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This review summarizes what the volcanology community has learned thus far from studying the deposits of pyroclastic currents (PCs) from the 1980 eruption sequence at Mount St. Helens. The review includes mass flow events during the May 18 eruption, including the lateral blast, the afternoon column collapse and boil-over PC activity, and some aspects of the debris avalanche. We also include a summary of PCs generated in the smaller eruptions following the climactic May 18 event. Our objective is to summarize the state of our understanding of PC transport and emplacement mechanisms from the combination of field and laboratory observations, granular flow experiments, and numerical modeling techniques. Specifically, we couple deposit characteristics, experiments, and numerical modeling techniques to critically address the problems of (1) constraining conditions in the flow boundary zone at the time of deposition; (2) the influence of substrate roughness and topography on PC behavior; (3) the prevalence, causes, and consequences of substrate erosion by PCs; and (4) the reconstruction of PC transportation and sedimentation processes from a combination of geophysical and sedimentological observations. We conclude by providing opportunities for future research as our field, experimental, and numerical research techniques advance.

Highlights • Tephra are abundant in NE North America, with 36 unique units deposited between ∼14,000 and the present day. • Source volcanoes are in the Cascades, Alaska, Kamchatka, Kuriles and potentially Japan. • Updated Bayesian modelled ages are presented for key proximal correlative eruptions and newly described tephra. • The tephra link paleoenvironmental records from this region to the Far East, Greenland and Europe. • Correlated source eruption volumes vary widely; this alone cannot explain recorded ash distribution trends. Lakes and bogs in northeastern North America preserve tephra deposits sourced from multiple volcanic systems in the Northern Hemisphere. However, most studies of these deposits focus on specific Holocene intervals and the latest Pleistocene, providing snapshots rather than a full picture. We combine new data with previous work, supplemented by a broad review of the characteristics and ages of potential source regions and volcanoes, to develop the first composite tephrostratigraphic framework covering the last ∼14,000 years for this region. We report new cryptotephra records from three ombrotrophic peat bogs—Irwin Smith (Michigan), Bloomingdale (New York), and Sidney Bog (Maine)—as well as new analyses and age models from previously reported sites, Nordan's Pond Bog (Newfoundland) and Thin-Ice Pond (Nova Scotia). A new tephra (Iliinsky) from the NGRIP and GRIP ice cores is also presented as it can be correlated to new data from these terrestrial records and helps validate radiocarbon age models. We identify 21 new tephra in addition to the 15 already known, several of which cover the entire region – the White River Ash east, Newberry Pumice, Ruppert (NDN-230), and Mazama. For the first time we find Mount St. Helens Yn (ca. 3660 cal yr BP) and a set P tephra (∼3000–2550 cal yr BP), and confirm the presence of Jala Pumice from Volcan Ceboruco, Mexico, and KS1 from Ksudach volcano, Kamchatka. We describe new “ultra-distal” tephra, including the early Holocene KS2 eruption, and propose correlations to volcanoes Iliinsky and Shiveluch of Kamchatka, and Ushishir of the Kurile Islands. Not all of these tephra represent large eruptions, with several plausible correlations to sub-Plinian events. Using Bayesian age-modeling, we present new age estimates for the newly described tephra, for tephra with previously poor age control, and for several proximal correlatives. Overall, we demonstrate northeastern North America's importance for providing transcontinental linkages between paleoenvironmental records and providing insights into ash distribution from different styles and sizes of eruptions.

Volcanic ash deposits (tephra) in western Canada are instrumental in providing independent chronologic control for many archaeological and paleoenvironmental sites. In Alberta, tephra are a key chronologic tool in a region where radiocarbon dates are often unreliable because of the prevalence of carbonate-rich bedrock and other “old carbon” sources, such as coal. However, many studies using tephra for age control, particularly archaeological projects, identify tephra simply through field characteristics or light microscopy. In both Alberta and British Columbia, many radiocarbon dates that were used to date key tephra deposits were bulk conventional ages on peat and lake sediments, which are not always reliable. These factors have led to uncertainty in the age and number of Bridge River and Mount St. Helens (MSH) set Y tephra present in the region and incomplete distribution maps. New major-element geochemical analyses from archaeological and sedimentary sites across south-central Alberta, complemented by new analyses of tephra from British Columbia and Saskatchewan, refine the distribution of the Bridge River, MSH Yn and Mazama tephra. New geochemical data, radiocarbon dates, and a detailed overview of proximal MSH set Y stratigraphy and geochemistry show that only one MSH layer, Yn, is present in this region, rather than two MSH set Y tephra as previously suggested. Additionally, re-assessment of age data combined with new geochemical analyses confirm that there is also only one Bridge River tephra. A Bayesian modelled age estimate is determined for MSH Yn based on new AMS dates on the tephra and vetted existing conventional ages, providing a revised age estimate for MSH Yn of 3805–3535 cal BP (mean of 3660 cal BP).

