A lava cave is any cave

Cave

A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...

 formed in volcanic rock

Volcanic rock

Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...

, although in typical usage it is reserved for caves formed by volcanic processes, which are more properly termed volcanic caves. Sea caves

Sea caves

A sea cave, also known as a littoral cave, is a type of cave formed primarily by the wave action of the sea. The primary process involved is erosion. Sea caves are found throughout the world, actively forming along present coastlines and as relict sea caves on former coastlines...

, and other sorts of erosional and crevice caves, may be formed in volcanic rocks, but are unrelated to volcanic processes and typically form long after the volcanic rock was emplaced.

Types

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
  Slope
  In mathematics, the slope or gradient of a line describes its steepness, incline, or grade. A higher slope value indicates a steeper incline....
  
  , terrain
  Terrain
  Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
  
  , and viscosity
  Viscosity
  Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
  
  , different kinds of lava tubes can form. Multilateral tubes are those that form paralleling
  Parallel (geometry)
  Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...
  
  , 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 channel
  Lava channel
  A lava channel is a stream of fluid lava contained within marginal zones of static lava or levees. The initial channel may not contain levees per se, until the parental flow solidifies over what develops into the channel and creates simple levees. This initial levee allows for the building of a...
  
  s. 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 widley. 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
  Pressure ridge (lava)
  A pressure ridge is created in an active lava flow. Formation occurs when the outer edges and surfaces of the lava flow begin to harden. If the advancing lava underneath becomes restricted it may push up on the hardened crust, tilting it outward...
  
  , 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
  Bend, Oregon
  Bend is a city in and the county seat of Deschutes County, Oregon, United States, and the principal city of the Bend, Oregon Metropolitan Statistical Area. Bend is Central Oregon's largest city, and, despite its modest size, is the de facto metropolis of the region, owing to the low population...
  
  , are related to pressure ridges and the inflationary process. Liftup caves form on the edges of the pressure ridges or pressure plateaus
  Pressure ridge (lava)
  A pressure ridge is created in an active lava flow. Formation occurs when the outer edges and surfaces of the lava flow begin to harden. If the advancing lava underneath becomes restricted it may push up on the hardened crust, tilting it outward...
  
  . 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
  Hornito
  Hornitos are formed in lava flows and are small openings that release small quantities of lava. Similar to spatter cones, Hornitos are rootless meaning they don't sit on top of the main vent like spatter cones. Instead they are usually created by the slow upwelling of lava through a skylight in a...
  
  . Hornitos are OVC's that form atop lavatubes.

• Pit crater
  Pit crater
  A pit crater is a depression formed by a sinking of the ground surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. It is often found in chains or troughs. Several craters may merge into a linear alignment...
  
  s 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
  Hualalai
  Hualālai is a dormant shield volcano on the island of Hawaii in the Hawaiian Islands. It is the third-youngest and the third most active of the five volcanoes that form the island of Hawaii, following Kīlauea and the much larger Mauna Loa, and also the westernmost. Its peak is above sea...
  
   Volcano in Hawaii
  Hawaii
  Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
  
  , 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
  Craters of the Moon National Monument and Preserve
  Craters of the Moon National Monument and Preserve is a national monument and national preserve located in the Snake River Plain in central Idaho, U.S.A. It is along US 20 , between the small cities of Arco and Carey, at an average elevation of above sea level...
  
  ). 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
  Ape Cave
  Ape Cave is a lava tube located in Gifford Pinchot National Forest just to the south of Mount St. Helens in Washington state. Its passageway is the longest continuous lava tube in the continental United States, at . Ape Cave is a popular hiking destination with beautiful views of the Mount St....
  
  , 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).