Mount Mihara
Mount Mihara (三原山, Mihara-yama) is the central scoria cone capping Izu-Ōshima, a basaltic stratovolcano forming an island in Japan's Izu Islands chain within Tokyo Metropolis.[1] Rising approximately 150 meters above the floor of a 4-km-wide summit caldera to an elevation of 746 meters, it features a bowl-shaped crater about 800 meters in diameter and is composed primarily of scoria from explosive eruptions.[1][2] The volcano has a history of activity dating back centuries, with documented eruptions including the formation of the cone during the Great An'ei eruption of 1777-1778 and more recent events such as the 1950-1951 and 1986 eruptions.[1][2] The 1986 eruption, originating from fissures on the cone's flank, produced lava fountains up to 1,600 meters high, ash plumes reaching 16 kilometers, and prompted the evacuation of around 12,000 island residents.[1] As part of the Izu-Bonin volcanic arc, Mount Mihara exemplifies ongoing subduction-related volcanism, with monitoring continuing due to its potential for future activity.[1][3]
Geography and Geology
Location and Topography
Mount Mihara is an active volcano situated at the center of Izu Ōshima, the largest island in the Izu Islands chain, which belongs to Tokyo Metropolis, Japan. The island lies at the northern tip of the Izu-Bonin volcanic arc in Sagami Bay, approximately 120 kilometers south of central Tokyo. Its geographic coordinates are 34.72°N, 139.39°E.[1] Izu Ōshima spans 11 by 13 kilometers, forming a broad, low-relief stratovolcano with a roughly circular outline and diverse volcanic terrain including lava flows, pyroclastic deposits, and over 40 subsidiary cones along NNW-SSE rift zones. Mount Mihara constitutes the island's topographic prominence, reaching an elevation of 758 meters above sea level as its highest point.[1][4] The volcano features a central scoria cone rising about 150 meters within a 4-kilometer-wide summit caldera, characterized by a summit crater roughly 800 meters in diameter and a base spanning 1,200 meters. This structure overlays older caldera formations, with the cone's steep slopes and crater rim defining the island's central highland amid gentler surrounding basaltic plains and coastal lowlands.[1][2]Volcanic Structure and Composition
Mount Mihara constitutes the active central cone of Izu-Ōshima volcano, a basaltic stratovolcano characterized by a summit caldera approximately 3-4 km in diameter.[5] The cone itself is a scoria cone rising about 150 meters above the caldera floor, with a summit diameter of 800 meters and a base diameter of 1,200 meters.[2] This structure formed through repeated explosive eruptions producing scoria and pyroclastic materials, culminating in the development of a pit crater at the summit.[6] The volcanic edifice of Mount Mihara is built primarily from layers of scoria, lava flows, and volcaniclastic deposits accumulated since its emergence within the caldera around the 7th century CE.[1] Geophysical surveys indicate a plumbing system involving shallow magma chambers beneath the cone, facilitating episodic ascent of basaltic magma.[6] Shallow water-saturated layers and high-conductivity zones near the water table suggest hydrothermal alteration influencing the volcano's structural stability.[7] Petrographically, the rocks are dominated by low-K, arc-type tholeiitic basalts, including olivine basalt and pyroxene-olivine basalt varieties.[3] Whole-rock analyses from eruptive products show SiO₂ contents ranging from 51.0 to 54.0 wt% and alkali oxides (Na₂O + K₂O) from 2.0 to 2.6 wt%, confirming a mafic, basaltic composition typical of island arc settings.[8] While predominantly basaltic, some historical eruptions have incorporated minor andesitic components, though these are subordinate to the tholeiitic basalt framework.[3] Fumarolic gases and isotopic studies reveal ongoing magmatic degassing, with variations in carbon and helium isotopes indicating interaction between mantle-derived magma and crustal fluids.[9]Eruption History
Formation and Prehistoric Activity
Izu-Ōshima volcano, encompassing Mount Mihara as its central cone, originated from subduction-related magmatism in the Izu-Bonin arc, with initial activity commencing approximately 40,000 to 50,000 years ago through submarine eruptions that facilitated island emergence.[3] The earliest deposits, part of the Senzu Formation, primarily consist of phreatomagmatic ejecta, lahars, and coarse pyroclastics indicative of explosive underwater and shallow-water interactions.[10][3] Pre-caldera growth involved the accumulation of the Older Ōshima Formation around 20,000 years ago, featuring scoria cones, basaltic lava flows, and over 100 pyroclastic layers deposited at roughly 150-year intervals, reflecting recurrent Strombolian-style eruptions and effusive activity that built the stratovolcanic edifice.[10][3] This phase transitioned to more intense explosivity, culminating in caldera formation approximately 1,300–1,700 years ago via a Plinian-style phreatic eruption that produced the Sashikiji Formation, including high-velocity pyroclastic density currents and widespread tephra fallout.[10][3] Mount Mihara emerged as the post-caldera summit cone through layered deposits of scoria, bombs, and lavas from subsequent prehistoric summit and flank eruptions within the 4-km-wide caldera, supplemented by over 40 aligned parasitic cones along NNW-SSE rift zones that contributed to the volcano's overall structure prior to 7th-century historical records.[1][3] These early post-caldera events, part of formations like Nomashi and Yuba, involved basaltic explosive and effusive phases at intervals of 100–150 years, establishing Mihara's basaltic composition and topographic prominence.[3]Historic Eruptions Prior to 20th Century
The documented eruptive history of Mount Mihara, the central cone of Izu-Ōshima volcano, includes multiple explosive events prior to 1900, primarily characterized by ash emissions, scoria ejection, and occasional lava flows, with volcanic explosivity indices (VEI) ranging from 2 to 3 for most recorded activity.[1] These eruptions contributed to the volcano's basaltic stratovolcano morphology, with parasitic cones and altered topography evident from historical accounts.[5] A table summarizing key pre-20th century eruptions, drawn from geological records, highlights the frequency and nature of activity:| Date | Eruption Type | VEI | Key Features and Impacts |
|---|---|---|---|
| 1552 Oct 7–15 | Explosive, lava flows to sea, ash, scoria | 3 | Initial historical record; flows entered ocean. |
| 1684 Feb–1690 | Explosive, lava flows to sea, ash, scoria | 3 | Property damage reported on island. |
| 1777 Aug 31–1779 | Explosive, lava fountains, flows to sea, cinder cone formation | 3 | An'ei eruption; shaped modern Mihara cone with scoria, bombs, and extensive lava flows covering ~3 km²; pyroclastic falls and ash plumes affected settlements.[8] |
| 1783–1786 | Explosive, ash | 2 | Property damage; part of prolonged An'ei sequence with intermittent activity.[1] |
| 1789 | Explosive, ash | 2 | Evacuations, fatalities, and structural damage. |
| 1821 | Explosive, ash | 3 | Significant ash dispersal. |
| 1876 Dec 27–1877 Feb 5 | Explosive, lava flows | 2 | Flows altered local terrain. |