HR: 0800h
AN: V21A-0381    [Abstracts]
TI: Pahoehoe, 'A'a and Transitional Lava at Makapuu Point, SE Oahu
AU: * Yakos, N
EM: yakos@hawaii.edu
AF: G&G/SOEST, 1680 East-West Road, Honolulu, HI 96822, United States
AU: Gurioli, L
EM: gurioli@hawaii.edu
AF: G&G/SOEST, 1680 East-West Road, Honolulu, HI 96822, United States
AU: Harris, A
EM: harris@higp.hawaii.edu
AF: HIGP/SOEST, 1680 East-West Road, Honolulu, HI 96822, United States
AB: At Makapuu Point (SE Oahu, Hawaii) three lava flows over-lie each other in a 2.3-m-thick section. Although being emplaced on identical slopes, at identical distances from the likely source (the SE rift zone of Koolau volcano), and stack up one upon the other, the three flows are entirely different. The base of the sequence comprises pahoehoe, the middle section comprises a single 'a'a flow unit, and the top comprises a flow that is transitional between pahoehoe and 'a'a. We have carried out macroscopic field studies of this sequence, supported by laboratory-based microscopic studies of the vesicle and crystal populations to (1) define the differences between these three flow types, and (2) determine what makes a lava of the same composition emplaced at the same location so different in morphology. The pahoehoe forms a 0.5-m-thick pile of seven lobes. These have densities of 1.25-1.75 g m^-3 which, using a dense rock value of 2.65 g m^-3, yield vesicularities of 34-53 %. Macroscopically, the vesicles show a classic S-Type pahoehoe distribution, with a population of smaller spherical bubbles at each lobe periphery, and larger spherical bubbles towards each lobe center. The 'a'a flow comprises a surface and basal clinker, 0.42 and 0.23 m thick, respectively, separated by a 0.58-m-thick massive core. Densities are 1.57-2.09 g m^-3 giving vesicularities of 21-41 %. Locally, the direction of shear (as indicated by the long axis orientation of vesicles) opens around shear-parting structures internal to the core. Finally, the 0.5-m-thick transitional flow has 0.08-0.42 m thick upper and lower zones with paheohoe characteristics, and a 0.17-m-thick core with 'a'a characteristics. The base has a population of small, spherical, bubbles and has a density of 1.63 g m^-3 to give a vesicularity of 38 %. This basal layer also toes into the underlying 'a'a clinker. This lower zone is separated from the core by a sharp contact, with the core containing sheared vesicles and having a density of 2.1 g m^-3 and vesicularity of 21 %. Microscopic analysis of bubble shape, as well as the content of phenocrysts and microphenocrysts, will be used to further define the differences between these three flow types, and to examine the likely differences in cooling history and rheology, as well as effusion rate, which likely led to each flow inheriting their observed morphology.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8425 Effusive volcanism
DE: 8429 Lava rheology and morphology
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting