The roseau ash: Deep-sea tephra deposits from a major eruption on Dominica, lesser antilles arc

Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 10⁵ km² with a volume of 13 km³ (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km³ of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash.Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km³ of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment.     

A gigantic Bezymianny-type event at the beginning of modern volcan Popocatepetl

The history of volcan Popocatepetl can be divided into two main periods: the formation of a large primitive volcano — approximatively 30 km wide — on which is superimposed a modern cone (6–8 km in diameter and 1700m high). A major event of Bezymianny type marks the transition between these two dissimilar periods.The activity of the primitive volcano was essentially effusive and lasted several hundred thousands of years. The total volume of products ejected by the volcano is of the order of 500–600 km³. Its last differentiated magmas are dacitic.A gigantic debris flow (D.F.) spread on the southern side is related to the Bezymianny-type event which destroyed the summit area of the ancient edifice. An elliptical caldera ( 6.5 × 11 km wide) was formed by the landslide. Its deposits, with a typical hummocky surface, cover 300 km² for a volume of 28–30 km³. Numerous outcrops belonging to this debris flow show “slabs” of more or less fractured and dislocated rocks that come from the primitive volcano. These deposits are compared to two studied debris flows of similar extent and volume: the Mount Shasta and Colima's D.F.This eruption takes a major place in the volcanologic and magmatic history of Popocatepetl: pyroclastic products of surge-type with “laminites” and crude layers, ashflows, and pumiceous airfall layers are directly related to this event and begin the history of the modern volcano probably less than 50,000 years ago. In addition, a second andesitic and dacitic phase rose both from the central vent — forming the basis of modern Popo — and from lateral vents.The terminal cone is characterized by long periods of construction by lava flows alternating with phases of destruction, the duration of these episodes being 1000 to 2000 years. The cone is composed of two edifices: the first, volcan El Fraile, began with effusive activity and was partly destroyed by three periods of intense explosive activity. The first period occurred prior to 10.000 years B.P., the second from 10.000 to 8000 years B.P. and the third from 5000 to 3800 years B.P. Each period of destruction shows cycles producing collapsing pyroclastic flows or nuées of the St Vincent-type related to the opening of large craters, plinian air-fall deposits and minor lava flows. The second edifice, the summit Popo, produced lava flows until 1200 years B.P. and since that time, entered into an explosive period. Two cataclysmic episodes, each including major pyroclastic eruptions, occurred 1200 and 900–1000 years ago. During the Pre-Hispanic and historic times effusive activity was restricted entirely to the summit area alternating with plinian eruptions. Nevertheless, despite the quiet appearance of the volcano, the last period of pyroclastic activity which started 1200 years ago may not have ended and can be very dangerous for the nearby populations.     

Discharge of pyroclastic flows into the sea during the 1996–1998 eruptions of the Soufrière Hills volcano, Montserrat

Explosive eruptions of the Soufrière Hills volcano on the island of Montserrat in the West Indies generated pyroclastic flows that reached the sea on the east and southwest coasts between November 1995 and July 1998. Discharge of the flows produced two pyroclastic deltas off of the Tar and White River valleys. A marine geological survey was conducted in July 1998 to study the submarine extensions of both deltas. Detailed profiles of depth and sub-bottom structure were obtained using a CHIRP II/bubble pulser system. These profiles were compared with pre-eruption bathymetric data in order to identify areas of recent deposition and erosion. Deposition off the Tar and White River valleys was thickest nearest the coastline and deltas, and extended into deeper water up to 5 km from shore. The total volume of submarine pyroclastic deposits as of July 1998 was 73×10⁶ m³ DRE. Submarine pyroclastic deposits off the Tar River valley made up more than two-thirds of the total volume (55×10⁶ m³ DRE) and covered an area of approximately 5.0 km², which included the delta. The volume of submarine pyroclastic deposits in the White River area (18×10⁶ m³ DRE) is probably underestimated due to the lack of precise pre-eruption bathymetric data in areas greater than 2 km from shore. Growth of pyroclastic deltas at the mouths of the Tar and White River valleys continued to the edge of the submarine shelf where there was a steep break in slope. In the Tar River area pyroclastic material was distributed down the steep shelf break and into deeper water at least a few kilometers from shore. The material spread out radially, forming a submarine fan, where distribution was primarily controlled by bathymetry and slope.Editorial responsibility; J. Stix     

