How long does it take for magma to cool so you can walk on it?

rocky Margie

Hi Margie,

You've asked a good question, but first a definition: Volcanologists use the word "magma" for molten rock that is still under ground and hasn't yet erupted. Once it has erupted onto the surface, we call it "lava". It really hasn't undergone any fundamental change (it may be a little cooler), but using the two different names makes things clearer. Therefore your question is:
How long does it take for lava to cool before you can walk on it?
If it is pahoehoe lava (the smooth glassy type) it will support your weight after only 5-10 minutes since it stops moving. It will still be pretty hot and uncomfortable, and probably might melt your boots, but if you absolutely had to get across such a flow you could. 'A'a flows are a different story. They are the big rough-surfaced flows with a top surface made up of broken and spiny blocks called clinkers. You can walk on an 'a'a flow while it is still moving but it is not very pleasant. 'A'a is not very fluid so you don't have to worry about sinking in but because there are so many spaces between the clinkers for heat to escape, they are really hot. You would only cross a hot one if you absolutely had to.

There is an account in the Lyman Museum in Hilo that says that the old Hawaiian Kahuna (priests) would step on fresh pahoehoe toes so that the molten lava would squish out the sides, just to impress the common people!

I hope this gives you some ideas on how fast lava cools.

Sincerely,

Scott Rowland, University of Hawaii 

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