Not all caves are created in a like manner – some are born under tremendous pressure. Unlike caves carved by the slow passage of water over rock, Ape Cave is a lava tube. This feature was formed by the rushing of superheated lava flow through fissuring in the ground. Created by one of Mt. St. Helen’s eruption episodes about 1,900 years ago, it came into being rather suddenly.
During an eruption, channels of smooth-flowing (pahoehoe) lava sometimes crust over and form this sort of lava tube. When the eruption ends (or the lava gets diverted elsewhere), the molten rock drains away to leave partially empty conduits beneath the ground. As an expert once explained to me (with a great deal of loud speech and strong gesticulations) “lava erodes in all directions, even downward, leaving empty space above and below the flow – creating bitchin traps for the unwary.”
Those volcanologists… quite an excitable lot.
Incidentally, for some inexplicable reason, I have been down a wide variety of these structures during the last twenty years and discovered it is way fun to run amok inside. Take a flashlight.
Ape Cave is the largest contiguous lava tube in the Western Hemisphere, being two-and-a-half miles in length – we take our claims to fame when and where we find them. First explored in 1946 by a local Boy Scout troop called the St. Helens Apes, the cave was named in their honor (I remain skeptical about this explanation but local legend being what it is – there you go). You can only imagine my level of excitement when I discovered Ape Cave is but a short drive from where I live. Aha! Spelunking we shall go! Actually, there isn’t a lot of effort involved since lava tubes are large enough to walk ten abreast and two or more tall (should you find yourself wanting to perform acrobatics or give piggy-back rides).
Lava tubes often feature unusual features – usually created by hot gases being trapped in the tube which affects the stability of the tunnel walls and ceiling. These intermittent periods of re-heating form stalactites made from lava. In Ape Cave there is an unusual anomaly, a cooled ball of lava which descended from the main flow and became wedged between ledges. When the molten material drained away, the sphere was left perched ten feet overhead. It’s a bit disconcerting to look up and find it staring down at you. Over time, additional erosion and deposition has swept the tunnels clean of sediment but left behind unusual sculpted formations which are reminiscent of sand castles. Nifty cool. You should go and see it for yourself.