Rock Varnish on Hualalai and Mauna Kea Volcanoes, Hawai'i

Abstract

Tropical rock varnishes found on Hualalai and Mauna Kea Volcanoes, Hawai'i, vary systematically with time and environment. Radiocarbon dating of encapsulated organic matter, (K+ + Ca2+)/Ti4+ ratios, and Zn, Cu, and Ni trace element concentrations in rock varnish are consistent with lava flowages established by K-Ar and 14C dating, where samples are collected from arid microsites well away from the soil surface. However, inaccurate ages are obtained from rock varnish in subsurface locations and from sites with abundant lichens, cyanobacteria, and fungi that chemically erode varnish. In contrast with continental deserts, Hawaiian varnishes commonly interfinger with and are less common than rock coatings of amorphous silica. Laboratory experiments on Hawaiian rock varnishes indicate that K and Ca are preferentially leached relative to Ti over time and at higher temperatures. The location of in situ leaching has been identified in Hawaiian varnishes as porous textures without abundant detrital grains.