THE CORRELATION BETWEEN ALTERATION AND ROCK STRENGTH IN SUBAERIAL AND SUBMARINE VOLCANOES: INSIGHTS FROM MAUNA KEA (HAWAII) AND BROKEOFF MOUNTAIN (CALIFORNIA)

Hydrothermal alteration can weaken volcanic edifices and predispose them to large-scale collapse, however previous studies have not investigated how alteration mineralogy affects strength independent of other rock properties (e.g., porosity and fracture density). To test this, we investigated two suites of altered volcanics: (1) Hyaloclastites and flows obtained from the Hawaii Scientific Drilling Project's 3km-deep core hole on the flanks of Mauna Kea, and (2) Block-and-ash flows and lavas from Brokeoff Mountain, an ancestral composite volcano in the Lassen Volcanic Field. We used X-ray and backscattered electron maps in conjunction with component analysis techniques to determine modal mineralogy and porosity. We measured compressive and shearing strengths of samples with joint rock-fracture shear and unconfined compressive point loading tests.

The alteration mineralogy of the Mauna Kea hyaloclastites consists of palagonitized glass, grain-coating smectite, and pore-filling phillipsite and chabazite. The alteration occurs in three main zones: Incipient (14 volume % alteration minerals, 4% pore space, 1.8Mpa compressive strength); Smectitic (15% alteration, 17% pore space, 4.4Mpa strength); and Palagonitic (29% alteration, 5% pore space, 9.2Mpa strength). Unaltered basalts within the zones have porosities of 6% and strengths of ~ 210Mpa. The alteration at Brokeoff Mt. is characterized by the replacement of primary minerals and textures with clays and silica. We categorized the samples by clay type: Kaolinite/dickite (85% alteration, 31% pore space, 26Mpa compressive strength); Natroalunite (89% alteration, 11% pore space, 18Mpa strength); and Smectite (73% alteration, 27% pore space, ~2Mpa strength).

As anticipated, there is a negative correlation between the alteration mineral volume and unit strength of the Brokeoff Mt. and Mauna Kea samples. However, alteration appears to also affect rock strength through infilling of pore space. In both these suites, samples with the highest volume of alteration minerals and the lowest porosity are consistently stronger than those with a lower volume of alteration minerals and higher porosity.