Entrances

There are two common types of entrances to lava tubes. Either they are formed by the collapse of the cave’s ceiling, or they lead in from a roofed-over portion of a collapsed trench. If the collapsed entrance is relatively small in relation to the cave, roughly the diameter of the underlying passage, we tend to call it a skylight. Most of those in the aerial photo #1 are skylight sorts of entrances. The second photo is either a huge skylight of a portion of collapse in a trench depending on yur definition. But perhaps the most common type of entrance, especially in the mainland USA, is at the edge of a collapse along a passage segment, which may gives access to more cave passage after emerging into more collapse. In Hawaii, either of these collapse-type entrances is known as a puka. More rarely, entrances may be found in cliff faces, such as those occurring in the ongoing eruptions of Kilauea in Hawaii, where tubes carry the lava all the way to the ocean and sometimes pour out of entrances in the cliffs.

Though initially formed by collapse, entrances may be modified by later lava flows, such as shown in the vertical photo #7 below. Here, a red lava flow through the tube has almost overflowed one of the skylight entrances, but then drained back in and coated the walls with a new layer of red. Inside the cave passage these might be known as linings, especially where a collapse or contraction crack makes linings pull away from their underlayers.

Depending on the location of a lava tube and the age of the lava, the entrance zone may form a unique ecosystem that harbors plant life not seen on the nearby surface. Particularly in arid regions, the tube may offer a supply of moist air that can harbor such things as ferns and mosses. In the bottom photo, ferns are found beneath a surface practically devoid of vegetation. Even in the lush Hawaiian rainforest, a tube entrance may provide a habitat for plants not seen nearby on the surface, such as in photos 5 and 6.