Volcanoes in Japan
A caldera is a volcanic depression, the rim of which is circle or almost circle. Although stratovolcanoes have a small caldera, a caldera volcano is defined as a volcano that ejects a large volume (>10 km3) of dacitic-rhyolitic volcaniclastic products to make broad pyroclastic depositional surface and creates a caldera more than 10 km in diameter at the center. About ten Late Quaternary caldera volcanoes were formed in Hokkaido, northern Tohoku, and Kyushu. There is no caldera volcano between southern Tohoku and central Kyushu; however, Early Quaternary and Tertiary caldera volcanoes are found in this region. Most caldera volcanoes were formed on the basement less than 500 m in altitude, while many stratovolcanoes erupted in higher areas. Submarine calderas are found in the Izu-Bonin Arc. The largest caldera in Japan is Kussharo Caldera in eastern Hokkaido, the size of which is 23 × 24 km. The topography of the caldera is indistinct because Mashu Volcano (Mashu-dake) was formed at the eastern rim of the caldera. Aso Caldera in Kyushu, the most well-known caldera, has clear caldera landforms, the size of which is 24 × 18 km.
Fig. 8 Aso Caldera 
Viewed from the southwest. The highest post-caldera cone is Nakadake 1592 m high. Elevations range from 460 m to 570 m on the bottom of caldera and from 860 m to 1100 m on top of the rim.
[Formation and evolution]
The formation of caldera volcanoes begins by several eruptions
accompanied with felsic pyroclastic flows on a large scale. The volume
of pyroclastic deposits is several tens to hundreds of cubic
kilometers, forming broad pyroclastic depositional surface. The fourth
pyroclastic flow of Aso-san (Aso-4), for instance, reached 200-km away
from the caldera. Such discharge of a large amount of magma makes a
cavity in the magma chamber, resulting in the collapse of the chamber
roof. As another type of caldera formation, explosion may blow out the
ground to create a depression. The mechanism of caldera formation is
generally explained above, but these scenarios cannot be simply applied
to various caldera volcanoes. Many factors including the size and depth
of magma chamber, the shape and structure of chamber roof, and
tectonics are related to caldera formation.
Most of Japanese caldera volcanoes do not have a large stratovolcano before caldera formation like Crater Lake Caldera. After a caldera created, small conical volcanoes are formed in and around the caldera. These volcanoes are about one-tenth as large as stratovolcanoes described above. The small conical volcanoes have the same development stages as those of stratovolcanoes (Stages 1 to 4). Magma is dacitic-rhyolitic (SiO2: about 67%) in the stage of caldera formation, but it changes to basaltic-andesitic (SiO2: about 55%) at the start of formation of conical volcanoes. The magmatic evolution of caldera volcanoes is different from that of stratovolcanoes (from basaltic to rhyolitic). The reason is thought as follows: basaltic magma newly intrudes into a chamber of dacitic-rhyolitic magma, and induces explosive eruptions with pyroclastic flows to create a caldera. Subsequently, the basaltic magma spout out to form small conical volcanoes.
[Types of caldera structure]
A well-known model of collapse caldera is the Crater Lake-type caldera (Figure 10-1), which was created by eruption of dacitic or andesitic magma. Before the caldera was formed, a large andesitic stratovolcano, Mt. Mazama, erupted and ejected massive volcaniclastic material. In this model, a large magma chamber is near the surface under the volcano. Most large caldera volcanoes in Japan are classified into this type. However, these volcanoes did not have a large stratovolcano before the formation of caldera. It is thought that a great eruption started at a point in an area where small volcanoes were formed.
Aso-san has a large caldera (24 × 18 km). However, boring surveys in the bottom of the caldera indicated that the collapse area is smaller than the present caldera area and the bottom of deposits filling the caldera is at 800 m in depth. Accordingly, the center of caldera probably collapsed in a funnel shape (Figure 10-2). This structure was also presumed by a model based on Bouguer anomaly. This structure under the caldera is different from that of Crater Lake-type with a large magma chamber shallow under the volcano. The funnel-shaped structure is probably common in Japan.
Another model is the Valles-type caldera (Figure 10-3). This type is characterized by the bottom of caldera falling down without destruction, the central part of caldera re-uplifted to be dome, and lava domes fringing the caldera. In Kussharo Caldera, the largest in Japan, lava domes are distributed circularly. This suggests that Kussharo Caldera (Figure 11) is the Valles-type caldera.
|Fig. 10-1 Crater Lake-type caldera
Based on Howel Williams's classic diagrams.
|Fig. 10-2 Funnel-shaped structure
|Fig. 10-3 Valles-type caldera
Based on Smith and Bailey, 1968.
Fig. 11 Kussharo Caldera
Red marks: Lava domes
The lifespan of caldera volcanoes appears to be shorter than that of stratovolcanoes. The activities of Late Quaternary caldera volcanoes in Japan started 300,000 years ago or later. Small conical volcanoes also develop fast; these volcanoes usually reach the late stage in about 20,000 years.
Photo 2 Scoria cone (Aso Yonezuka)
Volcanoes formed by only one eruption are called monogenetic volcanoes, including lava domes and pyroclastic cones (e.g. scoria cone, pumice cone, and cinder cone). These volcanoes are usually small. In Japan, most lava domes are about 1000 m in diameter (base) and 200 to 300 m in height. The lava dome of Daisen, in Tottori Prefecture, western Honshu, is the largest, 4400 m in diameter and 760 m in height. Common pyroclastic cones are also about 1000 m in diameter (base) and 200 m in height.
Lava domes and scoria cones are often created parasitically in stratovolcanoes and caldera volcanoes. Fuji-san has about 60 scoria cones. Monogenetic volcanoes are also found as a group. There are about 30 monogenetic volcano groups in Japan. They are mainly distributed on the Sea of Japan, especially in northwestern and central Kyushu and northwestern Chugoku. On the Pacific side, monogenetic volcanoes have been formed only in the eastern Izu Peninsula, central Honshu. An area in which monogenic volcanoes are formed is generally in a weak-compressive or tensile stress field. Magma rises easily thorough open fissures in such areas. Therefore, when magma spouts out from the chamber again, it uses a different fissure (new vent), not the used vent. Thus, a monogenic volcano group is produced. Although each volcano in a group is small, the total volume of volcanoes formed by a series of eruptions may be equivalent to that of one stratovolcano.
Photo 3 Abu monogenetic volcanoes
The Abu monogenetic volcano group is in northern part of Yamaguchi Prefecture, the northwestern Chugoku region, consisting of about 40 volcanoes on the seafloor and land. The volcanic activity began about two million years ago and the latest eruption occurred 8800 years ago. Basaltic magma extruded since the initial stage but andesitic and dacitic magma started to erupt 400 thousands years ago. The last eruption formed a scoria cone. Volcanoes in the sea in the photo are lava plateaus. The second volcano from the left is basaltic and other volcanoes are andesitic. Generally, andesitic lava forms dome-like volcanic body (lava dome) because of its higher viscosity than that of basaltic lava. However, the andesitic volcanoes in the photo have similar form to the basaltic one. These are rare andesitic lava plateaus. A land in the foreground is part of a volcano (Kasa-yama). Kasa-yama was formed by andesitic lava eruption about 11 thousands years ago, and the scoria cone mentioned above was produced on Kasa-yama. This photo was taken on the top of the scoria cone.