There are a couple of questions that came to mind:
"What would lava look like, if it immediately froze over?" and, "Have we forgot how nature can still be unexplainable, and do things that defy
scientific reasoning as well?" Also, the black smooth top surface, reminded me of a thick layer of black ice. But then again, I could even be
wrong.
Here are a few links to lava tubes lava tube sink holes, and then you can compare them all. To me, anything is possible.
(Pisgah Crater in the Mojave desert)
quest.nasa.gov...
farm3.static.flickr.com...
Glacier caves occur in ice and under glaciers and are formed by melting.
Fracture caves are formed when layers of more soluble minerals, such as gypsum, dissolve out from between layers of less soluble rock.
Talus caves are the openings between rocks that have fallen down into a pile, often at the bases of cliffs.
(this next one is a Glacier Cave Opening)
www.photosfan.com...
Some caves are formed at the same time as the surrounding rock. These are sometimes called primary caves.
Lava tubes are formed through volcanic activity and are the most common 'primary' caves.
Lava caves, include but are not limited to lava tubes. Other caves formed through volcanic activity include rift caves, lava mould caves, open
vertical volcanic conduits, and inflationary caves.
Corrasional or erosional caves are those that form entirely by erosion by flowing streams carrying rocks and other sediments. These can form in any
type of rock, including hard rocks such as granite.
I find nature to be the most amazing and mysterious aspect of life. Proving time over, that anything is possible. And just when we all think we have
it all figured out....we really do not. The world alone will never fully be even 150% explored. It has been said that 98% of Earth has yet to be
explored and discovered. That 2% covers a lot already (to our human mind).