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USGS1988

Interrelations among pyroclastic surge, pyroclastic flow, and lahars in Smith Creek valley during first minutes of 18 May 1980 eruption of Mount St. Helens, USA

A devastating pyroclastic surge and resultant lahars at Mount St. Helens on 18 May 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank. The tributaries channeled the flows to Smith Creek valley, which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank. Stratigraphy shows that the pyroclastic surge preceded the lahars; there is no notable "wet" character to the surge deposits. Therefore the lahars must have originated as snowmelt, not as ejected water-saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano. In stratigraphic order the Smith Creek valley-floor materials comprise (1) a complex valley-bottom facies of the pyroclastic surge and a related pyroclastic flow, (2) an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris, and (3) deposits of secondary pyroclastic flows. These units are capped by silt containing accretionary lapilli, which began falling from a rapidly expanding mushroom-shaped cloud 20 minutes after the eruption's onset. The Smith Creek valley-bottom pyroclastic facies consists of (a) a weakly graded basal bed of fines-poor granular sand, the deposit of a low-concentration lithic pyroclastic surge, and (b) a bed of very poorly sorted pebble to cobble gravel inversely graded near its base, the deposit of a high-concentration lithic pyroclastic flow. The surge apparently segregated while crossing the steep headwater tributaries of Smith Creek; large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge. The unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough, brown muddy sand matrix like that of lahar deposits upvalley. This unit contains irregular friable lenses and pods meters in diameter, blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier. The hummocky unit is the deposit of a high-viscosity debris flow which formed when lahars mingled with the pyroclastic materials on Smith Creek valley floor. Overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines-poor gas-escape pipes and containing charred wood. The deposits are thickest in topographic lows along margins of the hummocky diamict. Emplaced several minutes after the hot surge had passed, this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides. ?? 1988 Springer-Verlag.

Bulletin of Volcanology↗

USGS1988

The potential for catastrophic dam failure at Lake Nyos maar, Cameroon

The upper 40 m of Lake Nyos is bounded on the north by a narrow dam of poorly consolidated pyroclastic rocks, emplaced during the eruptive formation of the Lake Nyos maar a few hundred years ago. This 50-m-wide natural dam is structurally weak and is being eroded at an uncertain, but geologically alarming, rate. The eventual failure of the dam could cause a major flood (estimated peak discharge, 17000 m3/s) that would have a tragic impact on downstream areas as far as Nigeria, 108 km away. This serious hazard could be eliminated by lowering the lake level, either by controlled removal of the dam or by construction of a 680-m-long drainage tunnel about 65 m below the present lake surface. Either strategy would also lessen the lethal effects of future massive CO2 gas releases, such as the one that occurred in August 1986. ?? 1988 Springer-Verlag.

Bulletin of Volcanology↗

USGS1986

Gas analyses from the Pu'u O'o eruption in 1985, Kilauea volcano, Hawaii

Volcanic gas samples were collected from July to November 1985 from a lava pond in the main eruptive conduit of Pu'u O'o from a 2-week-long fissure eruption and from a minor flank eruption of Pu'u O'o. The molecular composition of these gases is consistent with thermodynamic equilibrium at a temperature slightly less than measured lava temperatures. Comparison of these samples with previous gas samples shows that the composition of volatiles in the magma has remained constant over the 3-year course of this episodic east rift eruption of Kilauea volcano. The uniformly carbon depleted nature of these gases is consistent with previous suggestions that all east rift eruptive magmas degas during prior storage in the shallow summit reservoir of Kilauea. Minor compositional variations within these gas collections are attributed to the kinetics of the magma degassing process. ?? 1986 Springer-Verlag.

