"Stratigraphy and Hydrologic Conditions in the HSDP II Borehole: Implications for Ocean Island Stability"

Don Thomas, Director, Center for the Study of Active Volcanoes, University of Hawaii

Abstract:

The Hawaii Scientific Drilling Project has recently extended the HSDP II borehole to a depth of 3340 m. The borehole has been (nearly) continuously cored from the surface and provides a uniquely detailed record of the subsurface stratigraphy and hydrology of an ocean island volcano. The first kilometer of the stratigraphic section is dominated by subaerial pahoehoe and a'a lava flows along with rare soil and ash intervals and confirms subsidence of this portion of Hawaii Island by more than 1 km. The next kilometer of section comprises hyaloclastites (fragmental lavas that have been formed by interaction of lava flows with shallow seawater) that have undergone progressive induration with age and depth. Temperature surveys show a low temperature gradient in the first 500 m of this interval, consistent with high rates of fluid circulation, whereas the bottom-most 500 m shows a conductive temperature gradient with minimal circulation. At 1980 m, the first interval of pillow lavas was encountered and, from this depth to bottom-hole, the section consisted of alternating intervals of multiple pillow units inter-layered with progressively thinner hyaloclastite intervals. Throughout this section, the pillow units were found to be highly fractured with variable amounts of fracture filling secondary minerals whereas the hyaloclastites were fully indurated with clays and zeolites filling the pores of the compacted primary fragmental deposits.

The complex stratigraphy hosts an equally complex hydrologic system. The surface basal freshwater lens gives way to seawater saturated rocks within a few tens of meters of the surface. However, the interface between Mauna Loa and Mauna Kea rocks at 300 m depth, shows a second 150 m thick freshwater layer beneath a soil and ash horizon that marks this interface. This is underlain by a cold circulating seawater system that is gradually choked off by induration of the hyaloclastites at depths below 1500 m. Temperature and pressure spikes at the pillow intervals below 2000 m show, however, that the fractured pillow layers are serving as water bearing formations. Wellhead pressure measurements show that artesian pressures within these formations exceed local hydrostatic pressure by more than 10 bars and fluid compositions indicate that these are confined aquifers that are isolated from the ocean and are in communication with surface freshwater systems.

These findings suggest that the fresh ground water system within ocean island volcanoes may be substantially larger than has been generally recognized and that atypical hydrostatic pressures arising from e.g. intense rainfall events, could propagate to substantial depths within ocean island volcanoes and contribute to edifice instability.

Time: 2:00 - 3:00 PM
Date: April 11, Friday
Place: POST 127, UH Manoa Campus
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