Internal Structures and Growth Style of a Quaternary Subaerial Rhyodacite Cryptodome at Ogariyama, Usu Volcano, Hokkaido, Japan

Yoshihiko Goto,Akihiko Tomiya

doi:10.3389/feart.2019.00066

Abstract

This paper describes internal structures of a Quaternary subaerial rhyodacite cryptodome at Usu volcano, Japan, and examines the textural differences between subaerial and subaqueous cryptodomes. Ogariyama dome is one of the youngest subaerial cryptodomes in the world (<0.4 ka). The cross-section of this young cryptodome is visible because a vertical fault formed during the 1977–1978 eruption and cut through the center of the cryptodome, exposing its interior. The morphology of the cryptodome is scalene triangular in shape, with rounded corners in cross-section, and it is 150 m across and 80 m high. The internal structure of the dome is concentrically zoned, with a massive core, jointed rim, and brecciated border, all of which are composed of uniform, feldspar-phyric rhyodacite (SiO2 = 71–72 wt.%). The massive core (130 m across) consists of coherent rhyodacite that has indistinct, large-scale flow banding and rectangular joints that are spaced 50–200 cm apart. The jointed rim (8–12 m wide) surrounds the massive core and consists of coherent rhyodacite that is characterized by distinct rectangular joints that are 30–80 cm apart and radiate outward. The outermost brecciated border (7–10 m wide) comprises monolithological breccia, consisting of angular rhyodacite clasts (5–30 cm across) and a cogenetic matrix. These internal structures suggest that Ogariyama dome was formed by endogenous growth, involving continuous magma supply during a single intrusive event and simple expansion from its interior. The massive core formed by slow cooling of homogeneous rhyodacite magma. The jointed rim formed by fracturing of solidifying rhyodacite magma in response to cooling–contraction and dynamic stress driven by continued movement of the less viscous core. The brecciated border formed by fragmentation of the solidified rim of the dome in response to dynamic stress. The growth style of Ogariyama dome closely resembles that of subaqueous cryptodomes. However, the morphology and internal structures of Ogariyama dome differ from those of subaqueous cryptodomes, given its asymmetric morphology and absence of radial columnar joints and large-scale flow banding. These differences might reflect the well-consolidated and non-homogeneous physical properties of host sediment, slow cooling rate, and high viscosity of the Ogariyama dome.

Highlights

Cryptodomes are shallow-level intrusions that cause updoming of overlying sediments or other rocks (Minakami et al, 1951; McPhie et al, 1993)
The subaerial Ogariyama cryptodome on Usu volcano has a scalene triangular shape in cross-section and is 150 m across and 80 m high
The massive core formed by slow cooling of homogeneous rhyodacite magma

Summary

Introduction

Cryptodomes are shallow-level intrusions that cause updoming of overlying sediments or other rocks (Minakami et al, 1951; McPhie et al, 1993). They commonly form by the emplacement of silicic magma into poorly consolidated sediment (White et al, 2015). The 1980 eruption at Mount St. Helens in Washington, United States, produced a dacitic cryptodome (bulge) on the volcano flank and caused a catastrophic sector collapse (e.g., Lipman et al, 1981; Voight et al, 1983). Marine geophysical studies of the Bay of Naples, Italy (Milia et al, 2012), suggest the presence of cryptodomes beneath the sea floor

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