What Is the Kusatsu-Shirane Volcanic Group?
The name Kusatsu-Shirane refers collectively to three major peaks: Mount Motoshirane (2,171 m), Mount Shirane (2,160 m), and Mount Ainomine (2,110 m).
Each mountain has a different geological structure. Mount Shirane is a complex volcanic cone, Mount Ainomine is a single pyroclastic cone, and Mount Motoshirane consists of a chain of pyroclastic cones extending across the mountain.
Near the summits, trees are scarce and rugged volcanic rock dominates the landscape. This creates the distinctive, almost otherworldly scenery characteristic of high-elevation volcanic areas.
The Famous Yugama Crater Lake
One of the best-known sights in the broader Kusatsu-Shirane area is Yugama, the crater lake on Mount Shirane.
The lake measures approximately 300 meters in diameter and about 30 meters deep. Its striking emerald-green water creates a dramatic contrast with the surrounding volcanic terrain.
With a water pH of approximately 1.2, Yugama is an extremely acidic lake and is known as one of the most acidic crater lakes in the world.
Volcanic Landforms and Natural Environment
Mount Motoshirane has a distinctive landscape in which volcanic cones extend from northeast to southwest. The present terrain was formed through volcanic activity that took place approximately 1,500 to 5,000 years ago.
Five major craters are arranged around the summit area. Geological studies suggest that formations including the Old Motoshirane Pyroclastic Cone, New Motoshirane Pyroclastic Cone, Kagamiike Pyroclastic Cone, and North Kagamiike Pyroclastic Cone developed successively through different periods of volcanic activity.
In the past, hiking trails were established around these volcanic features. During summer, alpine flowers such as Japanese gentians, rhododendrons, and komakusa added color to the rugged terrain.
The slopes were also associated with the former Kusatsu International Ski Area, making the surrounding mountain region a popular destination for outdoor recreation in different seasons.
Volcanic Activity and the 2018 Eruption
Mount Motoshirane has a long history of volcanic activity. An eruption accompanied by a lava flow is thought to have occurred approximately 3,000 years ago, while volcanic activity at the North Kagamiike Pyroclastic Cone is believed to have continued until around 1,500 years ago.
More recently, an eruption occurred near Kagamiike on January 23, 2018. The event attracted considerable attention because it occurred without a long period of obvious precursors.
The eruption is considered to have been a phreatic eruption, in which heated groundwater or other underground water rapidly turns into steam and explodes. Several new craters formed, while volcanic rocks were thrown into the surrounding area.
The eruption also caused damage to nearby ski-area and ropeway facilities. Following the event, volcanic monitoring was strengthened, and volcanic alert information for the Shirane area around Yugama and the Motoshirane area came to be announced separately.
Current Access and Safety
Because of continuing volcanic hazards, access to the summit and crater areas is restricted. Conditions and restricted zones can change depending on volcanic activity.
Anyone planning to visit the Kusatsu-Shirane area should check the latest official volcanic warnings and access restrictions before traveling. Visitors should always follow posted instructions and remain outside restricted areas.
Scenic Attractions Around Mount Motoshirane
Although Mount Motoshirane and Mount Shirane are relatively close to each other, their landscapes have distinctive characteristics.
From the Kusatsu Onsen area, Mount Shirane can be seen rising with its pale volcanic slopes. Its snow-covered appearance in winter creates an especially striking mountain panorama.
Mount Motoshirane, by contrast, has a broader and somewhat gentler overall profile when viewed from a distance. Its extensive foothills and highland scenery contribute to an impressive sense of scale.
The Shiga-Kusatsu-Kogen Route
National Route 292, also known as the Shiga-Kusatsu-Kogen Route, passes through the mountain area between Mount Motoshirane and Mount Shirane.
When the road reopens in spring after the winter closure, enormous walls of accumulated snow can form along the roadway. This spectacular section is known as the “Snow Corridor.”
Viewpoints along the route provide expansive views toward Yoshigadaira, the mountains of the Joshin’etsu region, and, on clear days, even the distant Northern Alps.
Alpine Plants and the Four Seasons
The highlands around Mount Motoshirane are known for their alpine vegetation. In summer, komakusa, Japanese gentians, rhododendrons, and other mountain flowers brighten the landscape during the short alpine growing season.
As autumn arrives, the vegetation turns into colorful grassland foliage, transforming the highlands into shades of red, orange, and gold.
Winter brings a completely different landscape, with heavy snow covering the mountains and creating an expansive white wilderness.
Winter Nature Experiences
Depending on conditions and available programs, guided snowshoe tours may offer opportunities to experience the winter landscape safely.
However, the volcanic environment requires particular caution. Volcanic gases and other natural hazards are among the reasons that access to certain summit areas remains prohibited. Visitors should never enter restricted zones, even when weather conditions appear favorable.
A Mountain That Teaches Us About Nature’s Power
Mount Motoshirane combines dramatic volcanic scenery with the delicate beauty of an alpine environment. Its rugged terrain records thousands of years of volcanic activity, while its vegetation demonstrates how life can adapt to a challenging highland environment.
The history of the 2018 eruption also reminds visitors that the mountain remains an active volcanic environment. Its beauty should therefore be appreciated together with an understanding of the natural forces at work beneath the landscape.
Discover the Volcanic Landscape of Kusatsu-Shirane
A visit to the wider Kusatsu-Shirane region offers more than spectacular mountain scenery. From the emerald waters of Yugama and rugged volcanic formations to alpine flowers, seasonal landscapes, and the famous Snow Corridor, the area provides a fascinating introduction to Japan’s volcanic landscapes.
By learning about the geological history and volcanic activity of Mount Motoshirane, visitors can appreciate both the beauty and the power of the natural environment. Before traveling, be sure to check the latest access and volcanic safety information and enjoy the region responsibly.