Morphology and dynamics of inflated subaqueous basaltic lava flows

International audienceDuring eruptions onto low slopes, basaltic Pahoehoe lava can form thin lobes that progressively coalesce and inflate to many times their original thickness, due to a steady injection of magma beneath brittle and viscoelastic layers of cooled lava that develop sufficient strength to retain the flow. Inflated lava flows forming tumuli and pressure ridges have been reported in different kinds of environments, such as at contemporary subaerial Hawaiian-type volcanoes in Hawaii, La Réunion and Iceland, in continental environments (states of Oregon, Idaho, Washington), and in the deep sea at Juan de Fuca Ridge, the Galapagos spreading center, and at the East Pacific Rise (this study). These lava have all undergone inflation processes, yet they display highly contrasting morphologies that correlate with their depositional environment, the most striking difference being the presence of water. Lava that have inflated in subaerial environments display inflation structures with morphologies that significantly differ from subaqueous lava emplaced in the deep sea, lakes, and rivers. Their height is 2-3 times smaller and their length being 10-15 times shorter. Based on heat diffusion equation, we demonstrate that more efficient cooling of a lava flow in water leads to the rapid development of thicker (by 25%) cooled layer at the flow surface, which has greater yield strength to counteract its internal hydrostatic pressure than in subaerial environments, thus limiting lava breakouts to form new lobes, hence promoting inflation. Buoyancy also increases the ability of a lava to inflate by 60%. Together, these differences can account for the observed variations in the thickness and extent of subaerial and subaqueous inflated lava flows

Long term mobilisation of chemical elements in tephra-rich peat (NE Iceland)

This paper presents geochemical profiles of a tephra-bearing minerotrophic peat column from NE-Iceland obtained using various elemental analyses of the solid phase and the pore water. The influence of tephra grain size, thickness and composition of each tephra on the peat geochemistry was investigated. Interpretations are supported by a statistical approach, in particular by autocorrelation, and by microscopy observations. Minerotrophic peat geochemistry may be strongly dependent upon post-depositional mobilization and possible leaching of elements as demonstrated by Fe and trace metal concentration profiles. Chemical elements, and more specifically potentially harmful metals, can be slowly leached out of volcanic falls during their weathering and re-accumulate downwards. It is emphasised that a tephra deposit can act as an active geochemical barrier, blocking downward elemental movements and leading to the formation of enriched layers. In this study, the formation of poorly amorphous Fe phases above the Hekla 3 tephra is shown. These poorly crystalline Fe phases scavenged Ni. (c) 2008 Elsevier Ltd. All rights reserved

Enregistrement stratigraphique de l’holocène en baie d’Audierne : impact morphologique des tempêtes

Notre étude vise à comprendre l’enregistrement stratigraphique de l’Holocène en baie d’Audierne, un secteur de la côte française très exposé aux houles atlantiques. Des transects de forage et des suivis stratigraphiques sur coupes ont été effectués afin de déterminer l’environnement chronologique des dépôts holocènes et la manifestation de tempêtes. Les formations littorales actuelles sont essentiellement contemporaines de l’Holocène supérieur, surtout depuis l’époque romaine, avec quelques traces d’événements plus anciens. La formation des dunes s’est probablement initiée vers 5 000 ans cal. BP, mais les champs de dunes ne sont apparus que depuis 3 000 ans cal. BP. Le cordon littoral récent s’est formé de 800-1 100 AD à aujourd’hui ; il n’en subsiste actuellement que le revers. Les traces de tempêtes les mieux enregistrées, parfois associées à une rupture de cordon, semblent être liées à des épisodes de refroidissement instable après une période chaude. Certaines sont clairement liées à un contexte froid instable. Des tempêtes hivernales millénaires ont impacté le trait de côte immédiatement avant l’Anomalie Climatique Médiévale, dans un contexte voisin de l’actuel. Nous montrons que l’efficacité maximale en termes de retrait du trait de côte a été atteinte pendant le Petit Age Glaciaire, avec des séries d’événements étalés sur plusieurs jours ou semaines, probablement en relation avec une exportation sédimentaire vers la zone infratidale ou le large. Les formations dunaires de la baie d’Audierne sont une conséquence directe de la restauration des plages après les grandes tempêtes, souvent lors d’événements négatifs NAO secs et venteux en été.Our work deals with the Holocene stratigraphic record of the Bay of Audierne, a sector of the French coasts highly susceptible to the westerly swells. Drilling transects and stratigraphic descriptions have been performed to define the timing of both deposits and recorded storms. Present-day coastal formations essentially consist in Upper Holocene deposits, mostly from the Roman time, with discrete traces of older periods. The onset of the formation of dunes field dates from probably back to 5,000 cal. yr BP, but true dune field only exists since 3,000 cal. yr BP. The active coastal barrier is built between 800-1,100 AD until present; only the back barrier facies are preserved. Evidences of the best preserved storm barriers, with some breaching, seem to be related with unstable cooling events following a warm period. Millennial winter storms have strongly modified the coastline, just prior to the Medieval Climatic Anomaly, in a climate context similar to the present one. We demonstrate that the maximal efficiency for coastal retreat is reached during the Little Ice Age, with clustered events lasting several days or weeks, probably in association with a sediment export to the infratidal zone or offshore. Dunes fields in the Bay of Audierne trace some beach restoral after recurrent large storms, often in connection with the negative NAO mode, dry and windy in summer