Emplacement of small-volume pyroclastic flows at Laacher See (East-Eifel,Germany)

We distinguish three eruptive units of pyroclastic flows (T1, T2, and T3; T for trass) within the late Quaternary Laacher See tephra sequence. These units differ in the chemical/mineralogical composition of the essential pyroclasts ranging from highly differentiated phonolite in T1 to mafic phonolite in T3. T1 and T2 flows were generated during Plinian phases, and T3 flows during a late Vulcanian phase. The volume of the pyroclastic flow deposits is about 0.6 km³. The lateral extent of the flows from the source vent decreases from > 10 km (T1) to < 4.5 km (T3). In the narrow valleys north of Laacher See, the total thickness of the deposits exceeds 60 m.At least 19 flow units in T1, 6 in T2, and 4 in T3 can be recognized at individual localities. Depositional cycles of 2 to 5 flow units are distinguished in the eruptive units. Thickness and internal structure of the flow units are strongly controlled by topography. Subfacies within flow units such as strongly enriched pumice and lithic concentration zones, dust layers, lapilli pipes, ground layers, and lithic breccias are all compositionally related to each other by enrichment or depletion of clasts depending on their size and density in a fluidized flow. While critical diameters of coarse-tail grading were found to mark the boundary between the coarse nonfluidized and the finer fluidized grain-size subpopulations, we document the second boundary between the fluidized and the very fine entrained subpopulations by histograms and Rosin-Rammler graphs. Grain-size distribution and composition of the fluidized middle-size subpopulations remained largely unchanged during transport.Rheological properties of the pyroclastic flows are deduced from the variations in flow-unit structure within the valleys. T1 flows are thought to have decelerated from 25 m/s at 4 km to < 15 m/s at 7 km from the vent; flow density was probably 600–900 kg/m³, and viscosity 5–50 P. The estimated yield strength of the flows of 200– > 1000 N/m² is consistent with the divergence of lithic size/distance curves from purely Newtonian models; the transport of lithics must be treated as in a Bingham fluid. The flow temperature probably decreased from T1 (300°–500°C) to T3 (<200°C).A large-scale longitudinal variation in the flow units from proximal through medial to distal facies dominantly reflects temporal changes during the progressive collapse of an eruption column. Only a small amount of fallout tephra was generated in the T1 phase of eruption. The pyroclastic flows probably formed from relatively low ash fountains rather than from high Plinian eruption columns.     

Nuées ardentes of 22 November 1994 at Merapi volcano, Java, Indonesia

Nuées ardentes associated with dome collapse on 22 November 1994, at Merapi volcano traveled to the south–southwest as far as 6.5 km, and collectively accumulated roughly 2.5–3 million cubic meters of deposits. The damaged area comprises 9.5 km² and is covered by two nuée ardente facies, a conventional “Merapi-type”, valley-fill block-and-ash flow facies and a pyroclastic surge facies. The proximal deposits reflect the accumulation of dozens of nuées ardentes, with many subsidiary flow units. The distal deposits are more simply organized, as only a few individual events reached to distances >3.5 km. The stratigraphic relationships north of Turgo hill indicate that the surge deposits are a facies of particularly mobile nuées ardentes that also deposited channeled block-and-ash flow facies. They further suggest that the surge facies beyond the channel margins correlate laterally with a finer-grained sublayer locally developed at the base of the block-and-ash flow facies. Eyewitness reports suggest that the emplacement of the block-and-ash flow facies in the distal part of the Boyong river may have followed, by a short time interval, the destruction and deposition of the surge facies at Turgo village. The stratigraphy is in accord with the eyewitness reports. The surge facies was emplaced by a dilute surge current, detached from the same dome-collapse nuée ardente that, as a separate flow unit, subsequently emplaced the distal block-and-ash deposit in the Boyong valley. The detachment occurred at higher elevations, likely at or above the slope break at about 2000 m elevation. This flow separation enabled the surge current to shortcut over the landscape and to emplace its deposit even as the block-and-ash flow continued its tortuous southward movement in the Boyong channel. Dome-collapse nuée ardente activity formed the bulk of the eruption, which was accompanied by virtually no significant vertical summit explosive activity.     