Bulletin of Volcanology↗

USGS1987

Coastal lava flows from Mauna Loa and Hualalai volcanoes, Kona, Hawaii

A major carbonate reef which drowned 13 ka is now submerged 150 m below sea level on the west coast of the island of Hawaii. A 25-km span of this reef was investigated using the submersible Makali'i. The reef occurs on the flanks of two active volcanoes, Mauna Loa and Hualalai, and the lavas from both volcanoes both underlie and overlie the submerged reef. Most of the basaltic lava flows that crossed the reef did so when the water was much shallower, and when they had to flow a shorter distance from shoreline to reef face. Lava flows on top of the reef have protected it from erosion and solution and now occur at seaward-projecting salients on the reef face. These relations suggest that the reef has retreated shoreward as much as 50 m since it formed. A 7-km-wide "shadow zone" occurs where no Hualalai lava flows cross the reef south of Kailua. These lava flows were probably diverted around a large summit cone complex. A similar "shadow zone" on the flank of Mauna Loa volcano in the Kealakekua Bay region is downslope from the present Mauna Loa caldera, which ponds Mauna Loa lava and prevents it from reaching the coastline. South of the Mauna Loa "shadow zone" the - 150 m reef has been totally covered and obscured by Mauna Loa lava. The boundary between Hualalai and Mauna Loa lava on land occurs over a 6-km-wide zone, whereas flows crossing the - 150 m reef show a sharper boundary offshore from the north side of the subaerial transition zone. This indicates that since the formation of the reef, Hualalai lava has migrated south, mantling Mauna Loa lava. More recently, Mauna Loa lava is again encroaching north on Hualalai lava.

Hawaii↗

USGS1987

Hawaiian xenolith populations, magma supply rates, and development of magma chambers

Hawaiian volcanoes pass through a sequence of four eruptive stages characterized by distinct lava types, magma supply rates, and xenolith populations. Magma supply rates are low in the earliest and two latest alkalic stages and high in the tholeiitic second stage. Magma storage reservoirs develop at shallow and intermediate depths as the magma supply rate increases during the earliest stage; magma in these reservoirs solidifies as the supply rate declines during the alkalic third stage. These magma storage reservoirs function as hydraulic filters and remove dense xenoliths that the ascending magma has entrained. During the earliest and latest stages, no magma storage zone exists, and mantle xenoliths of lherzolite are carried to the surface in primitive alkalic lava. During the tholeiitic second stage, magma storage reservoirs develop and persist both at the base of the ocean crust and 3-7 km below the caldera; only xenoliths of shallow origin are carried to the surface by differentiated lava. During the alkalic third stage, magma in the shallow subcaldera reservoir solidifies, and crustal xenoliths, including oceanic-crustal rocks, are carried to the surface in lava that fractionates in an intermediate-depth reservoir. Worldwide xenolith populations in tholeiitic and alkalic lava may reflect the presence or absence of subvolcanic magma storage reservoirs. ?? 1987 Springer-Verlag.

Bulletin of Volcanology↗

USGS1987

Geothermometry of Kilauea Iki lava lake, Hawaii

Data on the variation of temperature with time and in space are essential to a complete understanding of the crystallization history of basaltic magma in Kilauea Iki lava lake. Methods used to determine temperatures in the lake have included direct, downhole thermocouple measurements and Fe-Ti oxide geothermometry. In addition, the temperature variations of MgO and CaO contents of glasses, as determined in melting experiments on appropriate Kilauean samples, have been calibrated for use as purely empirical geothermometers and are directly applicable to interstitial glasses in olivine-bearing core from Kilauea Iki. The uncertainty in inferred quenching temperatures is ??8-10?? C. Comparison of the three methods shows that (1) oxide and glass geothermometry give results that are consistent with each other and consistent with the petrography and relative position of samples, (2) downhole thermo-couple measurements are low in all but the earliest, shallowest holes because the deeper holes never completely recover to predrilling temperatures, (3) glass geothermometry provides the greatest detail on temperature profiles in the partially molten zone, much of which is otherwise inaccessible, and (4) all three methods are necessary to construct a complete temperature profile for any given drill hole. Application of glass-based geothermometry to partially molten drill core recovered in 1975-1981 reveals in great detail the variation of temperature, in both time and space, within the partially molten zone of Kilauea Iki lava lake. The geothermometers developed here are also potentially applicable to glassy samples from other Kilauea lava lakes and to rapidly quenched lava samples from eruptions of Kilauea and Mauna Loa. ?? 1987 Springer-Verlag.