Makushin stratovolcano, Alaska, produced three, highly explosive, andesitic eruptions between ~ 9292 and 6215 yBP. Those eruptions are informally named the CFE (“crater-forming eruption”), Nateekin, and Driftwood Pumice, and they deposited significant tephra fallout in the present-day port of Dutch Harbor and City of Unalaska area. The focus of this study is to examine the geochemistry and petrology of those eruptions to better understand Makushin volcano hazards, andesite petrogenesis and eruption triggering by mafic recharge processes. The CFE, Nateekin, and Driftwood Pumice samples range from basaltic andesite to dacite but are predominantly andesitic (SiO2 = 55.6 to 63.5 wt%). The CFE deposits are slightly compositionally stratified, with the top CFE samples slightly more mafic (55 to 60 wt% SiO2) than the basal deposits (58 to 60 wt% SiO2). Disequilibrium mineral compositions and textures in the CFE, Nateekin, and Driftwood Pumice samples, combined with two pyroxene thermometry and An-rich plagioclase microlites (An80) found only in the top of the CFE deposits, provide evidence for repetitive mafic recharge triggering those eruptions, consistent with prior studies. We compare the Makushin geochemical data with data from select satellite vents and cones in the Makushin Volcanic Field (MVF) from prior studies, to examine possible genetic relationships. The geochemical data and Rhyolite-MELTS models run at crustal storage conditions (2 kbar, fO2 = Ni-NiO, and 1.5 and 3.5 wt% H2O) indicate that no single parental magma supplies the MVF satellite cones and Makushin volcano. Instead, two component mixing models better fit the MVF geochemical array. Our Makushin results compare well with models of predominantly andesitic volcanoes that require mafic recharge to mobilize the andesites and trigger eruptions.

Large explosive eruptions of silicic magma can produce widespread pumice fall, extensive ignimbrite sheets, and collapse calderas. The surfaces of voluminous ignimbrites are rarely preserved or documented because most terrestrial examples are heavily vegetated, or severely modified by post-depositional processes. Much research addresses the internal sedimentary characteristics, flow processes, and depositional mechanisms of ignimbrites, however, surface features of ignimbrites are less well documented and understood, except for comparatively small-volume deposits of historical eruptions. The ~ 7700 calendar year B.P. climactic eruption of Mount Manama, USA, vented ~ 50 km 3 of magma, deposited first as rhyodacite pumice fall and then as a zoned rhyodacite-to-andesite ignimbrite as Crater Lake caldera collapsed. Lidar collected during summer 2010 reveals the remarkably well-preserved surface of the Manama ignimbrite and related deposits surrounding Crater Lake caldera in unprecedented detail despite forest cover. The ± 1 m lateral and ± 4 cm vertical resolution lidar allows surface morphologies to be classified. Surface morphologies are created by internal depositional processes and can point to the processes at work when pyroclastic flows come to rest. We describe nine surface features including furrow-ridge sets and wedge-shaped mounds in pumice fall eroded by high-energy pyroclastic surges, flow-parallel ridges that record the passage of multiple pyroclastic flows, perched benches of marginal deposits stranded by more-mobile pyroclastic-flow cores, hummocks of dense clasts interpreted as lag deposit, transverse ridges that mark the compression and imbrication of flows as they came to rest, scarps indicating ignimbrite remobilization, fields of closely spaced pits caused by phreatic explosions, fractures and cracks due to extensional processes resulting from ignimbrite volume loss, and stream channels eroded in the newly formed surface. The nine morphologies presented here illustrate a dynamic depositional environment that varied spatially and with time during the eruption, and show that multiple processes modified the ignimbrite after deposition, both during and after the eruption.

The oldest postglacial lapilli–ash tephra recognized in sedimentary records surrounding Mount Rainier (Washington State, USA) is R tephra, a very early Holocene deposit that acts as an important stratigraphic and geochronologic marker bed. This multidisciplinary study incorporates tephrostratigraphy, radiocarbon dating, petrography, and electron microprobe analysis to characterize R tephra. Tephra samples were collected from Tipsoo Lake and a stream-cut exposure in the Cowlitz Divide area of Mount Rainier National Park. Field evidence from 25 new sites suggests that R tephra locally contains internal bedding and has a wider distribution than previously reported. Herein, we provide the first robust suite of geochemical data that characterize the tephra. Glass compositions are heterogeneous, predominantly ranging from andesite to rhyolite in ash- to lapilli-sized clasts. The mineral assemblage consists of plagioclase, orthopyroxene, clinopyroxene, and magnetite with trace apatite and ilmenite. Subaerial R tephra deposits appear more weathered in hand sample than subaqueous deposits, but weathering indices suggest negligible chemical weathering in both deposits. Statistical analysis of radiocarbon ages provides a median age for R tephra of ∼10 050 cal years BP, and a 2σ error range between 9960 and 10 130 cal years BP.

