Ran across Suwanose-jima (Suwanosejima) in the Smithsonian GVP weekly summary and decided to take a look at it.
The volcanic island is in the same region as Kikai, though farther southwest along the island chain between the East China Sea and the Pacific Ocean. It is one of the Tokara Islands, part of the Ryukyu Volcanic Arc, and the Kagoshima Prefecture. Neighboring islands are Nakanoshima 21 km to the north, Tairajima 18 km to the west and Akusekijima 22 km to the south.
The island itself is around 8 km long and just under 28 km2, has a population near 50, and a small airport only around 600 m long, not long enough to support commercial traffic. There are less than 2,000 people living within 100 km of the island. Normal transportation to the island is a nine-hour ferry from Kagoshima on the mainland. It is the second largest island in the Tokara Islands. Similarly, the Tokara Islands are a subset of the larger Ryukyu island chain south from Kyushu to Taiwan.
Economic activity on the island is mainly agriculture, fishing and seasonal tourism. They are also evacuated on an irregular basis based on activity of the volcano. Local climate is classified as subtropical with a rainy season May – Sept.
Nomenclature of the volcano is also somewhat confusing, at least to me. The island is named Suwanose-jima which is also the name that comes up on most descriptions of the volcano. On the other hand, the Japanese have named each peak on the island as a separate volcano. From oldest to newest, these are Tondachidake, Nabedao and Otake. The first two volcanoes are on either end of the island and Otake grew through the saddle between the first two volcanoes. That saddle was below the surface of the ocean in the early stages of Otake’s growth. Activity on Otake has migrated a bit to the NE, creating a chain of craters on its SW flank. These are Bunka and Tongama craters. To make things yet more confusing, a major lava flow feature on the southern flanks of Nabedao on the southern part of the island is named Makkodai. There is a summit flank collapse amphitheater (caldera) on Otake named Sakuchi that empties to the east.
Photovolcanica did a trip to the island in 2010. At the time, public access was allowed no closer than 1 km of the crater, 500 m below the crater rim. The threat at the time was ballistic ejecta (bombs) from crater explosions. Interesting observation from them was the static buildup from ash clouds which could clearly be felt close to the crater. The path up the volcano ended some 400 m above sea level which was about the same point where vegetation stopped.
Suwanose-jima has active webcams. This link courtesy Volcano Discovery: https://webcams.volcanodiscovery.com/Suwanosejima
This link is courtesy Japan 1: http://volcams.malinpebbles.com/pubweb/Japan1.htm
A webcam time lapse of a 7/10/18 eruption sequence:
The island is a roughly circular with several volcanic centers. These have different names as we have seen at Unzen. Its summit is just short of 800 m above sea level, giving the island a total height of 1,400 m above the ocean floor 500 – 600 m below the surface.
Nabedao volcano is in the southern part of the island. Tondachidake volcano is in the northern part of the island. These are the oldest volcanic edifices and as such deeply eroded. Tondachidake is thought to be the oldest of the two due to more its extensive erosion. Most recent is Otake (sometimes written as O-Take) volcano in the central part of the island. Deposits from Otake cover a major portion of Suwanose-jima island.
Due to erosion, there is no crater left at the top of Tondachidake. There is a depression remaining on top of Nabedao. Effusive andesitic lava flows from Nabedao built an area called Makkodai on the southern end of the island. These lavas are around 100 m thick near the summit and over 80 m thick near the Makkodai location on the southern flank.
Mt. Otake is the center of the island. There is a horseshoe shaped amphitheater pointing east from Otake’s summit, where a flank collapse removed part of the active cone and crater. This is referred to as the Sakuchi Caldera. Recent deposits around Otake suggest the flank collapse took place during the 1813 eruption sequence.
There is a pyroclastic cone in Sakuchi Caldera with a 400 m crater on top of it. There are also well-preserved lava flows on Otake’s slopes. Two of them were deposited during the 1813 eruption. Additional lavas were deposited 1884 – 1885.
Finally, there are two lineaments in the southern part of the island that are perpendicular to one another that are classified as active faults.
The oldest two volcanoes are 50 – 60,000 years above the surface of the ocean. Otake built in the saddle between the two original volcanoes and was 400 – 500 m tall by 40 – 50,000 years ago. Large explosive eruptions continued until 10,000 years ago.
Otake volcano is mostly an andesitic structure. It appears to have grown in three phases, older, middle and younger. Deposits of Older Otake are at the bottom of the Sakuchi caldera. These are altered lavas and pyroclastics, some of which were deposited under water. This portion is not well known as it has mostly been covered by middle and younger deposits. The eruptive center has moved westward gradually.
Middle Otake deposits are on the NW shore and the SE shore. The eruptive center for these is near the location of the current summit. Deposits are mainly subaerial lava flows with minor pyroclastics. They date around 67,000 years.
