Volcanoes in Japan
Volcanic landform formation depends on types of volcanic eruption and products. Generally, low viscous magma including basaltic magma causes mild eruption with lava flow, and high viscous magma such as andesitic or rhyolitic magma causes explosive eruption with volcaniclastic (pyroclastic) material as well as lava. However, eruption modes are related not only to magma viscosity but also to extrusive rate and vent conditions. Since most of Japanese volcanoes are andesitic or rhyolitic, their eruptions are often explosive and mainly form stratovolcanoes (composite volcanoes) and caldera volcanoes. There is no broad basaltic lava field such as Craters of the Moon in the western US and a Hawaiian shield volcano in Japan.
Volcanic landforms in Japan include stratovolcanoes, small shield volcanoes, pyroclastic plateaus, caldera volcanoes, lava fields, lava domes, pyroclastic cones (mainly scoria), tuff rings, and maars. About 70% of Japanese volcanoes are stratovolcanoes. Although these landforms are formed independently, many volcanoes are composed of some landform types. Lava domes and scoria cones, for instance, are often found on a stratovolcano. Stratovolcanoes and caldera volcanoes are formed by repeated eruptions using the same vent or adjacent vents (polygenetic volcano). Since the Japanese island arcs are in compressive stress field, magma cannot easily rise to the surface. Therefore, magma repeatedly uses a vent that has been produced and forms such volcanoes.
Stratovolcanoes are conical, built up of layers of lava alternating with beds of volcaniclastic material. Most stratovolcanoes are 10 to 100 km3 in volume, but Fuji-san (Fuji Volcano) has an exceptionally large body with about 300 km3. Although Fuji-san and Yotei-zan represents a typical shape of stratovolcano, such excellent conical volcanoes are few actually.
Photo 1 Yotei-zan
Yotei-zan is a typical conical stratovolcano in southwestern Hokkaido, 1898 m in altitude, 1600 m in relative height, and 45 km3 in volume. The volcanic activity began at least 50000 years ago and the latest eruption was about 2500 years ago.
The forms of stratovolcanoes vary depending on eruption modes and erosion in their volcanic activity stages. Some stratovolcanoes, for example, have lava domes on the summits and some of the others such as Bandai-san and Mount St. Helens lost a part of their body by eruption. Moriya (1983, 1992) proposed the following model of evolution based on his studies for many stratovolcanoes.
Fig. 7 Evolution of stratovolcano
Based on Moriya, 1983
Stage 1: A conical volcano like Fuji-san is formed
by spewing basaltic or basaltic-andesitic lava and scoria. Eruptions
are mild rather than explosive in this stage (Strombolian eruption).
[Figure 7-A; e.g. Fuji-san, Yotei-zan, and Iwate-san]
Stage 2: Volcanoes eject andesitic lava. The lava with high viscosity does not flow far from the vent even on steep slopes, so the forefront of lava flow becomes a scarp. Moreover, lava almost solidified in the bottom of vent is blown out by explosion, flying and depositing around the vent (Vulcanian eruption). As a result, the slope surrounding the summit becomes steep. The body of volcano is gravitationally unstable because it is composed of loose light particles such as scoria and the summit area is steep. Eruption or earthquake, therefore, may trigger off collapse in the upper part of the body. The volcanic activity in this stage diminishes compared to Stage 1. The interval of eruptions is long and the volume of volcanic products per time is less than in Stage 1. [Figure 7-B and C; e.g. Shiretoko Io-zan and Iwaki-san]
Stage 3: Magma is dacitic with higher viscosity than andesitic magma. Volatile components do not easily escape out from magma owing to the high viscosity. Therefore, they rapidly expand by decompression during going up to the surface and the magma foams to cause explosive eruption (Plinian eruption). The summit is broken by eruption. Pyroclastic flow occurs to expand the volcanic skirt. The volcanic products consist mainly of pumice. [Figure 7-D; e.g. Asama-yama and Hokkaido Komaga-dake]
Stage 4: A small caldera is formed on the summit. Dacitic or rhyolitic lava domes are produced in this caldera. Eruption is explosive with pyroclastic flow and pumice fall deposits (Plinian eruption). [Figure 7-E; e.g. Akagi-san and Haruna-san]
The evolution process of stratovolcanoes can be explained with a diapir model mentioned above. A partially melted diapir ascends in the upper mantle and stops immediately under the lithosphere. The fluid (basaltic magma) derived from the diapir penetrates the crust and rises to the ground surface (Stage 1). The basaltic magma heats up the lower crust to produce dacitic magma. The dacitic magma mixes with the basaltic magma to generate andesitic magma which spouts out to the surface (Stage 2). The magma in a magma chamber changes to highly viscous magma over time and causes explosive eruptions (Vulcanian eruption and Plinian eruption) deforming the volcanic body (Stage 3). Eventually, the basaltic magma in the lower crust cools and solidifies after the provision of melt from the diapir terminates, and the remnant of dacitic/rhyolitic magma slowly rises to the surface to form lava domes (Stage 4).
Even though Japanese stratovolcanoes can be classified into any of
the stages, not every volcano follows the course above. For example,
some volcanoes such as Haku-san in central Honshu are mainly composed
of thick andesitic lavas, being in Stage 2, but no basaltic lavas have
been found in these volcanoes. One of the interpretations of this fact
is that the lavas disappeared by erosion before andesitic lava erupted
in Stage 2. Another explanation is that these volcanoes initiated
activity with eruption of andesitic magma, that is, they did not
experienced Stage 1.
Another case is Unzen-dake in Kyushu. The volcano is thought of a volcano in Stage 2, but has ejected dacitic lava rather than andesitic lava. Eruption of dacitic magma is commonly accompanied by pyroclastic fall and pyroclastic flow of pumice in Japanese volcanoes. However, Unzen-dake repeatedly formed thick lava flow and lava domes. Unzen-dake is located in an area dominated by tensile stress (Beppu-Shimabara Graben). It is thought that the mechanism of magma ascent differs from other regions in Japan because the crust in this area has many fissures.
As seen in the examples, the development of stratovolcanoes depends on local factors including tectonic settings, structure under the volcano, and magma characteristics.