RATES, PROCESSES, AND TIMESCALES OF BASALT WEATHERING IN THE TROPICS

Hawaii is an excellent natural laboratory for investigating the rates and processes of chemical weathering, largely being comprised of basalt bedrock with enormous variations in rainfall. The effects of geologic time on weathering can also be investigated due to the widely varying age of volcanism in each Hawaii’s several islands, including substrates in Kauai (~4 Ma), Oahu (~2 Ma), and Hawaii (~0.30 to 0.17 Ma). Understanding tropical CZ (critical zone) development is vital given the large populations in developing countries that rely on the CZ for food and other resources. In addition to the broad interest in the origin and growth of ocean islands, the rates and mechanisms of their denudation are equally important.

HVSR (horizontal to vertical spectral ratio) passive seismic soundings on the ~4 Ma Kauai Volcano indicate that ~60% of the variability in laterite thickness is due to gradients in mean annual rainfall, with other factors playing important but lesser roles. HVSR is particularly well suited for this environment as it has been shown to reliably detect the base of the weathering profile, is rapid (20 min/sounding), highly portable, and occupies a very small footprint when collecting data. Other array-based methods are challenging due to topography, vegetation, and land access problems.

Comparison of ~4 Ma Kauai and ~2 Ma Oahu weathering profiles suggests that the Oahu laterites are fully formed despite being half the age of Kauai. By contrast, the young laterites on Kohala (~170 to ~300 ka), Hawaii, exhibit greatly contrasting thicknesses, where coastal laterites are thick and interior laterites are thin in regions of equal rainfall. This suggests that early weathering on shield volcanoes produces wedge-shaped bodies that thicken toward the coast. With time, the thick end of the wedge propagates up slope such that a fully-developed, constant thickness laterite carapace can form in ~1 Ma. The development of increased weathering down slope depends on greater water-rock ratios as vertically infiltrating water is diverted laterally due to the high ratio of horizontal-to-vertical permeability. This is a very different view of laterite development, as a function of time and space, compared to the common assumption of a slowly downward migrating saprolite-bedrock interface.