Bulletin of Volcanology↗

USGS1987

New perspectives on the eruption of 1912 in the valley of ten thousand smokes, Katmai National Park, Alaska

New data extend our understanding of the 1912 eruption, its backfilled vent complex at Novarupta, and magma-storage systems beneath adjacent stratovolcanoes. Initial Plinian rhyolite fallout is confined to a narrow downwind sector, and its maximum thickness may occur as far as 13 km from source. In contrast, the partly contemporaneous rhyolite-rich ash flows underwent relatively low-energy emplacement, their generation evidently being decoupled from the high column. Flow veneers 1-13 m thick on near-vent ridge crests exhibit a general rhyolite-to-andesite sequence like that of the much thicker valley-confined ignimbrite into which they merge downslope. Lithics in both the initial Plinian and the ignimbrite are predominantly fragments of the Jurassic Naknek Formation, which extends from the surface to a depth of ca. 1500 m. Absence of lithics from the underlying sedimentary section limits to < 1.5 km the fragmentation level and the structural depth of the vent, which is thought to be funnel-shaped, flaring shallowly to a surface diameter of 2 km. Overlying the ignimbrite are layers of Plinian dacite fallout, > 100 m thick near source and 10 m thick 3 km away, which dip back into an inner vent <0.5 km wide, nested inside the earlier vent funnel of the ignimbrite. The dacite fallout is poor in Naknek lithics but contains abundant fragments of vitrophyre, most of which was vent-filling, densely welded tuff reejected during later phases of the 3-day eruption. Adjacent to the inner vent, a 225-m-high asymmetrical accumulation of coarse near-vent ejecta is stratigraphically continuous with the regional dacite fallout. Distensional faulting of its crest may reflect spreading related to compaction and welding. Nearby andesite-dacite stratovolcanoes, i.e., Martin, Mageik, Trident, and Katmai, display at least 12 vents that define a linear volcanic front trending N65??E. The 1912 vent and adjacent dacite domes are disposed parallel to the front and ca. 4 km behind it. Mount Griggs, 10 km behind the front, is more potassic than other centers, taps isotopically more depleted source materials, and reflects a wholly independent magmatic plumbing system. Geochemical differences among the stratovolcanoes, characteristically small eruptive volumes ( < 0.1 to 0.4 km3), and the dominance of andesite and low-SiO2 dacite suggest complex crustal reservoirs, not large integrated magma chambers. Linear fractures just outside the 1912 vent strike nearly normal to the volcanic front and may reflect dike transport of magma previously stored beneath Trident 3-5 km away. Caldera collapse at Mount Katmai may have taken place in response to hydraulic transfer of Katmai magma toward Novarupta via reservoir components beneath Trident. The voluminous 1912 eruption (12-15 km3 DRE) was also unusual in producing high-silica rhyolite (6-9 km3 DRE), a composition rare in this arc and on volcanic fronts in general. Isotopic data indicate that rhyolite genesis involved little assimilation of sedimentary rocks, pre-Tertiary plutonic rocks, or hydrothermally altered rocks of any age. Trace-element data suggest nonetheless that the rhyolite contains a nontrivial crustal contribution, most likely partial melts of Late Cenozoic arc-intrusive rocks. Because the three compositions (77%, 66-64.5%, and 61.5-58.5% SiO2) that intermingled in 1912 vented both concurrently and repeatedly (after eruptive pauses hours in duration), the compositional gaps between them must have been intrinsic to the reservoir, not merely effects of withdrawal dynamics. ?? 1987 Springer-Verlag.

Bulletin of Volcanology↗

USGS1989

Are extrusive rhyolites produced from permeable foam eruptions?

