1. Locality, landforms and geology of Iwo-jima Iwo-jima (or Iwo-zima, Sulphur Island) is a small volcanic island situated 1250 km south of Tokyo at 24°47' N and 141°20' E. This island is located in the southern extension of the Fuji volcanic zone, which includes the following volcanoes (from north to south) : Mt. Fuji, Izu-shichito, Myojinsho, Smith Island, Tori-shima, Sofu-iwa, and Nishinoshima (Fig. 1). Kita-Iwo-jima and Minami-Iwo-jima are volcanic islands 70 km north and 60 km south of Iwo-jima, respectively ; the alignment of these three islands is called Kazan-retto (Volcanic Islands). The volcanic islands of the Fuji volcanic zone together form the volcanic ridge (Shichito-Iwojima Ridge) of the Izu-Ogasawara (Bonin) island arc. East of the ridge, Ogasawara non-volcanic islands run parallel to it. Iwo-jima is 8 km long and has an area of 22 km2. The island consists of three parts : Motoyama, a shield-shaped truncated strato-volcano in the northeastern part ; Suribachi-yama pyroclastic cone at the southwestern end, and the Chidorigahara unconsolidated sandy isthmus connecting the two volcanoes (Fig. 2). Motoyama volcano is composed of alternating of tuff breccia and lava beds, both of which are of trachyte andesite. This volcano was built mainly under the sea, then was thrust above the sea-level by very rapid upheaval. Suribachi-yama is a scoria cone with a crater, under which lie lava flow and pyroclastic deposits. All these rocks are also trachyte andesite, like those of Motoyama. Chidorigahara isthmus was built by a supply of pyroclastics and by uplift (Fig. 3). The bathymetry around Iwo-jima shows that Moto-yama is the central cone in a caldera on a large volcanic edifice, whose basal diameter is about 40 km and height 1500-2000 m. This volcano is situated near the center of the submarine Kazanretto Ridge (or Iwo-jima Ridge), the length of which is about 600 km, from Nishinoshima in the north to Takanosu seamount at its southern end (Fig. 4). 2. Volcanic activity of Iwo-jima and its surroundings On the ground of Iwo-jima, there are many spots of solfatara, steam fumarole, hot spring, and hot ground (about 100°C ground temperature). About 15 times of small phreatic explosions have been recorded since 1888. A substantial heat discharge (about 1 × 108 cal/sec) is measured from the land on the island this recent value is nearly the same as the first estimate of the value in 1922. Records of eruptions, including submarine ones, in the historical age on the whole Shichito Iwo-jima Ridge, exclusive of Izu-shichito volcanoes, are listed in Table 1. 3. Rocks of Iwo-jima The thickness of the crust around Iwo-jima is estimated to be about 15 km from seismic prospecting, and it is thought that the lower part of the crust beneath Iwo-jima is in a state of partial melting from the speed of elastic waves and heatflows. The rocks of Iwo-jima both above and beneath the sea are of alkali trachyte or trachy andesite. All the volcanoes in Fuji volcanic zone, except for Iwo-jima and a few submarine volcanoes (Fukutokuokanoba and Minami-hiyoshi) are made of non-alkalic rocks. This special characteristics of the rocks of Iwo-jima and the very rapid uplift of this island are the most noteworthy phenomena of Iwo-jima. 4. Research on Iwo-jima 1) Researches before 1960 Before the Second World War, studies were done on the island's topography, geology, petrology, volcanic activity, heatflow and the unusual uplift by KIKUCHI (1888), PETERSON (1891). TSUJIMURA (1917), HONMA (1925), KOZU and WATANABE (1928), TOYOSHIMA (1932), TSUYA (1936), IWASAKI (1937) and others. Studies were also carried out on the submarine volcanic activities and the appearrence and disappearrence of volcanic islands in 1904 and 1914 near Minami-Iwo-jima by SATO (1905), WAKIMIZU (1907), OGURA (1914), TERADA (1914) and others.
The crustal structures of the basins of Shikoku, Parece Vela (West Mariana) and West Philippine are oceanic in character resembling those of the Pacific Ocean basin. And the crustal structures of the ridges of Izu-Mariana Arc, Kyushu-Palau Ridge and Ryukyu Arc are different from those of basins. The Izu-Mariana Arc and the Kyushu-Palau Ridge are different from the Ryukyu Arc in point of lacking a granitic layer.
Iwo-jima volcano is a stratovolcano of 1500-2000 m in elevation, having a caldera of about 10 km in diameter. Marine terraces on the island, classified into more than 20 treads, indicate the rate and mode of uplift during the past several centuries. Distribution of active faults, concentrated in Chidorigahara isthmus, are related to the mode of Chidorigahara elliptical uplift. Comparison among the 1934, 1945 and 1981 bathymetries shows that uplift has occurred at shallow waters around the island but it has not reached the caldera rim.
