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Volcanoes
Contents
Photo
1 Fuji-san
- Generation and types of magma [p. 1]
- Scale and life of volcanic activity [p. 2]
- Volcanic landforms [p. 3]
- Distribution [p. 6]
Volcanic activities are limited mainly in divergent and convergent
boundaries of plates and hot spots. For example, volcanoes of mid-ocean
ridges are on the divergent boundaries, volcanic island arcs are on the
convergent boundaries, and Hawaii volcanoes are formed on a hot spot.
Since tectonic settings and the mechanism of magma generation are
different among these regions, these produce varieties in the scale of
volcanic activity, magma characteristics, eruption types, and volcanic
landforms. Such variations are also found within subduction zones and
even in Japan.
In this section, Japanese Quaternary volcanoes including magma, volcanic
landforms, and distribution are described.
Locations of volcanoes referred in this section are shown in
Figure 6.
Also, most of these volcanoes are listed in a catalog of Quaternary
volcanoes with their photos in the Geological Survey of Japan website. The address is
http://riodb02.ibase.aist.go.jp/strata/VOL_JP/EN/index.htm.
Generation and types of magma
Partial melting
Magma is generated by partial melting in the mantle. Melting in the mantle requires at least one of the following conditions: increase in temperature, reduction of pressure, and addition of volatile matter such as H2O. In subduction zones, a subsiding plate brings substances containing H2O (hydrous mineral) into the mantle, so H2O is thought to play an important role in partial melting.
Fig. 1 Volcanoes and depth
contours of focuses [
]
A clear border, called a volcanic front, between volcanic and non-volcanic zones often occurs in the trench side of an island arc. The depth of a subducting plate underneath a volcanic front is usually 100 to 150 km; in the Northeast Japan Arc, the depth of the Pacific Plate under the volcanic front is about 110 km. Many earthquakes in the arc-trench system occur near the surface of subducting plate. Therefore, depths of the surface of subducting plate can be estimated from the distribution of these focus depths (Wadati-Benioff zone). Magma is supplied most abundantly at a volcanic front, making volcanoes on the volcanic front most active. From the presence of volcanic front, partial melting is thought to start above the surface of plate at this depth. However, there are also different models of magma generation in island arcs. An observation using seismic wave for the deep underground structure of the Northeast Japan Arc revealed a low velocity area distributed parallel to the subducting plate in the mantle wedge. A mantle wedge is between the subducting plate and the lithosphere. The low velocity area shows a high temperature area. This suggests that magma rise obliquely from the deep area in the mantle wedge rather than from under the volcanic front.
Fig. 2 Low seismic wave velocity area in
northeast Japan (Tohoku)
V: Seismic wave velocity
Q: Q factor (seismic attenuation)
L: Low, H: High
Vol: Volcanoes
C: Crust, M: Moho
J: Sea of Japan
Based on Hasegawa and Matsumoto, 1995.
Mantle diapir
A mass with density lower than the surrounding mantle, produced by increasing temperature or partial melting, ascends by buoyancy. This mass is called a mantle diapir (hereinafter “diapir”, Figure 4). A diapir may be solid if it is yielded only by increasing temperature, but its rising induces partial melting because of decompression. The diapir stops moving up immediately below the plate. The diapir does usually not intrude the plate itself because the diapir solidifies within the plate of which the density and temperature are lower than those of the diapir. Magma in the crust is fluid separated from the diapir, including bubbles and crystallized minerals. Generally, magma easily rises in regions dominated by tensile stress, such as mid-ocean ridges, while difficultly moves upward in regions dominated by compressive stress, such as Japan. In mid-ocean ridges, a massive amount of magma is provided to fill gaps produced by which plates move away from each other. About 75% of magma on the surface of the earth has been spouted out from mid-ocean ridges.
Types of magma
Igneous rocks or magma are differentiated by chemical components. In a common classification using silica (SiO2) content as an index, the igneous rocks are classified into ultramafic (< 45% silica), mafic (45-52%), intermediate (53-65%), and felsic (> 65%). Basalt and gabbro are in the mafic class, andesite and diorite in the intermediate class, and rhyolite and granite in the felsic class.
Another classification uses the amount of Na2O and K2O (alkali). The alkali content increases with increasing silica. Volcanic rocks are divided into alkali rocks and subalkalic rocks. Moreover, there are two series indicating the composition change in differentiation subalkalic magma: the tholeiitic rock series and calc-alkali rock series. The composition of magma changes on the process of solidification called crystallization differentiation. As subalkalic magma evolves, iron (FeO+Fe2O3) accumulates in the tholeiitic magma, and silica accumulates in the calc-alkali magma. Volcanic rocks including basalt, andesite, and rhyolite can be classified into the alkali/subalkalic class and the tholeiitic/calc-alkali class: alkali basalt or subalkalic basalt and so on. This classification with alkali (Na2O+K2O) is important because it is related to magma generation and tectonic characteristics of volcanic areas.
