The 1888 Eruption of Bandai-san and Colored Lakes
Bandai-san is a volcano in the central area of Fukushima Prefecture, southern Tohoku. The 1888 eruption led to the large collapse of the northern part of volcanic body. This eruption is well-known because there were records of the course of eruption and detailed post-eruption surveys, often introduced in some volcanological books outside Japan. The large collapse largely changed the landforms of and around Bandai-san and formed many lakes and swamps. At present, the devastated area became luxuriant forests with a diversity of living things and a famous tourist resort due to the beautiful landscapes and impressive colored lakes and swamps including the Goshikinuma lakes.
Photo 1: Bandai-san viewed from north 
A swamp in the foreground is Nakase-numa, one of the lakes and swamps in the depositional surface of debris formed by the 1888 eruption.
Outline of Bandai-san
Fig. 1: Location of Bandai-san (on Google map)
Bandai-san is located near the volcanic front of the Northeast Japan Arc (about 20 km west of the front). There are active volcanoes including Azumayama and Adatarayama around Bandai-san. Bandai-san is a stratovolcano composed of pyroxene andesite lava and volcaniclastic materials. The volcanic activity started at least 300 thousand years ago and is still active. However, the last magmatic eruption occurred 25 thousand years ago, and after that, all eruptions were caused by steam. Bandai-san has three peaks (Obandai [1819 m], Kushigamine [1636 m], Akahaniyama [1430 m], but before the 1888 eruption, the volcano had one more peak (Kobandai [1760 m]) which collapsed because of the eruption. The Kushigamine and Akahaniyama volcanoes were formed 250 to 230 thousand years ago, the Obandai 36 thousand years ago, and Kobandai 24 thousand years ago. Therefore, four stratovolcanoes built Bandai-san.
Fig. 2: 3D map of Bandai-san viewed from northwest 
Heights are stretched 1.4 times. The distance from Aka-numa to the Goshikinuma lake area is about 3.5 km.
The 1888 eruption
On July 15, 1888, strong earthquake occurred around 7:30 A.M. and an
eruption began at 7:45 A.M. A black column rose to a height of 1200 to
1500 meters. Subsequently, explosions continued 15 to 20 times,
dispersing volcanic ash and rock fragments on the foot of Bandai-san.
The last big explosion occurred northward to break Kobandai. Debris of
Kobandai rushed down the north slope (debris avalanche). It was only one
minute from the first explosion to the debris avalanche. After the
collapse, the volcano rumbled with many minor explosions for 30 to 40
minutes. The ash clouds expanded like an umbrella, reaching a height of
4000 to 5200 meters. Volcanic ash and lapilli blown toward
east-southeast fell on a fan-shaped area and reached the Pacific coast.
A hot rain burned a lot of people at the foot of Bandai-san, which was
probably water that coagulated in the air and contained hot volcanic
ash. Moreover, the explosions caused a violent blast, a dense mixture of
vapor and air with rock fragments and sand. The blast knocked down trees
along valleys and destroyed houses. The blast also damaged people in the
eastern foot; they were stripped of their clothes, their skin and hair
were torn by twigs and so on, or they were hit by stones. The eruptive
activities completed in two to three hours and quiet came back in the
evening. The eruption buried five villages around Bandai-san, killed 461
people, and caused considerable damage to houses and livestock.
The 1888 eruption was steam explosion and had no juvenile material derived from magma. There was no marked precursor of the eruption, such as developing cracks and a bulge of the volcanic edifice, though rumble and weak earthquakes were often observed one week before the eruption. Therefore, residents around Bandai-san did not notice the impending eruption. In the morning of that day, visitors were in a hot spring near the summit to bathe. Some of them survived the eruption.
This eruption is often compared with the 1980 eruption of Mount St. Helens because both eruptions are characterized by a large failure of the flank, avalanche, and blast. However, the eruption of Mount St Helens was magmatic, and the volume of volcanic products and the scale of debris avalanche were much larger than those of Bandai-san. The distribution area and volume of landslide-debris avalanche, for example, was 34 km2, 1.2 to 1.5 km3 for Bandai-san and 64 km2, 2.5 km3 for Mount St. Helens. An eruption leading to debris avalanche caused by steam may be called the Bandai-type eruption.
A horseshoe-shaped crater open north was formed at the former peak of Kobandai. The debris avalanche carved an avalanche valley on the flank. The debris settled with a relatively flat surface on the northern foot of Bandai-san and dammed up rivers (drainage system) to form many lakes. Various sized hummocky hills are found on the deposit area and swamps and marshes are also on depressions. Debris flows and avalanches took place in 1938 and 1954. These resulted from failures of the debris slope that were formed in the 1888 eruption. (see Fig. 2)
Photo 2: Depositional surface of debris 
The photo was taken at the head of avalanche valley (the top of ski slope). White arrows shows hummocky hills. A large lake is Lake Hibara.
An English paper describing the 1888 eruption is available on UT Repository in the University of Tokyo website (Sekiya, S. and Kikuchi, Y., 1890, The eruption of Bandaisan. Sci. Imp. Univ. Tokyo, Japan, 3, 91-171). To get the paper, click a file name “tssj013_2_001.pdf”. Good sketches of Bandai-san at and after the eruption are attached to the end of this paper.