If you've never walked through a lava-tube, you are in for a Bucket List experience. There are many in Hawai'i (of course), but there are others in the Pacific Cascades of America and even in Idaho. I have a permanent dent in my forehead, a trophy obtained while climbing through Upper Ape Cave, on the south side of Mount St Helens, in Washington, State.
WEAR HELMETS. Do as I say, not as I did.
I’m writing a sci‐fi novel and would like to know what kind of rock makes up a lava tube? As far as I can tell with my feeble mind, it’s basaltic rock, is this right? I’ve tried searching the internet and can’t find a definite answer. Can you help me?
Thanks ahead, J.R.M.
A: Yep, you're right. Higher-silica lava like rhyolite and dacite don't make tubes - but crusty domes instead. It helps if you understand how lava tubes are formed. I've walked through lava-tubes in Hawai'i and Mount St Helens, and you can see that everything - even the "bathtub rings", is basalt. At Pu'u O'o, part of the East Rift Zone of Kilauea, a friend used a police speed-gun to clock the yellow-glowing basalt magma at 40 kph (25 miles per hour) as it shot through the active tube past a skylight. As lava pours down a slope, it finds the natural drainages (or makes its own) and follows them. The lava on the edges of these flowing, yellow-red rivers starts to cool, and then starts to crust over. When the cover is complete, you have a lava-tube that is now insulated from the (relatively) cold air, and liquid lava can now maintain its heat and travel farther. After the the hydraulic pressure stops from above, the tube drains, empties out, and cools off. The inside isn't perfectly smooth, either; there are lots of irregularities, and these are fascinating. They give subtle insights on how magma flows - and "paints" and drips and leaves "bathtub rings" in the walls of the tube.
The roof can be up to 7 meters tall, and boulders and cold lava in the hot lava path get incorporated into the flow or partially dam it. There are parts of the roof that break off - exposing skylights - and these slabs travel down as solid chunks (at least for awhile) in the lava. I have a permanent dent in my right forehead from making a right turn into one of these lava "blades" hanging out of a roof in the Ape's Cave lava tube at Mount St Helens. In my office I have a USGS cap soaked in dried blood from this experience: sort of a trophy, and a reminder to be more careful.