Landslides reflect landscape instability that evolves over meteorological and geological timescales, and they also pose threats to people, property, and the environment. The severity of these threats depends largely on landslide speed and travel distance, which are collectively described as landslide “mobility”. To investigate causes and effects of mobility, we focus on a disastrous landslide that occurred on 22 March 2014 near Oso, Washington, USA, following a long period of abnormally wet weather. The landslide's impacts were severe because its mobility exceeded that of prior historical landslides at the site, and also exceeded that of comparable landslides elsewhere. The ∼8×106 m3 landslide originated on a gently sloping (

Catastrophic dense granular flows, such as occur in rock avalanches, debris flows and pyroclastic flows, move as fully shearing mixtures that have approximately 60 vol.% solids and tend to segregate to form coarse-grained fronts and leveed channels. Levees restrict spreading of unconfined flows and form as coarse particles that become concentrated in the top of the flow are transported to the front and then advect to the sides in the flow head. Channels from which most material has drained away down slope are commonly lined with fine-grained deposit, widely thought to remain from the tail of the waning flow. We show how segregation in experimental dense flows of carborundum or sand (300–425 μm) mixed with spherical fine ballotini (150–250 μm), on rough slopes of 27–29°, produces fine-grained channel linings that are deposited with the levees, into which they grade laterally. Maximum runout distance is attained with mixtures containing 30–40% sand, just sufficient to segregate and form levees that are adequately robust to restrict the spreading attributable to the low-friction fines. Resin impregnation and serial sectioning of deliberately arrested experimental flows shows how fines-lined levees form from the flow head; the flows create their own stable ‘conduit’ entirely from the front, which in a geophysical context can play an important mechanistic role in facilitating runout. The flow self-organization ensures that low-friction fines at the base of the segregated channel flow shear over fine-grained substrate in the channel, thus reducing frictional energy losses. We propose that in pyroclastic flows and debris flows, which have considerable mobility attributable to pore-fluid pressures, such fine-grained flow-contact zones form similarly and not only reduce frictional energy losses but also reduce flow–substrate permeability so as to enhance pore-fluid pressure retention. Thus the granular flow self-organization that produces fine-grained channel linings can be an important factor in facilitating long runout of catastrophic geophysical flows on the low slopes (few degrees) of depositional fans and aprons around mountains and volcanoes.

Field, geochronologic, and geochemical evidence from proximal fine-grained tephras, and from limited exposures of Holocene lava flows and a small pyroclastic flow document ten–12 eruptions of Mount Rainier over the last 2,600 years, contrasting with previously published evidence for only 11–12 eruptions of the volcano for all of the Holocene. Except for the pumiceous subplinian C event of 2,200 cal year BP, the late-Holocene eruptions were weakly explosive, involving lava effusions and at least two block-and-ash pyroclastic flows. Eruptions were clustered from ∼2,600 to ∼2,200 cal year BP, an interval referred to as the Summerland eruptive period that includes the youngest lava effusion from the volcano. Thin, fine-grained tephras are the only known primary volcanic products from eruptions near 1,500 and 1,000 cal year BP, but these and earlier eruptions were penecontemporaneous with far-traveled lahars, probably created from newly erupted materials melting snow and glacial ice. The most recent magmatic eruption of Mount Rainier, documented geochemically, was the 1,000 cal year BP event. Products from a proposed eruption of Mount Rainier between AD 1820 and 1854 (X tephra of Mullineaux (US Geol Surv Bull 1326:1–83, 1974)) are redeposited C tephra, probably transported onto young moraines by snow avalanches, and do not record a nineteenth century eruption. We found no conclusive evidence for an eruption associated with the clay-rich Electron Mudflow of ∼500 cal year BP, and though rare, non-eruptive collapse of unstable edifice flanks remains as a potential hazard from Mount Rainier.

