142 research outputs found
Virtual Geosites as Innovative Tools for Geoheritage Popularization: A Case Study from Eastern Iceland
No full textIn this paper, we have adopted a modern, cutting-edge methodology to make geoheritage sites (geosites) available and explorable worldwide, through both immersive and non-immersive virtual reality, particularly suitable also in COVID-19 times. In doing this, we have focused our attention on five different outcroppings, shallow magma bodies in Iceland: such geological objects, although being often underestimated, are, on the contrary, very suitable for geoheritage popularization purposes. These outstanding outcrops have been transformed in virtual outcrops (VOs) through UAV-based photogrammetry 3D modelling, and have been uploaded on a brand-new, dedicated online resource (GeoVires Virtual Reality Lab for Earth Sciences) which is accessible worldwide for Earth Science teaching and communication. As already stressed above, the choice of these Icelandic shallow magma bodies has been suggested by the fact that such geological objects, although extraordinarily challenging both in terms of geotourism and teaching, are seldom the object of attention from the international scientific community. The five VOs are defined here as virtual geosites (VGs) because they are, indeed, geosites that are fully accessible with a smartphone, a tablet, or a PC; moreover, each is provided with a detailed description and notes available during 3D exploration. Our work could represent a model for future, similar efforts aimed at popularizing Earth Sciences and making geoheritage available to a broad public through VGs
Measurements of Ovis ammon antiqua and Hemitragus bonali from the Middle Pleistocene site of Arago Cave (Tautavel)
No full textRaw measurements of the bones and teeth of the small bovids (Ovis ammon antiqua and Hemitragus bonali) from the Arago Cave (Middle Pleistocene, France). References for the measurements are available in the atlas of measurements (file #1).
The data are the result of the PhD thesis of Rivals (2002):
Rivals, F., 2002. Les petits bovidés pléistocènes dans le bassin méditerranéen et le Caucase. Etude paléontologique, biostratigraphique, archéozoologique et paléoécologique. PhD thesis, Université de Perpignan, Perpignan
Interplay between inherited rift faults and strike-slip structures: Insights from analogue models and field data from Iceland
No full textDifferentiating lava slip planes from tectonic faults: A key issue in structural geology
No full textThe correct geological and structural survey of rock successions containing lava deposits may be biased by the possible confusion between tectonic faults and slip planes produced by lava flow sectors moving at different velocities during emplacement. Microtectonic features on slip planes resulting from lava emplacement, here named "lava slip planes", can have a very similar appearance to slickensides formed under tectonic deformation. In order to come up with criteria to distinguish between the two types of structures, slip planes of various extent from recent trachytic lava flows on Ischia Island (Italy), and basaltic lavas at Mt. Etna (Italy) and along the Northern Volcanic Zone in Iceland, have been studied in detail in the field, sampled, and analysed also with the Scanning Electrone Microscope. Strike-slip planes dominate along flow margins while reverse slip planes dominate near eruptive centers. Normal slip planes can also be found at the sides and at the front of lava flows. Slip planes form at thermal boundaries that enable plastic/brittle deformation and favour the development of shear zones. The rheological properties of the lava should be such that the material has developed a shear strength. Although the best criterion to recognize a lava slip plane is the evaluation of its confinement to a single depositional unit, this approach can be rarely used, due to the frequently limited (decimetre- to meter-sized) outcrop of slip planes. The best criterion, which can be used directly in the outcrop as well as by means of a microscope, is the individuation of burrs, which are sub-millimeter to centimeter-size portions of the slip surface that get dragged away in the direction of movement of the missing block
Dyke to sill deflection in the shallow heterogeneous crust during glacier retreat: part I
