Many lava tube passages have a smooth, thin, metallic-looking coating over the darker, coarser basalt underneath. Glaze frequently covers all the surfaces in a passage. The formation of glaze is a matter of dispute. Some feel that it represents remelting of the basalt from hot flows and gases in the tube. The Allreds (1998) suggest that glaze is “the continuous to discontinuous coating of magnetite crystals growing on either segregated or parent lava. Glaze grows after the segregated lava seeps or bubbles out”. They found that the silvery luster of glaze is due to tiny facets of magnetite on the surface. Glaze can become red when oxidized, as near a skylight, or greenish when high in pyroxene.

Glaze is also used somewhat more generically to describe any lining with a smooth surface.

AUTHOR: Dave Bunnell