Cataclysmic Eruption Research Articles

Of Two Minds About Toba's Impact

Researchers gathered at a meeting last month to probe the impact on modern humans of the cataclysmic eruption of Indonesia's Mount Toba about 74,000 years ago—and to ponder whether modern humans had made it to Asia by the time the volcano blew.

Mineralogy of loess and volcanic ash eolian mantles in Pacific Northwest (USA) landscapes

Eolian mantles consisting of Quaternary loess, Holocene volcanic ash, and mixtures of the two are prominent features of most landscapes in the inland Pacific Northwest region of the USA. Soils of the loess-mantled Palouse region of eastern Washington and northern Idaho exhibit regional mineralogical trends related to mean annual precipitation and age. Clay content increases and CaCO 3 decreases from areas of lower to higher precipitation. Clay mica is the dominant clay mineral in soils formed in the youngest loess (ca. < 15 ka). In the next older loess paleosol (ca. 40 ka), vermiculite is dominant, indicating a time-dependent mica-to-vermiculite weathering sequence. Smectite and kaolin also occur to a lesser extent in soils of the region. This mineralogical signature provides a useful tool in identifying paleosols and interpreting the complex pedogenesis occurring in local Palouse landscapes. More recently, Holocene volcanic ash has influenced many mid- and high-elevation forested soils. Relatively thick mantles of volcanic ash from the cataclysmic eruption of Mount Mazama 7600 years ago have undergone slight-to-moderate weathering, giving rise to Andisols (Andosols). Environmental conditions generally promote development of allophanic properties across the region, with allophane and ferrihydrite being dominant mineral components. Non-allophanic mineralogy is much less extensive in ash mantles of the region, but is favored in two pedogenic environments — E horizons of podzolized soils and horizons supporting vigorous growth of bracken fern.

Gas transport model for the magmatic system at Mount Pinatubo, Philippines: Insights from ( 210Pb)/( 226Ra)

Measurements of 226Ra– 210Pb disequilibria in eruptive products have the potential to track the accumulation or loss of volatiles in magmatic systems on timescales of decades because the intermediate nuclide, 222Rn, follows the gas phase. We present measurements of 210Pb– 226Ra disequilibria for whole-rock samples representing a time sequence of tephra through the June 15, 1991 cataclysmic eruption of Mount Pinatubo volcano, Philippines. Mount Pinatubo volcano is a dacitic system that did not significantly vent gases at the surface prior to eruption, and we can, therefore, isolate the 210Pb– 226Ra disequilibria due to gas accumulation without complications due to 222Rn loss during degassing. Pyroclastic samples have ( 210Pb)/( 226Ra) 0 ranging from 1.01 to 1.10; averaging 1.06. A sample of the post-climactic dome has ( 210Pb)/( 226Ra) 0 = 1.12. Previous uranium-series degassing studies have suggested that 210Pb excesses are created by rapid volatile transport (carrying the intermediate daughter 222Rn) and subsequent volatile accumulation and decay of 222Rn to 210Pb. However, bubbles in viscous dacite magma cannot rise at speeds needed to provide a flux of 222Rn large enough to cause measurable disequilibria in the 210Pb– 226Ra system. In addition, there is little evidence for magmatic sources large enough to supply a rapid flux of 222Rn. Therefore, we present a model in which 210Pb– 226Ra disequilibria is established during basaltic recharge of the Pinatubo reservoir. The relatively low viscosity of basaltic magma allows for differential gas motion and the production of 210Pb excess in localized basaltic melt. Transport of volatiles and 210Pb-rich basalt through a crystal matrix and the formation of bubble plumes in the dacitic reservoir produces a mixed magma with 210Pb excess. Through this mechanism, the timescale of gas transport and accumulation is constrained, not by the half-life of 222Rn (3.8 days), but rather by the half-life of 210Pb (22.6 years). Bubble plume motion preserves disequilibria and creates a zone of eruptable dacite with 210Pb excess alleviating the need for gas transport on very short time-scales. Using the rate of decay of 210Pb coupled with published trace element data, we present a quantitative investigation of this new conceptual model and propose that changes in 210Pb values with time may suggest changing conditions of magma supply at volcanoes.