Hazard assessment at volcanic fields: the Campi Flegrei case history

Volcanological analysis of the 10 000 yr –1538 explosive activity at Campi Flegrei shows that the most common explosive eruptions are characterized by the emplacement of flow or surge deposits, originating from the interaction between magma and shallow and/or sea water. The minimum volumes of pyroclastic products range between 0.04 and 0.7 km³; the proximal areas covered by these products range from 3–4 to 40–50 km². The pyroclastic flow and surge deposits occurring inside the caldera have been strongly controlled by pre-existent morphology; because of this, the area of present Napoli city was blanketed by approximately 5 m of pyroclastic deposits, during the last 5000 yr.Previous analysis suggests that the presence of even very low topographic obstacles may influence pyroclastic density current run out such that future eruptive deposits would mainly be confined inside the caldera rim. We suggest that a future eruption at Campi Flegrei would not seriously involve the urbanized area of Napoli city located on the hills. On the contrary, the plains located on the eastern side of the caldera (Fuorigrotta, Bagnoli) would be the most damaged area.     

A geomorphological reconnaissance of the submarine part of the East Rift Zone of Kilauea Volcano,Hawaii

More than half of the intensely active East Rift Zone of Kilauea Volcano crops out underwater along the crest of the submarine Puna Ridge. I present multibeam bathmetry of the entire ridge, near-bottom photographic and sonar observations of the plunging crest of its deeper distal half, and seismic profiles across the ridge tip and the adjacent structural moat. Analysis of large-scale relief, small-scale topography, and superficial rock types indicates that the rift zone is actively propagating across the moat but is probably a superficial structure that does not penetrate the underlying oceanic crust, that its tip is covered with large lava flows erupted at high rates and is surrounded with extensive debris flow deposits, and that the axial topography at depths of 2–4 km is dominated by gaping fissures and collapse pits, showing a preponderance of intrusive rather than extrusive events. Some aspects of this central-volcano rift zone, such as its geometry at small lateral offsets, resemble those at interplate rift zones along fast-spreading mid-ocean rises, but the great contrast in lithosphere thickness results in fundamental structural differences.     

Two-dimensional kinematic and rheological modeling of the 1912 pyroclastic flow,Katmai, Alaska

Pyroclastic flow emplacement is strongly influenced by eruption column height. A surface along which kinetic energy is zero theoretically connects the loci of eruption column collapse with all coeval ignimbrite termini. This surface is reconstructed as a two-dimensional energy line for the 1912 Katmai pyroclastic flow in the Valley of Ten Thousand Smokes from mapped flow termini and the runup of the ignimbrite onto obstructions and through passes. Extrapolation of the energy line to the vicinity of the source vent at Novarupta suggests the eruption column which generated the ignimbrite eruption was approximately 425 m high. The 1912 pyroclastic flow travelled about 25 km downvalley. Empirical velocity data calculated from runup elevations and surveyed centrifugal superelevations indicate initial velocities near Novarupta were greater than 79–88 m s^–1. The flow progressively decelerated and was travelling only 2–8 m s^–1 when it crossed a moraine 16 km downvalley. The constant slope of the energy line away from Novarupta suggests the flow was systematically slowed by internal and basal friction. Using a simple physical model to calculate flow velocities and a constant kinetic friction coefficient (Heim coefficient) of 0.04 derived from the reconstructed energy line, the flow is estimated to have decelerated at an average rate of –0.16 m s^–2 and to have taken approximately 9.5 minutes to travel 25 km down the Valley of Ten Thousand Smokes. The shear strength of the flowing ignimbrite at the moraine was approximately 0.5 kPa, and its Bingham viscosity when it crossed the moraine was 3.5 × 10³ P. If the flow was Newtonian, its viscosity was 4.2 × 10³ P. Reynolds and Froude numbers at the moraine were only 41–62 and 0.84–1.04, respectively, indicating laminar, subcritical flow.     

