There are many types of lava caves, with these being the most notable:
- Lava tubes are the most common and most extensive kind of lava cave. Lava tubes usually form in pahoehoe lava flows, though exceptions exist. As the lava is emitted from the vent area, it spreads in the path of least resistance. The outer layers of the lava harden, while the interior forms horizontal conduits that channel the advance of the flow. These conduits are the beginning stages of lava tubes that serve to insulate the heat from the lava which then provides a way for the lava flow to advance far and wide. Dependent upon the slope, terrain, and viscosity, different kinds of lava tubes can form. Multilateral tubes are those that form paralleling, often branching and anastomosing tubes. Multilevel tubes are those that sit directly on top or underneath another tube, sometimes above or below several tubes. Some lava flows hold a mixture of multilevel and multilateral tubes. One other form a lava tube is the tube-in-tube which can form inside lava tubes if the linings of the walls are weak enough to lean inward, forming a new floor above the old. Tube-in-Tubes are generally noted to form during the last lava draining through the main lava tube.
- Ice caves Some lava tubes are referred to as ice caves on maps, not because they are made in ice, but because they harbor ice.
- Surface tubes are small drained rivulets, or runners of the same highly fluid lava that flows in lava channels. They are formed on an existing hardened surface, and most are too small to enter. They are created by flowing lava that turns itself inside out. Sometimes referred to as "toes," they are thought to be instrumental in the growth (lengthwise) of lava tubes. They usually form when vents, channels, or reservoirs of lava overflow. They are very shallow and typically reside within the first few feet under the surface. Some surface tubes can connect to lava tubes deeper below the surface. Surface tubes typically have a uniform wall thickness and semi-circular cross section, flat side down against the surface on which they formed. Branching is common and broadly dendritic networks are not unusual. Widths range from a decimeter to several meters. Length depends primarily on an uninterrupted supply of lava and ranges widely. Surface tubes are far more numerous than is generally realized because most are subsequently buried.
- Inflationary caves tend to be small chambers which form when lava under pressure pushes up surface rock from below and then drains back down. One surface expression of this is the pressure ridge, a fractured lobe of hardened lava which is uncommonly hollow. 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. Rarely, the latter may be big enough to qualify as a cave. Inflated caves can be mistaken for lava tubes as well, for inside they can share a lot of the same characteristics.
- Liftup caves, a phrase coined by Lawrence Chitwood, a geologist of Bend, Oregon, are related to pressure ridges and the inflationary process. Liftup caves form on the edges of the pressure ridges or pressure plateaus. Specifically, when a concave edge of the ridge or plateau begins to inflate leaving a void underneath. They are shelters of no more than five to ten feet in length, though longer ones have been discovered up to thirty feet long.
- Open vertical conduits or OVCs are vertical passages through which lava rose to the surface then receded. Their mouth is usually, though not necessarily, at the top of a vent structure like a spatter cone, spatter ridge, or hornito. Hornitos are OVC's that form atop lavatubes.
- Pit craters form when magma that doesn’t quite reach the surface drains to form a void, and the ground above it slumps. These huge open-air pits with their sheer walls are analogous to some of the large shafts 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 130m-deep pit crater leads into an open vertical volcanic conduit, with a total depth of 268 m.
- Rift or fissure caves, which form along volcanic rift zones and eruptive fissures, or in fractures associated with volcanic activity. These are tectonic in formation, caused by “stress in lava during and after solidification.” They may also be the site of fissure eruptions, and the walls covered with spatter. Notable rift caves include Crystal Ice Cave, formed in Idaho’s Great Rift (and now part of Craters of the Moon National Monument and Preserve). Caves in the Great Rift are known up to 800 feet deep.
- Lava mold caves or sometimes erroneously called "lava casts", form when lava flows around trees or even large dead animals. 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. Such caves are known from Washington (USA), near Ape Cave, and most notably from Japan in the Yoshida-tanai area. 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 (USA).
Read more about this topic: Lava Cave
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