Leeniea:
The fast-moving lava has no water content - it is way too hot. The water (if any) is driven off explosively as steam. Fast moving lava is either 1) very hot and thus fluid and/or 2) very low in silica content...that is, it represent almost mantle (as opposed to crustal) material. I think we are seeing a little window into the Earth's mantle. Very cool (I mean hot!).