This material was written by Scott Rowland in Hawaii:

As for lava flows, most study has been done on basaltic lavas. They occur in two basic types, 'a'a and pahoehoe. 'A'a flows are thicker (usually 3-5 meters). They have slightly higher viscosities but move faster. They are produced by high effusion eruptions. Pahoehoe flows are thinner (usually <1 meter) and consist of thousands and thousands of flow units called toes. The total flow can be many meters thick, and composed of all of these toes. Pahoehoe flows are associated with low effusion eruptions and they move slowly.

'A'a and Pahoehoe flows can be the same length and volume, but of course it will take a pahoehoe flow longer to reach the same length. Pahoehoe flows are characterised by smooth glassy surfaces which remain intact while the lava flows beneath the cooled skin. 'A'a flows have jumbled rough surfaces. The surfaces get constantly torn apart while the flow rapidly flows along and pretty soon it has cooled to a viscosity that can no longer flow; instead it breaks into the jagged clinkers.

Pahoehoe and `a`a are Hawaiian words that have been adopted by the volcanological community. They don't have any meaning other than the names of the two lavas.

Other types of lavas include block lava, which has a surface of large angular blocks, and rhyolite lava. These two types are associated with lava chemistries other than basalt. They tend to be very thick (10-200 meters) and slow moving.

There are many chapters in many books on these subjects. Hopefully this will give you a start and by finding one of these books you can get some better examples.

Chuck Wood, University of North Dakota

Other Categories        Other Questions
To VolcanoWorld