Intrusive dike complexes, cumulate cores, and the extrusive growth of Hawaiian volcanoes Ashton F. Flinders,1 Garrett Ito,2 Michael O. Garcia,2 John M. Sinton,2 Jim Kauahikaua,3 and Brian Taylor2 Received 19 April 2013; revised 3 June 2013; accepted 4 June 2013; published 3 July 2013. [1] The Hawaiian Islands are the most geologically studied hot-spot islands in the world yet surprisingly, the only largescale compilation of marine and land gravity data is more than 45 years old. Early surveys served as reconnaissance studies only, and detailed analyses of the crustal-density structure have been limited. Here we present a new chainwide gravity compilation that incorporates historical island surveys, recently published work on the islands of Hawai‘i, Kaua‘i, and Ni‘ihau, and >122,000 km of newly compiled marine gravity data. Positive residual gravity anomalies reflect dense intrusive bodies, allowing us to locate current and former volcanic centers, major rift zones, and a previously suggested volcano on Ka‘ena Ridge. By inverting the residual gravity data, we generate a 3-D view of the dense, intrusive complexes and olivine-rich cumulate cores within individual volcanoes and rift zones. We find that the H¯ana and Ka‘ena ridges are underlain by particularly high-density intrusive material (>2.85 g/cm3) not observed beneath other Hawaiian rift zones. Contrary to previous estimates, volcanoes along the chain are shown to be composed of a small proportion of intrusive material (3.0 g/cm3) were detected under the majority of volcanoes, with volumes ranging from 2.85 g/cm3; Table 1). An upper bound estimate was made by decreasing the density isosurface for intrusive complexes from 2.85 g/cm3 to 2.80 g/cm3, approximately equivalent to changing from a 60% to 40% dike concentration and equal to the cutoff between intrusive and extrusive basalt used by Moore [2001]. Using this constraint and calculating an intrusive/ extrusive ratio across the entire chain, results in a mean increase to only 30% intrusive material. [13] Our estimate of a low intrusive proportion (10–30%) is contrary to prior conclusions that the Hawaiian Islands are built through predominately endogenous growth, with previous estimates that intrusions account for 65–90% of the total volume [Francis et al., 1993; Dzurisin et al., 1984]. These estimates were based on geologically short-term observations of the active K¯ılauea Volcano, specifically heat loss over the Kupaianaha lava pond (1986–1992 [Francis et al. 1993]) and uplift of K¯ılauea summit (1956–1983 [Dzurisin 3371 FLINDERS ET AL.: EXTRUSIVE GROWTH OF HAWAIIAN VOLCANOES Table 1. Volcanic Features and Their Cumulate Cores/Intrusive Complexesa Volume Isosurface Avg. Core/I.C.a Vol. Olivine Content Core/I.C. Volume Isosurface Avg. Core/I.C.a Vol. Olivine Content Core/I.C. Volcano/Rift (km3) (g/cm3) (g/cm3) (km3)b (%) (% of Volcano)c Volcano/Rift (km3) (g/cm3) (g/cm3) (km3)b (%) (% of Volcano)c L¯oih‘i 1,700 2.85 2.89 10 9–12 1 K¯ılauea 31,600 2.85 2.88 1,700 6–7 5 Mauna Loa 74,000 2.85 2.91 7,500 12–16 10 3.00 3.03
Posted on: Sun, 08 Sep 2013 05:51:00 +0000
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