Pahoehoe, aa, and block lava

Abstract

Macdonald describes the surface features and cross-sections of pahoehoe and aa. He distinguishes fast-moving sheet pahoehoe from slower-moving tube-fed pahoehoe. He also distinguishes pahoehoe toes (concentric structure) from pillow lava (radial structure) in cross section, and includes a table listing other distinguishing criteria. He contrasts the vesicle shapes of aa with those of pahoehoe. He states that pahoehoe commonly turns to aa, but that aa is never observed to grade back into pahoehoe. He contrasts block lava with both aa and pahoehoe, noting that true block lava is not found as basalt. He lists 14 facts regarding the relationship between aa and pahoehoe, both static and flowing. He notes as important the relationship between viscosity and flowage. He concludes that aa has the same chemistry and viscosity as pahoehoe, but has lost gas, such that agitation or stirring of the lava is important in effecting the transition from pahoehoe to aa. (Here Macdonald uses the making of fudge as an analogy.)