I've read in several texts that there is no (major) chemical
difference in aa and pahoehoe. Yet other references claim they are
compositionally different. What is your opinion?
Bob Holder
Hi Bob,
My opinion is that there isn't a chemical difference between the two.
The fact that you can watch right before your eyes pahoehoe make a
transition into 'a'a pretty much means that there can't be chemical
differences.
The reason why they are so different looking is that one of them ('a'a)
is caused to keep flowing after it has cooled to the point that it has a
viscosity that no longer allows it to flow easily, whereas the other
(pahoehoe) comes to a stop first and cools through high viscosities to a
solid without being disturbed.
The only difference that I could think of is that since
'a'a flows do not have an insulating skin, they tend to react with the
atmosphere to a greater degree while they're stil hot. This tends to
oxidize some of the iron. In fact, many 'a'a flows are noticeably red
(as in oxidized not as in glowing) only an hour or two after coming to a
stop.
I'd be interested to see what chemical differences have been considered
to differentiate the two.
Sincerely,
Scott Rowland
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