Notes on the 1902 eruption of Santa María volcano,Guatemala

The first historic activity of Santa María volcano, Guatemala constituted one of the ten largest historic eruptions in the world, producing 5.5 km³ of debris. In hindsight, the six-month period before the October 1902 eruption was one of extremely abnormal seismicity in all of western Guatemala. The pyroclastis from the eruption were scattered widely over Western Guatemala and Southern Mexico and also caused world-wide atmospheric effects. The volcanics produced were of andesitic-dacitic composition, but there was wide variation from place to place in the sampled material — a fact apparently chiefly attributable to atmospheric-fractionation. There was apparently a change in chemistry of ash during the two-day eruption as well, the first, most voluminous ash was pumicious and white; later ash was finer, denser, darker, and slightly less silicious. The kinetic energy/thermal energy partition is determined to be similar to the value derived for Krakatoa,E _k/E _th ? 5.0 %. The thermal energy of the eruption was estimated at 4.2×10²⁵ ergs. The explosion crater left on Santa María’s southwest flank after the eruption had a volume equal to less than 0.5 km³, a small fraction of the volume of material erupted. The two-day 1902 blast has greatly overshadowed subsequent activity; extrusion of the Santiaguito dome, which has occurred since 1922 in the explosion crater, has produced about 0.7 km³ of dacite lava and 1.6×10²⁵ ergs of thermal energy in 48 years of activity.     

The 2005 eruption of Ilamatepec (Santa Ana) volcano,El Salvador

We report the stratigraphic sequence of the 2005 eruption of Ilamatepec volcano together with sedimentological and chemical analyses of its products.Structural and textural characteristics of the deposits indicate that the eruption was driven by a small-volume rhyolitic intrusion at shallow levels, which resulted first in the collapse of the existing hydrothermally altered fan of previous deposits inside the crater lake, driving phreatic explosions with launching of blocks on ballistic trajectories; later the magma interacted with lake waters producing several hydromagmatic pyroclastic density currents (PDCs). These flows were energetic enough to knock down pine trees up to distances of 1.8 km from the crater in the E-NE sector of the volcano. Finally, ejection of ballistic blocks that landed on previously emplaced, wet pyroclastic density current deposits, caused the generation of a lahar that flowed down the steep eastern flank toward the El Jabillal gully. Subsequent lahars occurred as a result of intense rain caused by hurricane Stan.Radiocarbon ages on paleosols and charcoal fragments, separating previous volcanogenic sequences, indicate that similar eruptions have occurred more frequently in the past centuries, than previously thought.The new data confirms that Ilamatepec volcano is one of the most active volcanoes in El Salvador. Nevertheless, more detailed studies of the eruptive sequence of Ilamatepec volcano are mandatory to establish future eruptive patterns.     

Paleomagnetic constraints on eruption patterns at the Pacaya composite volcano,Guatemala

Pacaya volcano is an active composite volcano located in the volcanic highlands of Guatemala about 40 km south of Guatemala City. Volcanism at Pacaya alternates between Strombolian and Vulcanian, and during the past five years there has been a marked increase in the violence of eruptions. The volcano is composed principally of basalt flows interbedded with thin scoria fall units, several pyroclastic surge beds, and at least one welded tuff. Between 400 and 2000 years BP the W-SW sector of the volcano collapsed producing a horseshoeshaped amphitheater (0.65 km³) and providing a window into the cone's infrastructure. Lava flows and tephra exposed in the amphitheater are more then 200 m thick and when combined with flows erupted recently represent between 30 and 40% of the cone's history. Pacaya is ideally suited for a paleomagnetic study into the timing and duration of eruption episodes at a large, composite volcano. We drilled 27 paleomagnetic sites (25 aa flows, 1 dike, and 1 welded tuff) from four lava-flow sequences with between 4 and 14 sites per sequence. The four sequences represent initial through historic activity at Pacaya. We resolved, what appear to be, 22 time-independent paleomagnetic sites by averaging together directions from successive sites where the sitemean directions were indistinguishable at the 95% level of confidence. However, mean-sequence directions of individual lava-flow sequences yielded unusually high Fisher precision parameters (k=44–224) and small circles of 63% confidence (a₆₃=1.6–6.1°) suggesting as few as three or four time-independent sites were collected. This indicates that activity as Pacaya is strongly episodic and that episodes are characterized by voluminous outpouring of lavas. Modelling the data using Holocene PSV rates confirms this and shows that differences in within-sequence directions (6–11.5°) are consistent with emplacement of lava-flow sequences in less than 100 years to as many as 300 years. Relatively larger differences in directions (18–23°) between subjacent lava-flow sequences indicates that repose is at least 300–500 years and could be even longer.     