The main portion of Iwo-jima island consists of lava flows and pyroclastic rocks of trachy andesite, erupted and deposited in shallow sea, except the upper part of Suribachiyama, in 2600-2800 y.B.P. The island have been uplifting from about 800 y.B.P., followed by active faulting and change in coast lines.
The hole with the depth of 150 m was drilled at Motoyama area. From ground surface to the depth of 24.7 m, pyroclastic rock was found, and trachy andesite was found under the pyroclastic rock. Beneath these, additional two units of this type were recognized. The temperature at the bottom of the hole was 167 °C. According to electrical logging, under the sea level, the self-potential was changed from possitive to negative, and the resistivity index was about zero.
Since the first recorded phreatic explosion occurred in 1889, eruptions took place fifteen times in Iwo-jima. Recently, Iwo-jima volcanoes has been studied from geophysical standpoint including seismometrical, geodetic and geothermal investigations. Most of phreatic explosions has been accompanied with high active stage of shallow earthquakes. But explosions are not always accompanied by a swarm of shallow earthquakes.
Eight temporal seismic observations ranging 2 to 90 days in 1968-1978 revealed that the seismicity of the island is usually very low inspite of its large and continuous upheaval and that the focal region is limitted in its topographical caldera with depth lower than 3-4 km. Microtremor related to a thermal activity is only observed near the active fumaroles on the island and another kind of tremor which is closely related to the surface geologic features is commonly observed on the whole island. We propose an upheaval model which consists of two P10 (cos θ) type pressure sources which are located beneath the Chidori-gahara with depth in 1-2 km and the north-east coast of the island with depth 3-4 km.
An amount of upheaval and its horizontal distribution were measured using topographic maps, aerial photographs, data of levelling and so on. Since 1952, the general increase in the upheaval rate was found, and in some places, its rate exceeded ca. 30 cm/year. Recently, the upheaval rate decreased, and in some place, subsidence was found.
At the Iwo-jima, ground-tilt observation, precise geodetic survey and sea level observation have been carried out and relationship between volcanic activities and crustal deformation have been studied. The maximum value of upheaval which was geodetically measured was about 1.9 meters during 8 years from 1976 to 1984.
Bouguer anomalies on Iwo-jima generally become lower and lower toward the center of the island, which means that the shallow underground of the island consists of low density-volcanic ejecta. At the Motoyama area, relatively high Bouguer anomalies are found. This may show a high density-heat source, considering the extensive geothermal manifestations in the area. At the Chidorigahara area, relatively low Bouguer anomalies are found. This may show an existence of lower density-loose volcanic ash and fine cinder.
Gravity anomalies on Iwo-jima are analysed by two-dimensional Talwani's method. These results reveal that the Iwo-jima belongs to a caldera accompanied by low gravity anomalies. A low density part beneath the central part of Iwo-jima is thought to be significant for understanding the extraordinary crustal elevation and the cause of the reversed magnetic anomaly pattern just over the island.
A land magnetic survey on the island, a three component aeromagnetic survey and geological/geophysical investigation by a survey vessel around the island were conducted in 1980 and 1981. These geomagnetic studies suggested that Iwo-zima volcano is composed of a non-magnetic central portion and the other highly magnetized majority. The computed intensity of magnetization of the latter is roughly 1 × 10-2 emu/cc. Chemical and mineralogical studies on the dredged rock samples in the sea around Iwo-zima judged them as trachy andesites. Most of these samples show weak NRM, while the other few exceptions show relatively strong NRM according to the result of magnetic study.
The main features of volcanic activities of Iwo-jima are unusual uplift of land and extensive geothermal activities. The rate of total heat discharge is estimated to be an order of 108 cal/sec, which is extremely large for the thermal output from a geothermal area. Part of such a large heat discharge rate must be transferred upward by high enthalpy magmatic gas. This deduction is consistent with geochemical data.
Electric surveys were performed along four survey lines by three methods such as apparent resistivity by dipole-dipole, array, spontaneous potential and frequency effect of induced polarization in the island of volcano, Iwo-jima. It is concluded that combination of those methods is effective to survey complex geologic structure, convection of hydrothermal fluid and distribution of alteration zones in volcanic and hydrothermal areas.
The seismic wave velocity structure in the Iwo-jima volcanoes area, having three layers of which velocities are as follows : first layer (upper) is 1 km/sec (Vp), second layer is 2.3 km/sec (Vp) and third layer is 4.0 km/sec (Vp). The Ohmori coefficient, k, for earthquakes in this island area is inferred to be 2.5 from the seismic prospecting.
The temperature of fumarolic gas ranged from 100 to 128 °C. These gas usually contained few HCl and SO2, and contained a small amount of H2S. These facts indicated that acidic gas components were absorbed into the aquifer during the cause of ascent. Hot springs may be classified into three types according to their chemical composition : 1) volcanic thermal water type ; 2) sea water type ; and 3) type of mixture of 1 and 2.δD and δ18O of waters collected from this island are high. The rocks from Iwo-jima are all trachy andesite with the SiO2 content of 54-58 %, and with the Na2O+K2O content of 9-10 %.