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Roger Weller, geology instructor

by D Betz
Physical Geology
Joann Deakin, instructor
Fall 2013
click here for another tour of Ape Cave  
                                                                  Exploring Ape Caves

                                                                                                                            Cougar, Washington


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As a young teenager, I was lucky enough to climb Mount Saint Helens the summer before it underwent a dramatic transformation. The year was 1979, and I spent a couple of summer days hiking and exploring the vast natural wonders that surrounded Mt. St. Helens.

The first day, I hiked to Dogs Head, on the southwestern side of the mountain, and fished the many streams that connect down below the glaciers. I remember the scenery being so lush and green. Old growth timber and natural trout in the smallest of streams, soon they would all be a distant memory. The next day was spent hiking the Ape caves, a task that I was reluctant to do at first, because a slight fear of claustrophobia. I soon learned it was well worth the trip.



The Apes that give their name to the two lava tubes found outside of Mt St Helens were not primates at all, they were the members of a 1950s outdoor club who explored the massive tubes. They called themselves the Mount St. Helens Apes, and the lava tubes became known as their caves. The tubes are long tunnels in the thick lava beds; they run roughly parallel to the surface of the land. The lower tube is the easier one of the two to hike because of its relatively flat, gentle slope. The upper tube is larger and much rockier and is hiked by the more adventurous of cavers due to the approximately 27 boulder piles and an 8-foot high lava fall that can be scaled in this section.  It is not possible to hike the caves entire length because of the small space that separates the two sections. Even in the summer, the tubes were a constant, cool 42 degrees, so if planning a trip to hike the caves, remember to pack a jacket and a good flashlight or lantern. There are many places in the cave that never see the sunlight. 


     Ape Cave lava tube was formed in a kind of basalt flow that is known as a Pahoehoe flow. (A Hawaiian term for basaltic lava that has a smooth, hummocky, or ropy surface). In this type of volcanic flow, the molten lava on the inside is drained away, leaving the outer crust in place. While the 1980 eruptive episode was intense, it produced no new lava tubes. The eruption that produced the caves occurred roughly 2000 years ago when fissures opened high on the volcano. Charcoal samples from two localities under the lava tubes yielded Carbon-14 ages of 1,860+/-250 and 1,925+/-95 years[1]. Fluid basalt, similar to the famous eruptions of Kilauea Volcano in Hawaii, flowed mostly to the south, overrunning the valleys of the Toutle and Lewis Rivers. Even though lavas flowed down several sides of the volcano, they are best seen on slopes above the Lewis River, (Depicted by the yellow line on the map.)

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     The Mount St. Helens National Volcanic Monument encompasses one of the largest networks of caves in the Pacific Northwest, (sixty four lava tubes had been identified as of 1994[2]). Due to the vast amount of caves found in this area, geologists have named this rock unit the "Cave Basalt." The Cave Basalt Flow is almost a mile wide and about nine miles long. Similar lave tubes can be found in Oregon, California, and Idaho. In the United States, Ape Cave is the longest lava tube measuring 12,810 feet long, and the third longest lava tube in North America. (2). Lava tubes are frequently obstructed by blocks of solidified rock rafted along in the lava stream. These rocks commonly wedge at narrowing part of the tube forming jambs of lava blocks or as isolated pieces such as

The “Meatball” or “Lava Ball" in Ape Cave.  

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     Another interesting feature near the caves is the “Railroad tracks” a shoulder or levee that formed along the side of the lava flow. As the fluid lava drained out of the tube, the levee remained.


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Lava stalactites and stalagmites were formed as the lava flow cooled.
Flow marks can be seen throughout the walls and floor of the cave.

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     If you compare the volume of the Cave Basalt with eruptions that have occurred in the history of the Hawaiian Islands, it is evident that the eruptions have spanned months to a years. The relatively quiet eruptions were probably not as astonishing as the initial eruptive phase; Lava most likely would have flowed from fissures and was carried away from the vents through these lava tubes.  Tubes such as this not only directed lava but also allowed lava for travel many miles without significant heat loss. These lava tubes operated as an addition of the vent, allowing the lava flow to travel great distances.

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     The Because Ape Cave is so large and has a so much space; it has an air pressure system which causes cave winds. The wind is caused by differences in air temperature inside and outside the cave. This effect causes the caves to “breathe” from the fluctuations in air pressures. 


      During the winter months, the warm cave air rises like smoke in a chimney, and pours from the upper entrances. This chimney effect reverses itself during the summer when cool cave air drains down- slope and rushes out through lower openings or cracks. Cave wind in Ape Cave has been measured at seven (7) miles per hour.[3]

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4.       From: Pringle, 1993, Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity: Washington Department of natural Resources Division of Geology and Earth Resources Information Circular 88, p.64.

 Photos were found at:

[1] (Greeley and Hyde, 1972)

[2] Caves of Mount St. Helens - Guidebook

[3] "Caves of Mount St. Helens - Guidebook")