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Japan Volcanoes and Volcanics

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Map, Major Volcanoes of Japan

Human settlement of Japan can be traced for tens of thousands of years, and an unbroken line of emperors from 660 BC. Japan's first documented historical eruption was from Aso, its most prolific volcano, in 553 AD, the year after Buddhism was introduced from Korea. A fixed capital was first established in 710. By the time of Japan's largest historical eruption (Towada, 915 AD), 17 Japanese volcanoes had been documented in eruption, more than the rest of the world combined (including 10 in Europe). It was not until 1626, however, that history recorded an eruption from Japan's northern island of Hokkaido, and it was not formally made part of Japan until 1868. A feudal system had dominated all of Japan from 1192, but in 1868, 14 years after the nation was first opened to western trade, the Emperor Meiji overcame shogun power.

To the south, the Mariana Islands were populated from 1500 BC and explored by Spaniards in the 15th century AD, but the islands did not come under Spanish colonial rule until 1668. The first historical eruption was documented the following year. The northern volcanic islands were sold to Germany in 1898, occupied by Japan between the two World Wars, and named a Trust Territory by the UN in 1947 administered by the U.S. The islands became a self-governing US commonwealth in 1975. ...

The volcanoes of this region are unusually explosive, and include Kikai, which produced one of the earth's largest Holocene explosive eruptions about 6,300 radiocarbon years ago. No other region has documented more large explosive eruptions (VEI >= 4), or approaches its total of 41 AD eruptions of this magnitude. ...

Most volcanoes in this region result from subduction of westward-moving oceanic crust under the Asian Plate. In the Izu-Marianas chain, however, the crust to the west is also oceanic, forming island arcs where volcanoes are largely basaltic but far more explosive than oceanic hotspot volcanoes.

(This region) ... has the largest number of submarine volcanoes, mostly extending down the Izu-Marianas arc, and the largest number of reported submarine eruptions. The many reports of water discoloration over submarine vents have also contributed to this region's record number (180) of eruptions preceded by a question mark, indicating uncertainty that the eruption actually took place. ...

As with earthquakes, volcanic activity is linked to plate-tectonic processes. Most of the world's active above-sea volcanoes are located near convergent plate boundaries where subduction is occurring, particularly around the Pacific basin. ...

In 1991, two volcanoes on the western edge of the Philippine Plate produced major eruptions. On June 15, Mount Pinatubo spewed ash 40 km into the air and produced huge ash flows (also called pyroclastic flows) and mudflows that devastated a large area around the volcano. Pinatubo, located 90 kilometers from Manila, had been dormant for 600 years before the 1991 eruption, which ranks as one of the largest eruptions in this century. Also in 1991, Japan's Unzen Volcano, located on the Island of Kyushu about 40 kilometers east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapses of this active dome generated destructive ash flows that swept down its slopes at speeds as high as 200 kilometers per hour. Unzen is one of more than 75 active volcanoes in Japan; its eruption in 1792 killed more than 15,000 people--the worst volcanic disaster in the country's history.

The Azuma volcanic group consists of a cluster of stratovolcanoes, shield volcanoes, lava domes, and pyroclastic cones. The complex was constructed in two E-W rows above a relatively high basement of Tertiary sedimentary rocks and granodiorites. Volcanic activity has migrated to the east, with the Higashi-Azuma volcano group being the youngest. Historical eruptions, mostly small phreatic explosions, have been restricted to Issaikyo volcano at the northern end of the Higashi-yama group.

One of Japan's most noted volcanoes, Bandai-san (1,189 meters) rises above the north shore of Lake Inawashiro. The Bandai complex is formed of several overlapping stratovolcanoes, the largest of which is O-Bandai. O-Bandai volcano was constructed within a horseshoe-shaped caldera that formed about 40,000 years when an earlier volcano collapsed, forming the Okinajima debris avalanche, which traveled to the southwest and was accompanied by a plinian explosive eruption. Four major phreatic eruptions have occurred during the past 5,000 years, two of them in historical time, in 806 and 1888. Seen from the south, Bandai presents a conical profile, but much of the north side of the volcano is missing as a result of the collapse of Ko-Bandai volcano during the 1888 eruption, in which a debris avalanche buried several villages and formed several large lakes.

Nearly a century ago, the north flank of Bandai Volcano in Japan collapsed during an eruption quite similar to the May 18, 1980 eruption of Mount St. Helens. After a week of seismic activity, a large earthquake on July 15, 1888, was followed by a tremendous noise and a large explosion. Eyewitnesses hear about 15 to 20 additional explosions and observed that the last one was projected almost horizontally to the north. ...

