On a clear day from Konosu City in Saitama, the active volcano Asamayama on the border between Gunma and Nagano can be seen, sometimes with a white plume of smoke billowing from the summit. Usually, Asamayama goes through periods of low activity, when only gases and thin smokes issue from the crater, and mild activity, when a continuous white stack of noxious cotton tumbles into the wind. Yet every few years, the volcano coughs and rumbles, and grey ash boils into the sky while glowing lava splatters around the summit of the mountain. And then, of course, there are the medium and large scale eruptions that set local communities into high alert in case there is a repeat of 1783.
The area around Asamayama has been volcanically active for millions of years. To the southeast and south are the extinct volcanoes of Harunasan, Akagiyama, and Myogisan. To the north is a chain of large and small volcanoes including the dormant Azumayasan, and the still potentially dangerous Kusatsu Shiranesan. In the distance, one can see Nantaisan near Nikko, Fujisan, Yatsugatake, and Japan’s second and third highest volcanoes Ontakesan and Norikuradake.
Asamayama is the name of the present day volcano situated partially in the crater of an older volcano, Kurofuyama. According to the pictures in an information book about Asama, the original stratovlcano was formed tens of thousands of years ago. The eastern side of the volcano was then destroyed in a major eruption some 20 thousand years ago, leaving behind the western rim, which is now Kurofuyama. From about 20 to 16 thousand years ago, the Hotakeiwa volcano grew out of the partially destroyed crater, but this volcano destroyed itself in another major lava eruption around 13 thousand years ago. Then from 10 thousand years ago, a new cone began to grow where Hotakeiwa was. The rim of this new cone is now Maekakeyama. Since then, yet another cone has built up in the crater of Maekake, forming an even higher new summit. For climbers, the summit of the Maekakeyama rim is the officially marked summit, at 2,524 metres. However, the newer crater rim reaches 2,568 metres on the east side. This area is out of bounds; signs and information everywhere state that there is no entry permitted to the area around the crater due to the hazards of noxious gases and potential eruptive activity.
As an active volcano, Asamayama has eruptive activity warning levels. Level Zero is for a mountain that has shown no activity for a long time. Asama is never at zero. At Level One, the volcano is quiet and the possibility of an eruption is low. The volcano is mildly active at Level Two and there is danger of ejecta and ash from the crater. At Level Three, the volcano is experiencing a small to medium sized eruption and one should keep a two to three kilometre distance from the crater. Lava may spray or flow from the crater and small-scale pyroclastic activity, such as pyroclastic surges and flows, may occur.
Even when the volcano is quiet, gases regularly issue through fractures and vents by advection. Inside the crater are fumaroles where sulphur and salts precipitate on sulphur chimneys. The most common volcanic gas is water vapour, however, carbon dioxide, sulphur dioxide, and hydrogen sulphide are the ones to be most wary of around Asamayama. In high concentrations, all three are lethal. Sulphur dioxide has that rotten egg smell familiar around hot springs. Hydrogen sulphide is similar but more difficult to detect because after continued exposure one doesn’t notice the smell anymore. The molecule forms a bond with iron, thereby blocking oxygen from binding with haemoglobin in the blood. At around 300 ppm, the concentration becomes lethal, however, people working in areas where hydrogen sulphide concentrations occur set personal safety detectors as low as 10 ppm.
Since records were kept, Asamayama has shown a very active history with every century producing some medium to large scale eruptions, earthquakes related to volcanic activity, and numerous minor eruptions. The most notable eruption was in July of 1783, when a huge volume of lava flowed north from the crater.
Ash, mud, fallen trees, and debris plugged the Tone River, causing flooding and forcing the river to find a new route. The river previously emptied into Tokyo Bay but currently empties farther north at Choshi City in Chiba, with Ibaraki on the north side of the river. Ash from the volcano spread around the world and, in conjunction with an Icelandic volcano, caused European sunsets to be exceptionally red. Since the 1900s, the volcano has experienced medium and small scale eruptions several times, most recently in the 1970s and 1980s. In September of 2004 a minor eruption period began and continued through November. Then in February of 2009, another minor eruption period began and continued through March. During both eruptions ash rained on local communities and lava could be seen spraying from the crater.
