Introduction to the Landforms and Geology of Japan

GLGArcs

Volcanoes in Japan

Most volcanoes in the world erupt on divergent and convergent boundaries of plates and hot spots. For example, volcanoes of mid-ocean ridges are distributed along the divergent boundaries, those on island arcs are arranged along the convergent boundaries, and in Hawaii, volcanoes are formed on a hot spot. Since tectonic settings and the mechanism of magma generation vary with location, various types of volcanic activity are observed in these regions. In Japan in a subduction zone, many different types of island arc volcanoes have been formed. This section explains Japanese Quaternary volcanoes including magma, volcanic landforms, and distribution.

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 (Volcanoes of Japan)    

Generation and types of magma in subduction zones

Mantle wedge

An oceanic plate obliquely dives into the upper mantle under island arcs. Part of the plate in the mantle is called a slab. The mantle on the slab is wedge-shaped, called a mantle wedge (Figure 1). Observations using seismic wave for the deep structure under island arcs (e.g. the Northeast Japan Arc) in Japan revealed a high temperature area parallel to the slab in the mantle wedge. An upward flow brings the hotter mantle from the depths into the mantle wedge. The slab going down drags the bottom part, several tens kilometers thick, of the mantle wedge because the slab contacts with it. In the upper part of the wedge, an obliquely upward flow occurs as a counter to the downward flow.

Model of magma generation
Fig. 1 Model of magma generation in the subduction zone

Process of magma generation
Condition of partial meltingFig. 2 Conditions of partial melting
S: Solidus temperature
Sw: Solidus temperature when water is added
1 to 3 correspond with numbers in the text.

Magma is molten rock, generated by partial melting in the mantle or crust. The upper mantle is composed of peridotite and is solid because temperatures of peridotite are lower than a temperature at which peridotite begins to melt (solidus temperature) at any depth (Figure 2). However, the rock of the mantle partially melts in the following cases: 1) when a temperature in an area increases to above a solidus temperature, 2) when a pressure decreases (a solidus temperature becomes lower with decrease in pressure), or 3) when volatile matter such as water is added to the mantle (such matter reduces solidus temperatures).

At mid-ocean ridges (divergent plate boundary), peridotite of the mantle rises and partially melts due to reduced pressure. Magma generation in subduction zones is more complex than at mid-ocean ridges. Water brought into the mantle by subsiding plate plays an important role in partial melting.

Rocks of oceanic plates, including basalt, sedimentary rock, and some peridotite under the crust, contain water. With the subduction of the oceanic plate, water in sediments on the oceanic crust is squeezed out under a trench. Hydrous minerals of basalt in the slab release water into the mantle wedge at several tens kilometer depths when the crystal structure of minerals is transformed by high pressure. Released water is incorporated in minerals of peridotite but partial melting does not occur at the depths, temperatures at which are below the solidus temperature (about 1000°C). The peridotite containing water is taken to deeper part by a downward flow and the hydrous minerals release water at about 110 km and 170 km depth because of high pressure. Amphibole and chlorite are dehydrated at 110 km and phlogopite is dehydrated at 170 km. The released water moves upward and causes partial melting to generate magma. When partial melting occurs in an area in the mantle, the density of the area becomes lower than that of the surrounding mantle. Such relatively light mass is called a mantle diapir (hereinafter “diapir”, Figure 3). Diapirs can ascend because the mantle (asthenosphere) has flowability. The diapirs stop immediately below the plate (lithosphere) because they cannot intrude the rigid lithosphere. The melt (basaltic magma) in the diapirs penetrate into the lower crust.

DiapirFig. 3 Conceptual diagram of diapir and formed volcano
The figure was created based on Fujii, 1997 

Volcanoes are formed in zones over the slab at 110 km depth and 170 km depth. The total volume of volcanoes or magma in the zone on the trench side is greater than in the zone on the backarc side. The amount of water released from amphibole and chlorite at 110 km depth is more than that of water from phlogopite at 170 km depth. Therefore, diapirs are more produced under the zone on the trench side. In the Northeast Japan Arc, many volcanoes have erupted in the zone on the trench side and the total volume of volcanoes is about three times as large as that in the zone on the backarc side. 

Generally, magma easily rises in regions dominated by tensile stress, such as mid-ocean ridges, while it 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.

