Pillow flows of rhyolite rheoignimbrite exposed in the Fukube Falls, Hakusan-Shirakawago White Road

The Fukube Falls in the upper Jadani River exposes successive layers of essentially subaerial ignimbrite of the 70 Ma 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 bifurcating pillow lobes indicate that the western, central and eastern pillows flowed southward, eastward and southwestward, respectively. The pillow lobes range from 1 to more than 9 m in height and from 1 to 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 magma temperature is estimated from 782 °C to 810 °C, depending on the water content, based on the hornblende geothermometer. Analogue experiments on pillow lobe height constrain the upper and lower limits of extrusion rate and viscosity of the rheoignimbrite. 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 viscosities ranging from 10^4.5 Pa·s to 10^6.8 Pa·s, and extrusion rates from 2.4 × 10⁻³ m³/s to 3.1 × 10³ m³/s for point sources and from 2.4 × 10⁻³ m²/s to 3.1 × 10² m²/s for line sources. Such viscosities require water contents of the magma more than 1.4 mass%, suggesting the hydrous welding of ignimbrite that was initially extruded on land and was emplaced under water-saturated conditions.