Basaltic lava flows exposed to semiarid weathering on the island of Hawaii develop hydrous silica coatings that thicken and incorporate detrital material with age. The nearly pure silica coatings on the younger rocks are colorless, but basaltic detritus and its weathering products, incorporated into coatings on older flows, produce colors ranging from brown to red. Age, rate of supply of material, and the mechanical stability of flow surfaces dictate the color and thickness of coatings.
Fresh, glassy pahoehoe surfaces support thin (as much as 5 µm, clear silica coatings but break down in a few years to expose the vesicular underlying material. At first, coloring is concentrated in the open vesicles, yielding a dark brown color: a mixture of red vesicles and projections of black basalt. With age, thicker accumulations of silica incorporate layers of oxidized basaltic material. The dark basaltic material in Kilauea and Mauna Kea coatings is probably supplied by large explosive eruptions, but at Mauna Loa and Kohala, the source could be locally derived basaltic weathering products. The clear hydrous silica layers are probably the product of leaching of locally derived basaltic dust.
*Present address: Jet Propulsion Laboratory, Pasadena, California 91109.