25 August 2008
We’ve talked about how lava becomes molten, now let’s discuss how it behaves once it erupts. As liquid rock erupts from a volcanic vent it is glowing hot and can be very fluid. But, it cools rapidly, and as it does so it behaves more like rock and less like a liquid. Depending on the rate of eruption (among other things) there are two main types of lava flow: a’a and pahoehoe.
A’a is the Hawaiian word for “stony or rough lava” and also “burn” and “blaze”. A’a flows form when there is a lot of lava coming out of the volcanic vent and they tend to move much faster than pahoehoe flows. As you can see, a’a has a very rough texture. As the lava flows, its outer surface cools and quickly becomes brittle, but the inertia of the flow, which is still mostly liquid on the inside, is so great that it just breaks the brittle outer shell and plows forward. As the flow advances it repeatedly breaks any part of itself that cools and becomes too brittle, with the result being a tall (several meters) jagged tongue of lava, covered with loose chunks or “clinkers”. A’a tends to plow over obstacles like a bulldozer due to its inertia.
Pahoehoe flows form when the lava erupts slowly from the volcanic vent. In this case, the lava flows slowly, and the surface exposed to the air (or the ground) cools and acts as insulation, keeping the underlying lava liquid. Eventually the insulating surface breaks, and a little bit of molten lava oozes out, and the flow advances, as shown in the picture above. Pahoehoe tends to form “smooth” lobes and ropy textures, rather than the jagged, clinkery surface of a’a’. Take a look at the foreground of this photo from my recent Hawaii field work to get an idea of what a fresh pahoehoe flow looks like (click for full resolution).
Pahoehoe flows often are fed by breakouts from lava tubes which also serve to insulate the flowing lava and keep it molten, but are much larger than the individual little lobes of a pahoehoe flow. Lava tubes can be several meters across, and they keep the lava very hot and fluid. When an extra burst of lava comes out of the vent, or if a tube gets blocked the lava breaks out, cools and behaves like a slow pahoehoe flow. This doesn’t always happen though, and when I was visiting the active flow, the tube flowed directly to the ocean. In the picture above, the plume in the background is steam where the lava is pouring into the sea.
I was disappointed that we didn’t get to see lava flowing at the surface, but we did find a skylight: a hole in the roof of the lava tube. It was very eerie to peer down into that inferno and watch liquid rock rushing silently along directly under my feet.
One of the interesting things about pahoehoe flows is that they are not very violent. This has some strange effects when it reaches an obstacle. Unlike a’a, which bulldozes everything it its path, pahoehoe just encircles obstacles and then “inflates” as more lava fills in underneath the solidified crust. When pahoehoe touches a tree, the tree is cool enough to solidify the first lava that touches it. Of course, then the tree burns up leaving a cylindrical hole where the trunk was. Here’s a photo that I took of a tree mold where the top surface of the lava was cool enough that most of the tree did not burn up when it fell over:
As with all categories that people come up with to define the natural world, there are some lava flows that are not clearly a’a or pahoehoe. I spent a long time carefully picking my way over a flow that looked as if someone had shattered a pahoehoe flow, jumbled it up and then dumped it out. There are also flows where lava that has been sitting and cooling for some time breaks out and oozes like toothpaste. It’s too viscous to be pahoehoe and too slow and smooth to be a’a.
Finally, I have been ignoring all the different types of rock formed when lava cools in midair before it hits the ground. That may be the topic for another day…