Generation Of Pyroclastic Flows Research Articles

Using a discrete element approach to model lava dome emplacement and collapse

Lava dome collapses can lead to explosive activity and pyroclastic flow generation which makes them one of the most deadly consequences of volcanic activity. The mechanisms linked to a collapse are however still poorly understood and very few numerical models exist that investigate the actual collapse of a lava dome after emplacement. We use a discrete element method implemented in the modelling software Particle Flow Code to investigate lava dome growth, but also go further to test the stability of the dome under the following conditions: increased internal pressure; switch in extrusion direction caused by partial cooling of the dome; and extrusion of lava onto variable underlying topography. We initially show the morphology development of a growing lava dome, and how the rheological boundary between core and talus evolves throughout the lifetime of a dome and with varied solidus pressures. Through visualisation of strain accumulation within the lava dome we show superficial rockfall development due to interaction with topography, whereas large deep-seated failures occur when the dome is exposed to internal overpressures. We find that a switch in extrusion direction promotes a transition from endogenous to exogenous dome growth and leads to lava lobe formation. We demonstrate that lava dome collapse exhibits many features similar to common landslides and by investigating strain patterns within the dome, we can use numerical modelling to understand features that elude field observations.

Generation of pyroclastic flows by explosive interaction of lava flows with ice/water-saturated substrate

We describe a new type of secondary rootless phreatomagmatic explosions observed at active lava flows at volcanoes Klyuchevskoy (Russia) and Etna (Italy). The explosions occurred at considerable (up to 5 km) distances from primary volcanic vents, generally at steep (15–35°) slopes, and in places where incandescent basaltic or basaltic-andesitic lava propagated over ice/water-saturated substrate. The explosions produced high (up to 7 km) vertical ash/steam-laden clouds as well as pyroclastic flows that traveled up to 2 km downslope. Individual lobes of the pyroclastic flow deposits were up to 2 m thick, had steep lateral margins, and were composed of angular to subrounded bomb-size clasts in a poorly sorted ash–lapilli matrix. Character of the juvenile rock clasts in the pyroclastic flows (poorly vesiculated with chilled and fractured cauliflower outer surfaces) indicated their origin by explosive fragmentation of lava due to contact with external water. Non-juvenile rocks derived from the substrate of the lava flows comprised up to 75% in some of the pyroclastic flow deposits. We suggest a model where gradual heating of a water-saturated substrate under the advancing lava flow elevates pore pressure and thus reduces basal friction (in the case of frozen substrate water is initially formed by thawing of the substrate along the contact with lava). On steep slope this leads to gravitational instability and sliding of a part of the active lava flow and water-saturated substrate. The sliding lava and substrate disintegrate and intermix, triggering explosive “fuel–coolant” type interaction that produces large volume of fine-grained clastic material. Relatively cold steam-laden cloud of the phreatomagmatic explosion has limited capacity to transport upward the produced clastic material, thus part of it descends downslope in the form of pyroclastic flow. Similar explosive events were described for active lava flows of Llaima (Chile), Pavlof (Alaska), and Hekla (Iceland) indicating that this type of explosions and related hazard is common at snow/ice-clad volcanoes and sometimes happens also on fluid-saturated hydrothermally altered slopes.

Impact of the eruptive activity on glacier evolution at Popocatépetl Volcano (México) during 1994–2004

Distribution, rate and magnitude of the glacier changes caused by eruptive activity over 1994–2001 at Popocatépetl Volcano were determined. Digital-elevation models computed by scanned photographs within a digital photogrammetric workstation and the subsequent DEM comparison allowed the determination of glacial changes. The results show that drastic changes occurred in glacier geometry and morphology over 1996–2001: 53% of the glacier surface area was lost due to melting processes. The most important loss occurred over 2000–2001 when 19% of the glacier-covered area disappeared. The glacier volume losses increased over time; during 1999–2000 the largest loss was observed. However, in February 2001, the glacier showed an apparent small increase in volume. Volcanic processes disturbed the mass balance of the glacier, thereby accelerating the ablation processes. The eruptive behavior of Popocatépetl was characterized during this time by alternating periods of explosive events and low activity. Pyroclastic flow generation, ejection of incandescent material, and tephra fall affected the glacier. The tephra volume, distribution and remobilization produced considerable changes on glacier evolution. Tephra deposition on the glacier surface was the most frequent process. The irregular distribution and thickness of tephra provoked differential ablation, and tephra remobilization processes took place on the glacier surface. Together, these processes incised the glacier surface over time. Based on these results and observations, a model of glacier evolution is proposed, involving an adjustment phase, thinning phase, areal-retreat phase, and fragmentation phase. A complex interplay of factors and processes took place between eruptive activity and the glaciers at Popocatépetl Volcano. The glacier evolution and subsequent extinction were induced by the eruptive behavior over the years. While not the only process at work, eruptive activity played the primary role in accelerating retreat and as a consequence in glacier extinction.

