CompuServe Messages

As Mr. Trust noted, it's primarily temperature although the amount of certain gases present is also a factor. I can readily imagine that the two are related. Blowing off of those gases would tend to cool the lava down. I'm going by memory from a presentation at the Hawaii Volcanoes National Park, so I can't as specific as you might like, but I believe that pahoehoe has a higher gas content, and I know it's hotter. The presentation included a film, and you can see the difference in the flow rates. Pahoehoe flows quite rapidly, while a`a sort of oozes and has a crumbly looking leading edge.