Igneous Speleothems? Dark goings on inside lava tubes.

On the island of Maui, and also Big Island, in Hawaii there are lava tubes.  We visited the biggest one on Maui for a bit of a look around. Lava tubes are, as the name might suggest, tubes made of lava! When lava flows flow downhill the outer surfaces cool, solidify and harden as they're exposed to the atmosphere. Typically the top surface of the flow is put under stress from the flowing lava beneath and it breaks up and rejoins the flow.  Thus many lava flows form their own levees which channel the flow downhill. This is fairly normal. Sometimes, however, the top surface doesn't break up and it too cools and hardens forming a solid crust around the flow.  Since rock is a fairly good insulator the inside of the flow remains warm and gooey and continues to flow through underneath the crust.  If all of the flow comes out at the far end then you're left with a hollow structure known as a lava tube. Overtime more flows might flow through the tube, cooling and thickening the walls and cutting deeper down into the bedrock beneath, so these things can become quite large with all sorts of erosional and depositional features you might expect from caves made by the erosional power of water

Lava tubes even have speleothems (that's stalactites and stalagmites) although they form in a completely different way to the limestone ones which I study.

Descent into the lava tubes

The way down

Chocolate brown stalactites

Stalactites are pretty common, formed when the dropping lava levels form drips on the upper surface, that cool and solidify.  It is even possible that proper calcite speleothems begin to grow over the top of them from water running down through the cave system, getting through cracks formed as the walls shrank as they got cooler. But the lava tubes are generally too recent to get any decent paleoclimate data from as the stalactites haven't had long enough to get very big.

Stalagmites!

Stalagmites can also form, but this is much rarer. The ones in the photogragh above are about forty centimetres tall. These stalagmites also form from dripping lava, typically from a fresh lava flow over the top of the lava tube seeps through cracks in the tube and drops down into the empty tube.

The green and coppery colours that you may just be able to make out in this photograph (its a lot more obvious in real life) are chemolithotrophs.  They are organisms (typically bacteria) that get their energy source not from the sun (either directly or indirectly as most of life does) but from the rocks.  These guys literally feed off of the rocks and use chemical reactions of the compounds inside to generate energy.  Plus there are all sorts of former surface organisms that have become specialised to caves, gone white, and lost their eyes and such like, but these are quite rare, as you may imagine.

Typically the last lava flow to flow through the tube is a piddly little affair. It runs down through the centre of the tube, the sides of it cool and form levees which channel the flow.  As a result you sometimes get these bowling alley type formations running through the middle of the tubes.

Lava tubes are pretty damn cool to visit, hopefully the limestone ones that I'll be off to in five weeks time or so will be just as entertaining.