Iwo Gusuku (Okinawa Prefecture): A Thorough Explanation of the Active Volcano on Iwo Torishima Island and its Historical Role
Iwo Gusuku is an active volcano located in the southeastern part of Iwo Torishima Island, the northernmost island of Okinawa Prefecture. Iwo Torishima, the only active volcanic island in Okinawa Prefecture, has two main volcanic bodies: Iwo-dake in the northwest and Gusuku (Mae-dake) in the southeast. Of these, Gusuku volcano is attracting attention as a more volcanologically newer volcanic body. This article provides a comprehensive explanation of Iwo Gusuku’s geological characteristics, historical background, and current volcanic activity.
Geographical Location and Overview of Iwo Gusuku
Iwo Torishima is an uninhabited island located approximately 190 kilometers north-northeast of Naha City and approximately 65 kilometers west of Tokunoshima Island in Kagoshima Prefecture. The island has a circumference of approximately 7 kilometers and an area of approximately 2.5 square kilometers, with a long, narrow shape extending from northwest to southeast. It is approximately 3 kilometers long and 1 kilometer wide, and is known as a unique volcanic island located at the southernmost tip of the Kirishima volcanic belt. Iwo-gusuku is located at the southern tip of the island and is also known as “Maedake.” This volcanic body exhibits the form of a tuff ring (tuff ring) with a crater approximately 500 meters in diameter, and a flattened lava dome exists within the crater. Weak fumaroles are visible on the northern crater wall, indicating that volcanic activity is still ongoing.
While Okinawa Prefecture is generally thought to have no active volcanoes, Iwo-torishima is the only active volcanic island in the prefecture, possessing a unique natural environment distinct from other parts of Okinawa. This island is designated as a constantly monitored volcano by the Japan Meteorological Agency and plays a part in the nationwide active volcano monitoring system.
Geological Characteristics of Iwo-gusuku
Structure and Formation Process of the Volcanic Body
Iwo-torishima is classified as a triple-rim volcano with two outer rims and a central crater cone. Iwo-dake Island is composed of two main volcanic bodies: Iwo-dake in the northwest and Gusuku-zan in the southeast. Volcanologically, Gusuku-zan is clearly a younger volcanic body.
Gusuku-zan has a tuff ring structure formed by the accumulation of pyroclastic materials and is thought to have been formed by a phreatomagmatic eruption. Inside the 500-meter-diameter crater, there is a later-formed flattened lava dome, the presence of which tells the story of a complex history of volcanic activity.
Geological observations of the crater wall reveal traces of multiple eruptions, with alternating layers of volcanic ash, lapilli, and lava flows. This geological evidence indicates that Gusuku-zan has experienced intermittent volcanic activity over a long period.
Comparison with Iwo-dake
Iwo-dake, the other major volcanic body on Iwo-Tori-shima Island, is a lava dome located in the northwest of the island, with a crater approximately 500 meters in diameter at its summit. All recorded eruptions in historical times occurred at the crater of Iwo-dake, with records of eruptions in 1664, 1768, and 1903.
On the other hand, while no clear historical eruption records have been confirmed for Gusuku Volcano, its relative youth and ongoing fumarolic activity suggest that future volcanic activity cannot be ruled out. The geological fact that Gusuku is a younger volcano is important knowledge for considering future volcanic activity.
The lava composition of both volcanoes ranges from andesitic to dacitic, indicating the involvement of relatively viscous magma. This characteristic suggests the possibility of explosive eruptions, underscoring the importance of volcanic monitoring.
History of Iwo Gusuku and the Ryukyu Kingdom Era
Importance as a Sulfur Source
As the name Iwo-Tori-shima (Sulfur Bird Island) suggests, this island has long been known as a sulfur source. During the Ryukyu Kingdom era, sulfur was an extremely important strategic material as a raw material for gunpowder, and Iwo-Torishima Island was one of its main sources.
During the Ryukyu Kingdom era, the “Tomari-gashira-torikata,” established under the Tomari-jito (local administrator), governed Iwo-Torishima Island along with Tomari Village. People resided on the island for sulfur mining, and the mined sulfur was supplied to the Ryukyu Kingdom government and also played an important role as a trade item with China and mainland Japan.
Sulfur mining mainly took place around Mount Iwo, but it is possible that sulfur sublimated from fumaroles around the Gusuku volcano was also collected. Sulfur deposits exposed on the surface by volcanic activity were a valuable resource that could be extracted relatively easily even with the technology of the time.
Meaning of the name “Iwo-Gusuku”
The word “Gusuku” means “castle” in the Okinawan language and generally refers to the ruins of castles from the Ryukyu Kingdom era. However, “Iwo-Gusuku” on Iwo-Torishima Island is used as a place name referring to the volcanic body itself, not a castle.