Recent lahars at Volcán de Colima (Mexico): Drainage variation and spectral classification

Volcán de Colima is the most active volcano in Mexico, and represents a high risk for more than 500,000 people. In 1998 the volcano renewed its activity, with the extrusion of a lava dome and subsequent lava and block and ash flows. During the recent period of activity pyroclastic products did not directly affect villages around the volcano, however, several lahars did. We used LIDAR topographic coverage, ASTER and LANDSAT images for the recognition of morphological changes in the drainage system and lahar detection. For lahar delineation we applied principal components analysis and canonical classification (Tasseled Cap) in order to perform a supervised image classification using the maximum likelihood rule algorithm. LAHARZ (objective delineation of distal debris flow hazard zones) has been used and tested using two topographic datasets with different resolutions, which provided evidence of the importance of high-resolution topographic coverage in hazard assessment. Finally a hazard map for lahars is presented, showing that several villages and ranchos can be affected. In contrast, due to morphological changes produced by products of the intense explosive activity, other populated areas such as La Yerbabuena, are outside of the high risk zone. They could be affected by lahars only in case of cataclysmic eruptions such as the 1913 Plinian event.

Lead isotopes by LA-MC-ICPMS: Tracking the emergence of mantle signatures in an evolving silicic magma system

At Long Valley (LV) model Sr isotope phenocryst ages and absolute U–Pb zircon ages from precaldera Glass Mountain (GM) and caldera-related Bishop Tuff (BT) rhyolites show that these crystals track >1 Myr of evolution of a voluminous rhyolite magmatic system. In detail, strong disparities between the different age populations complicate ideas for a unified model for rhyolite generation, differentiation, and storage. To better elucidate the age discrepancies a new in situ Pb isotope technique has been developed to measure the compositions of 113 individual LV feldspars (mainly sanidine) and their host glasses by UV laser ablation MC-ICPMS. Given sufficient signal the accuracy and precision of this technique approaches that of double-spike thermal ionization mass spectrometry. The utility of our technique for many geologic materials is, however, limited to determining Pb isotope ratios that include 206Pb, 207Pb, and 208Pb, but exclude 204Pb. New zircon 238U– 206Pb crystallization ages were also obtained for two older Glass Mountain domes. A >1.5‰ difference between the Pb isotope compositions of feldspars from older (1.7–2.2 Ma) precaldera Glass Mountain (GM) rhyolites and younger LV rhyolites, including the BT, is found. The Pb isotope data for feldspars and their host glasses lie along a regional trend line between young basalts and evolved crust compositions, spanning ∼15% of that isotopic difference, and show a secular change towards increasing mantle contribution. Most feldspars have Pb isotope compositions that are similar to their host glasses and, as such, there persists an apparent >100 k.y. difference between Sr model feldspar ages and zircon ages for some GM rhyolites. Collectively, the feldspars define a Sr–Pb isotope mixing curve. Evidence for mixing complicates the interpretation that the Sr isotope data solely reflect radiogenic ingrowth. Where isotopically heterogeneous feldspar populations occur, there is greater uncertainty about the veracity of the Sr model ages. Specifically, we find no Pb isotope evidence that BT feldspars grew from older GM-like magmas. The distinct Pb isotope signatures for individual rhyolites and their feldspars support evidence based on zircon dating that LV volcanism did not erupt from a single long-lived magma chamber but rather tapped a number of different magmas. Moreover, contrary to the conventional model of gradual build-up prior to cataclysmic eruption, secular changes in the U–Pb age constraints on magma residence times and the magmas’ distinct Pb isotopic compositions suggest that, at Long Valley, eruptive volumes increase with shorter magma residence time and correlate with greater mantle input. Evidently, the plumbing and therefore activity at Long Valley was influenced by the evolving interaction between source and crustal magma system.

