High School
Teacher's Guide
to Craters of the Moon
Chapter Three:
Cave Management
The Indian Tunnel System
Chapter Three:
Cave Management
The Indian Tunnel System
About 2,100 years ago magma poured from a fissure where the Spatter Cones now stand. Sheets of red-hot lava raced down slope, in some places forming swift channels. The rivers of liquid rock sometimes crusted over, and the movement of lava continued below a hardened surface. These tubes transported enormous quantities of lava from the eruption source to the edge of the flow.
The volume of liquid rock was so great that the tubes could not contain it all. Lava burst through the roof of the tube system and formed huge ponds on the surface. Eventually, the flow of lava inside the tubes subsided and lava beneath the hardened surface of the ponds drained back into the tubes. With no lava to support the crust, the ponds collapsed, forming a series of bowl-shaped depressions called sinks.
From the sinks area, the flow advanced to the southwest. Lava tubes more than 30 feet high formed in the thicker sections of the flow, while many smaller tubes branched off to distribute lava to an ever-widening flow front. When the eruption waned, the lava drained from the tubes, adding a new layer to the lava fields beyond. Eventually the flow, called the "Blue Dragon Flow," covered an area of 100 square miles.
The lava tubes which make up the Indian Tunnel system all formed during the eruption of the Blue Dragon Flow. They represent a network of interconnected passageways which helped distribute the lava from its source up to 18 miles away.
The formation of tubes is a complex process dependent on the chemical and physical properties of the lava, eruption rate, and topography. In the simplest method of tube formation, the surface of the pahoehoe, exposed to the cool air, hardens. Lava continues to flow beneath this crust. As the lava supply diminishes, the conduit drains, leaving a hollow lava tube.
Often, though, rather than flowing as a broad sheet, lava becomes restricted in channels. Lava channels usually form in the fastest moving part of a flow along older lava channels and tubes, stream beds, rifts, or other depressions.
Once a channel stabilizes, a roof may form in a variety of ways. A scum may develop and fuse to the channel sides. This crust thickens through overflows on the surface and accretion of cooled lava on its underside. A crust may develop on channel edges and merge like a zipper in the center of the channel. In some channels, crustal plates form, tear loose, and drift along the surface of the flow. Eventually they fuse to one another and the channel sides, forming a roof. In turbulent lava flows splashing, spattering, and overflow along the channel form lava levees. These may continue to build until they arch over the channel and join.
The roof of a lava tube shrinks as it cools, causing numerous cracks that weaken the roof and may cause it to collapse. A roof that collapses while still hot and in a semi-plastic state will sag, creating a bowl-like depression. A roof that has completely hardened will fracture and tumble into a pile of broken rubble on the tube floor. If a tube roof survives the cooling process, it is very stable and may stand for many thousands of years.
Curbs
Ridges on the walls indicate places where the draining river of lava
temporarily held at a constant level. Lava deposits built up on the
walls like a bathtub ring.
Stalactites
Lava stalactites are not created by water, but in the birth of a lava
tube. The receding lava river may leave molten rock on the ceiling, or
the heat within the tube may cause it to remelt. The molten rock drips
and hardens, forming numerous small lava stalactites.
Mineral deposits
There are mineral deposits on the ceilings of many lava tubes, primarily
sulfate compounds. Geologists are not in total agreement about the
source of these minerals. They were deposited by either volcanic gases
during the eruptions, percolating surface water since the eruptions
ceased, or a combination of these two processes.
Ice and water
During the spring months, runoff from melting snow seeps through the
porous cave roofs. At night, the temperature drops and dripping water
freezes into ice stalactites and stalagmites. In most cases, the summer
heat destroys these ice formations, but in a few caves, ice remains all
year. Caves with an impermeable rock or ice floor collect water from
seepage and condensation, creating permanent water holes.
Plants
Most plants cannot survive without sunlight. With its many ceiling
collapses, Indian Tunnel provides suitable habitat for lichens and
mosses on the cave walls. In older tubes, soil builds up on the cave
floor beneath openings and other plants may grow.
Wildlife
Due to the lack of water, food, and light in most caves, animal life is
scarce. However, there are some creatures which take advantage of the
shelter the caves provide. The lava tubes are home to the blind leiodid
cave beetle, an insect whose range is restricted to lava tubes in
southern Idaho. While several bat species live in the monument, it is
unusual to see them in the caves along this trail. You may see other
winged creatures, though. Violet-green swallows, mountain bluebirds,
great-horned owls, and rock doves all use the rocky ledges around cave
entrances as nesting sites. Bushy-tailed woodrats commonly build their
nests in lava tubes. A variety of other birds, squirrels, chipmunks,
foxes, snakes, and other creatures occasionally enter the caves.
Archeological artifacts
Several archeological sites are located near caves. Indian Tunnel
derives its name from the rock circles and scattered artifacts found
near one of the entrances.
The lava tubes are subject to freeze-thaw action, wind and water erosion, and invasion by the plant roots, but the greatest threat to the caves comes from people. The caves area is the most heavily visited part of the monument. Because the resources they contain are irreplaceable, it is critical that they receive adequate protection. The National Park Service has a Cave Management Program which guides decisions about the type of use to be permitted in each cave in the monument. The program is designed to maximize both access to caves and protection of their resources.
The first step in the cave management process is collecting information. In the summer of 1993, a student intern spent 3 months visiting nearly 80 different lava tubes. He inventoried the contents of each cave, noting any significant plants, animals, ice, water, archeological and historic artifacts, rock formations, mineral deposits, or safety hazards. He sketched maps of the caves, photographed their entrances, and used a Global Positioning System (GPS) device to obtain the exact coordinates and elevation of the cave. A GPS device receives and differentiates multiple satellite signals to accurately determine ground location to within 10 meters.
Based on this information, the National Park Service has limited access to a few of the lava tubes at Craters of the Moon. The reasons for the closures vary. Arco Tunnel can be entered only after receiving a permit. An unprepared caver could easily become lost in the complex network of passages in this cave. Groups interested in visiting Arco Tunnel can get the key at the Visitor Center after filling out a permit and receiving a cave map. This enables the National Park Service to see that those venturing into Arco Tunnel have adequate equipment and knowledge to complete the trip safely.
Crystal Pit, like Arco Tunnel, has a locked gate. It is permanently closed not only to park visitors, access of park staff and researchers is also severely limited. There is no way to enter this cave except by rapelling into it and landing on the floor, which is covered with delicate crystal formations. It is thus impossible to enter the cave without causing permanent damage.
There are several caves in the monument which are closed to protect groups of Townsend's big-eared bats. This bat is a Category 2 species under the Endangered Species Act, meaning that listing as endangered or threatened may be appropriate but there is insufficient data to support such action. Townsend's big-eared bats are very sensitive to any disturbance.
The rest of the lava tubes in the monument are open to the public. Those along the Caves Trail area can be easily visited. Indian Tunnel, the largest lava tube, is accessible without a flashlight and receives thousands of visitors each year. The other caves in the area are dark and cooler, require some scrambling, and are more challenging to visit. Most of the lava tubes away from the Caves Trail are in rugged terrain requiring long hikes across the lava, and receive very few visitors.
The National Park Service will provide information upon request about all of the caves except those that are closed to the public. Only a handful of caves are closed to protect visitors, delicate mineral deposits, or sensitive wildlife.
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