Melting the crust to generate andesitic and dacitic magma

In island arcs including Japan, andesitic magma and dacitic/rhyolitic magma are common. In the lower crust, basaltic magma coming from a diapir heats up the surrounding crust to melt and yield dacitic magma. A mixture of the basaltic magma and the dacitic magma produces andesitic magma. The evolution of common volcanoes in island arcs corresponds with this generation process of magma. Part of basaltic magma derived from the diapir erupts to form a basaltic volcano, and then an andesitic volcano is produced on the basaltic volcano. Finally, the basaltic magma in the lower crust solidifies after the provision of magma from the diapir terminates and the remnant of dacitic magma slowly rises and erupts. Crystallization differentiation* also changes basaltic magma to dacitic magma in a magma chamber.

* In a magma chamber, minerals crystalize out and settle in the sequence of crystallizing point from highest to lowest with magma cooling. Accordingly, the composition of the remaining melt gradually changes. This process is known as Bowen’s reaction series explaining the derivation of intermediate and felsic magmas from a mafic (basaltic) magma.

Peculiar magma generation

When a very young and hot plate subducts, it may melt at shallow depths. There is an old volcanic zone, called the Setouchi volcanic zone, lying from western Kyushu through Seto Inland Sea and the Kii Peninsula to eastern Aichi Prefecture. The volcanic activity in the zone occurred in the Late Miocene. At that time, a newborn plate, the Philippine Sea Plate subducted under southwest Japan. The temperature of the slab was estimated to be several hundred hotter than that of a common (older) slab at 50 km depth, which was able to melt the subducting plate. Therefore, the Setouchi volcanic zone is closer to the Nankai trough than the current volcanic front. This magma generation is uncommon and the chemical characteristics of magma differ from common magma in subduction zones. Generally, magma is not generated by melting of a subducting plate as described above.

Types of magma

Igneous rocks or magma are classified 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 of subalkalic magma: the tholeiitic rock series and calc-alkali rock series. The composition of magma changes in 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. The classification with the amount of alkali (Na2O+K2O) is important because it is related to magma generation and tectonic characteristics of volcanic areas.

Magma generated by partial melting in the mantle is basaltic. Basaltic magma is divided into several types depending on the conditions of magma generation such as pressure, as mentioned above. For example, alkali basalt magma is produced under high pressure condition and the tholeiitic basalt magma under low pressure condition. Alkali basalt magma is primarily generated after the start of partial melting, and then subalkalic (tholeiitic) basaltic magma is produced when the proportion of melt increases to 20 to 30%. A large amount of tholeiitic basalt is found in mid-ocean ridges. This basalt is relatively uniform in lithological and chemical properties compared to that of other regions, referred to as mid-oceanic-ridge basalt (MORB). In rift zones such as East African Rift Valley, alkali basalt occurs as well as subalkalic basalt. Basalt yielded on hot spots is rich in alkali.

In island arcs, the volcanic rocks of calk-alkali series are common. Calk-alkali magma is generated in areas where the crust is melted in large volume. In the Japanese Islands, tholeiitic basaltic rocks constitute most volcanoes on the thin oceanic crust (Izu-Bonin Arc), and andesite, dacite, and rhyolite of calk-alkali series compose volcanoes on the thick continental crust (Northeast Japan Arc and Southwest Japan Arc). This volcanic rock distribution is related to the melting of the lower crust and the rock types of the crust. In Quaternary volcanoes from Hokkaido to the Tohoku regions, tholeiitic basaltic rocks are distributed on the Pacific side and alkali basaltic rocks on the Sea of Japan side; the amount of alkali in these rocks continuously increases from the Pacific side toward the Sea of Japan.  

Scale of volcanic activity

As mentioned above, magma constantly rises to mid-ocean ridges. At a hot spot, mantle plume coming from the deep mantle provides a large amount of magma. In these places, large scale volcanoes are formed and their durations of volcanic activity are very long, from one million to 100 million years. In island arcs, the amount of provided magma is much less than that in mid-ocean ridges and hot spots. The volume of individual volcanoes is several hundred cubic kilometers and the lifespan is several hundred thousand to one million years.

Table 1: Scale of volcanic activity
Table 1: Scale (order) of volcanic activity

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