The eruption of Mount St. Helens from 2004 to 2006 has comprised extrusion of solid lava spines whose growth patterns were shaped by a large space south of the 1980-86 dome that was occupied by the unique combination of glacial ice, concealed subglacial slopes, the crater walls, and relics of previous spines. The eruption beginning September 2004 can be divided (as of April 2006) into five phases: (1) pre-dome deformation and phreatic activity, (2) initial extrusion of spines, (3) recumbent spine growth and repeated breakup, (4) southward extrusion across previous dome debris, and (5) normal faulting of the phase 4 dome to form a depression, a shift to westward extrusion and overthrusting of earlier phase 5 products. Overall, steady spine extrusion gradually slowed from 6 m 3 /s in November 2004 to 0.6 m 3 /s in February 2006. Thermal camera data show that phase 1 activity included low-temperature thermal features, such as fumaroles, fractures, and ground warming related to rapid uplift, as well as deformation in the south moat of the crater. The relatively cold (

The process of lava-dome emplacement through a glacier was observed for the first time as the 2004-6 eruption of Mount St. Helens proceeded. The glacier that had grown in the crater since the cataclysmic 1980 eruption was split in two by the new lava dome. The two parts of the glacier were successively squeezed against the crater wall. Photography, photogrammetry, and geodetic measurements document glacier deformation of an extreme variety, with strain rates of extraordinary magnitude as compared to normal temperate alpine glaciers. Unlike such glaciers, the Mount St. Helens crater glacier shows no evidence of either speed-up at the beginning of the ablation season or diurnal speed fluctuations during the ablation season. Thus there is evidently no slip of the glacier over its bed. The most reasonable explanation for this anomaly is that meltwater penetrating the glacier is captured by a thick layer of coarse rubble at the bed and then enters the volcano's groundwater system rather than flowing through a drainage network along the bed. Mechanical consideration of the glacier-squeeze process also leads to an estimate for the driving pressure applied by the growing lava dome.

A helicopter-mounted thermal imaging radiometer documented the explosive vent-clearing and effusive phases of the eruption of Mount St. Helens in 2004. A gyrostabilized gimbal controlled by a crew member housed the radiometer and an optical video camera attached to the nose of the helicopter. Since October 1, 2004, the system has provided thermal and video observations of dome growth. Flights conducted as frequently as twice daily during the initial month of the eruption monitored rapid changes in the crater and 1980-86 lava dome. Thermal monitoring decreased to several times per week once dome extrusion began. The thermal imaging system provided unique observations, including timely recognition that the early explosive phase was phreatic, location of structures controlling thermal emissions and active faults, detection of increased heat flow prior to the extrusion of lava, and recognition of new lava extrusion. The first spines, 1 and 2, were hotter when they emerged (maximum temperature 700-730°C) than subsequent spines insulated by as much as several meters of fault gouge. Temperature of gouge-covered spines was about 200°C where they emerged from the vent, and it decreased rapidly with distance from the vent. The hottest parts of these spines were as high as 500-730°C in fractured and broken-up regions. Such temperature variation needs to be accounted for in the retrieval of eruption parameters using satellite-based techniques, as such features are smaller than pixels in satellite images.

The 2004-5 eruption of Mount St. Helens, still ongoing as of this writing (September 2006), has comprised chiefly lava dome extrusion that produced a series of solid, fault-gouge-mantled dacite spines. Vertical aerial photographs taken every 2 to 4 weeks, visual observations, and oblique photographs taken from aircraft and nearby observation points provide the basis for two types of photogeologic maps of the dome—photo—based maps and rectified maps. Eight map pairs, covering the period from October 1, 2004, through December 15, 2005, document the development of seven spines: an initial small, fin-shaped vertical spine; a north-south elongate wall of dacite; two large and elongate recumbent spines ("whalebacks"); a tall and elongate inclined spine; a smaller bulbous spine; and an initially endogenous spine extruded between remnants of preceding spines. All spines rose from the same general vent area near the southern margin of the 1980s lava dome. Maps also depict translation and rotation of active and abandoned spines, progressive deformation affecting Crater Glacier, and distribution of ash on the crater floor from phreatic and phreatomagmatic explosions. The maps help track key geologic and geographic features in the rapidly changing crater and help date dome, gouge, and ash samples that are no longer readily correlated to their original context because of deformation in a dynamic environment where spines extrude, deform, slough, and are overrun by newly erupted material.

