The NSS Bulletin - ISSN 1090-6924
Volume 40 Number 4: 118-122 - October 1978
A publication of the National Speleological Society

Strata of Lava Tube Roofs
Russell G. Harter

Abstract

Several previous investigators have observed hot, flowing, lava developing roofed channels, and at least five distinct modes of roof formation have been described. Each roofing mechanism results in a first layer of lava (the initial roof stratum) that spans the lava channel. Additional strata may be added on top of the initial roof stratum-as overflows from the tube, or as separate flows of lava. Ceiling linings, deposited against the underside of the initial roof stratum, also add to the toal roof thickness. Lava cave roofs can, therefore, include three different types of strata.

Cooling causes the lava to shrink, which has two most important effects: strata tend to part from one another, and individual strata break into rough blocks. Fractures caused by cooling stresses within lava tube roofs result in rockfalls that expose the various roof layers. Characteristic textures on surfaces of roof strata, and the relative position of the strata, are key features that can be used to determine the type of strata present.

The time requied for an initial roof stratum or an overlying flow unit to solidify can be approximated by comparing the stratum thickness with published measured thickness of solidified crust on Hawaiian lava lakes.

With appropriate exposures, the stratigraphic methods presented here can be used to find the sequence of events that contructed a lava tube roof. Application of these techniques, illustrated by examples from California, Washington, Oregon, and Idaho, results in new information regaring the formation history of lava tubes.

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