Lava cave
The outer layers of the lava harden, while the interior forms horizontal conduits that channel the advance of the flow.
Length depends primarily on an uninterrupted supply of lava and ranges widely.
[4] Inflationary caves tend to be small chambers that form when lava is pressurized and pushes exterior rock.
In some cases, volcanic gases may exert pressure on solid or semi-solid lava and form what is basically a bubble of thin rock called a blister.
Pressure ridges are fractured lobes of hardened lava and may occasionally be hollow.
These huge open-air pits, with their sheer walls, are analogous to some of the large shafts that formed by solution, and typically require a roped descent for exploration.
While most have no extension beyond the visible floor, others may have entrances into adjacent (now empty) magma chambers, such as was seen when the crater of Mauna Ulu in Hawai`i Volcanoes National Park was explored by a team of Swiss cavers.
In Na One, a pit crater on Hualālai Volcano in Hawaii, a narrow opening at the bottom of an 430 feet (130 m)-deep pit crater leads into an open vertical volcanic conduit, with a total depth of 880 feet (270 m).
The engulfed material eventually burns or decays away, but ends up leaving a hollow space with the original shape.
Usually these are not very large but can get somewhat complex where groups of fallen logs were touching, and may then form caves that go in several directions where the resulting voids intersect.
Elephant mold caves are known from the Nyiragongo volcano in Africa, and one in the shape of a Tertiary-age rhinoceros is known from Blue Lake, Washington.
