Estructura y organización de las coladas submarinas: características de las lavas almohadilladas de edad cretácica que afloran en la Cordillera Vasco- Cantábrica

Abstract

In the Basque-Cantabrian Basin, an important submarine volcanic activity of alkaline character was developed during the upper Cretaceous. This vulcanism was related to a rift and/or transform fault in the continental crust associated to the opening of the North Atlantic ocean. Pillow lava flows are noteworthy among the other volcanic materials by their volume and excellent preservation state. The lava flows are formed by the pile up of small flow-and cooling units, i.e. tubes or lava tubes, characterized by: i) coarse cylindrical morphology with abundant constrictions, ii) diameter less than 1 meter in a transversal section, iii) smooth or striated surface, iv) concentric and/or radial internal structure, and iv) the branches and direction changes during the outflow. Lava flows/tubes shape and surface characteristics depend on the viscosity, effusion rate and the thickness of quenched crust during growth.
The Tubes are moted directly on feeder dykes or are connected in tabular flows. The expanding and advancement of the tubes was the result of stretching or breaking of the quenched surface crust and spreading of the molten lava from the interior. Stretching features and cracks appear mainly at the flow front, but lobes of lava developed from the top and the flanks of the tubes are not uncommon. Only scarce pillowed lavas are truly isolated magma sacks separated from their sources. Related to the tabular flows and the biggest pillow lavas, some breccias were occasionally formed by the gravitational collapse of the roof of the draining tunnels.