Injection and arrest of dykes: implications for volcanic hazards

Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are commonly assumed to be feeders to lava flows. The actual proportion of dykes reaching the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. This follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes, but also the probability of the injected dykes being arrested. This paper presents field data on several thousand dykes from Iceland and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, it is shown that, for common loading conditions, the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested. The implication is that during periods of unrest in volcanoes, the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber.     

Frasnian–Famennian crisis in the Malaguide Complex (Betic Cordillera,Spain)

Devonian sediments of the Malaguide Complex potentially could include the Frasnian–Famennian boundary, one of the five greatest Phanerozoic biotic crises. Conodont biofacies and microfacies of carbonate clasts from a pebbly mudstone underlying Tournaisian radiolarites allows identification, for the first time in the Malaguide Complex, of Devonian shallow marine environments laterally grading to deeper realms. The clasts yielded Frasnian conodont associations of the falsiovalis to rhenana biozones, with six biofacies that reveal different environmental conditions in their source areas. Source sediments were dismantled and redeposited within the pebbly mudstone, whose origin is tentatively related to one of the events that are associated worldwide with the Frasnian–Famennian crisis. The latter is recorded, in two equivalent Malaguide pelagic successions, by stratigraphic discontinuities, and it was, probably, tectonically and/or eustatically controlled, as in other Alpine‐Mediterranean Paleotethyan margins.     

Changes in heat wave characteristics over Extremadura (SW Spain)

Theoretical and Applied Climatology - Heat wave (HW) events are becoming more frequent, and they have important consequences because of the negative effects they can have not only on the human...     

Large Earthquakes at the Ibero-Maghrebian Region: Basis for an EEWS

Large earthquakes (M_w > 6, I_max > VIII) occur at the Ibero-Maghrebian region, extending from a point (12ºW) southwest of Cape St. Vincent to Tunisia, with different characteristics depending on their location, which cause considerable damage and casualties. Seismic activity at this region is associated with the boundary between the lithospheric plates of Eurasia and Africa, which extends from the Azores Islands to Tunisia. The boundary at Cape St. Vincent, which has a clear oceanic nature in the westernmost part, experiences a transition from an oceanic to a continental boundary, with the interaction of the southern border of the Iberian Peninsula, the northern border of Africa, and the Alboran basin between them, corresponding to a wide area of deformation. Further to the east, the plate boundary recovers its oceanic nature following the northern coast of Algeria and Tunisia. The region has been divided into four zones with different seismic characteristics. From west to east, large earthquake occurrence, focal depth, total seismic moment tensor, and average seismic slip velocities for each zone along the region show the differences in seismic release of deformation. This must be taken into account in developing an EEWS for the region.     

Geochemical mobility of arsenic in the surficial waters from Argentina

The presence of arsenic (As) in surface water constitutes an important environmental risk, where mobility and adsorption processes are responsible for its behavior in the sediment–water interface. Therefore, the assessment of adsorption, mobility and water availability of arsenic in freshwater sediments, with agricultural, livestock and urban soil uses was performed. Arsenic concentrations in sediments ranged from 5.4 to 15.9 mg kg^?1 (total) and 2.8 to 6.5 mg kg^?1 (labile), and those of iron and manganese were 11,563–23,500 and 140.6–662.1 mg kg^?1, respectively. The As levels in water were significantly lower than those of sediments. Results would suggest that As co-precipitation and adsorption on Fe oxides are probably the major route of immobilization, determining its low lability. Manganese did not present an outstanding contribution to the retention, and cation-exchange capacity, pH and organic matter of sediments did not show an influence on the mobility of As.     

Stratigraphy and paleomagnetism of a 3-km-thick Miocene lava pile in the Mjoifjördur area,eastern Iceland

We have carried out stratigraphic mapping in the Upper Miocene basalt lava pile around the fjords Mjoifjördur and Seydisfjördur, eastern Iceland. The mapping is based on conventional methods including the use of interbasaltic clastic horizons and petrographically distinct lava groups. These units are also used to provide correlations with the Nordfjördur area south of Mjoifjördur. We present a 3-km composite stratigraphic column for the area between Mjoifjördur and Seydisfjördur. The geology of this area shows some differences from the classical model of Walker for the structure of eastern Iceland partly due to the fact that most of Mjoifjördur is not in the vicinity of central volcanoes. Detailed laboratory measurements of remanent magnetization were carried out on oriented core samples from 363 lavas in 10 selected profiles. The local paleomagnetic polarity stratigraphy supports correlations made on the basis of other criteria. Over 20 geomagnetic reversals are recorded in the eastern Iceland lava pile in a period approximately 13-10 Ma ago. The geomagnetic field during this period averages to a central axial dipole field, and its overall statistical properties resemble those obtained in earlier surveys in Iceland.     

Surface stresses associated with arrested dykes in rift zones

Many theoretical models predict that arrested dykes may generate major grabens at rift-zone surfaces. Arrested dyke tips in eroded rift zones, however, are normally not associated with major grabens or normal faults that could be generated by dyke-induced stresses ahead of the tips, and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant. Similarly, there are feeder dykes, as well as dykes arrested a few metres below the surface, that do not generate faults or grabens at the surface. Here I propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust. Numerical models presented here show that abrupt changes in Young's moduli, layers with high dyke-normal compressive stresses (stress barriers), and weak, horizontal contacts have large effects on the dyke-induced stress fields. For the models considered, the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift-zone surface. The only significant dyke-induced surface tensile stresses (2 MPa) in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension. That tensile stress, however, peaks above the ends of the weak horizontal contact, which, in the model considered, occur at distances of 4 km to either side of the dyke, and shows no simple relation to the depth to the dyke tip. Thus, for a layered crust with weak contacts, straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results.Editorial responsibility: A. Woods