The generation and collapse of a foam layer at the roof of a basaltic magma chamber

摘要: Basaltic volcanoes erupt in several different regimes which have not been explained. At Kilauea (Hawaii), eruption can take the form of either fire fountaining, where gas-rich jets propel lava clots to great heights atmosphere, or quiet effusive outflow vesicular lava. Another regime is commonly observed at Stromboli, large gas slugs burst intermittently vent. In an attempt provide a unifying framework for these regimes, we investigate phenomena induced by degassing reservoir empties into small conduit. Laboratory experiments are done cylindrical tank topped thin vertical tube. Working liquids silicone oils and glycerol solutions range viscosity surface tension. Gas bubbles generated bottom with known bubble diameter total flux. The rise through accumulate foam layer roof. Depending on behaviour this layer, three be distinguished: (i) steady horizontal flow leading bubbly conduit; (ii) alternating build-up collapse single, pocket; (iii) partially coalesced larger pockets intermittent slug These natural counterparts basaltic volcanoes.A simple theory proposed explain (ii). contact roof deform under action buoyancy forces, developing flat areas whose size increases as function thickness. Maximum deformation corresponds critical thickness hc = 2σ/eρlgR, σ coefficient tension, ρl liquid density, g acceleration due gravity, R radius e volume fraction foam. determined balance between input from below output conduit, proportional (μl Q/e2 ρlg)¼, μl Q A necessary sufficient condition that it exceeds value hc. given physical properties, occurs when flux depends viscosity, tension size. Experimental determinations time two events agreement theory.