Results 1 - 10 of 733
Results 1 - 10 of 733. Search took: 0.028 seconds
|Sort by: date | relevance|
[en] The determination of CO2 and H2O content of magma prior to the eruption and collected in crystallised components give us an indication more or less accurate about the depth and the chemical composition of silicated melt. The two components CO2 and H2O are deduced from glassy inclusions trapped in minerals during crystal growth. Development and improvement of nuclear microanalyses were performed, carbon content is easily analysed, analysis of hydrogen is possible but need more time and more experiments
[en] The extensive lava piles of Iceland contain a unique record of geomagnetic field variations spanning the past ~ 16 Ma. Since the 1950s, palaeomagnetic data have been obtained from over 9400 Icelandic lavas. We have compiled all palaeomagnetic data currently available and developed a publicly accessible database ( http://www.icepmag.org ). The data within the database are primarily palaeodirections, with a relatively smaller number of palaeointensity data (8936 entries contain direction only, 218 intensity only, and 337 both direction and intensity). In addition, the database contains a wide range of metadata, including geochronological information, site details, and laboratory methods. The search interface of the database allows users to search for data using a range of customisable filters (e.g. by publication, geological age, location, laboratory method, palaeomagnetic statistics) and to quickly visualise and download the matching search results. The data within the database have significant potential for understanding long-term palaeomagnetic field variations at high latitude, the behaviour of excursions and reversals, and geological mapping on Iceland. .
[en] Complete text of publication follows. There are two types of commonly used methods for determining absolute paleointensity from volcanic rocks: the Thellier-type method (Thellier and Thellier, 1959) and the Shaw-type method (Shaw, 1974). Most paleomagnetists have regarded the former method as the most reliable, but evidence is accumulating which indicates that the Thellier-type method is not always robust for historical basaltic samples. There have been a relatively increasing number of studies to clarify possible causes of incorrect Thellier paleointensity determinations obtained from historical lava flows. It seems that many authors prefer 'multi domain (MD) grains' as the predominant cause. However, apart from the MD grains, we think high-temperature (HT) oxidation states of titanomagnetite (TM) grains might also influence to resultant Thellier paleointensities: (1) low to middle degree of magnetostatic interactions are expected for low to middle HT oxidized TM grains (low to middle numbers of ilmenite lamellae) possibly resulting in about 10∼30 % high Thellier paleointensity; (2) high degree of magnetostatic interactions as well as acquisitions of thermochemical remanent magnetization (TCRM) are expected for middle to high HT oxidized TM grains (abundant ilmenite lamellae) possibly resulting in about 20∼70 % high Thellier paleointensity. This conjecture is based on the thorough experimental results obtained from the Hawaiian (Kilauea 1960) and Japanese (Sakurajima 1914 and 1946) historical lava flows. We will introduce the detail in the talk.
[en] Sample from lavas cored in the volcanic substratum of the atoll of Mururoa have been selected in order to carry out K-Ar datings to determine precisely the conditions for the building and the piling up rate. The ages for the upper 600 m range between 11.8 and 10.7 Ma. They indicate the presence of two volcanoes. The first, in the central part, was built up essentially prior to 11.3 Ma; afterwards, between 11.3 and 10.7 Ma its activity appears more scatter and trachytic lavas occurred during this ultimate period. A smaller, wholly basaltic volcano developed southwards and build up between 11.1 and 10.7 Ma. The age of the youngest lava flows (around 10.7 Ma) agrees with the position of Mururoa along a volcanic lineament linked to a hot spot the present location of which would be South-East of Pitcairn Island. It corresponds to a mean velocity of about 11 cm per year for the motion of the South-Pacific plate
[en] On the Alba Patera volcanic shield of Mars, a Hesperian flood-lava phase was followed by the extrusion of sheet lavas and tube-fed lavas emerging in many cases from the flanking fissures of rising domes. These events were followed by the eruption of additional sheet and tube-fed lavas from linear vents which formed complex flow fields. Later, Amazonian volcanism at Alba involved long, narrow flows from two complex summit calderas; the thermal energy outflow for some individual flows would have been substantially greater than the annual heat loss of the earth through volcanism, implying that the process of patera-building represented substantial Martian geological heat-loss during the planet's early volcanic-centralization stages. 40 refs