Lithic breccias in pyroclastic flow deposits on St. Kitts,West Indies

Lithic-rich breccias are described from within a sequence of young (2000–3000 yrs B.P.) scoria and ash flow deposits erupted from Mount Misery and an older pumice and ash flow deposit (ignimbrite) on St. Kitts. Cross sections constructed through pyroclastic flow fans in well-exposed sea cliffs 4–6 km from the vent show that the lithic breccias are lensoid deposits which seem to occur as channel-shaped accumulations (up to > 20 m thick and > 150 m wide) within flow units. The best-developed example infills a deeply incised channel cut into older flow units. The coarsest lithic breccias are clast supported and fines depleted and grade laterally and vertically through finer-grained, matrix-supported breccias into scoria and ash flow deposits. Coarse scoria-concentration zones mainly occur at the tops of scoria and ash flow units but also at the bases, and gas-segregation pipes are common. The lithic breccias are a type of body-concentration deposit as they pass laterally into normal scoria and ash flow deposits and, where best developed, clearly occur above a reversely graded basal shear zone or layer. Grain-size studies indicate the lithic breccias and parent flows are strongly fines depleted and were highly fluidized. We suggest this may be a feature of many Lesser Antillean pyroclastic flows because of increased turbulence-induced fluidization resulting from a high degree of surface roughness caused by the steep (up to 40 °) irregular slopes, densely vegetated sinuous gullies of the tropical volcanoes, and ingestion and ignition of large amounts of lush vegetation. Accumulation of batches of lithics concentrated in the highly fluidized flows began at the break in slope where flows moved from gullies across hydraulic jumps onto the outer coastal flanks. The accumulations of breccias continued to move and be channelled down the central parts of the flows. Initially, on crossing onto the lower slopes, some of these flows seem to have had very powerfully erosive, nondepositional heads, and in the extreme example a deep channel as long as 1–2 km may have cut through underlying flow units at least as far as the present coastline. Much of the overriding remainder of the flow then drained away laterally. Thin, fine-grained ash flow deposits may form a marginal overbank facies to the pyroclastic flow fans.     

An ignimbrite veneer deposit: The trail-marker of a pyroclastic flow

The term “ignimbrite veneer deposit” (IVD) is proposed for a new kind of pyroclastic deposit which is found associated with, and passes laterally into, Taupo ignimbrite of valley pond type in New Zealand. It forms a thin layer mantling the landscape over 15,000 km², and is regarded as the deposit from the trailing “tail” of a pyroclastic flow, where a relaxation of shear stress favoured the deposition of the basal part of the flow. The IVD differs little in grain-size from the associated ignimbrite, but it shows a crude internal stratification attributed to the deposition of a succession of layers, one after the passage of each pulse of the pyroclastic flow. It locally contains laterally-discontinuous lenses of coarse pumice (“lee-side lenses”) on the far-vent side of topographic obstacles. In nearvent exposures the Taupo IVD shows lensoid and cross-stratified bed-forms even where it stands on a planar surface, attributed to deposition from a flow travelling at an exceedingly high velocity.An IVD can be distinguished from a poorly sorted pyroclastic fall deposit because the beds in it show more rapid lateral variations in thickness, it may show a low-angle cross-stratification, and it contains carbonised wood from trees not in the position of growth; from the deposit of a wet base surge because it lacks vesicles and strong antidune-like structures and contains carbonised vegetation, and from a hot and dry pyroclastic surge deposit because it possesses a high content of pumice and “fines”.The significance of an IVD is that it records the passage of a pyroclastic flow, where the flow itself has moved farther on.     

An extremely large magnitude eruption close to the Plio-Pleistocene boundary: reconstruction of eruptive style and history of the Ebisutoge-Fukuda tephra, central Japan

An extremely large magnitude eruption of the Ebisutoge-Fukuda tephra, close to the Plio-Pleistocene boundary, central Japan, spread volcanic materials widely more than 290,000 km² reaching more than 300 km from the probable source. Characteristics of the distal air-fall ash (>150 km away from the vent) and proximal pyroclastic deposits are clarified to constrain the eruptive style, history, and magnitude of the Ebisutoge-Fukuda eruption.Eruptive history had five phases. Phase 1 is phreatoplinian eruption producing >105 km³ of volcanic materials. Phases 2 and 3 are plinian eruption and transition to pyroclastic flow. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations. In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic flow in all directions and resulted in more than 250–350 km³ of deposits. Thus, the total volume of this tephra amounts over 380–490 km³. This indicates that the Volcanic Explosivity Index (VEI) of the Ebisutoge-Fukuda tephra is greater than 7. The huge thickness of reworked volcaniclastic deposits overlying the fall units also attests to the tremendous volume of eruptive materials of this tephra.Numerous ancient tephra layers with large volume have been reported worldwide, but sources and eruptive history are often unknown and difficult to determine. Comparison of distal air-fall ashes with proximal pyroclastic deposits revealed eruption style, history and magnitude of the Ebisutoge-Fukuda tephra. Hence, recognition of the Ebisutoge-Fukuda tephra, is useful for understanding the volcanic activity during the Pliocene to Pleistocene, is important as a boundary marker bed, and can be used to interpret the global environmental and climatic impact of large magnitude eruptions in the past.     

