Figure F8. Periodic variation in average crustal thickness (wavelength ~ 80–100 km) (modified from Tamura et al., 2009). A. Dotted line = along-arc crustal structure (thickness of middle crust with VP = 6.0–6.8 km/s at 5–20 km depths) and solid squares = average SiO2 content of volcanic rocks sampled and dredged from the 16 Izu arc Quaternary volcanoes (Fig. F1B). The basalt-dominant island volcanoes produce small volumes of rhyolites referred to as R1 by Tamura et al. (2009). Rhyolite-dominant submarine volcanoes erupt mostly rhyolite that is compositionally distinct from R1, referred to as R2 by Tamura et al. (2009). B. Schematic crustal structure of the Izu volcanic front showing basalt-dominant island and rhyolite-dominant submarine volcanoes of South Hachijo, Myojin Knoll, and Myojinsho, which have thick and thin middle crust and erupt R1 and R2 rhyolite, respectively.
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