The first step to understanding the formation of the ice caves at Lava Beds National Monument in California is to understand how lava tubes form. The ice caves of the park are all set within lava tubes, and the special characteristics of the tubes preserve the ice within them.
How the lava tubes formed
The Medicine Lake volcano, source of most of the park’s lava tubes, is huge. However, it is not steep-sided and tall like many volcanoes. Instead, it is a shield volcano, the type that spreads over a broad territory and produces a gently sloped plateau.
Dormant now, the volcano was active at various times between about a million years ago and 1000 years ago. During eruptions, the volcano sent out enormous volumes of very fluid lava. The lava rolled down the gentle slopes of the volcano in tongues of molten rock, at temperatures of up to 2000 degrees Fahrenheit. As each tongue contacted surface rock and cold air, it cooled, forming a crust around the fiery flow of still fluid lava.
This crust insulated the molten rock within, so that it stayed hot and liquid and continued to flow downhill, sometimes for miles, finally leaving behind a solid tube of rock it had formed in its passage.
In the centuries since, erosion has pierced holes in the tops of many of the tubes, providing access to hundreds of caves. Some shelter bats or cave frogs. Some hold wild rock gardens of ferns, moss, or microscopic life. Others retain winter's chill. They are the fascinating ice caves.
How ice caves form
An ice cave is not a cave carved out of ice. The term for that is glacier cave. An ice cave is a cave in rock, one that preserves ice within it. That describes the ice caves of Lava Beds National Monument.
The insulating qualities of hardened lava helped create lava tubes. This insulation is also partly responsible for ice caves. The tubes that hold ice preserve winter’s cold all year. They are cold traps.
Some lava tubes with only a single entrance store cold air, because it sinks. Cold air is heavier than warm air, and in winter can sink into a cave opening. In summer it cannot be displaced by warm air, because warm air is lighter than cold and stays higher than the cave. All year round, the thick lava walls of the cave are excellent insulators, and preserve the winter temperature.
If warmer air does flow through it may melt the ice into oddly shaped knob formations called schmos. Most ice caves, though, have a single entrance and thick roof and walls.
Ice can floor a cave all year, like it does in the double-decker Skull cave. It can make a frozen pool like in Heppe tube. Or it can form beautiful icicles in winter, like in Sunshine cave and many others. Many of the tubes are well mapped, and strung out on a trail close to the visitor center. Rangers are also happy to tell visitors more about the ice caves of Lava Beds National Monument.