Pyroclastic flows and surges over water: an example from the 1883 Krakatau eruption

Pyroclastic deposits from the 1883 eruption of Krakatau are described from areas northeast of the volcano on the islands of Sebesi, Sebuku, and Lagoendi, and the southeast coast of Sumatra. Massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km. Granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the Krakatau islands. The distal deposits exhibit a decrease in sorting coefficient, median grain size, and thickness with increasing distance from Krakatau. Crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards. The deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 August at approximately 10 a.m. Energetic flows spread out away from the volcano at speeds in excess of 100 km/h and traveled up to 80 km from source. The flows retained temperatures high enough to burn victims on the SW coast of Sumatra. Historical accounts from ships in the Sunda Straits constrain the area affected by the flows to a minimum of 4x10³ km². At the distal edge of this area the flows were relatively dilute and turbulent, yet carried enough material to deposit several tens of centimeters of tephra. The great mobility of the Krakatau flows from the 10 a.m. activity may be the result of enhanced runout over the sea. It is proposed that the generation of steam at the flow/seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective. The buoyant distal edge of these ash-and steam-laden clouds lifted off into the atmosphere, leading to cooling, condensation, and mud rain.     

Submarine pyroclastic deposits formed during the 20th May 2006 dome collapse of the Soufrière Hills Volcano, Montserrat

The 20th May 2006 lava dome collapse of the Soufrière Hills Volcano, Montserrat, had a total non-dense rock equivalent (non-DRE) collapse volume of approximately 115?×?10⁶?m³. The majority of this volume was deposited into the ocean. The collapse was rapid, 85% of the mobilized volume being removed in just 35?min, giving peak pyroclastic flow flux of 66?×?10³?m³?s^?1. Channel and levee facies on the submarine flanks of the volcano and formation of a thick, steep-sided ridge, suggest that the largest and most dense blocks were transported proximally as a high concentration granular flow. Of the submerged volume, 30% was deposited from the base of this granular flow, forming a linear, high-relief ridge that extends 7?km from shore. The remaining 70% of the submerged volume comprises the finer grain sizes, which were transported at least 40?km by turbidity currents on gradients of <2°. At several localities, the May 2006 distal turbidity currents ran up 200?m of topography and eroded up to 20?cm of underlying substrate. Multiple turbidites are preserved, representing current reflection from the graben margins and deflection around topography. The high energy of the May 2006 collapse resulted in longer submarine run out than the larger (210?×?10⁶?m³) Soufrière Hills dome collapse in July 2003.     

Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado

The Pagosa Peak Dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous Fish Canyon Tuff (5000 km³) from the La Garita caldera at 28 Ma. The Pagosa Peak Dacite is thick (to 1 km), voluminous (>200 km³), and has a high aspect ratio (1:50) similar to those of silicic lava flows. It contains a high proportion (40–60%) of juvenile clasts (to 3–4 m) emplaced as viscous magma that was less vesiculated than typical pumice. Accidental lithic fragments are absent above the basal 5–10% of the unit. Thick densely welded proximal deposits flowed rheomorphically due to gravitational spreading, despite the very high viscosity of the crystal-rich magma, resulting in a macroscopic appearance similar to flow-layered silicic lava. Although it is a separate depositional unit, the Pagosa Peak Dacite is indistinguishable from the overlying Fish Canyon Tuff in bulk-rock chemistry, phenocryst compositions, and ⁴⁰Ar/³⁹Ar age.The unusual characteristics of this deposit are interpreted as consequences of eruption by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows. The inferred eruptive style may be in part related to synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. The Pagosa Peak eruptive sources are apparently buried in the southern La Garita caldera, where northerly extensions of observed syneruptive faults served as fissure vents. Cumulative vent cross-sections were large, leading to relatively low emission velocities for a given discharge rate. Many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow. Explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits, leading to fracture-dominated magma fragmentation at low vesicularity. Subsequent collapse of the 75×35 km² La Garita caldera and eruption of the Fish Canyon Tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the Pagosa Peak eruption.     