Volcanic hazards assessments at andesite stratovolcanoes rely on the assessment of frequency and magnitude of past events. The identification and correlation of proximal and distal andesitic tephra, which record the explosive eruptive history, are integral to such assessments. These tephra are potentially valuable stratigraphic marker beds useful to the temporal correlation and age dating of Quaternary volcanic, volcaniclastic and epiclastic sedimentary deposits with which they are interbedded. At Mt Ruapehu (New Zealand) and Mt Rainier (USA), much of the detail of the recent volcanic record remains unresolved because of the difficulty in identifying proximal tephra. This study investigates the value of geochemical methods in discriminating andesitic tephra. Our dataset comprises petrological and geochemical analyses of tephra that span the late Quaternary eruptive record of each volcano. Our data illustrate that andesitic tephra are remarkably heterogeneous in composition. Tephra compositions fluctuate widely over short time intervals, and there are no simple or systematic temporal trends in geochemistry within either eruptive record. This complexity in tephra geochemistry limits the application of geochemical approaches to tephrostratigraphic studies, beyond a general characterisation useful to provenance assignation. Petrological and geochemical data suggest that the products of andesite systems are inherently variable and therefore intractable to discrimination by simple geochemical methods alone. Copyright © 2006 John Wiley & Sons, Ltd.

Lahars occur during volcanic eruptions—or, less predictably, through other processes on steep volcanic terrain—when large masses of water mixed with sediment sweep down and off volcano slopes and commonly incorporate additional sediment and water. Because lahars are water-saturated, both liquid and solid interactions influence their behavior and distinguish them from other related phenomena common to volcanoes, such as debris avalanches and floods. The rock fragments carried by lahars make them especially destructive; the abundant liquid contained in them allows them to flow over gentle gradients and inundate areas far away from their sources. People in such distal areas commonly neither expect the danger nor anticipate the destructive power of lahars.

Lahars are volcanic debris flows that are dubbed primary when triggered by eruptive activity or secondary when triggered by other factors such as heavy rainfall after eruptive activity has waned. Variation in time and space of the proportion of sediment to water within a lahar dictates lahar flow phase and the resultant sedimentary character of deposits. Characteristics of source material and of debris eroded and incorporated during flow downstream may strongly affect the grain-size composition of flowing lahars and their deposits. Lahars borne on the flanks of two steep-sided stratocones in Ecuador exemplify two important lahar types. Glacier-clad Cotopaxi volcano has been a producer of primary lahars that flow great distances downstream. Such primary lahars include those of both clast-rich and matrix-rich composition—some of which have flowed as far as 325 km to the Pacific Ocean. Cotopaxi's last important eruption in 1877 generated formidable syneruptive lahars comparable in size to those that buried Armero, Colombia, following the 1985 eruption of Nevado del Ruiz volcano. In contrast, ash-producing eruptive activity during the past 15 years at Tungurahua volcano has generated a continual supply of fresh volcaniclastic debris that is regularly remobilized by precipitation. Between 2000 and 2011, 886 rain-generated lahars were registered at Tungurahua. These two volcanoes pose dramatically different hazards to nearby populations. At Tungurahua, the frequency and small sizes of lahars have resulted in effective mitigation measures. At Cotopaxi 137 years have passed since the last important lahar-producing eruption, and there is now a high-risk situation for more than 100,000 people living in downstream valleys.

The 2004-05 eruption of Mount St Helens exhibited sustained, near-equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes. Here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick-slip motion along the margins of the plug as it was forced incrementally upwards by ascending, solidifying, gas-poor magma. We formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma-plug system and that of a variably damped oscillator. Modelled stick-slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption, and they imply that magma pressure never deviated much from the steady equilibrium pressure. We infer that the volcano was probably poised in a near-eruptive equilibrium state long before the onset of the 2004-05 eruption.

A catastrophic lahar began on 30 October 1998, as hurricane precipitation triggered a small flank collapse of Casita volcano, a complex and probably dormant stratovolcano. The initial rockslide-debris avalanche evolved on the flank to yield a watery debris flood with a sediment concentration less than 60 per cent by volume at the base of the volcano. Within 2·5 km, however, the watery flow entrained (bulked) enough sediment to transform entirely to a debris flow. The debris flow, 6 km downstream and 1·2 km wide and 3 to 6 m deep, killed 2500 people, nearly the entire populations of the communities of El Porvenir and Rolando Rodriguez. These ‘new towns’ were developed in a prehistoric lahar pathway: at least three flows of similar size since 8330 14C years bp are documented by stratigraphy in the same 30-degree sector. Travel time between perception of the flow and destruction of the towns was only 2·5–3·0 minutes. The evolution of the flow wave occurred with hydraulic continuity and without pause or any extraordinary addition of water. The precipitation trigger of the Casita lahar emphasizes the need, in volcano hazard assessments, for including the potential for non-eruption-related collapse lahars with the more predictable potential of their syneruption analogues. The flow behaviour emphasizes that volcano collapses can yield not only volcanic debris avalanches with restricted runouts, but also mobile lahars that enlarge by bulking as they flow. Volumes and hence inundation areas of collapse-runout lahars can increase greatly beyond their sources: the volume of the Casita lahar bulked to at least 2·6 times the contributing volume of the flank collapse and 4·2 times that of the debris flood. At least 78 per cent of the debris flow matrix (sediment < −1·0Φ; 2 mm) was entrained during flow. Copyright © 2004 John Wiley & Sons, Ltd.

