On the significance of Caledonian pahoehoe, aa, and pillow lava from Bømlo, SW Norway

A subaerial metabasalt sequence containing pahoehoe flow units showing a transition to aa type occurs in Caledonian rocks on the island Bømlo, southwest Norway. The pahoehoe flow units occur as flattened tube-like bodies and are highly vesicular. It is proposed that most of the lava represents shelly pahoehoe, a type produced very close to the crater by the welling out of gas-charged lava. As the lava flowed away from the vent it graded into the beginning stage of aa type lava as the viscosity increased. The pahoehoe lava is overlain by pillow lava indicating local submergence of the area subsequent to the pahoehoe lava flow. The palaeo-flow directions have been estimated from the elongation of pillow lava and pahoehoe tongues and the direction of the pahoehoe-aa transition. The predominance of subaerial volcanics in this area is discussed in relation to older, thick submarine pillow lava sequences in other parts of the Norwegian Caledonides.