount Pinatubo climactic eruption deposits were substantially derived from coignimbrite clouds, and not only by a Plinian cloud, as generally thought. Coignimbrite ash-fall deposits are much richer in breathable Keywords: grain size, [PM.sub.]10, specific surface area, Pinatubo, pyroclastic deposits, Plinian, coignimbrite, volcanic hazards.

Products of two mud volcanoes from the distal part of the Mediterranean Ridge accretionary complex have been investigated regarding their B, C, and O stable isotope signatures. The mud breccias have been divided into mud matrix, lithified clasts, biogenic deposits, and authigenic cements and crusts related to fluid flow and cementation. Isotope geochemistry is used to evaluate the depth of mobilization of each phase in the subduction zone. B contents and isotope ratios of the mud and mud clasts show a general trend of B enrichment and decreasing [delta][sup.11]B values with increasing consolidation (i.e., depth). However, the majority of the clast and matrix samples relate to moderate depths of mobilization within the wedge (1-2 km below seafloor). The carbonate cements of most of these clasts as well as the authigenic crusts, however, provide evidence for a deep fluid influence, probably associated with the decollement at 5-6 km depth. This interpretation is supported by decollement at 5-6 km depth. This interpretation is supported by [delta][sup.13]C ratios of the crust, which indicate precipitation of C from thermogenic methane, and by the [delta][sup.11]B ratios of pore-water samples of mud-breccia drill cores. Clams (Vesicomya sp.) living adjacent to fluid vents have [delta][sup.11]B and [delta][sup.18]O values corresponding to brines known in the area, which acted as the parent solution for shell precipitation. Such brines are most likely Miocene pore waters trapped at deep levels within the backstop to the accretionary prism, probably prior to desiccation of the Mediterranean in the Messinian (6-5 Ma). Combining all results, deep fluid circulation and expulsion are identified as the main processes triggering mud liquefaction and extrusion, whereas brines contribute only locally. Given the high B contents, mud extrusion has to be considered a major backflux mechanism of B into the hydrosphere. Keywords: boron, stable isotopes, mud volcanism, accretionary complex, Mediterranean Ridge, TIMS.

Geologic hazards such as volcanism must be assessed when evaluating potential sites for the geologic disposal of high-level radioactive waste. The Japanese islands comprise one of the more volcanically active regions on Earth and the risk of volcanism to a geologic repository is being evaluated as part of the Japanese high-level radioactive waste-disposal program. One hazard that is being evaluated as part of volcanic risk assessment is the possible intersection of a repository by a dike, if a composite volcano were to form near a repository during the long (100 k.y.) performance period of the repository. In this paper we use the characteristics of a well-exposed radial dike system at the Summer Coon volcano in Colorado to define Monte Carlo simulations that estimate the probability of a dike intersection of a repository as a function of volcano distance, dike length and density, and repository area. The models indicate that the probability of intersection declines rapidly as a function of distance from a volcano, as dike density decreases due to the radial dike geometry. The probability of intersection for a mafic dike set with shorter average length declines more rapidly than that of a silicic dike set with longer average length. However, mafic dikes have a higher probability of intersecting a repository close to a volcano ( Keywords: geologic hazards, dikes, volcanology, radioactive waste repositories, Monte Carlo analysis, probability.

A discussion is presented regarding several hypotheses for the production of a dilute surge observed in New Zealand's Ruapehu volcano. Evidence is explored for a vertically stratified lahar and for a solitary wave originating in the Whangaehu River.

Hitherto sulfur flows have been recognized as lobate features similar in form to basaltic lava flows. However, we have discovered a self-combusting sulfur-flow mode that leaves an entirely different and unexpected deposit. In this mode, the flow is emplaced in a combusting state, so that all sulfur is burned away to leave a sulfur-free, thermally eroded trough. During the 4-hr-long event we observed, combustion of 0.6 m(super 3) of sulfur generated 2.4 tons of SO(sub 2). Once under way, combusting flows do not require eruption of a molten volume to maintain activity: They can generate supply volume by melting surficial sulfur along the flow path. Combusted flow features are widespread at Vulcano, Italy, indicating that this previously unknown emplacement mode may be common. Previous failure to recognize this flow style may account for the apparent rarity of sulfur flows. Our new findings overturn conventional thought on how sulfur flows are emplaced, interpreted, and considered, and may show them to be a common volcanic feature. Keywords: Vulcano volcano, sulfur flow, flow emplacement, combustion, sulfur dioxide, thermal erosion.

