Pahoehoe and Aa Basalt

Click the photo for the full-size version. This lava flow displays two textures of lava that have the same composition. The ragged, clinkery form on the left is called aa (or in more proper Hawaiian spelling, ‘a‘a). You pronounce it "ah-ah." Perhaps it has that name because the rough surface of the solidified lava can quickly cut your feet to ribbons, even with heavy boots. In Iceland, this kind of lava is called apalhraun.

The lava on the right is shiny and smooth, and it has its own name, like aa a Hawaiian word—pahoehoe. In Iceland, this kind of lava is called helluhraun. Smooth is a relative term—some forms of pahoehoe can have a surface as wrinkled as an elephant's trunk, but not at all jagged like aa.

What makes the exact same lava produce two different textures, pahoehoe and aa, is the difference in the way they have flowed. Fresh basalt lava is almost always smooth, liquid pahoehoe, but as it cools and crystallizes it turns sticky—that is, more viscous. At some point the surface can't stretch quickly enough to keep up with the movement of the flow's interior, and it breaks and shreds like the crust of a loaf of bread. This can happen simply from the lava growing cooler, or it can occur as the flow spills down a steep place making it stretch faster.

For photos of related rocks, see the volcanic rocks gallery.