Have you ever entered the odd hole in the ground, which is the entrance of the Ape Caves? Well if you have or haven’t, you may not know that these caves are actually a series of lava tubes. Lava tubes are found all over the world…on the Moon, Mars, and even Venus! But how are they formed? These strange tubes are formed by a flow of magma that cools on its edges and forms a somewhat smooth tunnel if the remaining hot magma can leave the channel. It is sort of crazy to think that hot magma inflates with heat and then the extremities cool, insulating the central part of the flow eventually allowing the magma to break out at the front of the lava flow, which on the odd occurrence that it happens – leaves an empty lava tube that people can walk through. Yielding the simple name of a lava tube.
Once the process is understood it is evident that lava tubes can only form in terrains where volcanoes and lava are present, but not all eruptions are the type that can form lava-tubes, or are “mafic lava”. This is why the Ape Caves are located right next to Mt. St Helens, but you don’t hear a lot about other caves in the area since few of the eruptions are of the right type of lava to form lava-tubes. The creation of a tube starts when a volcano erupts and iron and magnesium-rich (mafic) lava pours out and down the side of a volcano. The flowing lava will spread across the terrain and rapidly cool, but sometimes it will flow through a deep channel. Then it depends on the amount of lava and the composition, or stickiness of the lava (viscosity), in order for a tube to be formed. If there is enough lava in a channel, the edges of the flow will slowly start to cool down and form a hard thin crust. From there, the flow continues to slowly cool from the outside towards the center. Eventually, the lava on top of the flow will cool down, creating a roof. That usually insulates the inside of the tube so the center of the flow never actually cools off and dries up in the center of the tube. As the lava flow continues, lava will stick to the ceiling and drip to the bottom of the tube. While the lava drips from the ceiling, it will cool down to form lavacicles (lava version of stalactites). And while the lava empties out, it gets lower to the bottom of the tube, and the remaining lava at the bottom of the tube actually does cool down and creates the floor of the tube. So, the tube is left as a long, empty, and generally flat tunnel (Walker, 1991; Melville, 1994; Grimes, 1995).
The long, flat and dark tunnel is exactly what the Ape Caves are now, and the Ape Caves are an evacuated series of lava tubes. Though Mt. St Helens generally has explosive eruptions of ash, at some point in time had mafic-lava running down the side of the mountain in a channel, and that channel had to cool down which eventually formed a lava tube, and that tube happens to be the Ape Caves. Lava tubes are significant because it is an important geologic feature that can be used to determine how magma chambers below the volcano evolve over time and create wonderful features for society to enjoy and become excited about how volcanoes evolve and develop over time.