A new method of delineating lahar hazard zones in valleys that head on volcano flanks provides a rapid, objective, reproducible alternative to traditional methods. The rationale for the method derives from scaling analyses of generic lahar paths and statistical analyses of 27 lahar paths documented at nine volcanoes. Together these analyses yield semiempirical equations that predict inundated valley cross-sectional areas (A) and planimetric areas (B) as functions of lahar volume (V). The predictive equations (A = 0.05V 2/3 and B = 200V 2/3 ) provide all information necessary to calculate and plot in

The 3.8 km 3 Osceola Mudflow began as a water-saturated avalanche during phreatomagmatic eruptions at the summit of Mount Rainier about 5600 years ago. It filled valleys of the White River system north and northeast of Mount Rainier to depths of more than 100 m, flowed northward and westward more than 120 km, covered more than 200 km 2 of the Puget Sound lowland, and extended into Puget Sound. The lahar had a velocity of ≈19 m/s and peak discharge of ≈2.5×10 6 m 3 /s, 40 to 50 km downstream, and was hydraulically dammed behind a constriction. It was coeval with the Paradise lahar, which flowed down the south side of Mount Rainier, and was probably related to it genetically. Osceola Mudflow deposits comprise three facies. The axial facies forms normally graded deposits 1.5 to 25 m thick in lowlands and valley bottoms and thinner ungraded deposits in lowlands; the valley-side facies forms ungraded deposits 0.3 to 2 m thick that drape valley slopes; and the hummocky facies, interpreted before as a separate (Greenwater) lahar, forms 2–10-m-thick deposits dotted with numerous hummocks up to 20 m high and 60 m in plan. Deposits show progressive downstream improvement in sorting, increase in sand and gravel, and decrease in clay. These downstream progressions are caused by incorporation (bulking) of better sorted gravel and sand. Normally graded axial deposits show similar trends from top to bottom because of bulking. The coarse-grained basal deposits in valley bottoms are similar to deposits near inundation limits. Normal grading in deposits is best explained by incremental aggradation of a flow wave, coarser grained at its front than at its tail. The Osceola Mudflow transformed completely from debris avalanche to clay-rich (cohesive) lahar within 2 km of its source because of the presence within the preavalanche mass of large volumes of pore water and abundant weak hydrothermally altered rock. A survey of cohesive lahars suggests that the amount of hydrothermally altered rock in the preavalanche mass determines whether a debris avalanche will transform into a cohesive debris flow or remain a largely unsaturated debris avalanche. The distinction among cohesive lahar, noncohesive lahar, and debris avalanche is important in hazard assessment because cohesive lahars spread much more widely than noncohesive lahars that travel similar distances, and travel farther and spread more widely than debris avalanches of similar volume. The Osceola Mudflow is documented here as an example of a cohesive debris flow of huge size that can be used as a model for hazard analysis of similar flows.

