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Ape Cave is a lava tube. It was formed when Mount St. Helens erupted about 1900 years ago. |
| A lava flow, like this one in Hawaii, came down from high on the slopes. It traveled eight miles south to the Lewis River. |
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This type of smooth-flowing lava is called "pahoehoe" basalt. |
| Over several weeks, the lava flow began to cool and crust over on the surface. The lava beneath the surface continued to flow. |
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At the end of the eruption, the lava flowed out of the tube. It left behind Ape Cave as we see it today. |
| Ape Cave is 12,810 feet long--that's almost 2-1/2 miles long! It is the longest intact lava tube in the United States, and the second longest in the world. |
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No lava tubes were formed during the 1980 eruptions of Mount St. Helens. The volcanic material that came out was a different type of lava. |
| There are active volcanoes in Hawaii that are much less explosive than Mount St. Helens. We can study the formation of lava tubes there. |
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Geologists must carefully approach a skylight, or opening in a lava tube to photograph lava moving through an active tube. |
| Skylights form when small parts of lava tubes do not form, or when the tube collapses. |
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Ape cave was named in honor of a local youth group called the St. Helens Apes. They hiked and explored on Mount St. Helens. |
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