Rainfall‐induced volcanic activity on Montserrat

Dome‐forming volcanic eruptions cyclically extrude bodies of lava over several months, which then become gravitationally unstable and collapse, generating pyroclastic flows. On 29 July 2001 extreme rainfall over Montserrat coincided with a major collapse of the Soufrière Hills lava dome. We present rainfall and seismic records that demonstrate, for the first time, a relationship between intense rainfall and lava dome collapse, with associated pyroclastic flow generation. After seven months of little rain and a period of sustained dome growth, the onset of intense rain was followed within hours by dome collapse and pyroclastic flows. The large‐scale weather system responsible for the rain was identifiable in satellite images and predicted by meteorological forecasts issued 60 hours prior to the volcanic activity. It is suggested that weather prediction of intense rainfall be incorporated with existing geophysical and geochemical measurements to improve warnings of these hazardous events.

The eruption of Soufrière Hills Volcano, Montserrat (1995-1999): overview of scientific results

AbstractThe eruption of Soufrière Hills Volcano, Montserrat (1995-1999) has displayed a wide range of volcanic phenomena: growth of an andesitic lava dome, generation of pyroclastic flows by lava dome collapse and by fountain collapse in explosive eruptions, Vulcanian and sub-Plinian explosivity with accompanying tephra fall, entrance of pyroclastic flows into the sea, sector collapse with formation of a debris avalanche and a high-velocity pyroclastic density current, and generation of lahars. New phenomena include: cyclic patterns of ground deformation linked with shallow seismicity and eruptive activity; pyroclastic flows formed by rapid sedimentation from pyroclastic surges; and an unprecedented slow escalation of eruption intensity. Magma pulsations with timescales of hours to years have been recognized. Transitions from extrusive to explosive activity were triggered by major dome collapses. Relationships between magma ascent dynamics and geophysical signals have been elucidated. Ascending water-rich andesitic magma becomes Theologically stiffened by degassing and groundmass crystallization. Large magma overpressures are consequently developed in the upper conduit, causing shallow seismicity, radial patterns of ground deformation, and cyclic pulsations of eruptive activity. Lava dome growth involved heterogeneous deformation with formation of spines and lobes along shear zones. Collapse of the pressurized dome resulted in substantial pyroclastic surges forming above pyroclastic flows. Influx of hydrous mafic magma remobilized hot crystalline igneous rocks at depths of 5 to 6 km to form crystal-rich andesitic magma and triggered the eruption.

Marginal stability of atmospheric eruption columns and pyroclastic flow generation

Explosive volcanic eruptions frequently generate fall and flow deposits simultaneously, which can be attributed to a marginally stable atmospheric column in transitional conditions between the buoyant and collapse regimes. This behavior is reproduced by laboratory experiments and numerical simulations. Ten well‐documented eruptions are used to test theoretical models of explosive eruptions. Three types of deposits, fall, flow, and composite deposits made of intercalated flow and fall units, are observed in these eruptions. Estimates of mass discharge rate and initial volatile concentration in the magma are available for each eruptive phase. Using the simple assumptions that (1) the mass fraction of gas in the mixture is equal to the initial volatile content of magma and (2) jet expansion outside the vent is unconstrained by crater dimensions, theoretical predictions are not consistent with the data. Agreement between data and theory may be achieved by appealing to imperfect degassing of pyroclasts, which lowers the gas content of the erupted mixture. The effective amount of continuous gas phase carrying pyroclasts in suspension depends on the size distribution of pyroclasts. In coarse pyroclast populations a large amount of magmatic gas remains trapped in bubbles within the pyroclasts and is not involved in the bulk volcanic flow. A new regime diagram based on estimates of the effective gas content in the erupted mixture allows good agreement with the observations. For given mass flux and initial dissolved volatile content, changes of the size distribution of pyroclasts may have a strong effect on atmospheric column behavior.

