Extensive Ash Research Articles

Constraints on hydrothermal processes on basaltic edifices: inferences on the conditions leading to hydrovolcanic eruptions at Piton de la Fournaise, Réunion Island, Indian Ocean

The hydrothermal processes that occur on the active basaltic edifice of Piton de la Fournaise are investigated. The present-day volcanic activity concentrates in a west–east-oriented collapsed area, 13 km long and 6–8 km large, called Enclos Fouqué. Enclos Fouqué is open to the sea on its eastern side, while a horseshoe rim delimits its extension to the west, south and north. This forms a continuous cliff, 100–200 m high above the floor of Enclos Fouqué. Inside Enclos Fouqué, a 400 m high basaltic cone with two coalescent summit craters constitutes the most active area of the volcano. Beyond the western wall of Enclos Fouqué, a wide basaltic plateau called the Plaine des Sables stretches west for a few kilometers, until it reaches a cliff limiting its western part. To the north and south of this plateau, rivers notch the basaltic edifice, and along the slopes of the induced valleys springs flow. Geochemical data from these springs indicate hydrothermal activity within deep fractured media inside the plateau generated owing to west–east regional extension. On the floor of the Plaine des Sables, near the western wall of Enclos Fouqué, extensive ash deposits from a violent hydrothermal eruption have been recognized. Hydrothermal activity has also been detected in the deep (>100 m) porous layers of Enclos Fouqué, in an area surrounding the central cone. This circulation occurs in layers of vesiculated rocks that constitute a highly permeable and quasi-isotropic medium. Physical models are presented to illustrate the basic differences between the thermal regime of each hydrothermal system. We show that, in the periphery of the volcano, the fracture walls stabilize the hydrothermal circulation inside the fissure network, thus steadily draining the heat along the impermeable walls. Consequently, hot (230–240°C) rising convective currents with a finger shape, 150 m distant, reach the uppermost parts of the fracture. By contrast, inside the isotropic porous layers of the central area, the circulation is likely to be chaotic/turbulent because of the high permeability of the medium. This scatters the thermal energy through the whole porous media. Numerical models of the transient evolution of the hydrothermal circulation in the fractured medium are presented. They reveal that the fingers generated in the deep levels of the edifice move upward at a velocity of 15–150 m/yr, depending on the amplitude of the fracture permeability. The extensive character of the stresses in the Plaine des Sables, the velocity and the high temperature inside the hydrothermal flow constitute necessary conditions for the disruption of the fracture walls during boiling. Using a model of Germanovich and Lowell [J. Geophys. Res. 100 (1995) 8417–8434], these conditions are shown to be sufficient for the generation of huge (km 3 of deposits) hydrothermal eruptions in the periphery of the edifice, while only small (a few m 3 of deposits) hydrovolcanic events are shown to develop at the summit area. Accordingly, the ash deposits (0.6 km 3) recovered near the western wall of Enclos Fouqué are ascribed to the hydrothermal flow in fractures generated by the east–west extensional stresses.

Fluidized Bed Combustion of Leached Rice Straw

Leaching of inorganic constituents from biomass offers an approach to avoid extensive ash slagging, deposition, and bed agglomeration associated with many herbaceous materials including cereal straws. Leaching occurs naturally in the field when straw is exposed to rainfall, or it can be conducted under controlled conditions in industrial apparatus. Elemental composition and ash fusibility show that leaching can result in significant improvements in combustion characteristics of rice straw and other biomass. Bench-scale combustion in an atmospheric fluidized bed combustor confirms that the extensive bed agglomeration that occurs with combustion of untreated rice straw does not take place with naturally leached rice straw at combustion temperatures up to 900 °C. In addition, blending leached rice straw into a wood-based fuel blend does not increase but rather leads to a decline in ash deposit formation rate. The mechanism for a decline in deposition rate is not fully understood but may be related to the increased loading rate of ash in the furnace and possibly higher erosion rates of deposits that do form. Results confirm observations made during limited full-scale tests and further demonstrate that leached rice straw can be used as fuel without the rapid and undesirable ash deposition generally associated with untreated rice straw.

