Cueva de los Naturalistas on the Spanish island of Lanzaote is a wild lava cave that runs below the LZ 58 road about half-way between Masdache and La Vegueta. You can enter through a roof collapse on one side of the road and exit through another a few hundred meters into the lava flows on the far side of the road. It is an amazing place to explore with several branching passages. The most impressive thing about it is the fact you can still see the movement in the surface of the now petrified river of what was once 1200 degrees of moving molten rock. A short distance into the cave is a central pillar where you can clearly make out where the river parted and flowed around it. It is also very dry inside and care needs to be taken not to trip and fall as the lava is very sharp indeed, it will also destroy the soles of your shoes so go prepared with an old pair you don't mind ripping up a bit.
Lava caves are formed when a river of molten rock flowing along the land begins to cool. The much cooler air starts to solidify its surface first and forms a crust. The crust eventually hardens, thickens and then insulates the still moving molten river below. After a while the volcanic eruption ceases and the flow within slows and the level of the river lowers as the last of the lava passes through it. Eventually the stream inside cools and turns back into solid rock, leaving a hollow tube just below the surface.
The top shot was taken on my first visit to this cave and the others on a subsequent visit using the Pentax K1.
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