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Lava Flow (lava + flow)

Distribution by Scientific Domains

Earth and Environmental Science	88%

Selected Abstracts

Retrieving geomagnetic secular variations from lava flows: evidence from Mounts Arso, Etna and Vesuvius (southern Italy)

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002
Alberto Incoronato
Summary Mean directions of magnetization from Mounts Arso (Ischia Island, Gulf of Naples), Etna and Vesuvius lava flows have been determined based on very stringent linearity criteria. These indicate that, regardless of the source volcano, the lava flow mean directions of magnetization form a common path, the SISVC (Southern Italy Secular Variation Curve). This curve enables a reassessment of the age of eruption of several lavas. A date of AD 1169 is demonstrated to be the only possible time of emplacement for one Etna lava flow previously assigned an age of AD 812/1169. It is also demonstrated that two Etna lava flows, which, according to the literature, were emplaced in AD 1536 and 1595 respectively, were actually both emplaced around AD 1037. Three other Etna lava flows, one ascribed to AD 1566 and two to AD 1595, were actually emplaced between AD 1169 and 1284/85. The same time window also holds for a Vesuvius lava flow for which only an upper time threshold was previously available. Only one of the studied flows needs further sampling and analysis to verify whether this flow has been affected by a complete remagnetization or has an erroneous historical dating. The applied procedure seems to be the most appropriate one in carrying out palaeomagnetic surveys of lava flows, as also suggested by the broad agreement with some 17th and 19th century measurements of the geomagnetic field in Rome, relocated to Etna, and is likely to improve knowledge of past history of a volcano significantly. [source]

Magnetic field intensity study of the 1960 Kilauea lava flow, Hawaii, using the microwave palaeointensity technique

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2000
Mimi J. Hill
It is extremely valuable to study historic lava flows where the geomagnetic field at their time of extrusion is well known. In this study, two vertical sections, 16 m apart, have been sampled from the approximately 1 m thick 1960 Kilauea lava flow, Hawaii. Variations are seen in the rock-magnetic and palaeomagnetic properties between and within the two sections, indicating that there are small-scale lateral and vertical variations in the lava flow. The two sections showed different responses to microwave palaeointensity analysis. Section H6001 generally gave ideal linear behaviour on plots of natural remanent magnetization (NRM) lost against microwave-induced thermoremanent magnetization (TM,RM) gained, whilst the majority of samples from H6002 showed anomalous two-slope behaviour. When all plots were interpreted by taking the best-fitting line through all points, the flow mean intensity for H6001 was 31.6,±,3.6 ,T and that for H6002 was 37.1,±,6.4 ,T, compared with the expected intensity of 36 ,T. Additional historic flows need to be studied in order to ascertain whether this behaviour is typical of all lava, and whether it is best to always interpret NRM lost/TM,RM gained plots by taking the line of best fit regardless of shape. [source]

Petrology of the Yamato nakhlites

METEORITICS & PLANETARY SCIENCE, Issue 11 2005
N. Imae
They are paired cumulate clinopyroxenites. We obtained the intercumulus melt composition of the Yamato nakhlites and here call it the Yamato intercumulus melt (YIM). The YIM crystallized to form the augite rims, the olivine rims and the mesostasis phases in the cumulates. The augite rims consist of two layers: inner and outer. The crystallization of the inner rim drove the interstitial melt into the plagioclase liquidus field. Subsequently, the residual melt crystallized pigeonites and plagioclase to form the outer rims and the mesostasis. Three types of inclusions were identified in olivine phenocrysts: rounded vitrophyric, angular vitrophyric, and monomineralic augite inclusions. The monomineralic augite inclusions are common and may have been captured by growing olivine phenocrysts. The rounded vitrophyric inclusions are rare and may represent the composition of middle-stage melts, whereas the angular vitrophyric inclusions seem to have been derived from fractionated late-stage melts. Glass inclusions occur in close association with titanomagnetite and ferroan augite halo in phenocryst core augites and the assemblages may be magmatic inclusions in augites. We compared the YIM with compositions of magmatic inclusions in olivine and augite. The composition of magmatic inclusions in augite is similar to the YIM. Phenocrystic olivines contain exsolution lamellae, augite-magnetite aggregates, and symplectites in the cores. The symplectites often occur at the boundaries between olivine and augite grains. The aggregates, symplectite and lamellae formed by exsolution from the host olivine at magmatic temperatures. We present a formational scenario for nakhlites as follows: (1) accumulation of augite, olivine, and titanomagnetite phenocrysts took place on the floor of a magma chamber; (2) olivine exsolved augite and magnetite as augite-magnetite aggregates, symplectites and lamellae; (3) the overgrowth on olivine phenocrysts formed their rims, and the inner rims crystallized on augite phenocryst cores; and finally, (4) the outer rim formed surrounding the inner rims of augite phenocrysts, and plagioclase and minor minerals crystallized to form mesostasis under a rapid cooling condition, probably in a lava flow or a sill. [source]

Petrology, Mineralogy and Geochemisty of Antarctic Mesosiderite GRV 020175: Implications for Its Complex Formation History