Alkali Salt Ash Formation in Four Finnish Industrial Recovery Boilers

Combustion aerosol measurement methods were introduced and applied for extensive ash formation studies at four operating recovery boilers in Finland. Ash particle mass size distributions determined with a Berner-type low-pressure impactor downstream the heat exchangers were clearly bimodal with the fine mode at about 2 μm and the coarse mode above 3 μm aerodynamic diameter. According to SEM images, fine ash mode consists of individual, almost spherical 0.3−0.7 μm alkali salt particles and of agglomerates with few primary particles of similar diameter and shape. The degree of fine mode primary particle sintering increased when increasing boiler heat load. Coarse mode includes large agglomerates with up to thousands of 0.3−0.7 μm alkali salt primary particles and spherical silica particles. Ash particle main component was sodium sulfate as determined with X-ray diffraction. Sodium-to-sulfur molar ratio of ash particles calculated on the analyses results with an ion chromatography decreased from the upper fu...

Mafic pyroclastic flows at Santa Maria (Gaua) Volcano, Vanuatu: the caldera formation problem in mainly mafic island arc volcanoes

ABSTRACTAt Santa Maria Volcano (New Hebrides island arc), extensive ash and scoria flow deposits overlie the mainly effusive, pre‐caldera cone. Hydromagmatic features characterize these deposits, the composition of juvenile clasts ranges from basalt to acid andesite/dacite (SiO2= 51–63.6%) with a dominant basaltic composition. The stratigraphic position of this pyroclastic series and its spatial distribution around a 8.5 km × 6 km wide caldera provide evidence of a relationship between this series and the caldera formation. In addition, these pyroclastic deposits are co‐genetic to parasitic cones and lava flows developed along faults concentric to the caldera. Both series result from a compositionally layered magma reservoir, the subordinate differentiated magmas being the result of fractional crystallization from the basalts. A model of caldera formation which implies a large hydromagmatic eruption at the central vent and minor magma withdrawal by flank eruptions is proposed. This model emphasizes the importance of mafic hydroclastic eruptions in the caldera forming event and contradicts a model implying only quiet subsidence, a process often proposed for the formation of calderas in island are volcanoes of mainly mafic composition.

Geology of kilauea volcano

This paper summarizes studies of the structure, stratigraphy, petrology, drill holes, eruption frequency, and volcanic and seismic hazards of Kilauea volcano. All the volcano is discussed, but the focus is on its lower cast rift zone (LERZ) because active exploration for geothermal energy is concentrated in that area. Kilauea probably has several separate hydrothermal-convection systems that develop in response to the dynamic behavior of the volcano and the influx of abundant meteoric water. Important features of some of these hydrothermal-convection systems are known through studies of surface geology and drill holes. Observations of eruptions during the past two centuries, detailed geologic mapping, radiocarbon dating, and paleomagnetic secular-variation studies indicate that Kilauea has erupted frequently from its summit and two radial rift zones during Quaternary time. Petrologic studies have established that Kilauea erupts only tholeiitic basalt. Extensive ash deposits at Kilauea's summit and on its LERZ record locally violent, but temporary, disruptions of local hydrothermal-convection systems during the interaction of water or steam with magma. Recent drill holes on the LERZ provide data on the temperatures of the hydrothermal-convection systems, intensity of dike intrusion, porosity and permeability, and an increasing amount of hydrothermal alteration with depth. The prehistoric and historic record of volcanic and seismic activity indicates that magma will continue to be supplied to deep and shallow reservoirs beneath Kilauea's summit and rift zones and that the volcano will be affected by eruptions and earthquakes for many thousands of years.

Worldwide dispersal of ash and gases from earth's largest known eruption: Toba, Sumatra, 75 ka

The eruption of the Youngest Toba Tuff (YTT) occurred 75 ± 2 ka in North Sumatra (2°45′N, 98°45′E). The eruption produced at least 2800 km 3 of dense rock equivalent (DRE) rhyolite magma (7 × 10 18 g). Much of the volume of magma was preserved as a non-welded outflow sheet covering 20,000–30,000 km 2 and a thick, welded intracaldera tuff. At least 800 km 3 (2 × 10 18 g) of Toba ash was deposited in an extensive ash blanket over the Indian Ocean and Southern Asia. Detailed studies of the chemistry of minerals and glasses in pumices from the YTT enable estimation of the minimum masses of gaseous components released to earth's atmosphere during the eruption: H 2O:. The high eruption rate, with the entire event lasting perhaps 9–14 days make stratospheric venting of a significant fraction of these gases and associated silicate ash and aerosol particles likely. The masses of ash and gases released is nearly two orders of magnitude higher than any known historic eruption.