The physical volcanology of the 1600 eruption of Huaynaputina, southern Peru

Volcan Huaynaputina is a group of four vents located at 16°36'S, 70°51'W in southern Peru that produced one of the largest eruptions of historical times when ~11 km3 of magma was erupted during the period 19 February to 6 March 1600. The main eruptive vents are located at 4200 m within an erosion-modified amphitheater of a significantly older stratovolcano. The eruption proceeded in three stages. Stage I was an ~20-h sustained plinian eruption on 19–20 February that produced an extensive dacite pumice fall deposit (magma volume ~2.6 km3). Throughout medial–distal and distal parts of the dispersal area, a fine-grained plinian ashfall unit overlies the pumice fall deposit. This very widespread ash (magma volume ~6.2 km3) has been recognized in Antarctic ice cores. A short period of quiescence allowed local erosion of the uppermost stage-I deposits and was followed by renewed but intermittent explosive activity between 22 and 26 February (stage II). This activity resulted in intercalated pyroclastic flow and pumice fall deposits (~1 km3). The flow deposits are valley confined, whereas associated co-ignimbrite ash fall is found overlying the plinian ash deposit. Following another period of quiescence, vulcanian-type explosions of stage III commenced on 28 February and produced crudely bedded ash, lapilli, and bombs of dense dacite (~1 km3). Activity ceased on 6 March. Compositions erupted are predominantly high-K dacites with a phenocryst assemblage of plagioclase>hornblende>biotite>Fe–Ti oxides±apatite. Major elements are broadly similar in all three stages, but there are a few important differences. Stage-I pumice has less evolved glass compositions (~73% SiO2), lower crystal contents (17–20%), lower density (1.0–1.3 g/cm3), and phase equilibria suggest higher temperature and volatile contents. Stage-II and stage-III juvenile clasts have more evolved glass (~76% SiO2) compositions, higher crystal contents (25–35%), higher densities (up to 2.2 g/cm3), and lower temperature and volatile contents. All juvenile clasts show mineralogical evidence for thermal disequilibrium. Inflections on a plot of log thickness vs area1/2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system. The ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper-level winds, whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower-altitude wind field. Thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24–32 km and a total column height of 34–46 km. The estimated mass discharge rate for the ~20-h-long stage-I eruption is 2.4×108 kg/s and the volumetric discharge rate is ~3.6×105 m3/s. The pumice fall deposit has a dispersal index (Hildreth and Drake 1992) of 4.4, and its index of fragmentation is at least 89%, reflecting the dominant volume of fines produced. Of the 11 km3 total volume of dacite magma erupted in 1600, approximately 85% was evacuated during stage 1. The three main vents range in size from ~70 to ~400 m. Alignment of these vents and a late-stage dyke parallel to the NNW–SSE trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre-existing weaknesses. During stage I this fissure evolved into a large flared vent, vent 2, with a diameter of approximately 400 m. This vent was active throughout stage II, at the end of which a dome was emplaced within it. During stage III this dome was eviscerated forming the youngest vent in the group, vent 3. A minor extra-amphitheater vent was produced during the final event of the eruptive sequence. Recharge may have induced magma to rise away from a deep zone of magma generation and storage. Subsequently, vesiculation in the rising magma batch, possibly enhanced by interaction with an ancient hydrothermal system, triggered and fueled the sustained Plinian eruption of stage I. A lower volatile content in the stage-II and stage-III magma led to transitional column behavior and pyroclastic flow generation in stage II. Continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage III. No caldera collapse occurred because no shallow magma chamber developed beneath this volcano.

Reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits: 2. Application to the Pinatubo 1991 eruption

The granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic Plinian phase of the 1991 eruption at Pinatubo. The tephra fall deposits are composed of two units: layer C1, which corresponds to the first half of the climactic phase, and layer C2 the second half. The granulometric estimates of the expansion rate of the umbrella cloud for layers C1 and C2 are 7×1010 and 3×1010 m3/s, respectively, which agree with the observations of satellite images. These estimates indicate that the magma discharge rate decreased from 9×108 to 3×108 kg/s during the eruption. The grain‐size distribution at the top of the eruption column is characterized by depletion of coarse clasts, suggesting a distinct deceleration in the gas thrust region down to several tens of meters per second, particularly during the second half of the climactic phase. The total amount of layers C1 and C2 is estimated to be 3×1012 kg, and the very fine particles, which did not deposit in the accessible on‐land area, may occupy up to 60% of the total ejecta. The effective duration of the eruption estimated by the present methods (a few hours or less) is substantially shorter than a previous estimate from the real‐time observation based on infrasonic data (∼10 hours). This discrepancy may be attributed to the overestimation of the magma discharge rate due to the entrainment of the ambient air at the cloud top and/or to the underestimation of the total amount of the ejecta due to the effect of the contemporaneous Plinian activity and generation of pyroclastic flows. It is also suggested that the intensity of eruption was fluctuating with time so that the instantaneous observations such as cloud height in satellite images do not always show a quantitative agreement with the time‐averaged features predicted by the present methods.

A statistical study of trees damaged by the 22 November 1994 eruption of Merapi volcano (Java, Indonesia): relationships between ash-cloud surges and block-and-ash flows

On 22 November 1994, after 2 years of dome growth, Merapi volcano generated pyroclastic flows and surges. This eruption destroyed the south–southwest flank that had been intact for several decades. About 9.5 km 2 of forest were blown down. A study of tree damage, correlated with geological and eyewitness observations, is used here to reconstruct the dynamics of the pyroclastic surges and block-and-ash flows of the 22 November eruption. The main results are summarised in the form of maps showing blown-down trunk directions, and zones indicating the percentage of broken and downed trees. These maps provide evidence for various aspects of the emplacement of the 1994 flows: (1) pyroclastic flow generation from gravitational collapse of part of the active lava dome without significant initial explosion; (2) the control of topography (cliffs, hills, river channels) on damage distribution and range; (3) the genesis and decoupling of ash-cloud surges from block-and-ash flows.

The Taurewa Eruptive Episode: evidence for climactic eruptions at Ruapehu volcano, New Zealand

The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km3 of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone, where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic flow deposits). These eruptions identify a brief (<ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre.

Monitoring SO2 emission at the Soufriere Hills Volcano: Implications for changes in eruptive conditions

Correlation spectrometer measurements of sulfur dioxide (SO2) emission rates during the current eruption of the Soufriere Hills volcano, Montserrat, have contributed towards identifying different phases of volcanic activity. SO2 emission rate has increased from &lt;200 tonnes per day (td−1; &lt;2.3 kgs−1) in the early stages of dome growth to &gt;550 td−1 (&gt;6.4 kgs−1) after July 1996, with the uncertainty associated with any individual measurement ca. 30%. Significantly enhanced SO2 emission rates have been identified in association with early phreatic eruptions (800 td−1 (9.3 kgs−1)) and episodes of vigorous dome collapse and pyroclastic flow generation (900 to 1500 td−1 (10.4 to 17.4 kgs−1)). SO2 emission rate has proved a useful proxy measurement for magma production rate. Observed SO2 emission rates are significantly higher than those inferred from analyses of glass inclusions in phenocrysts, implying the existence of a S‐rich magmatic vapour phase.

Plinian eruptions at Glacier Peak and Newberry volcanoes, United States: Implications for volcanic hazards in the Cascade Range

Several fall deposits from Glacier Peak and Newberry volcanoes, both located in the Cascade Range, United States, have been studied to determine eruptive column heights, intensities, and volumes. The late Pleistocene eruptions of Glacier Peak ranged from small phreatic explosions to two Plinian eruptions that each erupted more than 1 km 3 of magma at intensities >10 8 kg/s, generating plumes with heights >30 km. At Newberry volcano, the last Plinian eruption (ca. 1300 14 C yr B.P.) had an intensity of 2.8 × 10 7 kg/s and a plume height of 18 to 21 km. About 0.1 km 3 of magma was erupted in the Plinian phase, followed by eruption of a pyroclastic flow and an obsidian lava flow. Combined with similar data from Mount St. Helens, Mount Rainier, and Mount Mazama (Crater Lake), these eruptions define the range of Plinian events that have occurred in the Cascade volcanic arc in the past 12 k.y. During this period there have been Plinian eruptions with plume heights between 11 and 55 km, intensities between 10 6 and 10 9 kg/s, and volumes between 0.01 and >5 km 3 of magma. All eruptions with intensities greater than or equal to 10 8 kg/s also produced large-volume pyroclastic flows and surges. Monitoring column height (intensity) during eruptions could help mitigate hazards because it may indicate pending generation of pyroclastic flows. In the Cascade Range, there have been at least 12 eruptions of >1 km 3 of tephra in the past 12 k.y., suggesting that eruptions of such magnitude occur about once every 1 k.y., although such frequencies vary greatly at each volcano. The volume of magma erupted in each event correlates with both column height (intensity) and magma composition, suggesting that eruptions of these volcanoes relate to the accumulation rate of magma in their reservoirs.

Eruption of Soufriere Hills Volcano in Montserrat continues

On July 18,1995, the Soufriere Hills volcano in Montserrat erupted for the first time in recorded history. The eruption began with intense fumarolic venting and phreatic explosions following 3 years of elevated seismicity. An andesite lava dome emerged on November 15, 1995, and continued to grow, with several periods of copious pyroclastic flow generation and an explosive eruption on September 17, 1996. The largest pyroclastic flows to date (on June 25, 1997) and a period of vulcanian explosions with fountain collapse (during early August 1997) indicate continued escalation of the eruption.

B22 プリニー式噴煙の挙動と火砕流発生の原因 : 樽前火山の降下火砕堆積物の粒径分布

B22 プリニー式噴煙の挙動と火砕流発生の原因 : 樽前火山の降下火砕堆積物の粒径分布

P27 溶岩ドームからの火砕流発生の条件 : 雲仙岳の場合

P27 溶岩ドームからの火砕流発生の条件 : 雲仙岳の場合

Crumbling of dacite dome lava and generation of pyroclastic flows at Unzen volcano

RECENT modelling of volcanic eruptions has shown that the efficiency of subsurface degassing of magmas determines whether magma erupts explosively or effuses quietly1,2. Slow uprise of magma is often accompanied by effective degassing, leading to the extrusion of lava flows and domes. Although lava dome extrusion is one of the less explosive modes of eruption, it is often accompanied by explosive pyroclastic activities3–5. The 1991 eruption of Unzen volcano provided an opportunity to observe at close range several types of small-scale pyroclastic flow (glowing avalanches) originating from lava domes. Most of the pyroclastic flows are of Merapi type, caused by blocks falling from a collapsing dome; others are of Pelean type originating in an explosion from the side of a dome. The lavas apparently show variable degrees of self-explosivity. We suggest that variable degrees of degassing of the magma produced a wide range of excess pore pressures in the extruded lava domes, resulting in both Merapi-type and Pelean-type pyroclastic flows from the domes.

Variations in column height and magma discharge during the May 18, 1980 eruption of Mount St. Helens

Abstract Peak eruption column heights for the B1, B2, B3 and B4 units of the May 18, 1980 fall deposit from Mount St. Helens have been determined from pumice and lithic clast sizes and models of tephra dispersal. Column heights determined from the fall deposit agree well with those determined by radar measurements. B1 and B2 units were derived from plinian activity between 0900 and about 1215 hrs. B3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs. A brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height (19 km) during deposition of B4. Variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass-balance calculations based on the volume of pyroclastic flows and coignimbrite ash. Peak magma discharge occurred during the period 1215–1630 hrs, when pyroclastic flows were generated by collapse of low fountains through the crater breach. Pyroclastic flow deposits and the widely dispersed co-ignimbrite ash account for 77% of the total erupted mass, with only 23% derived from plinian discharge. A shift in eruptive style at noon on May 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite. Increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw-up and tapping of deeper levels in the magma reservoir, as predicted by theoretical models of magma withdrawal. Return to plinian activity late in the afternoon, when magma discharge decreased, is consistent with theoretical predictions of eruption column behavior. The dominant generation of pyroclastic flows during the May 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable, convective eruption column to a collapsing one.