The permeable foam hypothesis is suggested by Eichelberger el al. (1986) to explain a major loss of water from rhyolithic magmas in the volcanic conduit. Evidence for the high-water content of the major portion of the magmas is herein examined and rejected. Eichelberger's hypothesis does not take into account the large (~2 orders of magnitude) viscosity change that would occur in the conduit as a result of water loss. It also requires that the permeable foam collapse and weld to form an obsidian that in thin section displays no evidence of the foam. An alternate hypothesis to explain the existence of small amounts of high water content rhyolite glasses in acid volcanoes is that rhyolite magmas are relatively dry (0.1-0.3% H2O) and that water enters the magma from the environment to produce a water-rich selvage which then is kneaded into the body of the magma. -Author

Bulletin of Volcanology↗

USGS1986

Internal structural variations in a debris-avalanche deposit from ancestral Mount Shasta, California, USA

Various parameters of the internal structure of a debris-avalanche deposit from ancestral Mount Shasta (size and percentage of block facies in each exposure, number and width of jigsaw cracks, and number of rounded clasts in matrix facies) were measured in order to study flow and emplacement mechanisms. Three types of coherent blocks were identified: blocks of massive or brecciated lava flows or domes, blocks of layered volcaniclastic deposits, and blocks of accidental material, typically from sedimentary units underlying Shasta Valley. The mean maximum dimension of the three largest blocks of layered volcaniclastic material is 220 m, and that of the lava blocks, 110 m. This difference may reflect plastic deformation of blocks of layered volcaniclastic material; blocks of massive or brecciated volcanic rock deformated brittly and may have split into several smaller blocks. The blocks in the deposit are one order of magnitude larger, and the height of collapse 1100 m higher, than the Pungarehu debris-avalanche deposit at Mount Egmont, New Zealand, although the degree of fracturing is about the same.This suggests either that the Shasta source material was less broken, or that the intensity of any accompanying explosion was smaller at ancestral Mount Shasta. The Shasta debris-avalanche deposit covered the floor of a closed basin; the flanks of the basin may have retarded the opening of jigsaw cracks and the formation of stretched and deformed blocks such as those of the Pungarehu debris-avalanche deposit. ?? 1986 Springer-Verlag.

Bulletin of Volcanology↗

USGS2000

A model for the magmatic-hydrothermal system at Mount Rainier, Washington, from seismic and geochemical observations

Mount Rainier is one of the most seismically active volcanoes in the Cascade Range, with an average of one to two high-frequency volcano-tectonic (or VT) earthquakes occurring directly beneath the summit in a given month. Despite this level of seismicity, little is known about its cause. The VT earthquakes occur at a steady rate in several clusters below the inferred base of the Quaternary volcanic edifice. More than half of 18 focal mechanisms determined for these events are normal, and most stress axes deviate significantly from the regional stress field. We argue that these characteristics are most consistent with earthquakes in response to processes associated with circulation of fluids and magmatic gases within and below the base of the edifice.Circulation of these fluids and gases has weakened rock and reduced effective stress to the point that gravity-induced brittle fracture, due to the weight of the overlying edifice, can occur. Results from seismic tomography and rock, water, and gas geochemistry studies support this interpretation. We combine constraints from these studies into a model for the magmatic system that includes a large volume of hot rock (temperatures greater than the brittle–ductile transition) with small pockets of melt and/or hot fluids at depths of 8–18 km below the summit. We infer that fluids and heat from this volume reach the edifice via a narrow conduit, resulting in fumarolic activity at the summit, hydrothermal alteration of the edifice, and seismicity.

Washington↗

USGS2000

Lava bubble-wall fragments formed by submarine hydrovolcanic explosions on Lō'ihi Seamount and Kīlauea Volcano

Glassy bubble-wall fragments, morphologically similar to littoral limu o Pele, have been found in volcanic sands erupted on Lō'ihi Seamount and along the submarine east rift zone of Kīlauea Volcano. The limu o Pele fragments are undegassed with respect to H 2 O and S and formed by mild steam explosions. Angular glass sand fragments apparently form at similar, and greater, depths by cooling-contraction granulation. The limu o Pele fragments from Lō'ihi Seamount are dominantly tholeiitic basalt containing 6.25–7.25% MgO. None of the limu o Pele samples from Lō'ihi Seamount contains less than 5.57% MgO, suggesting that higher viscosity magmas do not form lava bubbles. The dissolved CO 2 and H 2 O contents of 7 of the limu o Pele fragments indicate eruption at 1200±300 m depth (120±30 bar). These pressures exceed that generally thought to limit steam explosions. We conclude that hydrovolcanic eruptions are possible, with appropriate pre-mixing conditions, at pressures as great as 120 bar.