Behavioral patterns of Fuego volcano, Guatemala

The times of activity at Fuego (one of the most active volcanoes in the world) since 1800 correlate with the activity of other Central American volcanoes. Approximately 0.7 km³ of olivine-bearing, high-Al₂O₃ basalt has been erupted since 1932, and about 1.7 km³ has been produced during 450 years of historic records. A minimum of 13,000 years and a maximum of 100,000 years were required to build Fuego's cone of 50 km³. Within the recent cluster of activity since 1932, rates of magma production have increased to 0.5 m³/s and the trend has been toward more eruptions (shorter reposes) of progressively more mafic basalt. 47% of the eruptions occurred within 2 days of the fortnightly tidal maximum and 56% occurred within 2 hours of the semi-diurnal minimum of the vertical tidal gravity acceleration. Thus the maximum compressional component of the tidal cycles can trigger an eruption at Fuego. Eruptions with higher effusion rates produce larger volumes of materials, although they only last a few hours. The 20–70 year clusters of activity beginning at 80–170-year intervals are interpreted as reflecting the ascent of primary batches of magma. A deeper (8–16 km), larger (> 1 km³) primary chamber and a shallower (2–5 km), smaller (0.1 km³), dike-like secondary chamber best explain Fuego's behavioral pattern.     

The 1972–1974 eruption of klyuchevskoy volcano,Kamchatka

A new Klyuchevskoy volcano eruptive cycle encompasses terminal (March 30, 1972 to August 23, 1974) and lateral (August 23, 1974 to December, 1974) eruption stages. The terminal eruption stage resulted in lava flows and parasitic cones that formed on the south-western flank of the volcano. Eruption products are moderately alkalic high-alumina olivine-bearing andesite-basalts. The terminal eruption stage was accompanied by volcanic earthquakes and volcanic tremor. The lateral eruption was accompanied by explosive earthquakes. Volcanic tremor was the most useful prognostic sign indicating the onset of the lateral eruption. Eruptive mechanisms are discussed.     

Tarawera 1886, New Zealand — A basaltic plinian fissure eruption

New Zealand's biggest and most destructive volcanic eruption of historical times was that of Tarawera in 1886. The resulting scoria fall has a dispersal very similar in extent to that of the Vesuvius A.D. 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type. A new estimate of the volume (2 km³) is significantly greater than previous estimates. The basalt came mainly from a 7-km length of fissure, and emission and exit velocity were fairly uniform along at least 4 km of it, this is one of the few documented examples of a plinian eruption from a fissure vent. Primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min⁻¹. A model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1.5–3%. Violent phreatic explosions occurred in the southwestern extension of the fissure across the Rotomahana geothermal field, and it is thought that some of the water responsible for the power of the plinian eruption came from this source, though its amount was not sufficient to turn the eruption into a phreatoplinian one.     

The plinian fallout associated with Quilotoa's 800 yr BP eruption,Ecuadorian Andes

Large volcanic eruptions at dacitic or rhyolitic volcanoes often generate exceptional volumes of fine ash that mantles an area up to a million km². These eruptions are characterized by extreme fragmentation of the magma and hence extraordinary dispersal of ash and are categorized as plinian, ultraplinian, or phreatoplinian events. Large-volume co-ignimbrites or co-plinian ashes are often produced by such eruptions. High fragmentation indices of > 90% are attributed to the violent eruption of silicic magma, especially if augmented by fuel-coolant reactions produced when abundant external water interacts with the magma. The present study documents a case where the fine ash (≤ 1 mm diameter) fall deposit related to the plinian phase of the eruption comprises the overwhelming bulk – about 87 wt.% of the eruptive products. This is another example demonstrating the predominance of a widespread, fine-grained, co-plinian ash which follows the initial coarser lapilli fall. Historical eruptions at two other Andean volcanoes Quizapu, (Chile) and Huaynaputina, (Peru), and at Santa Maria, (Guatemala) and Novarupta, (Alaska) produced similar ash fall sequences.     

