into the air where it hardens and becomes solid rock. This molten rock that solidifies in the
air is known aspyroclastic material. In an igneous rock like pumice, small holes in the
solid rock show where gas bubbles were when the rock was still liquid lava.
Low-viscosity lava slides or flows down mountainsides. There is more than one type of low-
viscosity lava. The differences between them come from the lavas’ different composition
and different spots where they come to the surface. The type of igneous formations formed
depends on which type of lava it is. The three major categories are a’a, pahoehoe, and pillow
lava.
A’a Lava
A’a lava is the more viscous of the non-explosive lavas (Figure8.15). This lava forms a
thick and brittle crust which is torn into rough and jagged pieces. The solidified surface is
jagged and sharp. It can spread over large areas as the lava continues to flow underneath.
Figure 8.15: A’a lava flow. ( 13 )Pāhoehoe Lava
Pāhoehoe lava is less viscous than a’a lava, and flows more readily. Its surface looks more
wrinkly and smooth than the jagged a’a lava. Pāhoehoe lava flows in a series of lobes or
rounded areas that form strange twisted shapes and natural rock sculptures (Figure8.16).
Pāhoehoe lava can also form lava tubes beneath the ground (Figure8.17).