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

Cerro Quemado is a Holocene, andesitic to dacitic, exogenous dome complex located in a back-arc volcanic field 5 km west of Quezaltenango, Guatemala. Activity in the back-arc Almolonga volcanic field began prior to 84,000 yr B.P. with growth and collapse of an andesitic stratovolcano and has continued to the present with sporadic eruption of silicic domes and flows. The Cerro Quemado dome complex covers about 12 km 2 , has a volume of nearly 2 km 3 , and comprises at least 8 vents. Historic activity is limited to extrusion of a blocky lava flow during a six-month period beginning in January 1818. Cerro Quemado lavas are biotite-bearing hornblende andesite and dacite. The calc-alkaline lavas have a disequilibrium mineral assemblage that includes: three populations of zoned plagioclase phenocrysts and microphenocrysts, hornblende, biotite, magnetite, two pyroxenes, olivine, and quartz. In addition, spherical inclusions (2 mm to 30 cm) of basaltic andesite (SiO 2 56 wt. %) are ubiquitous in Cerro Quemado lavas. this petrographic evidence points to an origin by mixing of felsic and mafic magma for Cerro Quemado. In the past, Cerro Quemado typically erupted viscous blocky lava flows (up to 120 m thick and 2.5 km long) and plug domes. The presence of tephra and pyroclastic flow deposits, however, indicates that explosive volcanism has also occurred at the dome. About 1150 yr B.P., the west flank of the dome gravitationally collapsed, producing a debris avalanche that underlies 13 km 2 of the Llano del Pinal valley, and avalanche caldera, 1 km × 1.5 km × 90 m deep. The caldera and avalanche deposit have identical volumes of approximately 0.1 km 3 . The collapse apparently precipitated a hot, laterally directed blast that swept about 40 km 2 of rugged terrain that includes Llano del Pinal valley and the east slopes of nearby Siete Orejas to an elevation of 3100 m. A pyroclastic flow overlying the fines-poor lateral blast deposit apparently resulted from remobilization and flow age of hot blast deposits off the steep slopes of Siete Orejas. Subsequently, a small Peleean dome was emplaced in the upper part of the avalanche caldera. Following the catastrophic events of 1150 yr B.P., Cerro Quemado was dormant until 1818 A.D. Geologic mapping at Cerro Quemado volcano indicates that the principal volcanic hazards includes lava flow and exogenous dome growth, pyroclastic flow, debris avalanche, lateral blast, tephra fall, and mudflow. If Cerro Quemado erupts in the future Quezaltenango and environs (population more then 125,000) will be threatened.

Late Holocene dome-building eruptions at Mount Hood during the Timberline and Old Maid eruptive periods resulted in numerous dome-collapse pyroclastic fl ows and lahars that moved large volumes of volcaniclastic sediment into temporary storage in headwater canyons of the Sandy River. During each eruptive period, accelerated sediment loading to the river through erosion and remobilization of volcanic fragmental debris resulted in very high sediment-transport rates in the Sandy River during rain- and snowmelt-induced fl oods. Large sediment loads in excess of the river’s transport capacity led to channel aggradation, channel widening, and change to a braided channel form in the lowermost reach of the river, between 61 and 87 km downstream from the volcano. The post-eruption sediment load moved as a broad bed-material wave, which in the case of the Old Maid eruption took ~2 decades to crest 83 km downstream. Maximum post-eruption aggradation levels of at least 28 and 23 m were achieved in response to Timberline and Old Maid eruptions. In each case, downstream aggradation cycles were initiated by lahars, but the bulk of the aggradation was achieved by fl sediment transport and deposition. When the high rates of sediment supply began to diminish, the river degraded, incising the channel fi lls and forming progressively lower sets of degradational terraces. A variety of debris-fl ow, hyperconcentrated-fl ow, and fl uvial (upper and lower fl ow regime) deposits record the downstream passage of the sediment waves that were initiated by these eruptions. The deposits also presage a hazard that may be faced by communities along the Sandy River when volcanic activity at Mount Hood resumes.

[1] On 29 May 2006 a large rockfall off the Mount St. Helens lava dome produced an atmospheric plume that was reported by airplane pilots to have risen to 6,000 m above sea level and interpreted to be a result of an explosive event. However, subsequent field reconnaissance found no evidence of a ballistic field, indicating that there was no explosive component. The rockfall produced complex seismic and infrasonic signals, with the latter recorded at sites 0.6 and 13.4 km from the source. An unusual, very long-period (50 s) infrasonic signal was recorded, a signal we model as the result of air displacement. Two high-frequency infrasonic signals are inferred to result from the initial contact of a rock slab with the ground and from interaction of displaced air with a depression at the base of the active lava dome.

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[1] Geophysical granular flows display complex nonlinear, nonuniform, and unsteady rheologies, depending on the volumetric grain concentration within the flow: kinetic, kinetic-collisional, and frictional. To account for the whole spectrum of granular rheologies (and hence concentrations), we have used and further developed for geophysical-atmospheric applications a multiphase computer model initially developed by U.S. Department of Energy laboratories: (Geophysical) Multiphase Flow with Interphase Exchange. As demonstrated in this manuscript, (G)MFIX can successfully simulate a large span of pyroclastic phenomena and related processes: plinian clouds, pyroclastic flows and surges, flow transformations, and depositional processes. Plinian cloud simulations agree well with the classical plume theory and historical eruptions in the upper altitude of the cloud (HT) versus mass flux diagram. At high mass flux (>107 kg/s), plinian clouds pulsate periodically with time because of the vertical propagations of acoustic-gravity waves within the clouds. The lowest undercooled temperature anomalies measured within the upper part of the column can be as low as −18 K, which agrees well with El Chichon and Mt. St. Helens eruptions. Vertical and horizontal speed profiles within the plinian cloud compare well with those inferred from simple plume models and from umbrella experiments. Pyroclastic flow and surge simulations show that both end-members are closely tight together; e.g., an initially diluted flow may generate a denser basal underflow, which will eventually outrun the expanded head of the flow. We further illustrate evidence of vertical and lateral flow transformation processes between diluted and concentrated flows, particularly laterally from a turbulent “maintained over time fluidized zone” near source. Our comprehensive granular rheological model and our simulations demonstrate that the main depositional process is mainly a progressive vertical aggradation.