Worldwide dispersal of ash and gases from earth's largest known eruption: Toba, Sumatra, 75 ka

The eruption of the Youngest Toba Tuff (YTT) occurred 75 ± 2 ka in North Sumatra (2°45′N, 98°45′E). The eruption produced at least 2800 km 3 of dense rock equivalent (DRE) rhyolite magma (7 × 10 18 g). Much of the volume of magma was preserved as a non-welded outflow sheet covering 20,000–30,000 km 2 and a thick, welded intracaldera tuff. At least 800 km 3 (2 × 10 18 g) of Toba ash was deposited in an extensive ash blanket over the Indian Ocean and Southern Asia. Detailed studies of the chemistry of minerals and glasses in pumices from the YTT enable estimation of the minimum masses of gaseous components released to earth's atmosphere during the eruption: H 2O:. The high eruption rate, with the entire event lasting perhaps 9–14 days make stratospheric venting of a significant fraction of these gases and associated silicate ash and aerosol particles likely. The masses of ash and gases released is nearly two orders of magnitude higher than any known historic eruption.

Status of Two Endangered Fishes, Cyprinodon nevadensis mionectes and Rhinichthys osculus nevadensis, from Two Springs in Ash Meadows, Nevada

The largest remaining populations of the endangered Ash Meadows speckled dace, Rhinichthys osculus nevadensis, and two of the largest remaining populations of the endangered Ash Meadows Amargosa pupfish, Cyprinodon nevadensis mionectes, occur in Jack Rabbit and Big Springs. Although both springs contain large populations of C. n. mionectes and small, but apparently viable, populations of R. o. nevadensis, they also contain large populations of exotic fishes, crayfish and bullfrogs. Natural barriers exclude exotic fishes from the spring pool and part of the outflow of Jack Rabbit Spring. Below the barriers, sailfin mollies, Poecilia latipinna, predominate, with mosquitofish, Gambusia affinis, occurring in relatively small numbers. Both native fishes and the exotics, P. latipinna and G. affinis, occur throughout the Big Spring system. In Big Spring, the large, relatively still-water habitats present in the spring pool and wider outflow channel allow G. affinis to predominate. The rarity of R. o. nevadensis appears attributable to the abundance of exotic species. Notes on life history, population sizes and management of the endangered fishes are also provided. Ash Meadows is an approximately 16,200 ha oasis surrounded by the Amargosa Desert in southwestern Nevada. More than 20 springs and innumerable seeps within Ash Meadows provide a myriad of aquatic habitats in the midst of this otherwise arid region. Endemism of aquatic organisms is high, particularly among the molluscan and fish faunas. A species flock of snails, most of which are undescribed, is restricted to various springs in Ash Meadows (D. W. Taylor, pers. comm.). All five of the native fishes are endemic. Unfortunately, physical and biological alteration of the habitats has been pervasive. Numerous exotic species, such as mosquitofish, Gambusia affinis, sailfin mollies, Poecilia latipinna, bullfrogs, Rana catesbeiana, and crayfish, Procambarus clarkii, have become established in the larger springs (Deacon and Williams, 1984; Hardy, 1980). Largemouth bass, Micropterus salmoides, were introduced into Crystal Pool, the largest spring in Ash Meadows, but have subsequently disappeared. During the early 1960's, an illegal tropical fish farm resulted in the temporary introduction of such exotic predators as the arawana, Osteoglossum bicirrhosum (Soltz and Naiman, 1978). Physical habitat alteration has included draining of marshes, removal of extensive mesquite (Prosopis pubescens, P. glandulosa) and ash (Fraxinus velutina var. coriacea) thickets, leveling of the land surface and excessive groundwater pumping for agriculture, channelization of spring outflows, and concrete lining of certain spring pools. The impact of these changes on the five native fishes has been severe. Historically, the Ash Meadows poolfish, Empetrichthys merriami, inhabited the larger, lower-elevation springs in the valley (Miller, 1948). The species was last collected in 1942 and is now considered extinct (Miller, 1961). The

