Lava Caves are formed inside lava, which is molten rock.
They form before or while the lava cools down and solidifies, which means they
There are numerous processes how caves are formed in lava, some are extremely
are by far the most common lava caves.
This process happens very often, and results in extremely huge caves.
As a result lava tubes are the second most common type of caves on earth after karst caves.
Blisters are extremely common in lava, but blister caves not.
The reason is the definition of a cave, which says it must be big enough to be entered by a human.
Most blisters are smaller and do not qualify, big examples are extremely rare.
But there are some blister caves known from Africa, which are up to 100 m in diameter.
Pressure-Ridge Caves or inflationary caves
are formed by pahoehoe flows, where solidified crust on top of the flowing lava buckled crust appears as ridges several meters to a few tens of meters high and rather long.
If they overturn they may form caves.
The folds and ridges are pretty common, but mostly they are much smaller, forming parrallel rope like structures.
Those are called Pele's Hair on Hawai'i, or rope lava by geologists.
If the conditions are ideal they may become much bigger, and may form caves.
The caves are typically rather small and prolate.
Spatter Cone Chambers:
A spatter cone is a conic mountain, formed by the lava produced by a vent.
It is formed right aroung the mound of the vent, as most of the lava which is thrown out falls back vertically.
In the cone is a continuation of the vent, which may be even bigger in diameter or form a magma chamber.
After the eruption the lava may go back into the vent because of a reduction of pressure from below or because the rock shrinks when it cools.
The result is a huge cavern or shaft.
Sometimes it collapses and forms a caldera.
Megapillows are huge hollow pillows of lava.
Pillow lava is really common, it is the typical form lava solidifies if the eruption takes place under water.
The lava forms a hard crust as soon as it contacts the water, which is filled and expanded by the following lava, until the growth ends and the next pillow is formed beneath.
Sometimes the water enters the pillow through a crack, is vapourized and forms a steam bubble inside the pillow, which is inflated and forms a hollow pillow, which is much bigger than its filled companions.
Examples are reported from Italy being 14 by 2 meters in size.
A dyke is a cleft in the rock which is filled by lava from below.
The result are layers of basalt which run straight through all layering of the rock.
The lava close to the surface cools first and forms a sort of plug, but if then the pressure from below ceases the lava below this plug may recede slightly.
Such a cave, 17 m long, about 1.5 m wide and 1 m high with a very small entrance hole, is reported from Australia.
Sub-Crustal Drainage Lava Caves:
The name says it all, lava cools and starts to solidify, the still liquid lava below moves away and leaves a cave.
Both lava tubes and dyke caves are of this type, but there are much more.
A tree is the most common but probably not the only thing, which is meant here.
Trees are often covered by a lava flow, and if they are not burnt while inside the lava they decay rather fast afterwards and leave a hole, the negative mold of the former tree.
Sometimes they are big enough to crawl through, and thus a cave.
In general any plant, animal or other thing which decays rather small will leave such a cave.
There are numerous tree molds over the world, and there is even one cave which is thought to be the mold of a huge rhinoceros.