When lava is flowing, how long does it take to harden up?

rocky Valarie Maccarone

Hi Valerie,

Well, lava cools at different rates depending on how fast it is flowing. If it is flowing really fast, every time a cool crust develops, the motion of flowing breaks it apart and it has to form new crust. The new crust gets broken again, and on and on and on. All the broken pieces of crust are jagged and rough, and form the top layer of the flow. These are called 'a'a flows. Because 'a'a flows tend to be pretty thick (2-10 meters), their interiors cool very slowly once the flow comes to rest. An `a`a flow may be still warm a couple of weeks after it has stopped (as long as it hasn't been raining heavily. Only when the flow pretty much stops does it really begin to cool. When lava is flowing slowly, it can form a crust that gets carried along with the flow. In this case the crust is smooth and glassy, and this type of lava is called pahoehoe. Pahoehoe flows are made up of thousands of small flows called "toes" A single pahoehoe toe will be hard enough to walk on in about 20 minutes, and pretty much cooled in a couple of days. The thing is you never have just one toe, you have a whole pile of them so put together they will cool much more slowly. If the whole flow is 2-5 meters thick it can take just as long as an 'a'a flow to completely cool.

Pahoehoe and 'a'a are Hawaiian words that have been adopted by volcanologists all over the world. They don't have any other meaning other than the names of these types of lava. There are some stupid books that say "'a'a is what ancient Hawaiians used to say when they walked on 'a'a lava barefoot." This is a racist remark that has no basis in fact. Hawaiians were not so stupid that they would walk on 'a'a barefoot. Other equally stupid books say that "pahoehoe" means ropy or satiny in Hawaiian but that is not true either. They are just names for these types of lava.

I hope this gives you some idea of how lava flows behave.

Sincerely,

Scott Rowland, University of Hawaii 

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