Preludes to an Eruption

GEOLOGY The cataclysmic 79 A.D. eruption of Vesuvius buried Pompeii and Herculaneum, and thanks to careful observation of the event by Pliny the Younger, marked the beginning of the science of volcanology. Several million people now live on volcanic deposits surrounding Vesuvius, and it is one of the world's most thoroughly monitored volcanoes. Thus, understanding the pre-eruptive behavior of this volcano is crucial. Morgan et al. have analyzed the concentration profiles of barium across crystals in the 79 A.D. ash; the profiles reflect the time during which diffusion occurred in the magma between an abrupt phase of crystal growth or dissolution and the eruption. Because of the strong temperature dependence of the diffusion rates, the profiles also constrain the magma temperature. The data suggest that the eruptive magma was recharged several times in the decades leading up to the cataclysmic eruption, most noticeably around 20 years before. Such recharge may explain the origin of a major earthquake in 62 A.D. A separate intrusion likely triggered the eruption. — BH Geology 34 , 845 (2006).

Characterization of volcanic deposits and geoarchaeological studies from the 1815 eruption of Tambora volcano

http://dx.doi.org/10.17014/ijog.vol1no1.20066aThe eruption of Tambora volcano on the island of Sumbawa in 1815 is generally considered as the largest and the most violent volcanic event in recorded history. The cataclysmic eruption occurred on 11 April 1815 was initiated by Plinian eruption type on 5 April and killed more than 90,000 people on Sumbawa and nearby Lombok. The type plinian eruptions occurred twice and ejected gray pumice and ash, to form stratified deposits as thick as 40-150 cm on the slopes and mostly distributed over the district west of the volcano. Following this, at about 7 pm, on 11 April the first pyroclastic surge was generated and progressively became greater extending to almost whole direction, mainly to the north, west, and south districts from the eruption center. The deadliest volcanic eruption buried ancient villages by pyroclastic surge and flow deposits in almost intact state, thus preserving important archaeological evidence for the period. High preservation in relatively stable conditions and known date of the eruptions provide approximate dating for the archaeological remains. Archaeological excavations on the site uncovered a variety of remains were relieved by ground penetrating radar (GPR) to map structural remains of the ancient villages under the pyroclastic surge and flow deposits. These traverses showed that GPR could define structures as deep as 10 m (velocity 0.090 m/ns) and could accurately map the thickness of the stratified volcanic deposits in the Tambora village area.

Uplift, thermal unrest and magma intrusion at Yellowstone caldera

The Yellowstone caldera, in the western United States, formed approximately 640,000 years ago when an explosive eruption ejected approximately 1,000 km3 of material. It is the youngest of a series of large calderas that formed during sequential cataclysmic eruptions that began approximately 16 million years ago in eastern Oregon and northern Nevada. The Yellowstone caldera was largely buried by rhyolite lava flows during eruptions that occurred from approximately 150,000 to approximately 70,000 years ago. Since the last eruption, Yellowstone has remained restless, with high seismicity, continuing uplift/subsidence episodes with movements of approximately 70 cm historically to several metres since the Pleistocene epoch, and intense hydrothermal activity. Here we present observations of a new mode of surface deformation in Yellowstone, based on radar interferometry observations from the European Space Agency ERS-2 satellite. We infer that the observed pattern of uplift and subsidence results from variations in the movement of molten basalt into and out of the Yellowstone volcanic system.

Decadal‐scale change of infiltration characteristics of a tephra‐mantled hillslope at Mount St Helens, Washington