Hawai'i↗

USGS2001

Holocene geomagnetic secular variation recorded by volcanic deposits at Mount St. Helens, Washington

A compilation of paleomagnetic data from volcanic deposits of Mount St. Helens is presented in this report. The database is used to determine signature paleomagnetic directions of products from its Holocene eruptive events, to assign sampled units to their proper eruptive period, and to begin the assembly of a much larger database of paleomagnetic directions from Holocene volcanic rocks in western North America. The paleomagnetic results from Mount St. Helens are mostly of high quality, and generally agree with the division of its volcanic deposits into eruptive episodes based on previous geologic mapping and radiocarbon dates. The Muddy River andesite's paleomagnetic direction, however, indicates that it is more likely part of the Pine Creek eruptive period rather than the Castle Creek period. In addition, the Two-Fingers andesite flow is more likely part of the Middle Kalama eruptive period and not part of the Goat Rocks period. The paleomagnetic data from Mount St. Helens and Mount Hood document variation in the geomagnetic field's pole position over the last ~2,500 years. A distinct feature of the new paleosecular variation (PSV) record, similar to the Fish Lake record (Oregon), indicates a sudden change from rapid clockwise movement of the pole about the Earth's spin axis to relatively slow counterclockwise movement at ???800 to 900 years B.P.

Bulletin of Volcanology↗

USGS2002

Emplacement of the most recent lava flows on Hualālai Volcano, Hawai'i

A detailed field and petrologic study of the ca. 1800 a.d. flows at Hualālai Volcano documents at least two eruptive episodes, the Hu‘ehu‘e flow field ending in 1801, and the Ka‘ūpūlehu flow several decades earlier. The morphology and stratigraphy of the Ka‘ūpūlehu flow require an emplacement duration of several days to weeks. Based on a comparison with recent eruptive activity at Mauna Loa volcano, the eruption cannot have occurred at the anomalously high rate (10 4 –10 5 m 3 /s) proposed by previous workers. The hummocky flow surface of the later phase of the Hu‘ehu‘e eruption suggests a duration of months, based on a comparison with recent eruptive activity at Kīlauea Volcano. Although none of the ca. 1800 flows show evidence for extraordinarily fast emplacement or unusual fluid rheologies, both flows show unusual features. The abundant xenoliths for which the Ka‘ūpūlehu flow is famous were transported in numerous episodes of deposition and remobilization, during which they eroded the channel systems through which they traveled. Lava transport in proximal and medial regions of both flow fields was probably through lava tubes, as evidenced by preserved tubes and by the prevalence of pāhoehoe-lined channels that require thermally efficient transport of lava over great distances. Both flows also show abundant evidence for re-occupation of older cones and lava tubes, a characteristic that may typify infrequent eruptions of older volcanic systems. Although lava flows from Hualālai Volcano do not show anomalous eruptive behavior, they pose a substantial hazard for coastal communities of Kona.

