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Structure and organization of submarine basaltic flows: sheet flow transformation into pillow lavas in shallow submarine environments |
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| Authors: | M Carracedo Sánchez F Sarrionandia T Juteau J I Gil Ibarguchi |
| Institution: | 1. Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, Sarriena s/n, 48940, Leioa, Spain 2. Departamento de Geodinámica, Facultad de Farmacia, Universidad del País Vasco UPV/EHU, 01006, Vitoria, Spain 3. Professor Emeritus, Université de Bretagne Occidentale, Brest, France |
| Abstract: | Distal pillows occur associated with a sheet flow and megapillows in the me?akoz outcrops of the Basque–Cantabrian Basin (N Spain). Basaltic volcanic rocks are interbedded with Turonian sediments and depict typical features of shallow submarine emissions. An exceptional basaltic flow displays four types of morphology: (1) sheet lava with columnar jointing, (2) welded columnar breccia, (3) megapillows, and (4) pillow lavas with sparse megapillows. The field data from me?akoz combined with experimental and field data from the literature for similar volcanic facies can be integrated into a new propagation model for the transition from sheet flows to pillow lavas in underwater environments. At near vent high emission rates, lava flows develop a thin crust immediately after its emplacement and break at the front under the magma pressure allowing for the massive propagation of lava as a sheet flow. Increased cooling promotes thickening of the lava outer crust far from the vent while continuous supply of fresh magma increases the pressure onto the thick crust until its rupture. The lava emitted in small volumes from the flow front promotes the formation of megapillows and pillow lavas that are later on covered by the advancing sheet flow. The lava flow freezes progressively toward more distal parts, gradually increasing its viscosity until it stops. The crust temporarily holds the residual melt pressure increasing the volume of the flow distal section by inflation. Finally, the internal magma pressure breaks the crust and liberates lava at moderate-to-low flow rates producing pillows, while lava drainage inside the inflated sheet flow produces lava tunnels and gravitational collapse of the roofs by hydrostatic pressure to form breccias nurtured by columnar lava fragments. |
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