Search results for "rock"
showing 10 items of 1160 documents
Rhyolitic dykes of Paros Island, Cyclades
2005
Abstract The discovery of rhyolitic dykes from the NE part of the island of Paros is described here for the first time. The dykes that can be mapped for a length of ca. 1.1 km are striking about 6° and 38° NE. The width reaches up to 11m maximum. The rhyolitic rocks are porphyric with a fine-grained matrix of mainly feldspar, quartz, some biotites and opaques. K-feldspar, biotite and plagioclase occur as phenocrysts, and mafic-intermediate enclaves are common. At the chilled margin in contact to the gneissic country rocks the dykes are vitric. There, the enclaves have been preserved from contact reaction with the melt and alteration effects and show primary igneous minerals such as clinopyr…
On the growth of maars and diatremes and its relevance to the formation of tuff rings
1986
Small and large maars exist associated with small and large diatremes, respectively, their subsurface feeder structures. The problem of size and growth of maar-diatreme volcanoes is discussed from a phreatomagmatic point of view from field data, some geophysical data, and short-lived historic maar eruptions. A hydrostatic pressure barrier of usually about 20–30 bars is assumed to control the maximum depth level of explosive magma/groundwater interactions. Similar to the situation in submarine and subglacial volcanism, initial maar-forming water vapour explosions are therefore assumed to occur at shallow depth and to produce a small maar with a shallow diatreme. Because of limited availabili…
Allium aetnense (Amaryllidaceae), a new species from Sicily.
2013
A new species from Mt Etna (Sicily), Allium aetenense of A. sect. Codonoprasum, is described and illustrated. It is a diploid species (2n = 16) that grows on basaltic rocks in the mountain belt and shows close relationships with A. tenuiflorum of the Italian peninsula. Its morphology, karyology, leaf anatomy, seed coat microsculpturing, ecology and taxonomic position are examined.
Tracking dynamics of magma migration in open-conduit systems
2016
Open-conduit volcanic systems are typically characterized by unsealed volcanic conduits feeding permanent or quasi-permanent volcanic activity. This persistent activity limits our ability to read changes in the monitored parameters, making the assessment of possible eruptive crises more difficult. We show how an integrated approach to monitoring can solve this problem, opening a new way to data interpretation. The increasing rate of explosive transients, tremor amplitude, thermal emissions of ejected tephra, and rise of the very-long-period (VLP) seismic source towards the surface are interpreted as indicating an upward migration of the magma column in response to an increased magma input r…
Dendropoma lower intertidal reef formations and their palaeoclimatological significance, NW Sicily
1999
Abstract Most carbonate rocky shores of NW Sicily are marked by a coalescence of shells of the gastropod Dendropoma in a construction that is variably developed as a response to wave impact. Here, we review all the available information on these constructions and find that the fossil reefs are reliable sea-level indicators. The thickness of the reef samples never exceeds 30–40 cm below sea-level, whereas all 14 C dates fall within a range of few centuries. Some small fragments ejected by violent sea storms date back to 2500 years cal BP. No samples older than 6200 years cal BP have been detected so far. The present distribution of Mediterranean vermetid platforms should result from a northw…
Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast
1995
Abstract The reconstruction of the evolutive phases of the Mediterranean Spanish Central sector during the Late Quaternary is the aim of this work. From geomorphological, sedimentological and chronometrical studies, different morphostratigraphic units are determined. Results point out the development of several transgressive phases. The older one go back to isotope stage 7. The Eemian transgression is broadly recorded, and its coastline was near the present one. During the Flandrian episode, the sea surpassed the current coastline in the northern coastal plain, while in the meridional littoral erosive notches were formed between +1 and +2 m. During the regressive events, we find the prograd…
Some Considerations on 3-D and 2-D Numerical Models for the Assessment of the Stability of Underground Caves
2014
The application of numerical modeling to the analysis of the stability of both natural and man-made underground caves is rapidly increasing due to the availability of powerful numerical codes, that can account for either continuum or discontinuum behavior of the rock masses. Numerical methods allow to overcome traditional methods for cave stability analysis that assume too simplified geometrical, geological and geomechanical conditions. Further, they are also able to assess the potential failure mechanisms of underground systems. On the other hand, the application of numerical methods requires availability of a detailed geo-structural survey of the cave, as well as a proper geomechanical ch…
Geochemistry and volatile content of magmas feeding explosive eruptions at Telica volcano (Nicaragua)
2017
Telica volcano, in north-west Nicaragua, is a young stratovolcano of intermediate magma composition producing frequent Vulcanian to phreatic explosive eruptions. The Telica stratigraphic record also includes examples of (pre)historic sub-Plinian activity. To refine our knowledge of this very active volcano, weanalyzedmajor element composition and volatile content of melt inclusions fromsomestratigraphically significant Telica tephra deposits. These include: (1) the Scoria Telica Superior (STS) deposit (2000 to 200 years Before Present; Volcanic Explosive Index, VEI, of 2–3) and (2) pyroclasts from the post-1970s eruptive cycle (1982; 2011). Based on measurements with nanoscale secondary ion…
Chronology of the oldest supracrustal sequences in the Palaeoarchaean Barberton Greenstone Belt, South Africa and Swaziland
2016
Abstract Zircon age data for felsic metavolcanic rocks of the Sandspruit and Theespruit formations, the two oldest supracrustal components in the Palaeoarchaean Barberton Greenstone Belt, show that these two successions are time-equivalent and constitute one single volcanic event at ca. 3530 Ma. The Sandspruit felsic rocks are ubiquitously metasomatized, intensely deformed and intruded by, and tectonically interlayered with, ca. 3450 Ma granitoid sills that are probably part of the Theespruit Pluton. One metasomatized Sandspruit sample contains abundant metamorphic zircons with a weighted mean 207Pb/206Pb age of 3220.1 ± 1.6 Ma, reflecting a widespread metamorphic event in parts of the east…
Sm–Nd, Rb–Sr and Pb–Pb dating of silicic carbonates from the early Archaean Barberton Greenstone Belt, South Africa
1998
Abstract Two silicic carbonate sequences occurring in the Onverwacht Group (OG) and the overlying Fig Tree Group (FTG) of the early Archaean Barberton Greenstone Belt, South Africa, contain silicified dolomites with primary structures and textures. They occur with volcanic rocks dated by single zircons. The associated Onverwacht felsic volcanics have an age of ∼3440–3416 Ma, whereas the Fig Tree volcanics are ∼3225 Ma old. Therefore, the depositional age of both sequences is between 3440 and 3225 Ma. Pb-isotopic data for different parts of a single OG whole rock sample form a linear array corresponding to a Pb–Pb age of 2632±99 Ma. Sm–Nd and Rb–Sr isotopes were analysed in four whole rock s…