Search results for " ROC"
showing 10 items of 951 documents
Petrography and decay of a marly limestone in the cloister of a medieval cathedral in Sicily
2004
This paper deals with a significant process of decohesion of a marly limestone, taking place in the cloister of the medieval Cathedral of Cefalu, a pleasant town on the northern coast of Sicily. After desalination with deionised water and consolidation with ethyl silicate, the decay of the stone became faster. The aim of our study is to characterise the stony material and investigate the observed decay phenomena. The stone, that is a poor building material indeed, is characterised by means of petrographical, chemical and physical analyses on samples taken from the monument. Furthermore, experimental tests are performed in the laboratory in order to highlight the causes of incompatibility be…
Outgassing: Influence on speed of magma fragmentation
2013
[1] Predicting explosive eruptions remains an outstanding challenge. Knowledge of the controlling parameters and their relative importance is crucial to deepen our understanding of conduit flow dynamics and accurately model the processes involved. This experimental study sheds light on one important parameter—outgassing—and evaluates its influence on magma fragmentation behavior. We perform fragmentation experiments based on the shock tube theory at room temperature on natural pyroclastic material with a connected porosity ranging from 15% to 78%. For each sample series, we determine the initial pressure (P) required to initiate magma fragmentation (fragmentation threshold, Pth). Furthermor…
Explosive origin of silicic lava: Textural andδD–H 2 O evidence for pyroclastic degassing during rhyolite effusion
2014
A long-standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava. A widely cited idea is that the explosive-to-effusive transition manifests a two-stage degassing history whereby lava is the product of non-explosive, open-system gas release following initial explosive, closed-system degassing. Direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive, as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent. This explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed…
Evolution of depositional settings in the Torrey area during the Smithian (Early Triassic, Utah, USA) and their significance for the biotic recovery
2015
This work focuses on well-exposed Lower Triassic sedimentary rocks in the area of Torrey (south-central Utah, USA). The studied Smithian deposits record a large-scale third-order sea-level cycle, which permits a detailed reconstruction of the evolution of depositional settings. During the middle Smithian, peritidal microbial limestones associated with a rather low-diversity benthic fauna were deposited seaward of the tidal flat siliciclastic red beds. Associated with siliceous sponges, microbial limestones formed small m-scale patch reefs. During the late middle to late Smithian interval, the sedimentary system is characterized by tidal flat dolostones of an interior platform, ooid-bioclast…
Mercury contents and isotope ratios from diverse depositional environments across the Triassic–Jurassic Boundary: Towards a more robust mercury proxy…
2021
Abstract Mercury is gaining prominence as a proxy for large igneous province (LIP) volcanism in the sedimentary record. Despite temporal overlap between some mass extinctions and LIPs, the precise timing of magmatism relative to major ecological and environmental change is difficult to untangle, especially in marine settings. Changes in the relative contents of Hg in sedimentary rocks through time, or ‘Hg anomalies’, can help resolve the timing of LIP activity and marine extinctions. However, major questions remain unanswered about the fidelity of Hg as a proxy for LIP magmatism. In particular, depositional (e.g., redox) and post-depositional (e.g., oxidative weathering) processes can affec…
Taphonomy of insects in carbonates and amber
2004
Abstract The major taphonomic processes that control insect preservation in carbonate rocks (limestones, travertines and nodules) are biological: insect size and wingspan, degree of decomposition, presence of microbial mats, predation and scavenging; environmental: water surface tension, water temperature, density and salinity, current activity; and diagenetic: authigenic mineralisation, flattening, deformation, carbonisation. The major taphonomic processes that control the preservation of insects in fossil resins (amber and copal) are different, but can be considered under the same headings – biological: presence of resin producers, size and behaviour of insects; environmental: latitude, c…
Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations
2022
Abstract The Ordovician (∼487 to 443 Ma) ended with the formation of extensive Southern Hemisphere ice sheets, known as the Hirnantian glaciation, and the second largest mass extinction in Earth History. It was followed by the Silurian (∼443 to 419 Ma), one of the most climatically unstable periods of the Phanerozoic as evidenced by several large scale ( > 5 ‰ ) carbon isotope (δ13C) perturbations associated with further extinction events. Despite several decades of research, the cause of these environmental instabilities remains enigmatic. Here, we provide osmium (187Os/188Os) and lithium (δ7Li) isotope measurements of marine sedimentary rocks that cover four Silurian δ13C excursions. Osmi…
Fabric transitions from shell accumulations to reefs: an introduction with Palaeozoic examples
2007
One unresolved conceptual problem in some Palaeozoic sedimentary strata is the boundary between the concepts of ‘shell concentration’ and ‘reef’. In fact, numerous bioclastic strata are transitional coquina–reef deposits, because either distinct frame-building skeletons are not commonly preserved in growth position, or skeletal remains are episodically encrusted by ‘stabilizer’ (reef-like) organisms, such as calcareous and problematic algae, encrusting microbes, bryozoans, foraminifers and sponges. The term ‘parabiostrome’, coined by Kershaw, can be used to describe some stratiform bioclastic deposits formed through the growth and destruction, by fair-weather wave and storm wave action, of …
Evolution of the middle Paleozoic magmatism in the Chinese Altai: Constraints on the crustal differentiation at shallow depth in the accretionary oro…
2019
Abstract The Chinese Altai underwent intensive mafic and felsic magmatism in the middle Paleozoic, which was the most important magmatic event in the region. It is still unclear for the role of the event in formation of the crust that is thick (∼56 km) and has structure of three-layer stratification. In this study, the data for the middle Paleozoic mafic rocks and granites are comprehensively compiled and integrated with new geochronological, petrological, geochemical and Sr-Nd-Pb isotopic data for gabbroic rocks from Keketuohai area and tonalite and biotite granites from Habahe area. The middle Paleozoic mafic rocks exhibit two evolutionary trends, which can be defined as high-Al and low-A…
The tectonic domains of southern and western Madagascar
2019
Abstract Southern and western Madagascar is comprised of five tectonic provinces that, from northeast to southwest, are defined by the: (i) Ikalamavony, (ii) Anosyen, (iii) Androyen, (iv) Graphite and (v) Vohibory Domains. The Ikalamavony, Graphite and Vohibory Domains all have intermediate and felsic igneous protoliths of tonalite-trondhjemite-granodiorite-granite composition, with positive eNd, and low Sr and Pb isotopic ratios. All three domains are interpreted to be the products of intra-oceanic island arc magmatism. The protoliths of the Ikalamavony and Graphite Domains formed repectively between c. 1080–980 Ma and 1000–920 Ma, whereas those of the Vohibory Domain are younger and date …