The 1963–65 eruption of Irazú volcano,Costa Rica (the period of March 1963 to October 1964)

The 1963–65 eruption of Irazú, like all others of this volcano during the historic period, produced only ash and other fragmental ejecta without lava. The initial outbreak on March 13, 1963 started with a series of great explosions that hurled out much ash, blocks, and bombs, but the activity soon settled down to alternating periods of explosive cruptions and quiet emission of steam. Ash was deposited mostly along a zone that extended westward from the summit to and beyond the city of San Jose, 24 km away. The prolonged ashfall severely damaged dairy, vegetable, and coffee farms, and for a while made daily life in the affected cities extremely difficult. Accelerated runoff of rainwater from the ash-covered slopes of the volcano caused destructive floods, mudflows, and landslides. The climax of the cruption probably occurred during December 1963 and January 1964, when ash and incandescent scoria were erupted voluminously and the magma rose to within 100 meters of the lip of the vent. Precise levelling along the highway to the summit in May 1964 by the Geographic Institute revealed the upper part of the volcano upheaved as much as 11 cm above levels determined in 1949. A repetition of the levelling in September 1964 showed a subsidence to approximately the 1949 configuration, indicating a distinct reduction of pressure in the magma chamber. Substantial amounts of pulverized wallrock were present in the ash along with fragments of scoria and pumice. Progressive caving of the vent walls, which enlarged the diameter of the vent from 200 meters to 525 meters, kept dropping wallrock down onto the exploding magma, and at times stopped the eruption for a day or two by plugging the vent. The scoriaceous and pumiceous bombs were porphyritic two-pyroxene olivine basaltic andesite, and their composition remained remarkably constant throughout the eruption. The ash section was about 2 meters thick, 800 meters downwind from the vent in June 1964. In the section, deposits of the rainy season could be distinguished by their well developed stratification from those of the dry season. A zone containing three persistent pumice horizons represents the climactic period of December 1963 to January 1964. The cloudburst of December 10, 1963 is recorded by a highly rilled surface, and the strong winds of the dry season of 1964 are indicated by a rippled lag deposit.     

Geology of Santa Fe island: The oldest galapagos volcano

Santa Fe Island was a volcanic center when it emerged 3.9 ± 0.6 m.y. ago. Later upfaulting of a horst along the central axis of the island dominates its present morphology. Santa Fe is made up of evolved transitional lavas that are not related by fractional crystallization alone. Source heterogeneties, differing degrees of melting, or open-system magma chambers may explain the observed trace element variations.Santa Fe, Baltra, and Española make up a geologic subprovince in the central Galapagos: they are older than the other islands, and their lavas are compositionally similar. At the time of their emergence, the three islands were in a tectonic setting similar to that of the young western and central Galapagos Island.     

Gravity changes accompanying volcanic activity at Pacaya volcano, Guatemala

Gravity changes of up to 1.2 ± 0.1 mgal (1 standard deviation) were measured at three points within 400 m of an active vent on Pacaya volcano, Guatemala during eleven days of January, 1975. For five continuous days gravity varied inversely with the average muzzle velocity of ejecta, the frequency of volcanic explosions, and the frequency of volcanic earthquakes. The gravity changes are most reasonably interpreted as the product of intravolcanic movements of magma with masses one to two orders of magnitude larger than any flow ever erupted from the volcano. However, elevation changes and/or combination of elevation and mass distribution changes could also have been an important factor in effecting the observed gravity variations. Because we lack elevation control on the gravity stations, we are unable to unequivocally conclude which factor or which combination of factors produced the gravity changes. The study indicates the possibility of gravity monitoring of hazardous volcanoes as a predictive tool, and as an added means for investigating the internal mechanism of volcanic eruptions.     

