Mount Asama

Mount Asama has erupted many, many times.

People have written about its eruptions for more than 1,300 years.

Japan has more than 300 volcanoes because it lies along the Pacific mountain-building belt, the Karuizawa Tourist Association explains. Mount Asama is one of them, and it has often poured out lava and erupted plumes hundreds of meters high, sometimes more than 1,000 meters.

The oldest written record of one of its eruptions is from 686, and another was recorded in 1108. Very old eruptions left thick layers of volcanic material around its foot, soft rock that rivers have carved into valleys.

Karuizawa Tourist Association: Japan has more than 300 volcanoes along the Pacific orogenic belt; Asama has frequently discharged lava since the start of the historical period and erupted plumes hundreds of meters, sometimes more than 1,000 m, high, with cinders, ashfall and blasts. Records begin in 686 and include 1108.

Pyroclastic flows from the Pleistocene to ancient times are deposited thickly at Oiwabara (Karuizawa) and Kaikoen (Komoro), reaching Nakagomehara (Saku); like Kyushu's Shirasu Plateau, this soft, easily eroded deposit has been carved into U-shaped valleys, many used for rice paddies.

In 1783, Mount Asama had a very big eruption.

Hot lava flowed down the north side. Now it is a field of big black rocks.

The biggest eruption of Mount Asama in the written record lasted from May to August 1783. At its end, red-hot lava flowed 12 kilometers north from the crater, in a stream up to 4 kilometers wide.

The flow is made of huge blocks of a black volcanic rock called andesite. More than 220 years later, the rocks have hardly weathered and few plants grow on them. Today the lava field is a natural park.

Karuizawa Tourist Association: the most massive historically recorded explosion was the great eruption of May to August 1783; the last phase produced a lava flow 12 km long and up to 4 km wide from the crater to the north, of huge andesite blocks, the largest part of the 150 million tons of ejecta of the Tenmei eruption. The ash from the eruption caused a drop in temperature.

More than 220 years on, the lava has not weathered much and little vegetation has grown on it; it is now a natural park and visitor attraction in Kita-Karuizawa.

Forests grow on the lower slopes of the volcano.

In May, melting snow makes a white shape on the mountain that looks like a fish.

Around its foot, valleys cut into old volcanic deposits are used for rice paddies. In May, melting snow leaves a white pattern on the dark mountainside that looks like a climbing carp, a fish that is a local specialty.

The Karuizawa Tourist Association says farmers watched the snow shape to judge the weather and pick the best time to sow and plant rice.

Karuizawa Tourist Association: many of the U-shaped valleys in the volcanic deposits hold rice paddies (locals call them 'Tagiri' landforms). In May a snow pattern called the 'climbing carp' (Nobori-Koi) appears on the left middle of the mountain as the thick snow melts; Asama is the only mountain in the Saku region where snow patterns can be seen, and farmers used the shape to time the sowing and planting of rice.

The photo shows dense forest on the lower slopes below the bare cone.

Mount Asama is an active volcano.

Only go where the grown-ups in charge say it is safe.

Mount Asama is an active volcano. The Japan National Tourism Organization describes a hiking trail that starts at a mountain hut on the Nagano side.

Before any visit, follow the advice of local officials, the CDC says. Have a plan to leave or shelter, and protect your lungs and eyes from ash.

JNTO: an active volcano with a hiking trail from a mountain hut on the Nagano side. CDC volcano safety: follow local officials' advice; plan to evacuate or shelter; an emergency kit with respiratory protection and goggles; ash can harm the respiratory tract (an N95 respirator outdoors).