AbstractThe cataclysmic 1980 eruption of Mount St Helens radically reduced the infiltration characteristics of ∼60 000 ha of rugged terrain and dramatically altered landscape hydrology. Two decades of erosional, biogenic, cryogenic, and anthropogenic activity have modified the infiltration characteristics of much of that devastated landscape and modulated the hydrological impact of the eruption. We assessed infiltration and runoff characteristics of a segment of hillslope thickly mantled with tephra, but now revegetated primarily with grasses and other plants, to evaluate hydrological modifications due to erosion and natural turbation. Eruptive disturbance reduced infiltration capacity of the hillslope by as much as 50‐fold. Between 1980 and 2000, apparent infiltration capacities of plots on the hillslope increased as much as ten fold, but remain approximately three to five times less than the probable pre‐eruption capacities. Common regional rainfall intensities and snowmelt rates presently produce little surface runoff; however, high‐magnitude, low‐frequency storms and unusually rapid snowmelt can still induce broad infiltration‐excess overland flow. After 20 years, erosion and natural mechanical turbation have modulated, but not effaced, the hydrological perturbation caused by the cataclysmic eruption. Copyright © 2005 John Wiley &amp; Sons, Ltd.

ANDIC SOILS OF THE INLAND PACIFIC NORTHWEST, USA: PROPERTIES AND ECOLOGICAL SIGNIFICANCE

Holocene tephra from the cataclysmic eruption of Mount Mazama in southwestern Oregon is an important component of many soils east of Cascade Mountains in the Pacific Northwest region. This article examines ecologically important properties of these tephra-influenced soils. Characterization data from 886 soil horizons (384 pedons) of the region meeting criteria for andic soil properties or subgroup classification as andic or vitrandic intergrades in Soil Taxonomy were used to compare morphological, chemical, and physical properties. Mazama tephra typically comprises a significant portion of a surface mantle with textures ranging from silt loam in areas distal to the tephra source to very gravelly loamy coarse sand in proximal locations. Tephra has been mixed to varying degrees with other parent materials across the region. On average, volcanic glass only comprises 31.1% of the 0.05 to 2-mm fraction and has a bimodal distribution, suggesting that some tephra was deposited with significant quantities of dust or, that since deposition, considerable reworking and mixing of glass has occurred. Oxalate-extractable Al ranges from 0.04 to 5.4% and provides the best indicator of colloidal surface activity as indicated by P retention. Cation exchange capacity (CEC) determined at field pH (ECEC) averages 7.8 cmol c kg -1 and is less than one-third the CEC determined at pH 8.2, indicating considerable variable charge and relatively low capacity of these soils to store and supply nutrient cations. Mineralogical data suggest that a reduced pool of secondary Al may be limiting allophane formation in soil horizons with less tephra influence. Volumetric water-holding capacity of ash-influenced horizons is as much as twice that of underlying horizons and underscores the importance of tephra in seasonally dry, forested ecosystems of the Inland Northwest region.

Eruptive history of Fisher Caldera, Alaska, USA

New field, compositional, and geochronologic data from Fisher Caldera, the largest of 12 Holocene calderas in Alaska, provide insights into the eruptive history and formation of this volcanic system. Prior to the caldera-forming eruption (CFE) 9400 years ago, the volcanic system consisted of a cluster of several small (∼3 km 3) stratocones, which were independently active between 66±144 and 9.4±0.2 ka. Fisher Caldera formed through a single eruption, which produced a thick dacitic fall deposit and two pyroclastic-flow deposits, a small dacitic flow and a compositionally mixed basaltic-dacitic flow. Thickness and grain-size data indicate that the fall deposit was dispersed primarily to the northeast, whereas the two flows were oppositely directed to the south and north. After the cataclysmic eruption, a lake filled much of the caldera during what may have been a significant quiescent period. Volcanic activity from intracaldera vents gradually resumed, producing thick successions of scoria fall interbedded with lake sediments. Several Holocene stratocones have developed; one of which has had a major collapse event. The caldera lake catastrophically drained when a phreatomagmatic eruption generated a large wave that overtopped and incised the southwestern caldera wall. Multiple accretionary-lapilli-bearing deposits inside and outside the caldera suggest significant Holocene phreatomagmatic activity. The most recent eruptive activity from the Fisher volcanic system was a small explosive eruption in 1826, and current activity is hydrothermal. Late Pleistocene to Holocene magma eruption rates range from 0.03 to 0.09 km 3 ky −1 km −1, respectively. The Fisher volcanic system is chemically diverse, ∼48–72 wt.% SiO 2, with at least seven dacitic eruptions over the last 82±14 ka that may have become more frequent over time. Least squares calculations suggest that prior to the CFE, Fisher Volcano products were not derived from a single, large magma reservoir, and were likely erupted from multiple, compositionally independent magma reservoirs. After the CFE, the majority of products appear to have derived from a single reservoir in which magma mixing has occurred.