[en] We conducted a broadband magnetotelluric (MT) survey along a north–south transect across Unzen graben, Japan. The MT survey line is located ~ 2 km west of the most recent lava dome and consisted of 27 stations along a 9-km profile. We estimated the 3-D resistivity structure and correlated it with the seismic reflection structure obtained by the same survey line as in the present study. The best-fit resistivity structure shows an upper resistive layer underlain by a moderately conductive layer. The resistive layer, which is interpreted as a cold groundwater zone, is cut by four faults marked by their relatively high conductivity. The underlying layer, which is interpreted as a hydrothermal-water-rich layer, also shows relatively conductive values near the faults. By assuming that the faults are imaged as relatively conductive zones, we infer the dip and depth extent of fracture zones around the faults. Beneath the Chijiwa Fault, which is the longest and most active fault of Unzen graben, the dominant conductor (C1) has a width of 2 km and extends down to below 4 km depth. C1 corresponds to a zone of strong seismic reflection and is located close to one of the pressure sources causing surface deformation. In this study, we interpret C1 as a network of fractures generated by the Chijiwa Fault to which magmatic volatiles are supplied from a deeper pressure source. Given that C1 extends to a greater depth and its resistivity is lower than other conductive zones, it is possible that earthquakes have occurred repeatedly on the Chijiwa Fault. In the center of the study area, we identify a vertically oriented body of high resistivity (R1) that corresponds to a zone of low seismic reflectivity. We interpret R1 as a cooled dyke complex that may have acted as a volcanic conduit. .
[en] This report summarizes the current status of detailed petrographic studies of the basaltic lava flows identified in the XVII-XXV-L LVIII sectors, in the period 1970-1972; in the same are some conclusions concerning to the textural classification of these rocks. This paper attempts to standardize the criteria of petrographic description of these rocks, in order to establish bases or significant comparative petrographic s parameters
[en] The behavior of K, U and Th in lava at different space and time levels was studied by geodynamic methods. Examples of well-defined volcanic series bond up with characteristic magmatic processes were chosen for this purpose. Various cases were studied, corresponding to either general or particular problems and distributed over three of the large structural domains of the earth's crust: oceanic zone (oceanic islands, Afar rift); insular volcanic arcs (Japan, the lesser Antilles); Continental zone (mediterranean volcanism, basalts and associated derivatives). K, U and Th averages are given for certain of the commonest types of lava found on the earth's surface. Certain hypotheses on the genesis of magmas are confirmed or invalidated and a structural model is proposed for the sub-Afar layers
[fr]On a etudie les comportements de K, U et Th dans les laves a differents niveaux, aussi bien d'espace que de temps, en appliquant la methodologie geodynamique. Pour cela on a choisi des exemples de series volcaniques bien definies et liees a des processus magmatiques caracteristiques. On a etudie un certain nombre de cas correspondant soit a des problemes generaux soit a des problemes particuliers repartis dans trois des grands ensembles structuraux de l'ecorce terrestre: domaine oceanique (Iles oceaniques, Rift Afar); arcs volcaniques insulaires (Japon, Petites Antilles); domaine continental (volcanisme mediterraneen, basaltes et differencies associes). On donne des moyennes de K, U et Th pour certains types de laves parmi les plus repandues a la surface du globe. On confirme ou infirme certaines hypotheses genetiques des magmas et on presente un modele pour la structure des couches sous-Afar
[en] Complete text of publication follows. Samples from the 1960 lava flow on Hawaii Island were studied using the multi-specimen parallel difference pTRM method to validate the ancient field strength of ∼36 μT. Several sets of specimens were treated at different temperatures, and between 440 and 500C paleointensities between 32.2 and 34.2 μT were obtained. At 550C a significant overestimate was obtained which was clearly caused by thermal alteration processes. At 400C a similar overestimate was obtained, possibly the effect of cooling rate differences. The specimens used for these PI determinations were thermally stabilized and a full TRM was produced for synthetic PI experiments. These reproduced the laboratory field within error limits, if similar cooling rates were used for the full TRM and pTRM acquisition. Using a slower cooling rate for the TRM acquisition led to an overestimate of the PI. Systematic variations of the unblocking temperature spectra of different specimens were observed on the between drill-core level, while such variability was much smaller between specimens of one drill-core. Based on this observations a new protocol of the multi-specimen PI method presented, that is much less affected by such rock magnetic variability. The protocol is tested on synthetic TRM and natural TRM, using specimens from the 1960 lava flow.