On the other hand, historical documents such as the Shuri-Naha Port Folding Screen mention the name “Iwo Gusuku,” suggesting the possibility that it refers to a gusuku (castle) that served as a supply base for storing weapons and sulfur. This likely refers to a facility on Okinawa Island, not Iwo Torishima, and is thought to have functioned as a base for storing and managing sulfur mined on Iwo Torishima.
This duality in the name illustrates how important sulfur was as a strategic resource for the Ryukyu Kingdom. It reveals the historical context of sulfur mined on a volcanic island being strictly managed at military facilities on the main island and used for gunpowder production.
Life on Iwo Torishima
From the Ryukyu Kingdom era to modern times, people engaged in sulfur mining resided on Iwo Torishima. In the harsh natural environment of a volcanic island, people made their living by mining sulfur.
Life on the island was not easy; they likely faced many difficulties, including the dangers of volcanic gases, securing drinking water, and obtaining food. Nevertheless, the high economic value of sulfur continued to attract people to the island.
Sulfur mining continued after the Meiji era, but due to increased dangers such as the 1903 eruption of Mount Iwo and the decrease in demand for natural sulfur due to the development of the chemical industry, miners gradually withdrew from the island. Currently, Iwo-Torishima is an uninhabited island, but traces of past human activity remain in various parts of the island.
Current Volcanic Activity
Fumarole Activity and Volcanic Monitoring
At Iwo-Gusuku Volcano, weak fumarolic activity continues on the northern crater wall. Observations from 10,000 meters above ground in November 2019 confirmed small amounts of fumaroles from both the Iwo-dake crater and the Gusuku volcano crater.
These fumarolic activities indicate that a heat source still exists underground, and volcanic activity has not completely ceased. Changes in the temperature and chemical composition of the fumaroles are important indicators of fluctuations in magma activity, and are therefore regularly monitored.
The Japan Meteorological Agency (JMA) has designated Iwo-Torishima as a continuously monitored volcano and maintains 24-hour monitoring using observation equipment such as seismometers and tiltmeters. Regarding Gusuku Volcano, careful observation is also continuing to ensure that no changes in volcanic activity are overlooked, along with those of Iwo-dake.
Volcanic Eruption Warning and Forecast System
Since Iwo-Torishima is an uninhabited island, direct human casualties from an eruption would be limited. However, there are concerns about the impact on ships navigating the surrounding waters and aircraft flying overhead. Therefore, the JMA issues eruption warnings and forecasts according to the state of volcanic activity and provides information to relevant organizations.
Regarding Gusuku Volcano, there are no historical records of eruptions, but because the volcanic body is relatively young and fumarolic activity continues, the possibility of a future eruption cannot be completely ruled out. The latest volcanic activity status is regularly published through volcanic activity explanation materials.
The explanatory information regarding the volcanic situation provides detailed reports on the progression of seismic activity, changes in fumarolic activity, and the presence or absence of crustal deformation, offering expert assessments of the current state and future outlook of volcanic activity.
Positioning within the Nationwide Active Volcano Monitoring System
There are 111 active volcanoes throughout Japan, of which 50 are designated as continuously monitored volcanoes. Iwo-Torishima is one of these continuously monitored volcanoes and holds a special position as the only active volcano in Okinawa Prefecture.
Real-time information on Iwo-Torishima’s volcanic activity is posted on nationwide volcanic monitoring websites, allowing experts and the general public to access the latest information. Observation data for Gusuku Volcano, along with Iwo-dake, is also being accumulated and used for long-term trend analysis of volcanic activity.
As an active volcano located in the warm region of Okinawa Prefecture, Iwo-Torishima is a valuable research subject from a volcanological perspective. It continues to provide important information for considering the geological formation of the Ryukyu Islands and the southern limit of volcanic activity.
Natural Environment and Ecosystem of Iwo Gusuku
Unique Natural Environment
Iwo Torishima, including Iwo Gusuku, possesses a unique natural environment that differs significantly from other areas of Okinawa Prefecture. Sulfur gases and high-temperature fumaroles associated with volcanic activity hinder the establishment of normal vegetation, creating a distinctive landscape.
Around the crater, areas where the ground surface has turned yellow due to sulfur sublimation can be seen, creating a desolate, volcanic landscape. On the other hand, in areas with relatively little volcanic activity, subtropical vegetation has developed, forming an ecosystem unique to volcanic islands.
Along the coast, sea cliffs and wave-cut platforms formed by wave erosion of volcanic rock have developed, allowing for the observation of a unique topographical landscape created by volcanic landforms and marine erosion. These landforms are recognized as one of Japan’s 100 most beautiful topographical features and are of academic value.
Seabird Breeding Ground
The name Iwo Torishima (Seabird Island) originates from the fact that many seabirds inhabit the island. Fortunately, being an uninhabited island, it serves as a valuable breeding ground for seabirds, unaffected by human activity.
Seabirds such as red-footed boobies and streaked shearwaters visit the island for breeding, nesting in areas less affected by volcanic activity. For these seabirds, Iwo-Tori-jima is one of the important breeding bases in the East China Sea.
