Fukutoku-Oka-no-Ba submarine volcano information top
Release date: January 4, 2022 (JST)
Features of pumice rafts from Fukutoku-Oka-no-Ba submarine volcano
Since October, a large amount of pumice stones erupted from Fukutoku-Oka-no-Ba volcano have drifted to various parts of the Japanese archipelago. They have mainly been transported across the Pacific Ocean in large quantities to the Ryukyu Islands, including Okinawa Prefecture, but have also been observed since November in the sea around the Kanto region and the Izu Islands. However, some pumice stones found on the coast did not come from the August 2021 eruption of Fukutoku-Oka-no-Ba. Therefore, the characteristic physical features of the Fukutoku-Oka-no-Ba pumice are summarized below to allow even those who are not familiar with rock type determination to carry out pumice identification in the field.
1. Appearance
The color is mainly white to yellowish gray (Photos 1, 2, and 3). Some have dark gray stripes and a few are black (Photo 3). The black parts with a diameter of about 1 to 5 mm (lower left of Photo 1 and bottom of Photo 2) are a characteristic feature of Fukutoku-Oka-no-Ba pumice, which gives them a “chocolate chip cookie” appearance. Fukutoku-Oka-no-Ba pumice stones (left side of Photo 1) are generally more angular and rugged when compared with other pumice stones found on the coast (right side of Photo 1).
Photo 1 Pumice stones from the 2021 eruption of Fukutoku-Oka-no-Ba collected on the Ibu coast of Kunigami Village, Okinawa Prefecture (left side), and pumice stones originating from volcanoes other than Fukutoku-Oka-no-Ba (right side). Pumice stones not originating from Fukutoku-Oka-no-Ba (origin unknown) are regularly found on the coast of the Kanto region (e.g., Otake coast, Ibaraki Prefecture). The grid in the background is 5 mm.
Photo 2 Enlarged photos of pumice stones from the 2021 eruption of Fukutoku-Oka-no-Ba. The small holes (pores) on the surface vary in size and shape, and some appear large depending on the cross section, while others are stretched. The sizes of the characteristic black parts range from 1 to 5 mm in diameter, and are composed of collections of crystals (plagioclase, pyroxene, opaque minerals, etc. when viewed under a microscope) and brown glass. The grid in the background is 5 mm.
Photo 3 The various appearances of 2021 Fukutoku-Oka-no-Ba pumice stones collected from the Ibu coast in Kunigami Village, Okinawa Prefecture. Pumice stones that have washed ashore are mainly white to gray pumice (Photo 1), but others have different colors and internal structures. Some pumice stones are dark gray or black in color, whereas others incorporate dark gray parts within white pumice, or have striped mixtures of light and dark parts. The grid in the background is 5 mm.
2. Weight
Most of the pumice stones that have washed ashore in Okinawa Prefecture are smaller than 4 cm in diameter. Pumice stones with a slightly smaller diameter are likely to be found in the waters near the Kanto region, because these rocks are broken and worn down as they come into contact with each other while drifting. The Fukutoku-Oka-no-Ba pumice stones are glassy rocks with many small holes, and each piece is as light as a sponge.
3. Distribution at sea and on the beach
Pumice stones drifting in the sea are distributed in stretched-out bands or streaks. Because of this, the 2021 Fukutoku-Oka-no-Ba pumice stones are washed ashore together in a group, so they can often be continuously tracked along the coast. Note that it is difficult to distinguish between this year’s pumice and the pumice from the 1986 eruption of Fukutoku-Oka-no-Ba volcano because they are similar in terms of macroscopic observation and whole rock chemical composition.
Reference information
*Reference information is posted below:
Fukutoku-Oka-no-Ba Volcano 2021 Eruption and Drifting Pumice Disaster Information (Geological Survey of Japan Official Website): https://www.gsj.jp/en/hazards/volcano/fukutokuokanoba2021-e.html
IEVG (Research Institute of Earthquake and Volcano Geology）