Japanese volcanism and tectonics
Kyushu and southern Honshu

James S. Aber
GO 326/ES 767


Japan is an island-arc complex that began to develop in the Miocene. The complex consists of several continental terranes, along with accretionary material scraped off subducting plates, as well as younger volcanic rocks (Machida 2015). The islands lie at the junction of four converging plates, namely the Philippine, Eurasian, Pacific, and North American plates.

Simplified map of tectonic plates and boundaries in the vicinity of Japan. Four main plates converge along subduction zones (blue-toothed line): North America, Pacific, Philippine, and Eurasia. Arrows indicate direction and rate of movement (mm/year) relative to Africa, which is assumed to be stationary. Purple line = convergence, green line = transform, red line = spreading. K = Kyushu, H = Honshu. Adapted from original map by E. Gaba; obtained from Wikimedia Commons.

Kyushu and southern Honshu occupy the leading edge of the Eurasian plate next to the Nankai Trench and Philippine subduction zone, which dips westward below southern Japan. The West Japan Volcanic Zone (WJVZ) is developed in connection with this subduction, primarily in Kyushu, as well as southwestern Honshu and smaller volcanic islands to the south. Further complications include a spreading ridge system that extends from southern Kyushu southward to Okinawa and Taiwan. This spreading zone is marked in southern Kyushu by the Kagoshima graben and Sakurajima volcano.

Landsat 8 OLI image of Sakurajima and Kagoshima Bay, August 19, 2013, one day after an eruption produced ash and smoke that spread over the city of Kagoshima (left side). Lingering smoke continues to blow toward the west in this natural-color image. Adapted from USGS.

Volcanism in Kyushu is characterized by massive, explosive eruptions associated with collapse of large calderas. Large-volume (100s km3) ignimbrites (tuffs) and associated tephras derived from multiple volcanic centers blanket much of the island from these eruptions during the past one million years (Machida 2015). Subsequent post-caldera volcanoes have grown up in many of the calderas, and geothermal phenomena are widespread.

Kyushu volcanism
Central volcanic complex in the Aso caldera, central Kyushu. The Aso caldera formed during a series of four major eruptions, ~270,000 to 90,000 years ago. The last eruption (Aso-4) was the largest; total volume of all four eruptions is estimated at ~300 km3 (Nagaoka et al. 2015b). These post-caldera volcanoes have been active during the past 30,000 years. Inactive volcanic peak (left). Active steam venting from Nakadake (right). On September 14, 2015, Mount Aso erupted without warning.
Takachiho gorge exposes the Aso-4 pyroclastic flow, ~90,000 years old, which forms a welded ignimbrite (tuff) with columnar joints. Overview of narrow glen eroded by stream into the Aso-4 ignimbrite (left). Note people at upper right for scale. Close-up shot of distinctive columnar joints (right). Small rowboat for scale.
Sakurajima is among the most active volcanoes in the world with nearly continuous eruptions throughout recorded history. Major eruptions took place in 1476, 1779, 1914 and 1946. Once an island in Kagoshima Bay, the 1914 lava flow joined it to the land mass of Kyushu island. Overview (left) and close-up shot (right). The red structure is designed to slow down a landslide or lahar.
Waves crash across volcanic deposits (left) with Kaimondake volcano in the right background. Close-up view (right) of ignimbrite that consists of various rock fragments embedded in fine-grained matrix.

Tephra stratigraphy
Section exposing multiple tephras related to Sakurajima eruptions at Takatoga Pass, southern Kyushu. Dates for selected tephra layers in thousands of calendar years: a) 26, b) 24, c) 12.8, d) 10.6, and e) 7.3 (Moriwaki 2015). Dark zones are organic-rich paleosols buried between tephras.
Archaeologic site under excavation (left) in southern Kyushu. Cultural remains are typically found in buried soils (paleosols) between tephra layers. Detail of stratigraphy (right) shows dark organic-rich paleosol beneath a tephra.

Southeastern Honshu
Pacific coast at Shingu (left). Large waves were generated by an offshore typhoon. Mix of pebbles and cobbles on beach (right) includes volcanic, metamorphic and sedimentary rocks derived from diverse terranes, volcanoes, and accretionary-wedge sources. White rhyolite (upper right) is ~6 inches in diameter.

Geothermal energy
Modern geothermal power plant at Yamagawa in southern Kyushu generates electricity from steam-powered turbines. Since the earthquake and tsunami heavily damaged the Fukushima Daiichi nuclear plant in 2011, Japan has decided to place greater emphasis on solar, wind, and geothermal power in the future.
Left: former salt-production plant on southern Kyushu that operated 1944-64. Boiling water was used to evaporate sea water and precipitate salt. Right: hot-spring foot bath near Sakurajima volcano.

Tectonic hazards
Mount Fugen in the Unzen volcanic complex experienced a series of eruptions 1990-95, of which the pyroclastic flow on June 30, 1991 was most destructive and killed 43 people. Left: Mount Unzen (background), modern museum (left), and remains of destroyed building (right). Right: close-up view of destroyed building, which is preserved as part of the museum and a memorial to those who died.
Left: contemporary illustration of the 1792 eruption and collapse of Mayayama volcano in the Unzen volcanic complex. Debris flows into the adjacent sea generated a tsunami that killed an estimated 15,000 people, the largest volcano-related disaster in the history of Japan (Nagaoka et al. 2015a). Right: tsunami warning sign on the Pacific coast of Honshu at Shingu.

Large stained-glass panels in the train station at Kagoshima depict the view from Kagoshima across the bay toward Sakurajima volcano in the background. Approximately 5 m across.

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