Today Bandai Volcano and the area around it is a ski and vacation resort. The area is heavily vegetated, and the only signs of the catastrophic eruption are the horseshoe-shaped crater and the debris-avalanche hummocks. The trees "laid prostate on the ground in thousands" are nowhere to be found, and the blast deposit is not easily recognized.

Bandai 1888 Debris Avalanche -- Excerpt from Brantley and Glicken, 1986

The conical form of Fuji-san, Japan's highest (summit elev. 3,776 meters) and most noted volcano, belies its complex origin. The modern postglacial stratovolcano is constructed above a group of overlapping volcanoes, remnants of which form irregularities on Fuji's profile. Growth of the Younger Fuji volcano began with a period of voluminous lava flows from 11,000 to 8,000 years before present (BP), accounting for four-fifths of the volume of the Younger Fuji volcano. Minor explosive eruptions dominated activity from 8,000 to 4,500 BP, with another period of major lava flows occurring from 4,500 to 3,000 BP. Subsequently, intermittent major explosive eruptions occurred, with subordinate lava flows and small pyroclastic flows. Summit eruptions dominated from 3,000 to 2,000 BP, after which flank vents were active. The extensive basaltic lava flows from the summit and some of the more than 100 flank cones and vents blocked drainages against the Tertiary Misaka Mountains on the north side of the volcano, forming the Fuji Five Lakes. The last eruption of this dominantly basaltic volcano in 1707 ejected andesitic pumice and formed a large new crater on the east flank.

The Kikai Caldera is mostly submarine, with three islands and a few reefs representing part of the caldera rim and central cones. A double collapse structure for the caldera is inferred from submarine topography and a seismic profiling survey (K. Ono, written commun., 1983). Three cycles of voluminous silicic eruptions are known. The latest caldera-forming eruption, about 6,300 years B. P. (Before Present), is the largest known Holocene eruption on earth (bulk volume of products = 150 cubic kilometers). Pyroclastic flows from this eruption traveled at least 100 kilometers across the sea to southern Kyushu (Ui, 1973); airfall ash from the same eruption blanketed most of Japan (Machida and Arai, 1983). Postcaldera products are bimodal in composition, inlcuding many silicic vents and one basaltic vent.

The massive Unzen volcanic complex comprises much of the Shimabara Peninsula east of Nagasaki. A 30-40-kilometer-long, E-W-trending graben extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the peninsula. Fugen-dake and Mayu-yama volcanoes in the east-central portion of the andesitic-to-dacitic Unzen volcanic complex have been active during the Holocene. The Mayu-yama lava dome complex, which formed about 4000 years ago, was the source of a devastating 1792 AD debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugen-dake.

Unzen Information, 1792 Eruption, 1990's Eruption, etc.

Usu Volcano
Location: Hokkaido, Japan
Latitude: 42.53 N
Longitude: 140.83 E
Summit Elevation: 731 Meters
Volcano Type: Stratovolcano

Toya Caldera lies in the V-shaped intersection of the Kurile and Japan arcs, along both of which the Pacific plate is subducted northwesterly direction beneath the Eurasian plate at a rate of approximately 10 centimeters per year (Circum-Pacific Map Project, Northwest Quadrant Panel, 1981).

Toya Caldera formed by collapse about 110,000 years B.P., based on the age of pyroclastic deposits that have been related to the caldera-forming event (Ikumura and Sangawa, 1984; Machida and others, 1985; Ikeda and Katsui, 1986). Usu is a truncated stratovolcano with late-stage dacitic domes, located on the southern boundary of Toya Caldera. The main body of Usu was formed by eruptions of basaltic and andesitic magma (Usu somma lava). Then, 7,000 to 8,000 years ago, a violent explosion and a large debris avalanche formed a somma that is now largely filled by dacitic domes. Viscous dacitic magma has on occasion upheaved the land surface by more the 200 meters directly above a cryptodome (for example, Showa-shinzan ("Roof Mountain") in 1943-45).

Most of Toya Caldera is occupied by Lake Toya; the Naka-jima (or Nakano-shima) group of andesitic domes formed an island in the center of Lake Toya before Usu Volcano began to grow. Two additional domes have been identified by bathymetric and magnetic studies of the caldera lake (Nishida, 1984). On the basis of the same magnetic survey, Nishida (1984) proposes that the diameter of the Toya Caldera structure is only half that of the present topographic depression; if so, Usu lies outside the main structure of the caldera, on its modern topographic rim.

Two or more terraces have been identified along the Sobetsu River, in addition to possible marine terraces (Katsui and others, 1978b, p.386); these terraces might imply episodic uplift of the Usu area in recent geologic time.

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