Hawaiian Volcano Flank Stability Appraised From Strength Testing the Hawaiian Scientific Drilling Project's (HSDP) 3.1-km Drill Core

Strength results from limited testing of HSDP core samples reveal significant differences in their unconfined compressive and shear strength. The median strength values show a progressive increase from the incipient (2.4 MPa) to smectitic (4.2 MPa) to palagonitic (9.4 MPa) alteration zones. The strength differences include differences among hyaloclastites as a function of their alteration, as well as differences between hyaloclastites and the various forms of lava flows and intrusive bodies. The unconfined compressive strengths of extrusive submarine and subaerial lavas and intrusive rocks from the HSDP core samples are much greater than that of any of the hyaloclastites, and range from 82 to 150 MPa. The cohesive shear strengths of hyaloclastites increase successively with depth and type of alteration (from 0.9 MPa for the incipient zone to 3.2 MPa for the palagonite zone). Conversely, the frictional strengths of the hyaloclastites show a decrease with increasing depth and alteration zone (from 17.6° for hyaloclastites from the incipient zone to 13.7° for the palagonitic zone). The Mohr strength envelope changes from a linear relationship at shallow depth to become curvilinear with increasing depth and consequently shows higher cohesion and lower friction. Comparison of the three groups of the altered hyaloclastites showed that they had p values < 0.05, and that the mean strength results from the incipient, smectitic and palagonitic altered hyaloclastites were significantly different from each other. The greatest difference was calculated between the mean value of the palagonitic alteration and the mean values from the smectitic and incipient alteration though subsequent statistical testing showed that the means of the incipient and smectitic alteration were not significantly different from each other. The palagonitic mean remains significantly different from both the incipient and smectitic means. No statistically significant difference was found in comparing the means of the basaltic flows, intrusive and pillow lava values. The test results imply that shallow rotational slumps that develop within the upper few kilometers of spreading Hawai'ian volcanoes within low strength, poorly-consolidated, smectite-rich hyaloclastites are similar to those we have found from the incipient and smectitic alteration zones of the HSDP cores. Deeper slumps might be directed through over-pressured pillow lava units as a result of the stronger pillow lava units permitting deeper failure surfaces to develop. Petrographically the Mauna Kea hyaloclastites appear similar to those from actively spreading Hawai'ian shield volcanoes. Alteration processes apparently affect the strength of these hyaloclastites. In the shallower zones of incipient and smectitic alteration, hyaloclastites generally retain their high primary porosities. In the deeper, palagonitic zone of alteration, the hyaloclastites gain both compressive and shear strength, primarily through consolidation and zeolitic cementation. The marked strength contrast between hyaloclastites, and the lavas that overlie and underlie them is significant, and may be a primary factor in localizing the destabilization of the flanks of Hawaiian volcanoes.