Physical volcanology of the submarine Mariana and Volcano Arcs

Narrow-beam maps, selected dredge samplings, and surveys of the Mariana and Volcano Arcs identify 42 submarine volcanos. Observed activity and sample characteristics indicate 22 of these to be active or dormant. Edifices in the Volcano Arc are larger than most of the Mariana Arc edifices, more irregularly shaped with numerous subsidiary cones, and regularly spaced at 50–70 km. Volcanos in the Mariana Arc tend to be simple cones. Sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110° counterclockwise from that trend, suggesting a strong fault control on the distribution of arc magmas. Volcanos in the Mariana Arc are generally developed west of the frontal arc ridge, on rifted frontal arc crust or new back-arc basin crust. Volcanos in the central Mariana Arc are usually subaerial, large (> 500 km³), and spaced about 50–70 km apart. Those in the northern and southern Marianas are largely submarine, closer together, and generally less than 500 km³ in volume. There is a shoaling of the arc basement around Iwo Jima, accompanied by the appearance of incompatible-element enriched lavas with alkalic affinities. The larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos. If the magma supply (estimated at 10–20 km³/km of arc per million years at 18° N) has been relatively constant along the Mariana Arc, we can infer a possible evolutionary sequence for arc volcanos from small, irregularly spaced edifices to large (over 1000 km³) edifices spaced at 50–70 km. The volcano distribution and basal depths are consistent with the hypothesis of back-arc propagation into the Volcano Arc.     

Implications of long-term changes in valley geomorphology on the behavior of small-volume pyroclastic flows

Stratigraphic mapping in the lower 3km of the Vazcún Valley on the NE flank of Volcán Tungurahua (Ecuador) provides insight into the effects of long-term geomorphic changes on pyroclastic flow behavior. Exposures of deposits in the Vazcún Valley record activity over the last 2000years, during which time significant changes in the geomorphology of the valley have occurred. Two sets of terraces are present in the lower 2–3km of the valley, the older of which grades into a small debris fan at the mouth of the valley. Each terrace formed during a period of frequent activity that was separated by a long period of quiescence during which the Río Vazcún eroded a channel as deep as 40m reaching the previous base level. The pyroclastic flows from historical eruptions appear to have been largely contained within the channel that is cut through the higher terraces and debris fan. Their surface forms the lower terrace located upstream from the head of the debris fan. Thin pyroclastic deposits exposed within the city of Baños are mostly related to ash cloud surges that detached from the main flows as they slowed down within the channel. The lower reach of the present channel of the Rio Vazcún is very sinuous and deeply incised into the two sets of terraces. The winding channel would severely impede the mobility of future pyroclastic flows resulting in the deposition of thick deposits. Detachment of an overriding ash cloud surge could also occur in this region. Such a surge could be more likely to surmount the channel banks and travel over the surface of the terraces and debris for up to several kilometers from the channel.     

Transport and deposition of pyroclastic material from the ∼1000 A.D. caldera-forming eruption of Volcán Ceboruco, Nayarit, Mexico

The complex eruption sequence from the ∼1000 A.D. caldera-forming eruption of Volcán Ceboruco, known as the Jala Pumice, offers an exceptional opportunity to examine how pyroclastic material is transported and deposited from pyroclastic density currents over variable topography. Three main pyroclastic surge deposits (S1, S2, and S3) and two pyroclastic flow deposits (Marquesado and North-Flank PFDs) were emplaced during this eruption. Pyroclastic surge deposits are massive, planar, or cross-bedded, poor-to-well sorted, and display fluctuations in thickness, median diameter, sorting, and lithology as a function of distance, topography, and flow dynamics. Marquesado pyroclastic flow deposits reveal lateral variations from massive, poorly sorted deposits located within 5 km of Ceboruco to planar bedded, moderately well sorted deposits located >15 km away over the nearly horizontal topography to the south of Ceboruco. North-Flank pyroclastic flow deposits also reveal lateral variations from massive, poorly sorted deposits located within 4 km of Ceboruco to planar bedded, moderately well sorted deposits located 8 km away atop an escarpment that steeply rises 230 m from the northern valley floor. Field observations, granulometric analyses, component analyses, and crystal sedimentation calculations along flow-parallel sampling transects all suggest that both surges and flows were density stratified currents, where deposition occurred from a basal region of higher particle concentration that was supplied from an overlying dilute layer that transports particles in suspension. This supports the idea of a transition between “flow” and “surge” end members with variations in particle concentration. Topography greatly affects the transport and depositional capacity of the pyroclastic density currents as a result of “blocking”, either by topographic obstacles or by abrupt breaks at the base of volcano slopes, whereas the origin of Jala Pumice surge deposits (phreatomagmatic versus magmatic) appears to have little impact on their flow dynamics. Editorial responsibility: A.W. Woods This revised version was published in February 2005 with corrections to the title. An erratum to this article is available at .     