The Ilopango Caldera, located 10 km east of San Salvador, has erupted voluminous silicic pyroclastics four times in the last 57,000 years. The present caldera has a quasi-rectangular shape and is fi lled by Lake Ilopango. This paper provides a detailed description of a segment of the intracaldera stratigraphy at Ilopango caldera, with emphasis on the San Agustin Block Unit. Physical volcanology, petrology, and geochemistry establish the depositional environment and eruptive conditions of the intracaldera sequence and help to model the emplacement of the San Agustin Block Unit. The intracaldera stratigraphy comprises a sequence of pyroclastic density currents, unconformably overlain by lacustrine sediments and conformably overlain by the San Agustin Block Unit. A new radiocarbon age on wood near the top of the Lacustrine Unit indicates that a lake was present ≥43,670 years ago. The intracaldera sequence displays abundant evidence of emplacement in a subaqueous environment. The San Agustin Block Unit comprises a basal Fine Ash facies and an overlying Pumice Breccia facies. The basal Fine Ash facies is a hydromagmatic layer containing pumiceous and blocky angular glass shards, aggregates of fi ne ash and phenocryst fragments, and phenocrysts with a fi ne ash coating. The overlying Pumice Breccia facies is composed of pumice clasts up to three meters in length. The pumice clasts display a series of jointing textures indicative of hot emplacement and rapid cooling. These two facies suggest an initial subaqueous explosive eruption in which a vesiculated silicic melt fragmented upon contact with the water. When the magma had degassed suffi ciently, the eruption style evolved to subaqueous dome growth that spalled quenched pumice clasts from a moderately vesiculated carapace.

Experimental investigation of granular flows containing particles of several sizes and moving down slopes shows that segregation of coarse-grained, irregularly shaped particles induces a fingering instability at the propagating front. The size-segregation mechanism involves percolation of small particles downward and a corresponding migration of large ones toward the flow surface. Large particles at the flow surface experience velocities that are greater than average so that they migrate forward and begin to collect at the flow front. In the case of dry cohesionless flows, the instability depends upon these large particles at the flow perimeter being more angular and thus more resistant to flow than the smaller rounder ones in the interior. A simple analytical model predicts the fingering instability when friction of the flow front is greater than that of the following flow. The presence of viscous liquid inhibits both size-segregation and the development of the instability. Fluidization of dry flows permits segregation of large particles to flow perimeters, thus increasing permeability and permitting a similar instability that owes its development to the dry frictional perimeter that surrounds a partly fluidized interior. (c) 1999 American Institute of Physics.

Clay-poor lahars of late Holocene age from Mount Rainier change down the White River drainage into lahar-derived fl uvial and deltaic deposits that fi lled an arm of Puget Sound between the sites of Auburn and Seattle, 110–150 km downvalley from the volcano’s summit. Lahars in the debris-fl ow phase left cobbly and bouldery deposits on the walls of valleys within 70 km of the summit. At distances of 80–110 km, transitional (hyperconcentrated) fl ows deposited pebbles and sand that coat terraces in a gorge incised into glacial drift and the mid-Holocene Osceola Mudfl ow. On the broad, level fl oor of the Kent Valley at 110–130 km, lahars in the runout or streamfl ow phase deposited mostly sand-sized particles that locally include the trunks of trees probably entrained by the fl ows. Beyond 130 km, in the Duwamish Valley of Tukwila and Seattle, laminated andestic sand derived from Mount Rainier built a delta northward across the Seattle fault. This distal facies, warped during an earthquake in A.D. 900–930, rests on estuarine mud at depths as great as 20 m. The deltaic fi lling occurred in episodes that appear to overlap in time with the lahars. As judged from radiocarbon ages of twigs and logs, at least three episodes of distal deposition postdate the Osceola Mudfl ow. One of these episodes occurred ca. 2200–2800 cal. yr B.P., and two others occurred ca. 1700–1000 cal. yr B.P. The most recent episode ended by about the time of the earthquake of A.D. 900–930. The delta’s northward march to Seattle averaged between 6 and 14 m/yr in the late Holocene.