Many Hawaiian lava flows solidify as pahoehoe throughout their extent, and a few flows solidify completely as aa. Most flows, however, consist of both pahoehoe and aa in widely varying proportions. In a given flow, pahoehoe upstream commonly changes to aa downstream, but aa lava flows do not change into pahoehoe flows. The explanation for this oneway change lies in the delicate balance between the initial gas content of the lava, the changes in lava viscosity, and the rate of deformation ("shear strain") of the lava during flow and cooling. Once this critical balance is upset, pahoehoe can change to aa.

Fluid lava erupted or flowing under water may form a special structure called pillow lava. Such structures form when molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly solidifies as irregular, tongue-like protrusions. This process is repeated countless times, and the resulting protrusions stack one upon another as the lava flow advances underwater. The term pillow comes from the observation that these stacked protrusions are sack- or pillow-shaped in cross section. Typically ranging from less than a foot to several feel in diameter, each pillow has a glassy outer skin formed by the rapid cooling of the lava by water. Much pillow lava is erupted under relatively high pressure created by the weight of the overlying water; there is little or no explosive interaction between hot lava and cold water. The bulk of the submarine part of a Hawaiian volcano is composed of pillow lavas.