As an active volcano within sight of my city of residence, Asamayama has held my interest since I came to Japan. I went with a friend in May of 2006 to Kurofuyama where we viewed a steady cloud of smoke rising out of the crater of Asama. At that time the eruption warning was at Level Two and we couldn’t venture within 800 metres of the crater. I returned with some other friends in February of 2007, this armed with gas masks that would protect us from hydrogen sulphide. But there was too much ice climbing up the cone and we had to give up. At the time, the volcano was at Level One. One friend from that adventure and I planned to visit again in February of 2009, however our plans were thwarted by another eruption. Finally we were able to climb up to the crater on Sunday, June 6, 2010.
We started up the trail at 3:00 AM and reached the rest house below Kurofuyama just before sunrise. After a break and some photography, we set out for the cone. During the climb, we enjoyed views of Kurofuyama, as well as views of Azumayasan and Kusatsu Shiranesan, and Yatsugatake with some peaks of the South Alps behind, and the Central Alps. As we neared the top, we also saw the North Alps spread out on the western horizon. There is a gap between an arm of the old crater rim leading up to Maekakeyama and the new cone. Here are two shelters, refuges in case of sudden eruption activity. This is also where a rope and signs warn that entry beyond is not permitted. As it was early, there was only one person who had come here ahead of us and he had gone up Maekake. We stepped over the rope and followed an obvious path to the crater. Then at last, after so many years of dreaming about being here, I stood looking into the gapping mouth of Asamayama – an active volcano!
We were excited but extremely wary of the potential dangers. When we smelled the sulphurous odour emitted from the crater we hastily stepped back from the edge. My friend put a towel over his mouth. Then a sound like a propane stove turned on full blast came from within the crater. Was the mountain waking up? The longer we stayed up there, the less threatened we felt. The smell was no worse than an onsen and the gas stove sound subsided again. I paid my respects to Asama with a small prayer of thanks (something I have learned to do around volcanoes ever since giving blood to Pele in Hawaii) and then we headed back down to the saddle between the route to the Maekakeyama summit and the cone at the top. Along the way we met two other men who also had in mind to go up to the crater. As we were to discover, many people crossed the green rope and went up to look into one of the orifices of the planet.
There was a volcano shelter nearby – a refuge in case of volatiles being cast out of the crater – and we stopped here to have a snack and consider our next move. I wanted to go up to Maekakeyama but my friend was more interested in exploring below the cliffs of the Maekake crater rim and the new cone. We set out amidst an impressive collection of various volcanic rocks and boulders of all sizes and many colours. But once at the southern end of the Maekakeyama crater rim, my friend thought we should continue going around the cone and try to get up to the crater again at the highest point. I was not keen on the idea of having to impose on Asama again, particularly since it would mean crossing below where the smoke was coming out; however, we saw other people walking right through the smoke and so there was no real reason not to try.
It wasn’t all that long before we stood at the crater rim for the second time that day and not far from the true highest point on the mountain. After shooting views from here, we followed the crater rim around to the north, and at the lowest point the wind changed direction and we found ourselves engulfed in gases so strong that out throats and nostrils burned. It was a short shuffle up the rocks back to where we had first come up earlier in the morning.
From here we went back to the shelters and were surprised to see it was only 11:20 in the morning. We broke for lunch here before going all the way up to the summit marker on Maekakeyama. Finally satisfied we had done all we could, we headed back down the mountain. The route below the crater walls took us through such a beautiful spring forest. We had not seen it when we had climbed up in the dark. The trip ended with a soak in the iron-red waters of Tengu Onsen. We were tired and sleepy from the long day after not having had any sleep the night before. But we were also very satisfied with our visit to the crater rim of Asamayama.