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[SVC34-12] Pillow lava of rhyolite rheoignimbrite
Keywords:rheoignimbrite, pillow lava, rhyolite, Futomiyama Group, critical strain rate
The 70 Ma Futomiyama Group consists of rhyolite ignimbrite associated with rhyolite lava flows and intrusive rocks. The Fukube Fall in the upper Jadani River exposes successive layers of welded ignimbrite of the upper Futomiyama Group, that intervenes a rheoignimbrite layer of giant pillows. The pillowed layer extends 320 m from east to west and varies in thickness from 15 m to 84 m. The direction, height, and width of the elongated pillow lobes were measured based on a 3D model constructed from 4K video captured by a drone. The bifurcating pillow lobes indicate that the western, central and eastern pillows flowed southward, eastward and southwestward, respectively. The pillow lobes range from 1–9 m in height and 1–13 m in width, with an average of 3.4±1.6 m and 4.2±2.1 m, respectively. The height/width ratio is 0.78, which is similar to the ranges for tholeiite pillows of the Recent Loihi Seamount, Hawaii, and larger than those of the Cretaceous Oman Ophiolite.
The obsidian glass of the pillow margin has a rhyolitic composition with 77 mass% SiO2. The minimum viscosity of the Fukube Fall magma at the hornblende-saturated temperature of 790 °C is estimated to be 106.8 Pa·s based on the 6.0 mass% water content and the rhyolitic composition. Analogue experiments on pillow lobe height constrain the upper and lower limits of extrusion rate and viscosity of the lava. In addition, the Devorah number, the ratio of rate of strain accumulation to the rate of strain relaxation, also gives the maximum viscosity and extrusion rate that allows for the formation of coherent pillows without fragmentation. Based on these constraints, the conditions of pillow lobe formation are lava viscosities ranging from 104.7 Pa·s to 106.8 Pa·s, and extrusion rates from 2.3X10-3 m3/s to 3.1X103 m3/s for point sources and from 2.3X10-3 m2/s to 3.1X102 m2/s for line sources. Such viscosities require water contents of the mama as high as 2.2 mass% to 6.0 mass%, suggesting the welding of ignimbrite under hydrous conditions.
The obsidian glass of the pillow margin has a rhyolitic composition with 77 mass% SiO2. The minimum viscosity of the Fukube Fall magma at the hornblende-saturated temperature of 790 °C is estimated to be 106.8 Pa·s based on the 6.0 mass% water content and the rhyolitic composition. Analogue experiments on pillow lobe height constrain the upper and lower limits of extrusion rate and viscosity of the lava. In addition, the Devorah number, the ratio of rate of strain accumulation to the rate of strain relaxation, also gives the maximum viscosity and extrusion rate that allows for the formation of coherent pillows without fragmentation. Based on these constraints, the conditions of pillow lobe formation are lava viscosities ranging from 104.7 Pa·s to 106.8 Pa·s, and extrusion rates from 2.3X10-3 m3/s to 3.1X103 m3/s for point sources and from 2.3X10-3 m2/s to 3.1X102 m2/s for line sources. Such viscosities require water contents of the mama as high as 2.2 mass% to 6.0 mass%, suggesting the welding of ignimbrite under hydrous conditions.