Pahoehoe vs aa
4 messages in this thread
I'm looking for a good distinction between pahoehoe and aa lava as found on
Hawaii. Is there any chemical difference? Or is it merely the texture caused
by the rate of cooling? TIA. Jim
I believe the temperature difference is the main factor. As pahoehoe cools, it
turns to an aa flow, then to a block flow. Usually only basalt forms pahoehoe,
occasionally andesitic flows will form pahoehoe, though andesites usually form
aa and block flows. Rhyolite (when not an ash flow or nuee ardente) generally
only forms block flows. Basaltic flows tend to be in excess of 1000 C;
andesitic between 850 C-1000 C; rhyolitic about 750 C (excluding ash flows).
It seems to me that temperature is the main control; but keep in mind that the
temperature prior to eruption is controlled in part by the composition of the
magma.
Jeffrey
Good answer on aa.
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.