Posteruption glacier development within the crater of Mount St. Helens, Washington, USA

The cataclysmic eruption of Mount St. Helens on May 18, 1980, resulted in a large, north-facing amphitheater, with a steep headwall rising 700 m above the crater floor. In this deeply shaded niche a glacier, here named the Amphitheater glacier, has formed. Tongues of ice-containing crevasses extend from the main ice mass around both the east and the west sides of the lava dome that occupies the center of the crater floor. Aerial photographs taken in September 1996 reveal a small glacier in the southwest portion of the amphitheater containing several crevasses and a bergschrund-like feature at its head. The extent of the glacier at this time is probably about 0.1 km 2. By September 2001, the debris-laden glacier had grown to about 1 km 2 in area, with a maximum thickness of about 200 m, and contained an estimated 120,000,000 m 3 of ice and rock debris. Approximately one-third of the volume of the glacier is thought to be rock debris derived mainly from rock avalanches from the surrounding amphitheater walls. The newly formed Amphitheater glacier is not only the largest glacier on Mount St. Helens but its aerial extent exceeds that of all other remaining glaciers combined.

Oxidized sulfur-rich mafic magma at Mount Pinatubo, Philippines

Basaltic fragments enclosed in andesitic dome lavas and pyroclastic flows erupted during the early stages of the 1991 eruption of Mount Pinatubo, Philippines, contain amphiboles that crystallized during the injection of mafic magma into a dacitic magma body. The amphiboles contain abundant melt inclusions, which recorded the mixing of andesitic melt in the mafic magma and rhyolitic melt in the dacitic magma. The least evolved melt inclusions have high sulfur contents (up to 1,700 ppm) mostly as SO4 2–, which suggests an oxidized state of the magma (NNO+1.4). The intrinsically oxidized nature of the mafic magma is confirmed by spinel–olivine oxygen barometry. The value is comparable to that of the dacitic magma (NNO+1.6). Hence, models invoking mixing as a means of releasing sulfur from the melt are not applicable to Pinatubo. Instead, the oxidized state of the dacitic magma likely reflects that of parental mafic magma and the source region in the sub-arc mantle. Our results fit a model in which long-lived SO2 discharge from underplated mafic magma accumulated in the overlying dacitic magma and immiscible aqueous fluids. The fluids were the most likely source of sulfur that was released into the atmosphere during the cataclysmic eruption. The concurrence of highly oxidized basaltic magma and disproportionate sulfur output during the 1991 Mt. Pinatubo eruption suggests that oxidized mafic melt is an efficient medium for transferring sulfur from the mantle to shallow crustal levels and the atmosphere. As it can carry large amounts of sulfur, effectively scavenge sulfides from the source mantle and discharge SO2 during ascent, oxidized mafic magma forms arc volcanoes with high sulfur fluxes, and potentially contributes to the formation of metallic sulfide deposits.

Volcanic eruptions recorded in the Illimani ice core (Bolivia): 1918–1998 and Tambora periods

Abstract. Acid layers of volcanic origin detected in polar snow and ice layers are commonly used to document past volcanic activity on a global scale or, conversely, to date polar ice cores. Although most cataclysmic eruptions of the last two centuries (Pinatubo, El Chichon, Agung, Krakatoa, Cosiguina, Tambora, etc.) occurred in the tropics, cold tropical glaciers have not been used for the reconstruction of past volcanism. The glaciochemical study of a 137 m ice core drilled in 1999 close to the summit of Nevado Illimani (Eastern Bolivian Andes, 16°37' S, 67°46' W, 6350 m asl) demonstrates, for the first time, that such eruptions are recorded by both their tropospheric and stratospheric deposits. An 80-year ice sequence (1918-1998) and the Tambora years have been analyzed in detail. In several cases, ash, chloride and fluoride were also detected. The ice records of the Pinatubo (1991), Agung (1963) and Tambora (1815) eruptions are discussed in detail. The potential impact of less important regional eruptions is discussed.

