The Formation of Basaltic Lava Flows

Abstract

The basaltic lava flows of Mount Lassen and the Hawaiian Islands have three distinguishable surface textures. Aa lava has a texture resembling that of broken scoria, block lava has broken cleanly after congealing, and pahoehoe has a relatively smooth surface. Aa lava with channels is usually found on the steeper slopes, while pahoehoe lava with tunnels is usually found on the flatter slopes of Hawaii. The block lavas of Mount Lassen resemble shrinkage polyhedrons, with one or more aa or pahoehoe faces, violently disturbed while cooling. Flow units can be distinguished in recent flows and in the ancient flow lavas composing valley walls. Mixed contacts between aa and pahoehoe apparently show the effects of pressure in addition to the former fluidity. The first lava out of a vent is first to congeal and may be left exposed near by, at both sides of the flow. This effect should be considered in sampling; later lava appears near the front of the flow. Under certain conditions, successively later lavas might b...