GEOPHYSICAL INVESTIGATIONS REVEAL THE MECHANISMS OF VERTICAL AND LATERAL GROWTH OF HAWAIIAN LATERITE PROFILES

The Hawaiian Islands permit investigation of tropical chemical weathering rates and processes on a single rock type (basalt). Critical zone (CZ) profiles can be investigated as a function of large rainfall gradients over differing time spans due to the age of the various islands, including Kauai (~4 Ma), Oahu (~2 Ma), and Hawaii’s Kohala Peninsula (~0.3 to 0.17 Ma). Understanding the tropical CZ is vital given the large populations that rely upon it.

The HVSR (horizontal-to-vertical spectral ratio) method permits rapid measurement of CZ thickness over broad areas. This is possible because the V_s of saprolite (~300 m/s) and underlying basalt (~500 m/s) are relatively constant as revealed by MASW (multichannel analysis of surface waves) surveys. HVSR has been shown to reliably detect the base of the weathering profile because the boundary is usually sufficiently sharp to produce pronounced horizontal amplifications. The method is rapid (~20 min/sounding), highly portable, and occupies a very small footprint. By contrast, MASW surveys are time-consuming and require a much larger footprint, which is often difficult given the topography, vegetation, and land access restrictions in the Islands.

HVSR soundings on older substrates like Kauai indicate that 60% to 80% of the variability in laterite thickness is due to gradients in precipitation, with errors in forward modeling, erosion corrections and variability in the original permeability structure of the volcanic sequence playing important but secondary roles. Basalts have higher horizontal than vertical hydraulic conductivity (K_h>K_v), and local variability in K_h:K_v likely drives much of the remaining variability in laterite thickness via differences in water-rock ratios during weathering.

By contrast, the young laterites on Kohala (~170 to ~300 ka) exhibit greatly contrasting thicknesses where coastal laterites are thick and interior laterites are thin, suggesting that early weathering on shield volcanoes produces wedge-shaped profiles near the coast. With time, the thick (coastal) end of the wedge propagates upslope such that a fully developed, constant-thickness laterite carapace can form in ~2Ma or less. The development of thickened coastal laterites on young substrates depends on greater water–rock ratios as vertically infiltrating water upslope is diverted laterally and increasingly down slope. This view of laterite development is very different compared to endmember models of continental weathering and CZ development, ,which are dominated by vertical growth.