Temporal gravity and elevation changes at Pacaya volcano, Guatemala

Repetitive gravity surveys at Pacaya Volcano from 1975 to 1979 revealed time-dependent changes in the gravity field, which although related to volcanic activity, could not be uniquely attributed to elevation changes or mass redistributions because elevation control was lacking. Elevation control was established in July 1979 using precision leveling. Relative elevation and gravity measurements in June and July of 1979, January 1980 and June 1980 indicate concurrent gravity and elevation changes contemporaneous with variations in eruptive activity. From June 1979 to January 1980, while fumarolic activity was dominant, relative to the most remote station, the volcano deflated by at least 195 mm and the gravity field increased by up to 221 μgal. From January 1980 to June 1980, preceding a Strombolian eruption beginning about June 1980, the volcano inflated by at least 19 mm and the gravity field decreased by up to 231 μgal. Gravity change maps for the intervals of January 1978 to June 1979, June 1979 to January 1980, and January 1980 to June 1980 show areas subject to repeated positive and negative gravity change. Some of those areas coincide with areas of maximum elevation change observed in the June 1979–January 1980 and January 1980–June 1980 intervals; however, gravity changes were observed in areas lacking elevation changes. Adjusting observed gravity changes for elevation changes using a free-air value of −3.086 μgal/cm does not substantially alter the pattern, position, or amplitude of the gravity changes. The relationship between gravity changes, elevation changes, and volcanic activity requires a mechanism producing gravity decreases with little inflation during times of increased eruptive activity, and producing gravity increases with subsidence during times of declining eruptive activity. Such a pattern of changes could be produced by a near-surface magma body in which high-density degassed magma is displaced volume for volume by low-density vesiculated magma during time of increased eruptive activity, and in which loss of gasses by fumarolic activity produces a density increase and a reduction in volume of the magma body during periods of declining eruptive activity. Such a pattern of changes could also be induced by a low-density, vesiculated magma body moving upward in the volcanic pile by piecemeal stoping where the high-density rocks of the volcano are replaced on a volume for volume basis by low-density magma during periods of increasing eruptive activity; and by later density increases and magma body volume reductions accompanying devolatilization and devesiculation during periods of declining eruptive activity. Simple density change and density contrast models involving shallow magma bodies at depths of 100 to 200 m indicate density changes or contrasts of about 0.4 g/cm³ could produce the gravity changes.     

The submarine eruption of the Bombarda volcano,Milos Island,Cyclades, Greece

The Milos volcanic field includes a well-exposed volcaniclastic succession which records a long history of submarine explosive volcanism. The Bombarda volcano, a rhyolitic monogenetic center, erupted ∼1.7 Ma at a depth <200 m below sea level. The aphyric products are represented by a volcaniclastic apron (up to 50 m thick) and a lava dome. The apron is composed of pale gray juvenile fragments and accessory lithic clasts ranging from ash to blocks. The juvenile clasts are highly vesicular to non-vesicular; the vesicles are dominantly tube vesicles. The volcaniclastic apron is made up of three fades: massive to normally graded pumice-lithic breccia, stratified pumice-lithic breccia, and laminated ash with pumice blocks. We interpret the apron beds to be the result of water-supported, volcaniclastic mass-How emplacement, derived directly from the collapse of a small-volume, subaqueous eruption column and from syn-eruptive, down-slope resedimentation of volcaniclastic debris. During this eruptive phase, the activity could have involved a complex combination of phreatomagmatic explosions and minor submarine effusion. The lava dome, emplaced later in the source area, is made up of flow-banded lava and separated from the apron by an obsidian carapace a few meters thick. The near-vertical orientation of the carapace suggests that the dome was intruded within the apron. Remobilization of pyroclastic debris could have been triggered by seismic activity and the lava dome emplacement. Published online: 30 January 2003 Editorial responsibility: J. McPhie     

Thermal-image-derived dynamics of vertical ash plumes at Santiaguito volcano,Guatemala

Vertical ash plumes were imaged at Santiaguito (Guatemala) using a thermal camera to capture plume ascent dynamics. The plumes comprised a convecting plume front fed by a steady feeder plume. Of the 25 plumes imaged, 24 had a gas thrust region within which ascent velocities were 15–50 m s⁻¹. A transition to buoyant ascent occurred 20 to 50 m above the vent, where ascent velocities declined to 4–15 m s⁻¹. Plumes that attained greater heights had higher heat contents, wider feeder plumes and higher buoyant ascent velocities.     

