Although there are ice caves scattered in the mountains of central and eastern Montana, the ice cave near Crystal Lake (above photo) and Big Ice Cave in the Pryors are two of Montana's most popular ice caves. Most (perhaps all) ice caves in the state are found in a rock formation called the Madison Limestone, which has been described as a 'geologic Swiss cheese" due to it's many caves and sinkholes. In fact, Montana's most famous cave, the Lewis and Clark Caverns, was formed in the same Madison Limestone as the two pictured above even though vast distances separate the three.
So what's an ice cave?. . .
Contrary to what you might think, the kind of ice cave featured here is not a cave formed in ice, but rather a cave that remains cold enough to contain ice all year long. As you saw in the video, smooth ice at least several inches thick covers the floor of Big Ice Cave. The air in caves is almost always cooler than air outside during summer, and warmer than outside air during winter because caves are well-insulated by the thickness of rock around them, however very few stay cold enough for ice to persist through the hot months of summer. So, what makes some caves so cold? There are two factors involved.
1. It's a density thing . . .
Whether you are studying geology, meteorology, oceanography, or astronomy, one of the most important underlying principles is "density". Density also plays a crucial role in the persistence of of ice in some of Montana's caves. Since molecules of cold air move slower than those of warmer air, colder air molecules tend to be closer together. This makes cold air denser (heavier for its volume) than warmer air. As a result, cold air tends to sink to the lowest location . . . In this case, the bottom of the cave.
2. Where do you keep your ice cream? . . .
When it comes to storing frozen foods, there are two basic types of freezer compartments. There is the kind located along the bottom, top, or next to the refrigerator compartment. To get frozen foods from these, you simply open a door. But as you do, the cold air inside flows out onto the floor and is replaced with warmer air from the room. After you shut the door, electricity is used to re-cool the air so your ice cream doesn't melt. These freezer compartments are like normal caves.
The second type of freezer compartment, which opens from the top, is much more efficient because when you open it the cold air does not easily escape. Sometimes called "chest freezers", "freezer chests", or "deep freezes", these are similar to ice caves. Ice caves are laid out in such a way that the entrance to the cave is located above the cave, so cold air doesn't flow out. Like the chest type of freezer, the opening to an ice cave is above the place where cold, dense air sits undisturbed.
Below: In the ice cave (left) there is no outlet for the cold, dense air from the previous winter. In the normal cave (right) cold air spills out and is replaced by warmer air throughout the spring and summer.
Terms: limestone, convection
Watch this video of my trip into the ice cave south of Lewistown in 2009.