Extensive ash falls in and around the sea of Japan from large late quaternary eruptions

Five widespread ashes of late Quaternary age have been recognized on land and also in deep-sea sediments in and around the Sea of Japan. All are favorable for determining the magnitude and type of the great eruptions, and for establishing a Quaternary chronology around Japan. Accurate determination of refractive indices of volcanic glass shards and phenocrysts, together with chemical data, have enabled successful characterization of the tephra layers and correlation between land and sea. Of the five tephras two are alkalic and are the products of major eruptions of two Korean Volcanoes: the Baegdusan—Tomakomai ash (B-Tm) and the Ulreung-Oki ash (U-Oki), The B-Tm ash is recognized at the top of most cores in the northern part of the Sea of Japan and also on Hokkaido and the northern part of Honshu, where a reliable archeological age is given around the 11th century. The U-Oki ash occurs between the Kikai-Akahoya and Aira-Tn ashes in cores from adjacent to the Ulreung-do to central Honshu, where three radiocarbon ages of around 9300 yr B.P. were obtained. The other three tephras are subalkalic and are correlated with large-scale eruptions producing voluminous pyroclastic-flows and calderas in Kyushu: the Kikai-Akahoya (K-Ah), Aira-Tn (AT) and Aso-4 ashes. The former two have been well described already but their fallout areas are partly revised in this manuscript. The AT ash is the most prominent marker, recognized not only on land of Japan but also in cores from the whole area of the Sea of Japan and part of the Pacific Ocean. The airfall Aso-4 ash, simultaneous with the Aso-4 pyroclastic-flow eruption is also described. It mantles very extensive areas and serves as an important time marker about 70,000 years ago.

Pleistocene Volcanic Ash Deposits in Utah

The major western Pleistocene volcanic ashes are reviewed with emphasis on their occurrence in Utah. The Pleistocene Bishop ash, Pearlette type 0 ash, Mazama ash, and several local ashes have been identified in Utah. The value of these extensive ash layers as stratigraphic markers depends on the ability to distinguish between them. Techniques employed to distinguish the ashes are discussed with emphasis on using the chemical composition of the volcanic glass shards. The major element chemistry of the glass portion of these ashes is presented together with that of other Cascade ashes and the Green Mountain Reservoir ash. It is noted that in the concentrations of major elements these ashes are all chemically similar to rhyolite, and only titanium, calcium, and iron are useful in separating the ash layers. The trace element con ten t (titanium, barium, manganese, iron, chlorine, rubidium, yttrium, zirconium, strontium, and niobium) of these ashes is more conclusive in identifying the ash layers. Some heretofore undescribed ashes from the east flank of the Oquirrh Mountains are not chemically similar to any other well characterized Pleistocene ash and are chemically distinct from Tertiary ashes occurring in the Salt Lake Group of the Bonneville Basin as well.

Late Pleistocene Biostratigraphy and Paleoclimate of Gulf of Mexico Deep-Sea Cores: ABSTRACT

Studies carried out on 25 of 55 piston cores collected during Kane surveys in the Gulf of Mexico show that detailed paleoclimatic curves can be determined for the late Pleistocene based on change in frequency of planktonic Foraminifera. Three carbonate-rich cores from the southwestern Gulf of Mexico have been examined in detail and show that most of the 28 species or forms distinguished in these cores show quantitative trends in response to paleoclimatic change. During warm intervals the Globorotalia menardii complex, Pulleniatina obliquiloculata, and Globorotaloides hexagona (interglacial) are characteristic whereas Globorotalia inflata and Globigerina falconensis indicate cool intervals. A total of 3 major warmings and 2 major coolings are recorded; these probably correlate with zones Z to V of Ericson and Wollin. In addition faunal changes of less magnitude reflect secondary temperature oscillations superimposed on the more marked (glacial-interglacial) climatic oscillations. The climatic oscillations are remarkably uniform in some cores if fairly constant sedimentation rates are assumed. Volcanic ash zones in 2 cores approximately correspond to the lowermost and uppermost boundaries of the last interglacial period. In one core a large increase in Orbulina universa coinciding with these ash zones may be due to temporary environmental changes associated with extensive ash deposition. End_of_Article - Last_Page 345------------