Geochemical trends of sequential lava flows from Meseta volcano, Guatemala

The Meseta and Fuego volcanoes closely overlap and collectively are known as the Fuego Volcanic Complex. Historic activity occurs exclusively at Fuego, the southern center, and consists of high-Al basalts. Meseta, the inactive northern center, is predominantly composed of basaltic andesites with minor basalt and andesite. A thick sequence of lava flows and dikes is exposed by a steep collapse escarpment on the east flank of Meseta. The upper 75% of the sequence was sampled from three interfingering stratigraphic sections consisting of 27, 10 and 4 lavas, respectively. Temporal geochemical trends of each section indicates a complex evolutionary history. A major trend toward more evolved compositions upward in the section is consistent with crystal fractionation. This trend is sharply interrupted by the youngest lavas which become distinctly more mafic in composition. Magma mixing is apparently the dominant magmatic evolution process that generated these lavas. The two trends have distinct Sr signatures that suggest a change in parental magma compositions. This abrupt change in composition is interpreted to signal high input rates of mafic magma into the subvolcanic magma chamber. These changes eventually led to sector collapse of Meseta volcano and deposition of the Escuintla debris avalanche. Eruptive activity then migrated to the Fuego volcano where historic activity is similar to that of Meseta immediately prior to its collapse.     

The Sarikavak Tephra, Galatea, north central Turkey: a case study of a Miocene complex plinian eruption deposit

The Sarikavak Tephra from the central Galatean Volcanic Province (Turkey) represents the deposit of a complex multiple phase plinian eruption of Miocene age. The eruptive sequence is subdivided into the Lower-, Middle-, and Upper Sarikavak Tephra (LSKT, MSKT, USKT) which differ in type of deposits, lithology and eruptive mechanisms.The Lower Sarikavak Tephra is characterised by pumice fall deposits with minor interbedded fine-grained ash beds in the lower LSKT-A. Deposits are well stratified and enriched in lithic fragments up to >50 wt% in some layers. The upper LSKT-B is mainly reversely graded pumice fall with minor amounts of lithics. It represents the main plinian phase of the eruption. The LSKT-A and B units are separated from each other by a fine-grained ash fall deposit. The Middle Sarikavak Tephra is predominantly composed of cross-bedded ash-and-pumice surge deposits with minor pumice fall deposits in the lower MSKT-A and major pyroclastic flow deposits in the upper MSKT-B unit. The Upper Sarikavak Tephra shows subaerial laminated surge deposits in USKT-A and subaqueous tephra beds in USKT-B.Isopach maps of the LSKT pumice fall deposits as well as the fine ash at the LSKT-A/B boundary indicate NNE–SSW extending depositional fans with the source area in the western part of the Ovaçik caldera. The MSKT pyroclastic flow and surge deposits form a SW-extending main lobe related to paleotopography where the deposits are thickest.Internal bedding and lithic distribution of the LSKT-A result from intermittent activity due to significant vent wall instabilities. Reductions in eruption power from (partial) plugging of the vent produced fine ash deposits in near-vent locations and subsequent explosive expulsion of wall rock debris was responsible for the high lithic contents of the lapilli fall deposits. A period of vent closure promoted fine ash fall deposition at the end of LSKT-A. The subsequent main plinian phase of the LSKT-B evolved from stable vent conditions after some initial gravitational column collapses during the early ascent of the re-established eruption plume. The ash-and-pumice surges of the MSKT-A are interpreted as deposits from phreatomagmatic activity prior to the main pyroclastic flow formation of the MSKT-B.