Lava tubes frequently show fantastic features, and I saw some really cool features while I was interning through the GeoCorps Program with the BLM at Craters of the Moon National Monument and Preserve. Some of the more decorative formations are the result of boiling gases inside the lava. Honestly, I hadn’t seen many of these formations before this internship, so it was very exciting!
It was interesting to learn more about lava: how the tube walls themselves cool, how secondary flows erode the original tube, how both cohesiveness and fluidity contribute to form features, and how the pressure in the flow creates different landscapes (like pressure ridges and tumuli.) Being able to observe a great quantity of lava over the course of three months was highly educational - even if I can’t cite lava facts of statistics, I know more about the characteristics of lava by seeing so much of it.
I know I haven’t given much information on the region’s geology itself, but that’s because I get too darned excited about lava tubes.
As the ceiling of a lava tube is cooling: first the exterior layers, and then the interior. Once the exterior layer has begun congealing, gases in the interior lava can boil, squeezing lava out through the exterior layers. (Kind of like a pasta machine.) As this lava drips down, the sides of the drip cool, leaving the liquid lava inside. This liquid lava can then flow to the bottom of the stalactite, creating a hollow space. Sometimes the last bit of the drip falls off the stalactite, other times it plugs the stalactite up. The left picture shows some stubby stalactites from Craters of the Moon, and the right picture shows some really delicate “soda straw” stalactites from near Mt. St. Helens.
If the lava inside these stalactites drips onto the ground, it can pile up to form a stalagmite. I didn’t see many in Idaho, but the ones I did see were really tall. Unfortunately, I didn’t have a camera that day, but I also saw some good ones in Bend this summer.
These are called “stalagpies” by the local cavers, but are more widely known as lava roses, especially when they have a clearly defined series of concentric rings. I think they look a bit gross, but they can form in a really nifty fashion: when lava boils from under a semi-cooled floor, the pressure of the gases pushes the lava up through the floor. As a result, these are also sometimes called “lava volcanoes.” These are identifiably by their “central conduit,” which can be easily seen in the bottom picture. These pictures are from a lava lake, so this explanation makes sense.
Because these don’t have a conduit, I think this is an example of the other way in which lava roses form: when larger clumps or sheets fall from the ceiling, and pile up, cooling, slumping, and cracking as they do so. (I think the right one is especially ugly – it resembles a miniature Horta.) These pictures are from a different cave than the previous lava roses – so a different origin is plausible.
These are lava helictites! These are created in a manner similar to the lava stalactites above, but the lava is pushed through weak spots in the developing crystal structure, forcing it into a twisted shape. Both lava and calcite helictites refuse to obey gravity.
If you want to learn more about lava caves and their features, here are some great resources:
The Virtual Lava Tube is an easily accessible resource, complete with beautiful pictures. This site is run by Dave Bunnell, editor of the National Speleological Society News. He also published the information in a book called Caves of Fire: Inside America's Lava Tubes, which is gorgeous.
Nomenclature of Lava Tube Features is an older article, but describes a greater number of features than the Virtual Lava Tube, including many different types of stalactites and pahoehoe lavas. It’s available in print form in the proceedings of the 6th International Symposium on Vulcanospeleology, and in illustrated form as An Illustrated Glossary of Lava Tube Features. (I wish I’d found the online copy earlier – I accidentally left my print copy in storage!)
An aside: nothing I say on this blog represents the opinion of Craters of the Moon National Monument and Preserve, the BLM, the NPS, the Geological Society of America, GeoCorps, or the National Speleological Society and its internal organizations. I will not disclose any cave locations, but if you wish to go caving in Idaho, please visit Craters of the Moon National Monument (NPS,) the Shoshone Field Office (BLM), or get in touch with your local caving club.