In the western U.S. there are two principal types of caves: solution caves in limestone and lava tube caves in basalt. Their origins could hardly be more different. Limestone caves are cavities slowly dissolved by acidic groundwater in bedrock laid down millions of years previously. Lava tube caves are cavities once occupied by flowing lava, in bedrock emplaced by and usually no older than the lava tube itself.

Lava tubes are conduits on or within a lava flow that carry lava to the advancing front of the flow. They form only in pahoehoe lava (pronounced pah hoy hoy), a highly fluid type that behaves much like water as long as it stays hot. However, it quickly cools and hardens when exposed to the atmosphere, becoming a strong and durable rock known as basalt.

Simply stated, tubes form initially by hardening of the outer surfaces of masses of lava, inside which the fluid lava continues to move. Because basalt is a good insulator, lava can travel great distances inside lava tubes, with only slight loss of heat. (Heat loss of about 3 degrees F. per mile has been measured in a Hawaiian lava tube.) That is why lava tubes are a principal means by which pahoehoe lava is spread thinly over wide areas.

HOW LAVA TUBES FORM

Lava tubes originate in two distinctly different ways: as surface tubes, or by roofing of a lava channel. Surface tubes form all at once, on an existing surface (hence the name). Roofing is an evolutionary process that forms a roof over an existing channel.

SURFACE TUBES

Surface tubes are the simplest form of lava tube. They are the hardened outer jacket of a lava lobe, or toe, resting on an existing surface. Lobes and toes are rounded, elongate pods of lava that emerge through the crusted exterior of active pahoehoe lava flows, and are a principal means of expansion of such flows. Surface tubes are semi-circular in cross section, flat side down. Their size depends primarily on an uninterrupted supply of lava and ranges from small blisters, to rambling sausage-like tubes with many branches. Seldom are they larger than a few feet in diameter, so by definition many are not caves (because they cannot be entered by a person).

LAVA TUBES

Nearly all lava tubes (other than surface tubes) begin with formation of a roof over a river of lava flowing in an established lava channel. Steady eruption of pahoehoe lava will inevitably result in formation of channels, because eventually there simply isn't enough lava to supply all parts of the growing lava field. Outer parts of the spreading lava field stagnate, harden, offer resistance, and lava flows where resistance is least. (The route of a channel may be controlled by pre-existing topography.) Lava rivers, like rivers of water, can meander, overflow their banks, build levees, erode their channel downward and, of primary importance to the formation of lava tubes, can "freeze over," or develop a roof. Unlike rivers of water, lava rivers are larger nearer the source.

If lava flow in a channel is moderate and steady, crusts build from each side of the channel to meet in the center, forming a roof, and a lava tube is born. Roofs are often broken and swept away by surges in the lava river. They may reform, or the pieces may drift along to a bend in the channel where they jam together, forming another roof. If a roof lasts long enough, it is further thickened and strengthened as lava congeals on its underside, or overflows adding to its top side. Once a roof is formed, the resultant tube becomes, in effect, an extension of the vent, through which lava can pass with very little loss of heat. What happens next depends on the duration and especially the regularity of lava flow. Irregular flow causes blockages and overflows, and creates short-lived lava tubes. Long lava tubes require eruptions of steady rate and long duration.

LAVA RIVER CAVE

Lava River Cave probably formed within a few hours after a brief volcanic eruption. In comparison to other geologic events, like the cutting of a canyon or the movement of a glacier, Lava River Cave formed in the briefest of moments. Since the cave appears today much as it did shortly after its formation, it is indeed a "frozen moment" in geologic time. Lava River Cave is a unique kind of cave known as a "lava tube." It is the longest cave of this kind in Arizona. Geologists believe Lava River Cave was formed sometime between 650,000 and 700,000 years ago when molten lava erupted from a volcano near the present day site of Michelbach Ranch. When the lava came to the surface its temperature was hotter than 2,000' F!

Lava Flow Features

Lava River Cave contains a variety of outstanding lava flow features. These features also used to exist outside the cave, but have long since been washed away by wind and rain, and overgrown by plants. The lava flow features in this cave include flow ripples, splashdowns, cooling cracks and lavasicles. Flow ripples can be observed on most of the floor throughout the last two thirds of the cave. This gives the floor the appearance of a frozen river. Actually the floor is a "frozen river" of lava which flowed through the cave shortly after the walls and ceiling hardened. Splashdowns appear to be rocks floating on the frozen river because they actually were. Shortly after the ceiling hardened a few rocks fell into the still flowing floor and floated downstream a little ways before the floor also hardened.

Cooling cracks are long cracks in the floor ceiling and walls. Some of these are six inches wide, three feet deep and over twenty feet long. These cracks formed as the lava cooled and hardened because lava shrinks when it cools.

Lavasicles are very small icicle-like formations which formed after the walls and ceiling hardened. For some reason a hot blast of gas shot through the tube shortly after it formed and partially remelted the walls and ceiling. This caused drips of remelted lava to form and quickly hardened into lavasicles.

Nature's Ice Box

Lava River Cave is an amazingly simple natural ice box. In fact, it could be considered the largest "refrigerator" in northern Arizona. On the hottest day of summer, when the temperature outside the cave is over 90 F, just inside the temperature is 35'F and sometimes there is ice! There are two reasons that temperatures within the cave remain so cold: 1. The lava rock which formed the cave is an excellent insulator. Its dark color and very dense composition prevent heat from traveling from the surface into the cave. 2. Lava River Cave is cold because of its general shape. The highest point of the cave is the entrance. Since hot air rises and cold air falls, cold air falls into the cave during winter and is trapped there throughout the summer.

Creatures That Like The Cold And Dark

Lava River Cave is occasionally used by animals and insects. These include crickets, beetles, porcupines, squirrels and bats. All of these creatures are shy, and since so many people visit Lava River Cave, it is rare to actually see them while in the cave. We do know that animals use the cave because their droppings can be observed beneath rocks and along the walls.

Animal droppings are probably not a wonderful lunch-time topic, but for the complete Lave River Cave experience, you might want to see if you can find and identify these three types during your visit:

A Few Words About Bats

Bats are truly amazing little creatures which unfortunately have an undeserved bad reputation. A lot of people think bats are filthy, disease-ridden, flying mice. This is not the case. Bats are actually very clean and no more likely to contract rabies than your household pet. They are very important to the environment since they are the only major night-flying predator of insects. In one night some individual bats can nearly eat their own weight in insects! Anyone who has had their share of mosquito bites ought to appreciate that.

Bats and other creatures which like to live in cold, dark caves are very vulnerable to human disturbance. Please remember to treat any creatures you observe within Lava River Cave with respect and give them lots of room.

NOTES ABOUT HIKING IN THE CAVE:

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   1. Watch for ice near the entrance. Once inside the cave, the rocks near the entrance are covered with ice most of the year. Please go slow and be careful not to slip!
   2. Wear long pants! The basalt is jagged and can easily scrape you if you brush against it. Also, the temperature in the cave is around 35 degrees, and we may be in there for an hour or more, which leads us to…
   3. Bring a warm sweatshirt or jacket! It will be cold in the cave.
   4. Watch your head!!! Because there is no lighting in the cave, it is easy to lose track of the height of the roof. Use your flashlight to illuminate the roof as well as the floor. Which brings us to…
   5. Bring a flashlight!!!