THE EARLY WEATHERING OF OCEAN ISLANDS: A SYNTHESIS OF GEOCHEMISTRY AND GEOPHYSICS, KOHALA PENINSULA, HAWAII, USA

Lateritic weathering profiles are widespread in the tropics and comprise an important component of the Critical Zone (CZ) where much of the world’s population lives. The Hawaiian Islands are an excellent natural laboratory for examining the tropical CZ where the bedrock composition (basalt) is uniform and climate (rainfall) varies greatly. The purpose of this study is to develop a model for early, yet deep weathering of ocean islands.

In parts of the Kohala Peninsula, Hawaii, a variably weathered, thick laterite profile is exposed along sea cliffs in a mesic climate. Laterite development includes the entire vadose zone, with a mineralogy of halloysite ± gibbsite and Fe-oxides on a 460 to 260 ka substrate. However, enhanced zones of weathering occur in gibbsite-rich horizons. Elsewhere at the base of the weathering front and on the rinds of core stones, initial and transient smectite clays are found.

Shear-wave velocity data derived from Multi-channel Analysis of Surface Waves (MASW) and common depth-point (CDP) seismic reflection profiles reveal an internal structure that complements the nearby sea cliff outcrop. In addition to identifying the depth of the weathering front, stiff horizons within the laterite correlate to high gibbsite abundances in MASW profiles. Parallel to paleo-lava flow directions, relict igneous stratigraphy is expressed as seaward dipping reflectors on CDP profiles, whereas perpendicular to flow, reflector geometry show lenticular bodies (relict flow packages) within laterite.

A geochemical and geophysical synthesis leads to a conceptual model for variable laterite weathering. Although there will be overall trend for the downward migration of the weathering front with time, zones of high initial permeability (e.g., rubble above or below a’a flows) are influenced by both downward and lateral fluxes of water, leading to enhanced weathering in what are now gibbsite-rich horizons. The dense cores of a’a flows with widely-spaced joints preserve core stones that weather inward where halloysite-rich saprolite gives way to smectite-rich rims. Thus, differential weathering is a natural consequence of textural variations in primary igneous stratigraphy. Our study shows how these variations are preserved in outcrop, in geochemical observations, and in geophysical profiles.