Short-term climatic impact of the 1991 volcanic eruption of Mt. Pinatubo and effects on atmospheric tracers

Abstract. Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulphur gases (mostly sulphur dioxide) above the tropopause, causing increases in the stratospheric aerosol optical depth that may be even larger than one order of magnitude. The e-folding particle lifetime in the stratosphere is much longer than in the troposphere (one year versus a few days) so that climatic perturbations in a timeframe of a few years are produced after major volcanic eruptions. A climate-chemistry coupled model is used here to study the dynamical effects of the radiative forcing due to stratospheric aerosols formed after the June, 1991 cataclysmic eruption of Mt. Pinatubo in the Philippines. It is shown that the dynamical perturbation is twofold: (a) the stratospheric mean meridional circulation is affected by local aerosol radiative heating (mostly located in the tropical lower stratosphere); (b) the planetary wave propagation in the mid- to high-latitude lower stratosphere is altered as a consequence of decreasing atmospheric stability due to the climatic perturbation. Dynamical results of the climate model are compared with available observations; a discussion is made regarding the similarities with the dynamical regime of the easterly phase of the equatorial quasi-biennial oscillation. Major findings of this study are: (a) radiatively forced changes in the stratospheric circulation during the first two years after the eruption may, to a large extent, explain the observed trend decline of long-lived greenhouse gases (CH4 and N2O, in particular); (b) the dynamical perturbation helps explain why simple photochemical studies of the ozone trends during 1991–1993 generally fail in reproducing the satellite observed feature consisting of a 2% additional global ozone depletion during 1993 with respect to 1992. In both cases we conclude that an increase in the mid- to high-latitude downward flux at the tropopause is the key factor for explaining the behaviour of these atmospheric tracers during 1991/92.

Contribution of mafic melt to porphyry copper mineralization: evidence from Mount Pinatubo, Philippines, and Bingham Canyon, Utah, USA

Mount Pinatubo in the Philippines, known for its cataclysmic eruption in 1991, hosts several porphyry copper deposits and active geothermal systems. An underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption. The eruption caused purging of the sulphur-rich fluid from the dacite to the atmosphere and extensive fracturing. Similar events took place at Bingham Canyon, Utah, site of the largest copper and gold deposit in North America at 38 Ma. The Bingham Canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling. Ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma, which consolidated to form the Bingham stock and its underlying magma chamber. Injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock.

Twenty‐one years later, Mount St. Helens still offers lessons

Mount St. Helens National Volcanic Monument, Washington State, October 18—During the drive up to Mount St. Helens on a brisk, clear, fall day, Christine Colasurdo noted that this region of the country has been shaped by volcanism.“This is a place where the soils have dropped from the sky” said Colasurdo. Her family had enjoyed a cabin along the mountain's Spirit Lake prior to a series of volcanic events that began in 1980—including earthquake‐triggered landslides, a lateral cataclysmic eruption, a vertical column of ash, and lahars and pyroclastic flows on May 18,1980. Events of that day alone lowered the 2,932‐meter‐high volcano by 398 meters, as 2.8 billion cubic meters of material removed from the mountain flattened 148,000 hectares of forest, killed 57 people, and completely changed the landscape. Ash from the volcano circled the Earth in 15 days.

A seismically constrained mass discharge rate for the initiation of the May 18, 1980 Mount St. Helens eruption

We calculate the vertical mass discharge rate from Mount St. Helens for the first few minutes of the May 18, 1980 cataclysmic eruption using a new method based on seismic constraints. The observed seismic waves indicate that the seismic source is a series of single forces. We model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet. The momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model. Only two parameters are necessary for the calculation: Mach number and sound velocity. The calculated mass ejected in the first 100 s is 1.6×1011 −4.6×1011 kg. Since the total blast deposit is ∼3.2×1011 − 4.1×1011 kg, one possible interpretation is that the directed blast had a significant (&gt;30%) vertical component.

