The surfaces produced by lava flows often reflect their internal movement in relation to their congealing crust. Sheet- and pahoehoe lava flows result in continuous surfaces. Aa- and block flows result in discontinuous surfaces. The terms "pahoehoe" and "aa" are Hawaiian words.
(Fig. 2.46) Pahoehoe lavas. Credit: Dr. Duncan Heron
Pahoehoe lavas are thin. They flow smoothly and are often formed by small volumes of hot, fluidbasalt. The higher the volume of lava emitted the faster the current. Pahoehoe flows move forwards in tongues or lobes and are characterized by a glassy, plastic skin. They may embrace obstacles at a rate of about 50m an hour. When the pahoehoe lava flow cools, it often solidifies to a smooth surface.
(Fig. 2.47) Sheet lavas. Credit: Mr.Mathot
Sheet lavas emerge from fissure systems forming flows commonly ranging between 10m and 30m in thickness. They flow out so fast that vast volumes of basalt are discharged over an enormous area. Featureless lava plateaus are formed. During the eruption of Roza, Oregon, 14 million years ago, 1500km3 of sheet lavas were produced in about a week.
(Fig. 2.48) Aa Flows. Credit: Dr. Duncan Heron
Aa flows are emitted from the vent at high rates ranging to 50km an hour, often with much lava fountaining. They are characteristic of viscous magmas. Aa flows are animated with sporadic bursts of energy. They may push down houses, walls and forests. However, the hallmark of aa lava flows is the very rough surface it produces when it cools and solidifies.
(Fig. 2.49) Block Lavas. Credit: Dr. Duncan Heron
Block-lavas are often emerged in a fairly viscous state. They tend to be both stronger and thicker than aa lava flows. The more silicic the magma, the shorter and stubbier is the flow. Block lavas move slowly at a rate ranging from 1 to 5 meter a day. When solidified, they are characterized by often cubic masses with relatively smooth faces. In comparison with aa lava flows the surfaces of block lavas are much less rough and pitted than aa lava flows.