Proximal stratigraphy and syn-eruptive faulting in rhyolitic Grants Ridge Tuff, New Mexico, USA

Proximal deposits of the 3.3 Ma Grants Ridge Tuff, part of a 5-km³ topaz rhyolite sequence, are composed of basal pyroclastic flow, surge, and fallout deposits, a thick central ignimbrite, and upper surge and fallout deposits. Large lithic blocks (≤2 m) of underlying sedimentary and granitic bedrock that are present in lower pyroclastic flow and fallout deposits indicate that the eruptive sequence began with explosive, conduit-excavating eruptions. The massive, nonwelded central ignimbrite displays evidence for postemplacement deformation. The upper pyroclastic surge deposits are dominated by fine ash, some beds containing accretionary lapilli, soft-sediment deformation features, and mud-coated lithic lapilli, indicating an explosive, hydromagmatic component to these later eruptions. The upper fall and surge deposits are overlain by fluvially reworked volcaniclastic deposits that truncate the primary section with a relatively planar surface. The proximal, upper pyroclastic surge and Plinian fall deposits are preserved only in small grabens (5–8 m deep and wide), where they subsided into the ignimbrite and were protected from reworking. The pyroclastic surge and fall deposits within the grabens are offset by numerous small normal faults. The offset on some faults decreases upward through the section, indicating that the faulting process may have been syn-eruptive. Several graben-bounding faults extend downward into the ignimbrite, but the uppermost, fluvially reworked tephra layers are not cut by these faults. The faulting mechanism may have been related to settling and compaction of the 60 m thick, valley-filling ignimbrite along the axis of the paleovalley. Draping surge contacts against the graben faults and brittle and soft-style disruption of the upper pyroclastic surge beds indicate that subsidence was ongoing during the emplacement of the upper eruptive sequence. Seismicity accompanying the late-stage hydromagmatic explosions may have contributed to the abrupt settling and compaction of the ignimbrite.     

Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan

During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km³ in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km³, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano.     

An Upper Pliocene coarse pumice breccia generated by a shallow submarine explosive eruption,Milos, Greece

The Filakopi Pumice Breccia (FPB) is a very well exposed, Pliocene volcaniclastic unit on Milos, Greece, and has a minimum bulk volume of 1 km³. It consists of three main units: (A) basal lithic breccia (4–8 m) mainly composed of angular to subangular, andesitic and dacitic clasts up to 2.6 m in diameter; (B) very thickly bedded, poorly sorted pumice breccia (16–17 m); and (C) very thick, reversely graded, grain-supported, coarse pumice breccia (6.5–20 m), at the top. The depositional setting is well constrained as shallow marine (up to a few hundred metres) by overlying fossiliferous and bioturbated mudstone. This large volume of fine pumice clasts is interpreted to be the product of an explosive eruption from a submarine vent because: (1) pumice clasts are the dominant component; (2) the coarse pumice clasts (>64 mm) have complete quenched margins; (3) very large (>1 m) pumice clasts are common; (4) overall, the formation shows good hydraulic sorting; and (5) a significant volume of ash was deposited together with the coarsest pyroclasts.The bed forms in units A and B suggest deposition from lithic-rich and pumiceous, respectively, submarine gravity currents. In unit C, the coarse (up to 6.5 m) pumice clasts are set in matrix that grades upwards from diffusely stratified, fine (1–2 cm) pumice clasts at the base to laminated shard rich mud at the top. The coarse pumice clasts in unit C were settled from suspension and the framework was progressively infilled by fine pumice clasts from waning traction currents and then by water-settled ash. The FPB displays important features of the products of submarine explosive eruptions that result from the ambient fluid being seawater, rather than volcanic gas or air. In particular, submarine pyroclastic deposits are characterised by the presence of very coarse juvenile pumice clasts, pumice clasts with complete quenched rims, and good hydraulic sorting.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: J. Donelly-Nolan