Bulletin of Volcanology↗

USGS2002

A conceptual model of the Mount Spurr magmatic system from seismic and geochemical observations of the 1992 Crater Peak eruption sequence

A conceptual model of the geometry and dynamics of the Mount Spurr magmatic system is developed using seismic, geochemical, and visual observations of the 1992 Crater Peak eruption sequence. The basis for this model is a new classification of all located seismic events and results from prior studies of seismology, geology, geochemistry, and geophysics of the Mount Spurr area. Significant seismic features of the 1992 eruption sequence include (1) a distinct swarm of volcano-tectonic (VT) earthquakes in August 1991 directly beneath the Crater Peak vent, (2) a caldera-wide increase in VT earthquakes, lasting 7 months, which preceded the 27 June eruption, (3) two shallow swarms of VT earthquakes that occurred on 5 June and 27 June, the latter immediately preceding the 27 June eruption, (4) a mix of VT, long-period (LP), and hybrid events at depths of 20-40 km, which began coincident with the onset of seismic unrest and reached a peak after eruptive activity ended, (5) a strong swarm of VT earthquakes that began as the 16-17 September eruption was ending, (6) a prominent swarm of VT earthquakes on 9-10 November at depths of 1 to 4 km beneath Crater Peak, and (7) a smaller swarm of VT earthquakes in late December 1992, which were located between 7 and 10 km depth. These seismic observations, combined with geological, geochemical, and geophysical data and observations, suggest a deep magmatic source zone for Crater Peak andesites at depths of 20-40 km, a smaller mid-crustal storage zone at about 10 km depth, and a conduit that extends to the surface. We infer that the magmas erupted in 1992 were generated at depths of 20-40 km and rose to the mid-crustal storage zone that fed all three 1992 eruptions. The 1992 eruption sequence may have terminated when additional magma solidified at shallow depths.

Bulletin of Volcanology↗

USGS2002

The 1999 eruption of Shishaldin Volcano, Alaska: Monitoring a distant eruption

Shishaldin Volcano, in the central Aleutian volcanic arc, became seismically restless during the summer of 1998. Increasing unrest was monitored using a newly installed seismic network, weather satellites, and rare local visual observations. The unrest culminated in large eruptions on 19 April and 22-23 April 1999. The opening phase of the 19 April eruption produced a sub-Plinian column that rose to 16 km before rapidly dissipating. About 80 min into the 19 April event we infer that the eruption style transitioned to vigorous Strombolian fountaining. Exceptionally vigorous seismic tremor heralded the 23 April eruption, which produced a large thermal anomaly observable by satellite, but only a modest, 6-km-high plume. There are no ground-based visual observations of this eruption; however we infer that there was renewed, vigorous Strombolian fountaining. Smaller low-level ash-rich plumes were produced through the end of May 1999. The lava that erupted was evolved basalt with about 49% SiO2. Subsequent field investigations have been unable to find a distinction between deposits from each of the two major eruptive episodes.

Bulletin of Volcanology↗

USGS2002

A tectonic earthquake sequence preceding the April-May 1999 eruption of Shishaldin Volcano, Alaska

On 4 March 1999, a shallow ML 5.2 earthquake occurred beneath Unimak Island in the Aleutian Arc. This earthquake was located 10-15 km west of Shishaldin Volcano, a large, frequently active basaltic-andesite stratovolcano. A Strombolian eruption began at Shishaldin roughly 1 month after the mainshock, culminating in a large explosive eruption on 19 April. We address the question of whether or not the eruption caused the mainshock by computing the Coulomb stress change caused by an inflating dike on fault planes oriented parallel to the mainshock focal mechanism. We found Coulomb stress increases of ???0.1 MPa in the region of the mainshock, suggesting that magma intrusion prior to the eruption could have caused the mainshock. Satellite and seismic data indicate that magma was moving upwards beneath Shishaldin well before the mainshock. indicating that, in an overall sense, the mainshock cannot be said to have caused the eruption. However, observations of changes at the volcano following the mainshock and several large aftershocks suggest that the earthquakes may, in turn, have influenced the course of the eruption.