The low-frequency earthquake swarms at Mount Pinatubo, Philippines: implications for magma dynamics

Mount Pinatubo's intense low-frequency (LF) earthquake swarms in 1992 provide insight into the volcano's shallow plumbing system and into the possible magmatic process that generate these unusual earthquakes. LF waveforms are remarkably uniform with spectral peaks at about 1 Hz. Recorded at Pinatubo 1 week prior to the June 1991 cataclysmic eruption, LF events reappeared 1 year later and a day before a new dome began to grow in the volcano's freshly-formed caldera. Seismicity with at least 23,000 LF events accompanied the 1992 dome growth, and both simultaneously ceased after 4 months. The magnitude range of the thousands of LF events is narrow at only 1.6 units, and show a more restricted range in size during any particular period within the crisis, defining a “group magnitude.” During two LF earthquake swarms, the group magnitude gradually varied, indicating a pervasive process affecting the mechanism of LF earthquake-generation. Hypocenters of Pinatubo's LF earthquakes define a vertical pipe extending from the crater floor to ∼4.2 km below sea level, coinciding with the column formed by high-frequency (HF) earthquakes prior to the 1991 cataclysmic eruption. A dike intrusion along the NW-trending Maraunot Fault is also suggested by the hypocenters. The temporal, amplitude and waveform features and hypocenter location of Pinatubo's LF earthquakes are distinctly different from those of HF events and indicate a close affinity of LF earthquakes to one another. LF events appear to be caused by interactions between more fluid magma intruding semi-ductile materials confined within the volcano's shallow plumbing system. This source mechanism can account for: the occurrence of LF earthquakes as swarms and multiplets; the limited magnitude range of the events; the temporally-changing group magnitudes; the obscurity of S-phase; the existence of hybrid LF–HF earthquakes; and the close relationship of LF earthquakes with dome extrusions. The 1992 LF seismicity at Pinatubo trails the 1991 paroxysmal eruption. Residual melt left in the shallow conduit following the 1991 eruption may have reached density and viscosity contrasts that allowed fluid fractions to intrude more viscous but still ductile part of the magma. This process may have produced the small-sized 1992 dome. By analogy, we suggest that the 1991 pre-eruptive dome and LF events may have involved viscous materials left by the ∼500 year BP eruption and intruded by the then ascending melt.

Petrologic and experimental evidence for the movement and heating of the pre-eruptive Minoan rhyodacite (Santorini, Greece)

Hydrothermal experiments combined with petrologic observations form the basis for a new two-stage model for the evolution of the pre-eruption Minoan magma chamber at Santorini, Greece. Ninety-nine percent of the erupted volume is two-pyroxene, rhyodacitic magma that had been stored at a temperature of ∼885 °C, based on magnetite-ilmenite and QUILF geothermometry. The rest of the volume is basaltic to andesitic magma, which occurs as 200 MPa. In addition, the composition of the plagioclase hosts (An56 ± 6) of the inclusions require temperatures of 825 ± 25 °C at pressures >200 MPa. This demonstrates that the Minoan rhyodacitic magma underwent a two-stage evolution, first crystallizing at ∼825∘C and >200 MPa, and then rinsing to a shallow ∼50 MPa storage region with a concomitant rise in temperature to ∼885 °C. We suggest that the episodic intrusion of mafic magmas provided the necessary heat and perhaps contributed to the ascent of the magma to shallow crustal depths where it reequilibrated before the cataclysmic eruption. Phase equilibria suggest that much of the heating of the rhyodacite occurred in the shallow storage region. Thermal budget calculations suggest that the rhyodacite magma could have been heated by intrusions of basalt rising at reasonable upwelling rates and injected into the storage zone over several hundred years. Preservation of amphibole in the mafic scoria indicate that injection of mafic magma continued up until days before the cataclysmic eruption, perhaps triggering the event.