A Complex Volcanic Mountain
Kusatsu-Shirane is a complex of pyroclastic cones formed through repeated volcanic activity. Numerous craters and lava vents are concentrated around the summit area.
Some of the craters were formed by eruptions approximately 5,000 years ago and are arranged in a distinctive northeast-to-southwest alignment.
Today, the upper mountain has a stark, almost barren appearance. Historical accounts indicate that the area around the craters was once much greener, even before an eruption in 1882. The contrast illustrates how volcanic activity has repeatedly transformed the landscape.
The Mysterious Crater Lake Yugama
One of the World’s Most Acidic Lakes
Yugama is a crater lake near the summit, measuring approximately 300 meters in diameter and around 30 meters deep. Its water temperature is roughly 18°C (64°F), while its pH is around 1.0, making it one of the most strongly acidic natural lakes in the world.
The extreme acidity is thought to result from volcanic gases containing hydrogen chloride and sulfur dioxide dissolving into the lake water and forming hydrochloric and sulfuric acids.
The lake has a striking cloudy blue-green appearance. Dissolved substances including iron ions and sulfur interact with light in particular ways, producing the remarkable emerald-green color for which Yugama is famous.
A Dramatic Contrast of White Crater Walls and Green Water
The brilliant lake contrasts dramatically with the pale, barren crater walls, creating a landscape that can resemble the surface of another planet.
Other volcanic craters and crater lakes, including Mizugama and Karegama, are found in the surrounding area. Together with visible volcanic gas and other geothermal phenomena, they provide a vivid reminder that Kusatsu-Shirane remains an active volcano.
Access to areas around Yugama may be restricted depending on volcanic conditions. Visitors should always check the latest official volcanic and access information before traveling.
The History of Sulfur Mining
The area around Yugama was once an important site for sulfur mining. Sulfur deposits that accumulated around the lake and its shores were extracted by mining companies from before World War II until around 1960.
Mining infrastructure included small rail carts and lifts used to transport sulfur, while offices and work buildings were established around the lakeshore.
Working in such an environment was extremely hazardous. Volcanic gases and steam posed serious risks to workers, and the mining operation was conducted under difficult conditions.
Today, remnants of the former mining operations remain as reminders of the industrial history of Kusatsu-Shirane and its relationship with the region’s volcanic resources.
A Long History of Volcanic Activity
Activity Dating Back Hundreds of Thousands of Years
Kusatsu-Shirane began its volcanic activity approximately 570,000 years ago. Numerous periods of eruption followed, gradually forming the complex volcanic landscape seen today.
Activity after approximately 18,000 years ago produced pyroclastic cones and lava flows associated with the Shirane volcanic complex, helping establish the basic form of the modern landscape.
Eruptions in Recorded History
Historical records document several eruptions, including a phreatic eruption in 1805. Further eruptions occurred during the Meiji, Taisho, and Showa periods of Japanese history.
In 1932, a volcanic mudflow caused casualties and demonstrated the potential hazards associated with the mountain’s volcanic activity.
More recently, on January 23, 2018, a sudden phreatic eruption occurred near the northern crater of Kagamiike on Moto-Shirane. Volcanic rocks ejected during the eruption caused deaths and injuries.
The event was particularly significant because the eruption occurred at a crater different from Yugama, which had traditionally been the main focus of volcanic monitoring. It highlighted the difficulty of predicting phreatic eruptions and the importance of monitoring the wider volcanic area.
Volcanic Alert Levels and Safety Measures
Japan’s Volcanic Alert Level system has been used at Kusatsu-Shirane since 2007, while the Japan Meteorological Agency continuously monitors volcanic activity.
Depending on the alert level and observed conditions, access to volcanic areas may be restricted and roads may be closed to protect visitors and local communities.
The Shiga-Kusatsu Road, National Route 292, in particular, may be subject to closures and other restrictions depending on volcanic activity. If volcanic earthquakes, tremors, or other unusual activity are detected, authorities can take immediate safety measures.
Because conditions can change, travelers should check current volcanic alerts, road conditions, and sightseeing restrictions immediately before visiting.
An Important Site for Scientific Research
Kusatsu-Shirane is also an important site for volcanological and earth science research. Scientists study the amount of volcanic gas released by the mountain and investigate the hydrothermal systems operating beneath the volcanic complex.
The chemistry of Yugama is particularly interesting. Its lake water contains relatively high concentrations of certain lanthanide elements, providing valuable data for research into the movement and distribution of chemical elements in volcanic environments.
For researchers, the mountain offers an opportunity to study the complex interaction between volcanic gases, groundwater, hot springs, and geothermal activity.
Scenic Routes and Access
Spectacular Views from the Shiga-Kusatsu Road
National Route 292, known as the Shiga-Kusatsu Road, is famous as a high-altitude scenic driving route. The road reaches elevations of around 2,000 meters and offers impressive views of the surrounding mountains.
A walking route from the rest area once provided access to the Yugama viewpoint in approximately 10 minutes. However, availability of the route and viewpoint depends on current volcanic regulations and safety conditions.
By Public Transportation
Local buses operate from Yudanaka Station on the Nagano Electric Railway and from Naganohara-Kusatsuguchi Station on the JR Agatsuma Line.
However, if volcanic gas concentrations become elevated, the Shirane Kazan bus stop may not be available for use. Travelers should check transportation information before setting out.
A Different Face in Every Season
In summer, alpine plants bloom across the highlands, while the clear mountain air provides expansive views. In autumn, colorful foliage spreads across the mountain slopes, creating a vivid contrast with the volcanic terrain.
During winter, the mountain is transformed into a vast snowy landscape. The harsh alpine environment creates challenging conditions, but it also reveals another side of Kusatsu-Shirane’s natural beauty.
Experience the Power of an Active Volcano
Kusatsu-Shirane Mountain is a place where visitors can experience the immense forces shaping the Earth. The emerald waters of Yugama, pale crater walls, volcanic gases, and rugged mountain scenery all reveal the continuing influence of volcanic activity.
Its relationship with Kusatsu Onsen is particularly fascinating. The same geothermal forces that shaped the volcano also contribute to the abundant hot spring resources for which Kusatsu is famous.
Visitors should always check the latest volcanic alerts, road conditions, and access restrictions before traveling and follow all safety instructions while in the area.
When conditions permit, a visit to Kusatsu-Shirane offers a memorable opportunity to see the dramatic landscape created by an active volcano and to better understand the powerful connection between volcanic activity and hot spring culture in Kusatsu.