Lava Types: Pahoehoe and Aa

Lavas, particularly basaltic ones, come in two primary types: pahoehoe (pronounced 'paw-hoey-hoey") and aa (pronounced "ah-ah"). Both names, like a number of volcanological terms, are of Hawaiian origin. A third type, pillow lava, forms during submarine eruptions. The adjacent picture of a dark pahoehoe flow on a lighter brown aa flow illustrates the difference between the two (photo from Galapagos, Islands Lost in Time by T. De Roy Moore, Viking Press, 1980). The difference in color is in this case is a reflection of age. The older aa in the photo has weathered and the iron in it has oxided somewhat, giving it a reddish appearance (even young aa flows are occasionally slightly brown or reddish, due to the oxidation that occurs during flow). The pahoehoe flow has a comparatively smooth or "ropy" surface. The surface of the aa flow consists of free chunks of very angular pieces of lava. This difference in form reflects flow dynamics.

Aa forms when lava flows rapidly. Under these circumstances, there is rapid heat loss and a resulting increase in viscosity. When the solid surface crust is torn by differential flow, the underlying lava is unable to move sufficiently rapidly to heal the tear. Bits of the crust are then tumbled in and coated by still liquid lava, forming the chunks. Sometimes the crust breaks in large plates, forming a platy aa. Pahoehoe forms when lava flows more slowly. Under these circumstances, a well-developed skin can form which inhibits heat loss. When a tear in the skin does form, it is readily healed. Both magma discharge rate and the steepness of the slope over which the lava flows affect the flow rate. Thus aa lavas are associated with high discharge rates and steep slopes while pahoehoe flows are associated with lower discharge rates and gentle slopes. The steep slopes of the large western Galapagos volcanos thus generally consist of aa, making ascent very difficult (and occasionally painful!). The less common pahoehoe flows on these volcanos are erupted from vents on the gently sloping apron or the caldera floor. Flows which begin as pahoehoe can convert to aa when a steep slope is encountered.