Deposits of Younger Otake are 40 – 50,000 years to present. For the most part, they are thick agglutinates, pyroclastic flows, pyroclastic falls, and some lava flows at the foot of the volcano. Total thickness of the agglutinates approach 150 m. There are three of these with the most recent from the 1813 eruption. Thick pyroclastic fall deposits cover the lower regions of Otake. The lower part of these are scoria and pumice fall deposits around 20 m thick. These date around 29,000 years ago. These are covered by several meters of volcanic sand dating around 10,000 years ago.
There are lava flows on Younger Otake. Some of these are thick dacites. Lavas on the east coast of the island in Sakuchi Caldera are andesites from Otake crater. A lobe of andesite was covered by 1813 pyroclastic deposits.
The Smithsonian GVP lists 27 eruptive periods since 1600. Four of these 1600 – 1889 are classed as VEI 4. All are from Otake. Since the turn of the century, everything has been VEI2 or smaller. The Wiki lists even more than that, with 156 eruptions in 2008 and 216 in 2009.
The 1813 Bunka Eruption was a large explosive eruption from the crater chain extending from the summit SSW. This eruption took place in three phases and erupted basaltic andesites. The first phase alternated phreatomagmatic and strombolian eruptions from fissure vents on the southern flank of Otake. The fissure propagated toward the summit crater and a sub-Plinian eruption (Phase II). A flank collapse took place in the last stage of the eruption (Phase III).
Most Phase I deposits are on the southern part of the island. There were several pyroclastic flows from multiple cone collapses that traveled all the way to the sea. The climatic part of Phase II enlarged the fissure eruption to the summit crater, produced a fire fountain, and a small pyroclastic cone. There were pyroclastic and lava flows from these craters as the eruption continued. Part of the pyroclastic cone that formed on a steep slope suffered a landslide during or just after the transition to Phase III. Phase III was a large collapse of the volcanic edifice. Debris avalanche from near Otake crater eastward left debris as thick as 10 m at the shore.
The 1884 – 1885 Meiji eruption began with an explosion followed by pumice, ash fall, and then lava flows all from Otake crater. The eruption lasted until Feb. 1885. Small portions of a pyroclastic cone were built during this eruption. Lava flows from this eruption covered a wide area toward the shoreline from the crater. Its surface appears to be pahoehoe lava.
Activity since 1885 has been small-scale Vulcanian, Strombolian and ash-producing eruptions. The Vulcanian eruptions are short explosions, ash plumes that will dust areas a few kilometers from the crater. Explosions also produce dense bread-crust bombs. Stombolian eruptions are intermittent emissions of fragmented magmas. Some of the eruptions produce continuous ash emissions.
The Otake Pyroclastic Cone is 600 m wide by 80 m tall. Its ash fall deposit covers the whole island with a few meters of ash. Eruption products are basaltic andesites to dacites.
Frequency of eruption activity in recent years has progressed to the point where since 1976 the volcano is classified as either active or quiescent. Active periods since 1976 are 1980 – 1984, 1989 – 1994, and 2000 – 2011. All of these had a high frequency of explosive activity and lavas splatters and bombs, though large scale lava effusion has not happened since the end of the 1885 eruption.
The active volcano supports fumaroles, geothermal activity and hot springs around Otake crater. There is an aquifer thought to exist at a shallow level beneath the crater.
Suwanose-jima collapsed several times in its history. It is probable that Otake will collapse again with subsequent debris flow. That collapse can be triggered by either an eruption like 1813 or distant tectonic earthquakes.
Suwanose-jima is a member of the Ryukyu volcanic arc which is in turn formed by the subduction of the Philippines Plate under the Eurasian (Yangtze) Plate. I covered a lot of what is going on in this part of Japan in the Kikai post last November. Take a look at the Tectonics section for more information. https://volcanohotspot.wordpress.com/2018/11/22/dome-inflation-at-kikai-caldera-japan/#more-22667
Essentially, the Ryukyu volcanic arc / volcanic islands are a back-arc string of subduction driven volcanoes. Photovolcanica ties the subduction of the Kyushu – Palau Ridge of the Philippine Sea Plate under Japan as cause of this volcano grouping. I respectfully disagree, as that line is pointed directly at Kyushu rather than nine hour ferry ride south along the string of Ryukyu subduction islands.
The Philippine Plate is nor particularly deep at this point at 80 km and with the Ryukyu Trench being some 120 km ENE of the island.
Suwanose-jima is a very active member of the Ryukyu Volcanic Arc. It has at least a 60,000-year history above the waves. While current activity is relatively benign, it has multiple VEI 4 eruptions within a 60-year period in the 19th Century. Like all active, explosive Japanese volcanoes, it should be treated with great respect.