Telescoping is the process of juxtaposing or overprinting early, deep mineralization, commonly of porphyry type, and late, shallow, generally epithermal styles of precious- and base-metal mineralization. Telescoping is attributed to synhydrothermal degradation of volcanic paleosurfaces, as a result of either rapid erosion under pluvial conditions or sector (and, less probably, caldera) collapse of the volcanic edifices. Paleosurfaces may be lowered easily by 1 km during the [approximately] 1 m.y. total life spans of hydrothermal systems, leading to the vertical compression of any contained ore deposits by at least 1 km. Sector collapse may be triggered by volcanic tumescence due to synmineralization intrusion, and it may be facilitated by hydrothermal weakening of volcanic edifices. Sector collapse causes extensive ingress of meteoric and/or ocean water to the magmatic environment and a decrease in confining pressure. The latter may induce hydrothermal brecciation, boiling and possible epithermal Au precipitation, and even accelerated efflux of magmatic fluids. Telescoped systems are believed to possess greater potential for the existence of both porphyry-type deposits at shallower than normal depths and giant ore deposits.

Open-channel hydraulic theory for flow of an inviscid fluid down a channel of changing slope and width yields a longtitudinal velocity profile that matches measured velocities of the August 7, 1980, pyroclastic flow at Mount St. Helens. The model is useful for prediction of zones of erosion, deposition, and hazards. It requires only topographic data, estimates of volumetric flow rate, and a Manning coefficient appropriate for the channel.

Sand-wave structures of both progressive (downstream migrating) and regressive (upstream migrating) types occur within the same pyroclastic-surge deposit at both Roccamonfina volcano (southern Italy) and Sugarloaf Mountain (Arizona, United States). Both 'wet' and 'dry' surge deposits are described that contain examples of regressive and progressive sandwave structures. At Sugarloaf Mountain progressive and regressive sand waves occur within metres of each other within the same bedset, indicating that penecontemporaneous deposition of the two different sand-wave structures occured in close proximity. On the basis of this evidence it is suggested that the migration direction of sand waves in these pyroclastic-surge deposits is controlled by the flow regime of the surge rather than the temperature and/or moisture content.

Catastrophic collapses of steep volcano flanks threaten many populated regions, and understanding factors that promote collapse could save lives and property. Large collapses of hydrothermally altered parts of Mount Rainier have generated far-traveled debris flows; future flows would threaten densely populated parts of the Puget Sound region. We evaluate edifice collapse hazards at Mount Rainier using a new three-dimensional slope stability method incorporating detailed geologic mapping and subsurface geophysical imaging to determine distributions of strong (fresh) and weak (altered) rock. Quantitative three-dimensional slope stability calculations reveal that sizeable flank collapse ([is greater than] 0.1 [km.sup.3]) is promoted by voluminous, weak, hydrothermally altered rock situated high on steep slopes. These conditions exist only on Mount Rainier's upper west slope, consistent with the Holocene debris-flow history. Widespread alteration on lower flanks or concealed in regions of gentle slope high on the edifice does not greatly facilitate collapse. Our quantitative stability assessment method can also provide useful hazard predictions using reconnaissance geologic information and is a potentially rapid and inexpensive new tool for aiding volcano hazard assessments. Keywords: landslide, volcano, debris flow, slope stability, hydrothermal alteration, Mount Rainier.

Catastrophic sector collapse occurs when a volcano becomes structurally unable to support its own load. One process particularly capable of weakening the edifice is hydrothermal activity. It can produce high pore pressures and alter strong rock to clays. Alteration can extend progressively over long periods ([is greater than] 100 yr), allowing deformation to develop slowly before collapse. An important finding is that structures produced by such deformation are recognizable and could permit collapse prediction. We present the case of Casita, Nicaragua, where hydrothermai activity has been weakening the edifice core, causing flank spreading, altering original constructional shape, and steepening flank slopes. One side is slumping outward, producing a crescentic scar with a basal bulge. We identify this feature as the site of a potential sector collapse, with conditions ripe for failure. Keywords: Casita, volcano, sector collapse, hydrothermal, gravitational spreading, Nicaragua.