GRADATIONAL WEATHERING OF MOLOKAI, HAWAII: GEOPHYSICAL STUDY OF HAWAIIAN LATERITIC WEATHERING PROFILES

Laterite weathering profiles (LWPs) comprise much of the Critical Zone developed over time through chemical weathering, where LWP thickness is a function of precipitation and time. The Hawaiian Islands are an excellent natural analogue to study chemical weathering rates due to a uniform bedrock (basalt) and large variations in rainfall. Using a rapid non-invasive horizontal-to-vertical spectral ratio (HVSR) method, LWP thicknesses can be determined to estimate chemical weathering. Studying the laterite weathering profiles of Oahu (~2 Ma), and Molokai (~1 Ma) reveal two profiles in varying developmental stages. With millions living in tropical regions of the world with volcanic substrates, understanding the basalt weathering process is vital to these areas.

Using the HVSR method, >150 soundings were collected on Molokai (63) and Oahu (99). Vs values of LWPS (~300 m/s) were determined by MASW (multichannel analysis of surface waves), as well as LWP thickness from geologic logs and outcrop. On Oahu, LWPs tend to be fully developed with a clear boundary between saprolite and underlying basalt visible on frequency spectra. On Molokai, the boundary between saprolite and basalt is less clear for a portion of the island. The HVSR frequency spectra are mostly featureless. MASW surveys on Molokai suggest that featureless HVSR spectra are due to a gradational weathering profile rather than sharp saprolite-bedrock boundaries commonly seen in Kauai, Oahu and parts of Hawaii (Kohala). These featureless spectra have been observed on the leeward side of the island with lower annual precipitation rates, while more mature LWPs with sharp boundary spectra are observed on the windward side of the island at upper elevations with higher annual precipitation rates. It can be speculated that the gradational nature of the LWP is attributed to the age of the island. With Molokai being ~1 Ma, the LWP has reached average depth, but has not completely gone through the weathering process.