Lava Tube Caves
- Excerpted from USGS and Wikipedia Publications
Crystal Ice Cave
Lava Beds National Monument is home to Crystal Ice Cave. This lava cave drops as deep as 150 feet below the ground and runs 960 feet long, and features towering ice stalactites, stalagmites, and two frozen waterfalls. Video courtesy of Midpines Media
A lava cave is any cave formed in volcanic rock, though it typically means caves formed by volcanic processes, which are more properly termed volcanic caves. Sea caves, and other sorts of erosional and crevice caves, may be formed in volcanic rocks, but through non-volcanic processes and usually long after the volcanic rock was emplaced. Specifically, a lava tube is a natural conduit formed by flowing lava which moves beneath the hardened surface of a lava flow. Tubes can drain lava from a volcano during an eruption, or can be extinct, meaning the lava flow has ceased, and the rock has cooled and left a long cave.
A lava tube is a type of lava cave formed when a low-viscosity lava flow develops a continuous and hard crust, which thickens and forms a roof above the still-flowing lava stream. Tubes form in one of two ways: either by the crusting over of lava channels, or from pāhoehoe flows where the lava is moving under the surface.
Lava usually leaves the point of eruption in channels. These channels tend to stay very hot as their surroundings cool. This means they slowly develop walls around them as the surrounding lava cools and/or as the channel melts its way deeper. These channels can get deep enough to crust over, forming an insulating tube that keeps the lava molten and serves as a conduit for the flowing lava. These types of lava tubes tend to be closer to the lava eruption point.
Farther away from the eruption point, lava can flow in an unchanneled, fan-like manner as it leaves its source, which is usually another lava tube leading back to the eruption point. Called pāhoehoe flows, these areas of surface-moving lava cool, forming either a smooth or rough, ropy surface. The lava continues to flow this way until it begins to block its source. At this point, the subsurface lava is still hot enough to break out at a point, and from this point the lava begins as a new "source". Lava flows from the previous source to this breakout point as the surrounding lava of the pāhoehoe flow cools. This forms an underground channel that becomes a lava tube.
Types of Lava Caves
Lava Beds National Monument. Photo by Bill Frantz, NSS
Lava tubes are the most common and extensive type of lava cave. Lava tubes usually form in pāhoehoe 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 longer distances.
Dependent upon the slope, terrain, and lava 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.
Some lava tubes are referred to as ice caves because they contain ice within. The natural thermal shield provided by the surrounding lava allows these caves to trap cold air during winter months. In some instances, these delicate environments create perfect conditions for exceptionally impressive ice crystalization.
Small Surface Tubes
Surface tubes are small drained rivulets, or runners of the same highly fluid lava that flows in lava channels. They form 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 where they formed. Branching is common and broadly dendritic networks are not unusual. Widths range from about four inches (a decimeter) to several yards (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 that form when lava is pressurized and pushes exterior rock. The lava may then later drain leaving an inflationary cave. 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. These blisters are at times big enough to qualify as a cave. Inflated caves can be mistaken for lava tubes because they often share many of the same characteristics. An example of inflationary caves can be found in pressure ridges. Pressure ridges are fractured lobes of hardened lava and may occasionally be hollow.
Lava Beds National Monument. Photo by Dave Bunnell.
Liftup Caves are related to pressure ridges and the inflationary process. Liftup caves can form on the edges of pressure ridges or pressure plateaus where the convex edge of a ridge or plateau begins to expand outward it commonly leaves a void below. Liftup caves are usually no more than 5-10 feet long, though longer ones have been discovered up to 30 feet.
Open Vertical Conduits
These are vertical passages through which lava rose to the surface then receded. They have a round or oval-shaped passage. Depths range from a few feet to at least 165 feet, and diameters range from less than a foot to 25 feet. Their interior consists of remelted lining, usually adorned with short stalactites. Conduits usually, though not necessarily, form at the top of a vent structure like a spatter cone, spatter ridge, or hornito. Hornitos are open vertical conduits that form atop lava tubes. One of the deepest and most spectacular OVCs known is Thrihnukagigur in Iceland. It drops 120 meters from the surface to the upper floor of the magma chamber.
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 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-deep pit crater leads into an open vertical volcanic conduit, with a total depth of 880 feet.
Lava Beds National Monument. Photo by Dave Bunnell.
Rift or fissure caves 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
Lava mold caves or sometimes erroneously called "lava casts", form when lava flows around trees (lava tree molds) 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 (US), 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.
Medicine Lake Highlands. Photo by Dave Bunnell, NSS
A broad lava-flow field often consists of a main lava tube and a series of smaller tubes that supply lava to the front of one or more separate flows. When the supply of lava stops at the end of an eruption or lava is diverted elsewhere, lava in the tube system drains downslope and leaves partially empty caves.
Such drained tubes commonly exhibit step marks on their walls that mark the various depths at which the lava flowed, known as flow ledges or flow lines depending on how prominently they protrude from the walls. Lava tubes generally have pāhoehoe floors, although this may often be covered in breakdown from the ceiling. A variety of speleothems may be found in lava tubes including a variety of stalactite forms generally known as lavacicles, which can be of the splash, shark tooth, or tubular variety. Lavacicles are the most common of lava tube speleothems. Drip stalagmites may form under tubular lava stalactites, and the latter may grade into a form known as a tubular lava helictite. A runner is a bead of lava that is extruded from a small opening and then runs down a wall. Lava tubes may also contain mineral deposits that most commonly take the form of crusts or small crystals, and less commonly, as stalactites and stalagmites.
Catacombs Cave, Lava Beds National Monument. Photo by John Woods, NSS
Lava tubes can be up to 50 feet wide, though most are narrower and some are quite larger. They run anywhere from 3 to 50 feet below the surface. Lava tubes can also be extremely long. One tube from the Mauna Loa 1859 flow enters the ocean about 31 miles from its eruption point, and the Cueva del Viento-Sobrado system on Teide, Tenerife island, is over 11 miles long, due to extensive braided maze areas at the upper zones of the system. One of the most accessible caves in California's Lava Beds National Monument is Sentinel Cave. At 3,280 feet, it offers an enjoyable experience for cavers new to lava tube exploration.
Extraterrestrial lava tubes
By far the largest known lava tubes in the Solar System are on Venus.
Lunar lava tubes have been discovered and have been studied as possible human habitats, providing natural shielding from radiation.
Martian lava tubes are associated with innumerable lava flows and lava channels on the flanks of Olympus Mons. Partially collapsed lava tubes are visible as chains of pit craters, and broad lava fans formed by lava emerging from intact, subsurface tubes are also common.