HR: 1330h
AN: T12B-0452 INVITED     [PDF]
TI: Frequent Landslides from Ko`olau Volcano: Results from Cores 1 \& 2 at ODP Site 1223
AU: * Garcia, M O
EM: mogarcia@hawaii.edu
AF: Dept. Geology-Geophysics, Univ. of Hawaii, Honolulu, HI 96822 United States
AU: Sherman, S B
EM: bean@higp.hawaii.edu
AF: Dept. Geology-Geophysics, Univ. of Hawaii, Honolulu, HI 96822 United States
AU: Moore, G
EM: gmoore@hawaii.edu
AF: Dept. Geology-Geophysics, Univ. of Hawaii, Honolulu, HI 96822 United States
AU: Goll, R
EM: goll@odpemail.tamu.edu
AF: Ocean Drilling Program, Texas A&M University, College Station, TX 77845 United States
AB: Landslides associated with the collapse of oceanic islands are an infrequent but potentially devastating natural hazard. The Hawaiian Islands are known to produce some of the largest landslides on Earth. The Nu`uanu slide largest of the Hawaiian slides; it removed ~40% of the Ko`olau volcano from the island of O`ahu. The purpose for drilling Site 1223 was to determine the depositional history, timing, thickness and hazards associated with the Nu`uanu landslide. Site 1223 was drilled 300 km northeast of the island of O`ahu on the axis of the Hawaiian Ridge, a topographic high 500 m high, to a depth of 41 m. Soft sediments were recovered to a depth of 11 mbsf in cores 1 and 2 at this site before hard rock (tuffs) was encountered. Six prominent dark gray, sandy layers are preserved in these cores. The basal contact of each sand layer is sharp and in some cases, irregular. In contrast, the upper contact of these layers is gradational with overlying yellowish-brown clay. The sandy layers range in thickness from 11 to 232 cm and are normally graded. The thickest sand unit is internally complex with numerous thin (0.5 to 2 cm thick) light and dark bands of varying grain size overlying a basal carbonate gravel. The sand layers contain abundant volcanic material including fresh glass, olivine, and plagioclase fragments. The composition of the glass grains are all tholeiitic and typical of Hawaiian shield volcanoes. However, the compositional range at any level of fractionation (i.e., MgO) is greater than observed for any Hawaiian shield. Most of the compositions are typical of Ko`olau, including the high SiO2-type. A few glasses have unusually high MgO (up to 12 wt%) indicating primitive magma compositions and very high quenching temperatures. A few others have high SiO2 (up to 60 wt%), which are very rare in Hawai`i. The vast majority of the glasses ($>$90%) are degassed ($>$0.03 wt% S) indicating that they were erupted subaerially. The sand layers are probably turbidites, the distal remnants of landslides derived primarily from the flanks of Ko`olau volcano. Timing of these landslides and whether any of the sand layers are related to the Nu`uanu slide remains uncertain. Preliminary work on radiolarian tests yield ages ranging from Quaternary to Early Eocene. The Eocene ages may represent reworking of older sediments. It is clear from the Site 1223 results that Hawaiian volcanoes can collapse repeatedly and that the debris from these slides can travel great distances across the ocean floor and over significant bathymetric highs.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3022 Marine sediments--processes and transport
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
SC: Tectonophysics [T]
MN: 2003 Fall Meeting