Mount Usu
Mount Usu (有珠山, Usu-zan) is an active stratovolcano located in Shikotsu-Toya National Park, Hokkaido, Japan. It is part of the Pacific Ring of Fire and is known for its frequent explosive eruptions and the formation of lava domes and cryptodomes.
Geological Setting and History:
Mount Usu lies on the southern rim of the Toya caldera, a large volcanic depression formed by a massive eruption around 110,000 years ago. Usu has been active for approximately 20,000 years. It is characterized by a complex geological history, marked by alternating periods of explosive eruptions and the quieter extrusion of viscous lava.
Key features associated with Mount Usu include:
- O-Usu: The older, main cone of Mount Usu, formed before the 20th century eruptions.
- Showa-Shinzan: A lava dome that dramatically rose from a farm field during an eruption in 1943-1945. Its rapid growth made it a symbol of volcanic activity and earned it recognition as a Special Natural Monument.
- Kompirayama: A smaller volcanic cone that formed during an eruption in 1910.
- Multiple Cryptodomes: Intrusions of magma beneath the surface that cause ground deformation. These cryptodomes have been significant precursors to several eruptions.
Eruptive History:
Mount Usu has experienced several significant eruptions in recorded history. Some of the notable eruptions include:
- 1663: A major explosive eruption.
- 1769: Another significant eruption.
- 1822: An eruption accompanied by the formation of a new lava dome.
- 1910: An eruption that formed Kompirayama.
- 1943-1945: The eruption that created Showa-Shinzan.
- 1977-1978: An eruption that caused significant ashfall and lahars, leading to evacuations of nearby communities.
- 2000: An eruption that prompted widespread evacuations and the formation of new craters.
Monitoring and Hazards:
Mount Usu is closely monitored by volcanologists due to its active nature and potential for future eruptions. Monitoring techniques include seismicity monitoring, ground deformation measurements (using GPS and InSAR), gas emission analysis, and thermal imaging.
Volcanic hazards associated with Mount Usu include:
- Explosive eruptions: Capable of producing ash clouds, pyroclastic flows, and lahars.
- Lava dome collapse: Leading to pyroclastic flows and surges.
- Ashfall: Disrupting transportation, agriculture, and infrastructure.
- Lahars: Mudflows composed of volcanic ash, debris, and water, which can inundate valleys and cause significant damage.
- Ground deformation: Caused by magma intrusion, potentially leading to structural damage.
Tourism and Access:
Despite its volcanic activity, Mount Usu is a popular tourist destination. The area offers opportunities for hiking, sightseeing, and learning about volcanology. The Usuzan Ropeway provides access to viewpoints near the summit, offering panoramic views of Showa-Shinzan, Lake Toya, and the surrounding landscape. The Mount Usu Volcano Village provides educational exhibits and information about the volcano's history and hazards.
Significance:
Mount Usu is a significant volcano both scientifically and culturally. Its frequent eruptions provide valuable opportunities for studying volcanic processes and improving eruption forecasting techniques. The volcano is also an important part of the local landscape and culture, attracting tourists and providing inspiration for artists and writers.