Types of Lava Flows
Pahoehoe (pronounced: pah-hoy-hoy)
Pahoehoe is a Hawaiian term for “smooth." A pahoehoe lava flow derives from a basaltic volcanic eruption that is extremely hot with a low viscosity. For this reason, the lava flows freely over a large surface area and takes a long time to cool. Types of pahoehoe lava flows are famous for their twisted, “ropey" texture. As the lava flow slowly cools, a smooth, delicate film forms over the surface. This film gets repeatedly broken by the hot, progressing bulk of the pahoehoe lava, creating a system of tubes, lobes, and ropes downhill. When the pahoehoe fully solidifies, it has the effect of a massive, rushing, black river flowing from the vent of the volcano.
Sheet lava is aptly named because it resembles a massive, black blanket of rock over a flat surface area. Like pahoehoe, sheet lavas are basaltic and wide-spreading. They emerge from fissure volcanic eruptions in large flat regions. As a sheet lava flow pools and begins to cool, the still molten interior will buckle the crust to create a mosaic of swirls, seams, and lobes.
Aa (pronounced: ah-ah)
Aa is a Hawaiian term meaning “rocky" or “ragged." Types of aa lava flows are characterized by their sharp edges, jagged surface, and heaping piles of crumbled volcanic rock. Unlike pahoehoe, aa is highly viscous and relatively low temperature, as the lava has lost its volcanic gases before the volcanic eruption. A heavy crust cools quickly over the surface of the aa flow, while fresh, hot lava bulldozes its way through and breaks the crust up into jagged piles of rock. This buildup can form a wall or a dam that contains the lava flow after awhile, preventing aa volcanic eruption from spreading over an extensive distance. Occasionally pahoehoe flows will graduate to aa as they move further from the vent and are allowed to cool. The eruption of Mauna Loa, Hawaii in 1984 was an example of an aa type of lava flow.
Block lavas emerge from a volcanic eruption highly viscous and moderately cooled. These types of lava flows tend to be thicker and heavier than aa flows but are often classified as the same. While aa is generally basaltic, block lava is composed of felsic rhyolite or andesite which is lighter in color, cools much faster, and travels slower than aa flows.