Abstract

With large populations living in tropical regions of the world with volcanic substrates, understanding basalt weathering processes is vital. The Hawaiian Islands are an excellent natural analogue to study chemical weathering rates due to a uniform bedrock (basalt), large variations in rainfall, and varying ages across the islands. Laterite weathering profiles (LWP) develop over time through chemical weathering, where LWP thickness is influenced by many factors, including precipitation and time. Using the rapid, non-invasive horizontal-to-vertical spectral ratio (HVSR) method, LWP thicknesses can be estimated to constrain chemical weathering rates. Studying the laterite weathering profiles developed from basaltic bedrock of varying ages on Oahu (~2 Ma), Molokai (~1 Ma) and Kohala, Hawaii (~0.3 Ma) reveals three profiles in varying developmental stages. Over 200 HVSR soundings were collected on Oahu, Molokai, and Kohala. Shear wave velocity values of LWPs were determined by MASW (multichannel analysis of surface waves), and LWP thicknesses verified from geologic logs and outcrop. Oahu has thick LWPs compared to the other islands and shows a trend of increasing thickness with increasing precipitation across the island. The Molokai LWP follows a trend similar to Oahu, with a noticeable difference of thicknesses (20-40 m) at similar precipitation thresholds. Molokai presented a unique case, where the shear-wave velocity (Vs) boundaries between laterite and basalt were gradational for ~43% of HVSR datapoints, resulting in featureless frequency spectra that could not reliably model laterite-basalt boundary depths. The gradational nature of the LWP of Molokai is attributed to the young age of the island, and primary permeability properties of the thick, post-shield alkalic lavas. Molokai has an aerially average weathering rate of 0.02 to 0.04 m/ka. Kohala HVSR data show a newly developed LWP with varying LWP thickness within the same precipitation isohyet. LWPs on Kohala show a unique trend where LWP is thickest along the coast and is wedge shaped thinning out towards higher elevations. Each island differs in age and has its own unique LWP trends, with older islands tending to have deeper, more developed LWPs at similar precipitation ranges.

Degree

MS

College and Department

Physical and Mathematical Sciences; Geological Sciences

Rights

https://lib.byu.edu/about/copyright/

Date Submitted

2021-12-02

Document Type

Thesis

Handle

http://hdl.lib.byu.edu/1877/etd11934

Keywords

Molokai, Hawaii, HVSR, erosion rates, weathering reactions, laterite thickness, laterite weathering profiles

Language

english

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