Using Spectroscopy to Understand Hawaii’s Hidden Geology

A recent study published in the Journal of Volcanology and Geothermal Research explored the Hawaiian subsurface using several spectroscopic techniques, including long-wave infrared (LWIR) spectroscopy, scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and bulk X-ray fluorescence (XRF) geochemistry (1). This study was led by Pooja Sheevam and Wendy M. Calvin of the University of Nevada, Reno. Their insights helped unveil new information about the mineralogical and geochemical transformations preserved in the PTA-2 drill core.

The PTA-2 drill core is located on the Big Island of Hawaii. The Big Island is the youngest and largest of the Hawaiian islands, and it is known for being the home to Hawaii Volcanoes National Park (2). The Big Island is known for its active volcanoes, as well as its diverse topography, with all but four of the world’s climate zones present on the island (2). Because of the amount of tectonic activity taking place on the Big Island, researchers have concentrated their efforts on learning more about the mineral composition of the basaltic rocks that exist on the island to learn more about the geochemical transformations that have taken place on Hawaii.

In their study, Sheevam and Calvin used a combination of laboratory and imaging techniques such as LWIR spectroscopy, SEM-EDS, and XRF to create high-resolution maps of mineral phases and identify zones of hydrothermal alteration. Sheevam and Calvin used high-resolution LWIR and short-wave infrared (SWIR) spectroscopy on both the thin and cut core sections. These methods allowed for the identification and quantification of primary minerals such as calcium-rich feldspars, pyroxenes, and olivine, which are typical constituents of shield-phase basalts (1). The duo also used SEM imaging and infrared (IR) spectroscopy to detect quartz in their samples.

Sheevam and Calvin also used petrographic imaging to uncover a variety of basaltic textures. These textures included fine-grained groundmass to prominent olivine phenocrysts (1). Combining these mineral textures with geochemical data, the researchers were able to infer key aspects of fluid-rock interaction. Surprisingly, the study found no evidence for significant elemental loss, suggesting that alteration occurred in a low water-to-rock ratio, closed system, which is an environment where chemical components remain largely in place (1).

Another key takeaway from this study is that the researchers theorized, based on their findings, that silica blobs moved during past hydrothermal activity (1). They came to this conclusion through their finding of small quartz and silica-rich blobs in two core samples. However, the researchers were not able to figure out the source of heat for the alteration (1). The researchers speculate in their study that the late-stage volcanic intrusions may have introduced localized heating; however, the researchers did not obtain conclusive evidence to confirm this (1). Additionally, no radiometric age dating was performed on the host rocks, leaving questions about the exact timing of deposition and alteration events (1).

The final major takeaway from this study is that it exposed the limitations of existing mineral spectral libraries. Variations in mineral orientation and complex mixtures are not accounted for in current databases (1). As a result, new spectral databases are needed to include the new spectra.

In the conclusion of their article, Sheevam and Calvin discussed future steps in this research. Their goal is to apply their methodologies to two other Hawaiian drill cores, which include KMA-1 from the Humu‘ula Saddle and HPF Well 10 on Lāna‘i Island (1). A combined analysis of all three cores, the authors state, will deepen our understanding of Hawaii’s geologic evolution, hydrologic processes, and potential geothermal resources (1).

By integrating advanced imaging and geochemical techniques, this study represents a significant leap in volcanic and geothermal research. Detailed analyses of deep subsurface cores in Hawaii have been limited. This study offered a glimpse at the mineralogical changes and geochemical processes occurring beneath the Hawaiian surface, using spectroscopy to improve understanding of subsurface hydrothermal systems (1).

References

1. Sheevam, P.; Calvin, W. M. Comprehensive characterization and geochemical alteration pathways of drill core from the Humu'ula Groundwater Research Project, Hawaii, USA: I. Pohakuloa Training Area. J. Vol. Geo. Res. 2025, 462, 108311. DOI: 10.1016/j.jvolgeores.2025.108311
2. Meet Hawaii, Hawai’I Island Fact Sheet. Meet Hawaii. Available at: https://www.meethawaii.com/media-room/hawaiian-islands-fact-sheets/hawaii-island/ (accessed 2025-05-06).