Macusanite Occurrence, Age, and Composition, Macusani, Peru

Research Article| May 01, 1970 Macusanite Occurrence, Age, and Composition, Macusani, Peru VIRGIL E BARNES; VIRGIL E BARNES Bureau of Economic Geology, The University of Texas at Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar GEORGE EDWARDS; GEORGE EDWARDS Shell Development Company, Houston, Texas 77001 Search for other works by this author on: GSW Google Scholar W. A McLAUGHLIN; W. A McLAUGHLIN Shell Development Company, Houston, Texas 77001 Search for other works by this author on: GSW Google Scholar IRVING FRIEDMAN; IRVING FRIEDMAN U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar OIVA JOENSUU OIVA JOENSUU The Marine Laboratory, University of Miami, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar Author and Article Information VIRGIL E BARNES Bureau of Economic Geology, The University of Texas at Austin, Texas 78712 GEORGE EDWARDS Shell Development Company, Houston, Texas 77001 W. A McLAUGHLIN Shell Development Company, Houston, Texas 77001 IRVING FRIEDMAN U.S. Geological Survey, Denver, Colorado 80225 OIVA JOENSUU The Marine Laboratory, University of Miami, Miami, Florida 33149 Publisher: Geological Society of America Received: 21 May 1969 Revision Received: 30 Jul 1969 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1970, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1970) 81 (5): 1539–1546. https://doi.org/10.1130/0016-7606(1970)81[1539:MOAACM]2.0.CO;2 Article history Received: 21 May 1969 Revision Received: 30 Jul 1969 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation VIRGIL E BARNES, GEORGE EDWARDS, W. A McLAUGHLIN, IRVING FRIEDMAN, OIVA JOENSUU; Macusanite Occurrence, Age, and Composition, Macusani, Peru. GSA Bulletin 1970;; 81 (5): 1539–1546. doi: https://doi.org/10.1130/0016-7606(1970)81[1539:MOAACM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Macusanite, originally believed to be a type of tektite because of its sculpture, is shown to be related to sillar of the Macusani region, Peru. K-Ar measurements establish identical Pliocene ages (4.2 m.y.) for macusanite and sillar and relate these deposits to the extensive ash flows of the southern Andes. These rocks are unique for glassy rocks in that lithium, boron, and arsenic contents are very high; cesium, rubidium, tellurium, fluorine, and tin are higher than normal; zinc, copper, chromium, and zirconium are lower than normal; and high-alumina minerals such as andalusite are present. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Radio frequencies for industrial and medical applications: Masimenko, P., and Polanskii, Yu Radio ( Moscow), 5, p. 56 (1963)