No full textDykes and sills occupy Mode I (extension), Mode II (shear), or hybrid mode fractures and most of the time transport and store magma from deep reservoirs to the surface. Subject to their successful propagation, they feed volcanic eruptions. Yet, dykes and sills can also stall and become arrested as a result of the crust's heterogeneous and anisotropic characteristics. Dykes can become deflected at mechanical discontinuities to form sills, and vice versa. Although several studies have examined dyke propagation in heterogeneous and anisotropic crustal segments before, the conditions under which dykes propagate in glacial-volcanotectonic regimes remain unclear. Here, we coupled field observations with 2D FEM numerical modelling to explore the mechanical conditions that encourage (or not) dyke-sill transitions in volcanotectonic or glacial settings. We used as a field example the Stardalur cone sheet-laccolith system, which lies on the Esja peninsula, close to the western rift zone, NW of the southern part of the Icelandic rift. The laccolith is composed of several vertical dykes that transition into sills and form a unique stacked sill 'flower' structure. Here, we investigate whether the Stardalur laccolith was formed under the influence of stresses caused by glacial retreat due to thickness variations (0-1 km) in addition to regional and local tectonic stresses (1-3 MPa extension or compression) and varied magma overpressure (1-30 MPa), as well as the influence of the mechanical properties of the lava/hyaloclastite contact. Our results show that the observed field structure in non-glacial regimes was formed as a result of either the mechanical (Young's modulus) contrast of the lava/hyaloclastite contact or a compressional regime due to pre-existing dykes or faulting. In the glacial domain, the extensional stress field below the ice cap encouraged the formation of the laccolith as the glacier became thinner (subject to a lower vertical load). In all cases, the local stress field influenced dyke to sill deflection in both volcanotectonic regimes
Dyke to sill deflection in the shallow heterogeneous crust during glacier retreat: part II
No full textChanges from dyke to sill propagation in the shallow crust are often caused by dissimilar layer properties. However, most previous studies have not considered the influence of glacial loading and unloading on dyke and sill deflection processes. Here, we attempt to collectively explore mechanical (layer stiffness) and geometrical (dyke dip, layer thickness) realistic parameters subject to two different magma overpressure values (namely 5 MPa and 10 MPa) that promote dyke-sill transitions in both non-glacial and glacial settings. To do this, we use as a field example, the Stardalur laccolith: a multiple stacked-sill intrusion located in SW Iceland. The laccolith lies near the retreating Langjökull glacier and was emplaced at the contact between a stiff lava layer and a soft hyaloclastite layer. We initially model two different stratigraphic crustal segments (stratigraphy a and b) and perform sensitivity analyses to investigate the likely contact opening due to the Cook-Gordon debonding and delamination mechanism under different loading conditions: magma overpressure, regional horizontal extension, glacial vertical load and a thin elastic layer at the stratigraphic contact. Our results show that contact opening (delamination) occurs in both non-glacial and glacial settings when the dissimilar mechanical contact is weak (low shear and tensile stress, zero tensile strength). In non-glacial settings, stiff layers (e.g., lavas) concentrate more tensile stress than soft layers (e.g., hyaloclastites/breccia) but accommodate less total (x–y) displacement than the surrounding host rock (e.g., soft hyaloclastites) in the vicinity of a dyke tip. Yet, a thicker hyaloclastite layer in the stratigraphy, subject to higher magma overpressure (Po = 10 MPa), may encourage dyke-sill transitions. Instead, in glacial domains, the stress conditions imposed by the variable vertical pressure of the ice cap result in higher tensile stress accumulation and displacement in stiff layers which they primarily control sill emplacement
Iceland, an open-air museum for geoheritage and earth science communication Purposes
No full textIceland is one of the most recognizable and iconic places on Earth, offering an unparalleled chance to admire the most powerful natural phenomena related to the combination of geodynamic, tectonic and magmatic forces, such as active rifting, volcanic eruptions and subvolcanic intrusions. We have identified and selected 25 geosites from the Snaefellsnes Peninsula and the Northern Volcanic Zone, areas where most of the above phenomena can be admired as they unfold before the viewers' eyes. We have qualitatively assessed the selected volcano-tectonic geosites by applying a set of criteria derived from previous studies and illustrated them through field photographs, unmanned aerial vehicle (UAV)-captured images and 3-D models. Finally, we have discussed and compared the different options and advantages provided by such visualization techniques and proposed a novel, cutting-edge approach to geoheritage promotion and popularization, based on interactive, navigable Virtual Outcrops made available online
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