Abstract

The Cave Basalt, a high-alumina pahoehoe flow containing numerous lava tubes, originated at the southeast flank of Mount St. Helens, southwestern Washington, and flowed down a stream valley incised in older pyroclastic flow deposits. In situ charcoal samples from two localities within lava tubes yield C14 dates of 1,860 ± 250 years B.P. and 1,925 ± 95 years B.P. Detailed survey of 9,125 m of lava tubes, correlated with surface geologic mapping, yields several geomorphic relations of basalt flows. Most of the lava tubes apparently formed between shear planes in laminar lava flow, although some tube sections show evidence that the tube roof formed by accretion of spattered lava in turbulent flow. Partial collapse of tube interiors reveals: (1) The wall separating the tube interior from the preflow country rock may be thinner than 25 cm. (2) Lava flows can erode the surface over which they flow. (3) Collapse of the tube interior can occur immediately after the tube has been drained of molten lava and before the walls cool completely. (4) Lava tubes may represent the thickest part of the lava flow, occupying topographic lows (stream channels). (5) Tubes can be modified extensively through accretion and erosion by later lava flows.

Many surface features of basalt flows are directly related to lava tubes. Pressure within the closed lava-tube system caused by out-gassing and hydrostatic pressure and overflow of lava from ruptured roof sections (or from channel overflow prior to roof formation) result in formation of a topographic high along many sections of the lava-tube axis. If roof rupture does not occur, tumuli may develop in weak areas of the roof, forming positive features (may be solid or hollow) 40 to 50 m in diameter and several meters high. Most of the hollow tumuli of the Cave Basalt collapsed, probably as a result of withdrawal of supporting lava during drainage of the lava tubes. Raised-rim craters found in many parts of the flow are associated with lava tubes and were probably formed by collapse of hollow tumuli.

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