While bird habitats are limited around the crater of Gusuku Volcano due to volcanic activity, seabirds can sometimes be observed on the crater rim during periods of calm volcanic activity. From the perspective of coexistence between volcanoes and ecosystems, it is also an interesting subject of research.
Access and Visiting Iwo-Gusuku
Access Restrictions and Safety Management
Iwo-Tori-jima is currently an uninhabited island and, being an active volcano, public access is strictly restricted. This is due to numerous dangers, including the risk of volcanic gas poisoning, the possibility of sudden eruptions, and the risk of falls due to the steep terrain.
In particular, the area around the crater of Gusuku Volcano is experiencing continuous fumarolic activity, potentially releasing toxic gases such as hydrogen sulfide. Approaching without specialized equipment and knowledge is extremely dangerous.
Landing for academic research or volcanic observation is only permitted with appropriate permission and sufficient safety measures in place. Landing for general tourism purposes is not permitted.
Observation from a Distance
While directly visiting Iwo-Torishima is difficult, the island can be viewed from surrounding islands such as Tokunoshima on clear days. Especially on clear days, it is possible to observe the distinctive silhouette of the two volcanic bodies, Iwo-dake and Gusuku Volcano, joined together.
Observation from an aircraft is also an effective method. Air routes connecting Naha and the Amami Islands allow for aerial views of Iwo-Torishima depending on the weather and flight path. From above, the arrangement of the two volcanic bodies, the appearance of the crater, and the overall shape of the island can be observed.
Aerial photographs and satellite images published by the Japan Meteorological Agency and research institutions are also valuable sources of information about Iwo-Gusuku. These images allow for detailed observation of the crater’s condition, fumarole activity, and vegetation distribution.
Research on Iwo Gusuku and Future Challenges
Importance of Volcanological Research
Iwo Gusuku is an important research subject for understanding the characteristics of volcanic activity in the Ryukyu Islands. Many questions remain to be answered, such as why active volcanoes exist in this region, the origin of the magma, and how future activity will unfold.
Through dating and petrological analysis of the volcanic body, the formation history and eruption history of the gusuku volcano are gradually becoming clear. However, detailed geological surveys are not easy due to the island being uninhabited and access being restricted.
The introduction of new survey methods, such as remote sensing technology and the use of unmanned aerial vehicles (drones), is expected to enable safer and more efficient research.
Volcanic Disaster Prevention and Information Dissemination
Although Iwo Torishima is an uninhabited island, fishing activities are conducted in the surrounding waters, and air and sea routes pass nearby. Understanding the potential impact of an eruption and the likely precursory phenomena is crucial for disaster prevention.
Regarding Gusuku Volcano, the lack of historical eruption records makes predicting its scale and style difficult. Eruption scenarios are being developed through detailed investigations of past eruption deposits and comparative studies with similar volcanoes.
Effective dissemination of volcanic activity information is also a critical issue. A system is needed to clearly explain specialized observation data and appropriately communicate it to relevant organizations and the general public.
Value as a Cultural Heritage Site
Iwo Gusuku possesses not only natural scientific value but also cultural heritage, conveying the history of the sulfur industry during the Ryukyu Kingdom era. The remains of sulfur mining sites and traces of daily life at the time are valuable resources for understanding the industrial history of Ryukyu.
Unraveling the historical meaning behind the name “Iwo Gusuku” and passing on the history of the relationship between volcanoes and humans to future generations is another important challenge. Further research is needed regarding the relationship between Iwo Gusuku, depicted in the Shuri-Naha Port Folding Screen, and the Gusuku volcano on the volcanic island.
While it differs in character from the Gusuku castles that are a World Heritage Site in Okinawa, Iwo Gusuku holds undeniable importance in understanding Ryukyu history from multiple perspectives.
Summary
Iwo Gusuku is an active volcano located in the southeastern part of Iwo Torishima Island in Okinawa Prefecture, characterized by a tuff ring structure with a crater 500 meters in diameter. As a volcanic body, it is younger than Iwo-dake in the northwest, and weak fumarolic activity continues to this day.
During the Ryukyu Kingdom era, Iwo Torishima Island flourished as an important sulfur source, and the name “Iwo Gusuku” may refer to both the volcanic body itself and the logistical facilities for storing sulfur. This duality reflects the history of sulfur being extremely important as a strategic material.
Currently, Iwo Torishima Island is uninhabited, but as Okinawa Prefecture’s only active volcano, it is continuously monitored by the Japan Meteorological Agency. As part of the nationwide active volcano monitoring system, changes in seismic and fumarolic activity are carefully monitored, and information on volcanic activity is regularly published.
The challenges surrounding Iwo Gusuku are diverse, encompassing volcanological research, disaster prevention measures, and cultural heritage protection. As a symbol of Okinawa’s unique natural environment and history, continued research and appropriate conservation are required. Iwo Gusuku will continue to be a valuable resource that conveys the history of the relationship between volcanoes and humans to the present day.
