MODELS OF HAWAIIAN VOLCANO GROWTH AND PLUME STRUCTURE - IMPLICATIONS OF RESULTS FROM THE HAWAII SCIENTIFIC DRILLING PROJECT

The shapes of typical Hawaiian volcanoes are simply parameterized, and a relationship is derived for the dependence of lava accumulation rat es on volcano volume and volumetric growth rate. The dependence of lav a accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the Hawaiian plume, with the eruption ra te determined from a specified radial dependence of magma generation i n the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit. The timescale of volcano growth i s t = 2R/v(plate) where R is the radius of the melting zone of the (ci rcular) plume and v(plate) is the velocity of the Pacific plate. The g rowth progress of a volcano can be described by a dimensionless time t ' = tv(plate)/2R, where t' = 0 is chosen to be the start of volcano gr owth and t' = 1 approximates the end of ''shield'' growth. Using a mel t generation rate for the whole plume of 0.2 km(3)/yr, a plume diamete r of 50 km, and a plate velocity of 10 cm/yr, we calculate that the li fetime of a typical volcano is 1000 kyr. For a volcano that traverses the axis of the plume, the ''standard'' dimensions are a volume of 57, 000 km(3), a summit thickness of 18 km, a summit elevation of 3.6 km, and a basal radius of 60 km. The volcano first breaches the sea surfac e at t' approximate to 0.22 when it has attained only 5% of its eventu al volume; 80% of the volume accumulates between t' = 0.3 and t' = 0.7 . Typical lava accumulation rates start out over 50 m/kyr in the earli est stages of growth from the seafloor, and level out at similar to 35 m/kyr from t' approximate to 0.05 until t' = 0.4. From t' = 0.4 to t' = 0.9, the submarine lava accumulation rates decrease almost linearly from 35 m/kyr to similar to 0; subaerial accumulation rates are about 30% lower, The lava accumulation rate is a good indicator of volcano age. A volcano that passes over the plume at a distance 0.4R off to th e side of the plume axis is predicted to have a volume of about 60% of the standard volcano, a lifetime about 8% shorter, and lava accumulat ion rates about 15-20% smaller. The depth-age data fur Mauna Kea lavas cored by the Hawaii Scientific Drilling Project are a good fit to the model parameters used, given that Mauna Kea appears to have crossed t he plume about 15-20 km off-axis. The lifetime of Mauna I(ca is estima ted to be 920 kyr. Mauna Loa is predicted to be at a stage correspondi ng to t' approximate to 0.8, Kilauea is at t' approximate to 0.6, and Loihi is at t' approximate to 0.16. The model also allows the subsurfa ce structure of the volcanoes (the interfaces between lavas from diffe rent volcanoes) to be modeled. Radial geochemical structure in the plu me may be blurred in the lavas because the volcanoes capture magma fro m a sizeable cross-sectional area of the plume; this inference is qual itatively born out by available isotopic data. The model predicts that new Hawaiian volcanoes are typically initiated on the seafloor near t he base of the next older volcano but generally off the older volcano' s flank.