The hydrothermal processes that occur on the active basaltic edifice of Piton de la Fournaise are investigated. The present-day volcanic activity concentrates in a west–east-oriented collapsed area, 13 km long and 6–8 km large, called Enclos Fouqué. Enclos Fouqué is open to the sea on its eastern side, while a horseshoe rim delimits its extension to the west, south and north. This forms a continuous cliff, 100–200 m high above the floor of Enclos Fouqué. Inside Enclos Fouqué, a 400 m high basaltic cone with two coalescent summit craters constitutes the most active area of the volcano. Beyond the western wall of Enclos Fouqué, a wide basaltic plateau called the Plaine des Sables stretches west for a few kilometers, until it reaches a cliff limiting its western part. To the north and south of this plateau, rivers notch the basaltic edifice, and along the slopes of the induced valleys springs flow. Geochemical data from these springs indicate hydrothermal activity within deep fractured media inside the plateau generated owing to west–east regional extension. On the floor of the Plaine des Sables, near the western wall of Enclos Fouqué, extensive ash deposits from a violent hydrothermal eruption have been recognized. Hydrothermal activity has also been detected in the deep (>100 m) porous layers of Enclos Fouqué, in an area surrounding the central cone. This circulation occurs in layers of vesiculated rocks that constitute a highly permeable and quasi-isotropic medium. Physical models are presented to illustrate the basic differences between the thermal regime of each hydrothermal system. We show that, in the periphery of the volcano, the fracture walls stabilize the hydrothermal circulation inside the fissure network, thus steadily draining the heat along the impermeable walls. Consequently, hot (230–240°C) rising convective currents with a finger shape, 150 m distant, reach the uppermost parts of the fracture. By contrast, inside the isotropic porous layers of the central area, the circulation is likely to be chaotic/turbulent because of the high permeability of the medium. This scatters the thermal energy through the whole porous media. Numerical models of the transient evolution of the hydrothermal circulation in the fractured medium are presented. They reveal that the fingers generated in the deep levels of the edifice move upward at a velocity of 15–150 m/yr, depending on the amplitude of the fracture permeability. The extensive character of the stresses in the Plaine des Sables, the velocity and the high temperature inside the hydrothermal flow constitute necessary conditions for the disruption of the fracture walls during boiling. Using a model of Germanovich and Lowell [J. Geophys. Res. 100 (1995) 8417–8434], these conditions are shown to be sufficient for the generation of huge (km3 of deposits) hydrothermal eruptions in the periphery of the edifice, while only small (a few m3 of deposits) hydrovolcanic events are shown to develop at the summit area. Accordingly, the ash deposits (0.6 km3) recovered near the western wall of Enclos Fouqué are ascribed to the hydrothermal flow in fractures generated by the east–west extensional stresses.

Ultrasonic attenuation in lead: MacKintosh, A.R. Proceedings of the Royal Society of London, 271, No. 1344, p. 88 (1963)

The hydrothermal processes that occur on the active basaltic edifice of Piton de la Fournaise are investigated. The present-day volcanic activity concentrates in a west–east-oriented collapsed area, 13 km long and 6–8 km large, called Enclos Fouqué. Enclos Fouqué is open to the sea on its eastern side, while a horseshoe rim delimits its extension to the west, south and north. This forms a continuous cliff, 100–200 m high above the floor of Enclos Fouqué. Inside Enclos Fouqué, a 400 m high basaltic cone with two coalescent summit craters constitutes the most active area of the volcano. Beyond the western wall of Enclos Fouqué, a wide basaltic plateau called the Plaine des Sables stretches west for a few kilometers, until it reaches a cliff limiting its western part. To the north and south of this plateau, rivers notch the basaltic edifice, and along the slopes of the induced valleys springs flow. Geochemical data from these springs indicate hydrothermal activity within deep fractured media inside the plateau generated owing to west–east regional extension. On the floor of the Plaine des Sables, near the western wall of Enclos Fouqué, extensive ash deposits from a violent hydrothermal eruption have been recognized. Hydrothermal activity has also been detected in the deep (>100 m) porous layers of Enclos Fouqué, in an area surrounding the central cone. This circulation occurs in layers of vesiculated rocks that constitute a highly permeable and quasi-isotropic medium. Physical models are presented to illustrate the basic differences between the thermal regime of each hydrothermal system. We show that, in the periphery of the volcano, the fracture walls stabilize the hydrothermal circulation inside the fissure network, thus steadily draining the heat along the impermeable walls. Consequently, hot (230–240°C) rising convective currents with a finger shape, 150 m distant, reach the uppermost parts of the fracture. By contrast, inside the isotropic porous layers of the central area, the circulation is likely to be chaotic/turbulent because of the high permeability of the medium. This scatters the thermal energy through the whole porous media. Numerical models of the transient evolution of the hydrothermal circulation in the fractured medium are presented. They reveal that the fingers generated in the deep levels of the edifice move upward at a velocity of 15–150 m/yr, depending on the amplitude of the fracture permeability. The extensive character of the stresses in the Plaine des Sables, the velocity and the high temperature inside the hydrothermal flow constitute necessary conditions for the disruption of the fracture walls during boiling. Using a model of Germanovich and Lowell [J. Geophys. Res. 100 (1995) 8417–8434], these conditions are shown to be sufficient for the generation of huge (km3 of deposits) hydrothermal eruptions in the periphery of the edifice, while only small (a few m3 of deposits) hydrovolcanic events are shown to develop at the summit area. Accordingly, the ash deposits (0.6 km3) recovered near the western wall of Enclos Fouqué are ascribed to the hydrothermal flow in fractures generated by the east–west extensional stresses.

Ultrasonic detection of fatigue cracks in low-frequency static load experiments: Rublev, Ya. A., and Danilov, Yu. S. Zavodskaya Laboratoriya, 29, No. 10, p. 1188 (1963)

The hydrothermal processes that occur on the active basaltic edifice of Piton de la Fournaise are investigated. The present-day volcanic activity concentrates in a west–east-oriented collapsed area, 13 km long and 6–8 km large, called Enclos Fouqué. Enclos Fouqué is open to the sea on its eastern side, while a horseshoe rim delimits its extension to the west, south and north. This forms a continuous cliff, 100–200 m high above the floor of Enclos Fouqué. Inside Enclos Fouqué, a 400 m high basaltic cone with two coalescent summit craters constitutes the most active area of the volcano. Beyond the western wall of Enclos Fouqué, a wide basaltic plateau called the Plaine des Sables stretches west for a few kilometers, until it reaches a cliff limiting its western part. To the north and south of this plateau, rivers notch the basaltic edifice, and along the slopes of the induced valleys springs flow. Geochemical data from these springs indicate hydrothermal activity within deep fractured media inside the plateau generated owing to west–east regional extension. On the floor of the Plaine des Sables, near the western wall of Enclos Fouqué, extensive ash deposits from a violent hydrothermal eruption have been recognized. Hydrothermal activity has also been detected in the deep (>100 m) porous layers of Enclos Fouqué, in an area surrounding the central cone. This circulation occurs in layers of vesiculated rocks that constitute a highly permeable and quasi-isotropic medium. Physical models are presented to illustrate the basic differences between the thermal regime of each hydrothermal system. We show that, in the periphery of the volcano, the fracture walls stabilize the hydrothermal circulation inside the fissure network, thus steadily draining the heat along the impermeable walls. Consequently, hot (230–240°C) rising convective currents with a finger shape, 150 m distant, reach the uppermost parts of the fracture. By contrast, inside the isotropic porous layers of the central area, the circulation is likely to be chaotic/turbulent because of the high permeability of the medium. This scatters the thermal energy through the whole porous media. Numerical models of the transient evolution of the hydrothermal circulation in the fractured medium are presented. They reveal that the fingers generated in the deep levels of the edifice move upward at a velocity of 15–150 m/yr, depending on the amplitude of the fracture permeability. The extensive character of the stresses in the Plaine des Sables, the velocity and the high temperature inside the hydrothermal flow constitute necessary conditions for the disruption of the fracture walls during boiling. Using a model of Germanovich and Lowell [J. Geophys. Res. 100 (1995) 8417–8434], these conditions are shown to be sufficient for the generation of huge (km3 of deposits) hydrothermal eruptions in the periphery of the edifice, while only small (a few m3 of deposits) hydrovolcanic events are shown to develop at the summit area. Accordingly, the ash deposits (0.6 km3) recovered near the western wall of Enclos Fouqué are ascribed to the hydrothermal flow in fractures generated by the east–west extensional stresses.

Origin of the Worzel Deep-Sea Ash

IN a recent paper1, Worzel announced the discovery of an extensive ash deposit at the bottom of the Pacific Ocean. Ewing, Heezen and Ericson2 have suggested in a companion paper that the deposit may well be of world-wide extent. If this should prove to be the case, the origin of the Worzel ash layer becomes a matter of extraordinary interest. The mass of a world-wide layer would be of the order of 1 × 1020 gm., which exceeds by a factor of ∼ 300 the yield of the largest volcanic eruption recorded during historic times (Tambora, 1815). Moreover, the areal distribution of volcanic ash is usually quite limited2. Worzel and Ewing et al. have therefore proposed the following events as possible sources of the ash: a single very large volcanic explosion, the simultaneous explosion of many volcanoes, a cometary collision, or some other catastrophic event involving bodies of cosmic origin.