Plinian and co-ignimbrite tephra fall from the

A study of pyroclastic deposits from the 1815 Tambora eruption reveals two distinct phases of activity, i.e., four initial tephra falls followed by generation of pyroclastic flows and the production of major co-ignimbrite ash fall. The first explosive event produced minor ash fall from phreatomagmatic explosions (F-1 layer). The second event was a Plinian eruption (F-2) correlated to the large explosion of 5 April 1815, which produced a column height of 33 km with an eruption rate of 1.1 × 108 kg/s. The third event occurred during the lull in major activity from 5 to 10 April and produced minor ash fall (F-3). The fourth event produced a 43-km-high Plinian eruption column with an eruption rate of 2.8 × 108 kg/s during the climax of activity on 10 April. Although very energetic, the Plinian events were of short duration (2.8 h each) and total erupted volume of the early (F-1 to F-4) fall deposits is only 1.8 km3 (DRE, dense rock equivalent). An abrupt change in style of activity occurred at end of the second Plinian event with onset of pyroclastic flow and surge generation. At least seven pyroclastic flows were generated, which spread over most of the volcano and Sanggar peninsula and entered the ocean. The volume of pyroclastic flow deposits on land is 2.6 km3 DRE. Coastal exposures show that pyroclastic flows entering the sea became highly fines depleted, resulting in mass loss of about 32%, in addition to 8% glass elutriation, as indicated by component fractionation. The subaqueous pyroclastic flows have thus lost about 40% of mass compared to the original erupted mixture. Pyroclastic flows and surges from this phase of the eruption are stratigraphically equivalent to a major ash fall deposit (F-5) present beyond the flow and surge zone at 40 km from the source and in distal areas. The F-5 fall deposit forms a larger proportion of the total tephra fall with increasing distance from source and represents about 80% of the total at a distance of 90 km and 92% of the total tephra fall from the 1815 eruption. The field relations indicate that the 20-km3 (DRE) F-5 deposit is a co-ignimbrite ash fall, generated largely during entrance of pyroclastic flows into the ocean. Based on the observed 40% fines depletion and component fractionation from the flows, the large volume of the F-5 co-ignimbrite ash requires eruption of 50 km3 (DRE, 1.4 × 1014 kg) pyroclastic flows.

The intensity of plinian eruptions

Peak intensities (magma discharge rate) of 45 Pleistocene and Holocene plinian eruptions have been inferred from lithic dispersal patterns by using a theoretical model of pyroclast fallout from eruption columns. Values range over three orders of magnitude from 1.6 × 106 to 1.1 × 109 kg/s. Magnitudes (total erupted mass) also vary over about three orders of magnitude from 2.0 × 1011 to 6.8 × 1014 kg and include several large ignimbrite-forming events with associated caldera formation. Intensity is found to be positively correlated with the magnitude when total erupted mass (tephra fall, surges and pyroclastic flows) is considered. Initial plinian fall phases with intensities in excess of 2.0 × 108 kg/s typically herald the onset of major pyroclastic flow generation and subsequent caldera collapse. During eruptions of large magnitude, the transition to pyroclastic flows is likely to be the result of high intensity, whereas the generation of pyroclastic flows in small magnitude eruptions may occur more often by reduction of magmatic volatile content or some transient change in magma properties. The correlation between plinian fall intensity and total magnitude suggests that the rate of magma discharge is related to the size of the chamber being tapped. A simple model is presented to account for the variation in intensity by progressive enlargement of conduits and vents and excess pressure at the chamber roof caused by buoyant forces acting on the chamber as it resides in the crust. Both processes are fundamentally linked to the absolute size of the pre-eruption reservoir. The data suggest that sustained eruption column heights (i.e. magma discharge rates) are indicators of eventual eruption magnitude, and perhaps eruptive style, and thus are key parameters to monitor in order to assess the temporal evolution of plinian eruptions.

Generation of pyroclastic flows and surges by hot-rock avalanches from the dome of Mount St. Helens volcano, USA

Several hot-rock avalanches have occurred during the growth of the composite dome of Mount St. Helens, Washington between 1980 and 1987. One of these occurred on 9 May 1986 and produced a fan-shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic-flow deposit. Laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash-cloud surge swept beyond the limits of the pyroclastic flow. Plumes that rose 2–3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event, but no explosion occurred. All the facies observed originated from a single avalanche. Erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater, and a small flood that carried juvenile and other clasts north of the crater. A second, broadly similar event occured in October 1986. Larger events of this nature could present a significant volcanic hazard.