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010
Linyan WANG
Abstract: GRV 020175 is an Antarctic mesosiderite, containing about 43 vol% silicates and 57 vol% metal. Metal occurs in a variety of textures from irregular large masses, to veins penetrating silicates, and to matrix fine grains. The metallic portion contains kamacite, troilite and minor taenite. Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides. Silicate phases exhibit a porphyritic texture with pyroxene, plagioclase, minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass. The matrix is ophitic and consists mainly of pyroxene and plagioclase grains. Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth; others are characterized by many fine inclusions of plagioclase composition. Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims. Olivine occurs as subhedral crystals, surrounded by a necklace of tiny chromite grains (about 2,3 ,m). Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature (,1000°C) and a closure temperature (,875°C). Molar Fe/Mn ratios (19,32) of pyroxenes are consistent with mesosiderite pyroxenes (16,35) and most plagioclase compositions (An87.5,96.6) are within the range of mesosiderite plagioclase grains (An88,95). Olivine composition (Fo53.8) is only slightly lower than the range of olivine compositions in mesosiderites (Fo55,90). All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase, it can be further classified as a type 3A mesosiderite. Mineralogical, petrological, and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion. Following primary igneous crystallization, the silicate underwent varying degrees of reheating. It was reheated to 1000°C, followed by rapid cooling to 875°C. Subsequently, metal mixed with silicate, during or after which, reduction of silicates occurred; the reducing agent is likely to have been sulfur. After redox reaction, the sample underwent thermal metamorphism, which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts, and homogenized matrix pyroxenes. Nevertheless, metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials. [source]

Evidence for two episodes of volcanism in the Bigadiç borate basin and tectonic implications for western Turkey

GEOLOGICAL JOURNAL, Issue 5 2005
Fuat Erkül
Abstract Western Turkey has been dominated by N,S extension since the Early Miocene. The timing and cause of this N,S extension and related basin formation have been the subject of much debate, but new data from the Bigadiç borate basin provide insights that may solve this controversy. The basin is located in the Bornova Flysch Zone, which is thought to have formed as a major NE-trending transform zone during Late Cretaceous-Palaeocene collisional Tethyan orogenesis and later reactivated as a transfer zone of weakness, and which separates two orogenic domains having different structural evolutions. Volcanism in the Bigadiç area is characterized by two rock units that are separated by an angular unconformity. These are: (1) the Kocaiskan volcanites that gives K/Ar ages of 23,Ma, and (2) the Bigadiç volcano-sedimentary succession that yields ages of 20.6 to 17.8,Ma. Both units are unconformably overlain by Upper Miocene-Pliocene continental deposits. The Kocaiskan volcanites are related to the first episode of volcanic activity and comprise thick volcanogenic sedimentary rocks derived from subaerial andesitic intrusions, domes, lava flows and pyroclastic rocks. The second episode of volcanic activity, represented by basaltic to rhyolitic lavas and pyroclastic rocks, accompanied lacustrine,evaporitic sedimentation. Dacitic to rhyolitic volcanic rocks, called the S,nd,rg, volcanites, comprise NE-trending intrusions producing lava flows, ignimbrites, ash-fall deposits and associated volcanogenic sedimentary rocks. Other NE-trending olivine basaltic (Gölcük basalt) and trachyandesitic (Kay,rlar volcanites) intrusions and lava flows were synchronously emplaced into the lacustrine sediments. The intrusions typically display peperitic rocks along their contacts with the sedimentary rocks. It is important to note that the Gölcük basalt described here is the first recorded Early Miocene alkali basalt in western Turkey. The oldest volcanic episode occurred in the NE-trending zone when the region was still experiencing N,S compression. The angular unconformity between the two volcanic episodes marks an abrupt transition from N,S collision-related convergence to N,S extension related to retreat of the Aegean subduction zone to the south along an extensional detachment. Thrust faults with top-to-the-north sense of shear and a series of anticlines and synclines with subvertical NE-striking axial planes observed in the Bigadiç volcano-sedimentary succession suggest that NW,SE compression was reactivated following sedimentation. Geochemical data from the Bigadiç area also support the validity of the extensional regime, which was characterized by a bimodal volcanism related to extrusion of coeval alkaline and calc-alkaline volcanic rocks during the second volcanic episode. The formation of alkaline volcanic rocks dated as 19.7,±,0.4,Ma can be related directly to the onset of the N,S extensional regime in western Turkey. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Retrieving geomagnetic secular variations from lava flows: evidence from Mounts Arso, Etna and Vesuvius (southern Italy)

Spatial patterns of microsite colonisation on two young lava flows on Mount Hekla, Iceland

JOURNAL OF VEGETATION SCIENCE, Issue 2 2008
N.A. Cutler
Abstract Questions: How does vegetation first establish on newly-formed lava substrates? Do very small (cm) and meso-scale (m) variations in the physical environment influence this process and subsequent vegetation development? Location: Mount Hekla, southern Iceland (64°00' N, 19°40' W). Methods: Data on vegetation structure and the incidence of ,safe sites' suitable for colonisation were collected from high and low points on the surfaces of lava flows emplaced during the 1991 and 2000 A.D. eruptions of Mount Hekla. Effects of flow age and meso-topographic position on vegetation structure (moss cover, patch density, stem length) were assessed by two-way analyses of variance. The distributions of colonisation events and available safe sites were analysed using point pattern techniques. Results: Rapid colonisation of the lava surface was observed, despite stressful environmental conditions. The 1991 and 2000 flows differed significantly in vegetation structure, but there were no significant differences in moss cover, patch density and stem length between ,high' and ,low' sites. Conclusions: Colonisation events are invariably associated with small-scale irregularities on the surface of the lava. The colonisation process appears to be spatially random. Development of the moss ,carpet' proceeds by vertical thickening and lateral growth and coalescence of moss patches that establish in ,safe sites'. This process is rapid, with close to 100% of available safe sites exploited within 20 years. Topographic position makes no difference to the very early stages of vegetation development and cannot be used to ,forecast' the later stages of development. [source]