Bulletin of Volcanology↗

USGS2002

Structure and physical characteristics of pumice from the climactic eruption of Mount Mazama (Crater Lake), Oregon

The vesicularity, permeability, and structure of pumice clasts provide insight into conditions of vesiculation and fragmentation during Plinian fall and pyroclastic flow-producing phases of the ~7,700 cal. year B.P. climactic eruption of Mount Mazama (Crater Lake), Oregon. We show that bulk properties (vesicularity and permeability) can be correlated with internal textures and that the clast structure can be related to inferred changes in eruption conditions. The vesicularity of all pumice clasts is 75–88%, with >90% interconnected pore volume. However, pumice clasts from the Plinian fall deposits exhibit a wider vesicularity range and higher volume percentage of interconnected vesicles than do clasts from pyroclastic-flow deposits. Pumice permeabilities also differ between the two clast types, with pumice from the fall deposit having higher minimum permeabilities (~5×10 –13 m 2 ) and a narrower permeability range (5–50×10 –13 m 2 ) than clasts from pyroclastic-flow deposits (0.2–330×10 –13 m 2 ). The observed permeability can be modeled to estimate average vesicle aperture radii of 1–5 µm for the fall deposit clasts and 0.25–1 µm for clasts from the pyroclastic flows. High vesicle number densities (~10 9 cm –3 ) in all clasts suggest that bubble nucleation occurred rapidly and at high supersaturations. Post-nucleation modifications to bubble populations include both bubble growth and coalescence. A single stage of bubble nucleation and growth can account for 35–60% of the vesicle population in clasts from the fall deposits, and 65–80% in pumice from pyroclastic flows. Large vesicles form a separate population which defines a power law distribution with fractal dimension D=3.3 (range 3.0–3.5). The large D value, coupled with textural evidence, suggests that the large vesicles formed primarily by coalescence. When viewed together, the bulk properties (vesicularity, permeability) and textural characteristics of all clasts indicate rapid bubble nucleation followed by bubble growth, coalescence and permeability development. This sequence of events is best explained by nucleation in response to a downward-propagating decompression wave, followed by rapid bubble growth and coalescence prior to magma disruption by fragmentation. The heterogeneity of vesicle sizes and shapes, and the absence of differential expansion across individual clasts, suggest that post-fragmentation expansion played a limited role in the development of pumice structure. The higher vesicle number densities and lower permeabilities of pyroclastic-flow clasts indicate limited coalescence and suggest that fragmentation occurred shortly after decompression. Either increased eruption velocities or increased depth of fragmentation accompanying caldera collapse could explain compression of the pre-fragmentation vesiculation interval.

Oregon↗

USGS2002

Long-period events and tremor at Popocatepetl volcano (1994–2000) and their broadband characteristics

Following an initial phreatic eruption on 21 December 1994, activity at Popocatepetl has been dominated by fumarolic emissions interspersed with more energetic emissions of ashes and gases. A phase of repetitive dome-building and dome-destroying episodes began in March 1996 and is still ongoing at present. We describe the long-period (LP) seismicity accompanying eruptive activity at Popocatepetl from December 1994 through May 2000, using data from a three-component broadband seismometer located 5 km from the summit crater. The broadband records display a variety of signals, with periods ranging in the band 0.04–90 s. Long-period events and tremor with typical dominant periods in the range 0.3–2.0 s are the most characteristic signals observed at Popocatepetl. These signals appear to reflect volumetric sources driven by pressure fluctuations associated with the unsteady transport of gases beneath the crater. Very-long-period (VLP) signals are also observed in association with LP events and tremor. The VLP signals which accompany LP events display Ricker-like wavelets with periods near 36 s, whereas VLP signals associated with tremor waveforms typically show sustained oscillations at periods ranging up to 90 s. The spectra and particle motion patterns remain similar from event to event for the majority of LP and tremor signals analyzed during the time span of this study, suggesting a repeated, non-destructive activation of a common source. Hypocenters determined by phase pick analyses of selected LP events recorded by the seven-station, permanent Popocatepetl short-period network suggest that the majority of these events are confined to a source region in the top 1.5 km below the crater floor. The repetitive occurrences of VLP signals with closely matched waveform characteristics are consistent with a non-destructive reactivation of at least two sources. One source appears to coincide with the main source region of LP seismicity, whereas the second is a deeper source whose activity appears to be intimately linked with episodes of monochromatic tremor.

Bulletin of Volcanology↗