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Megata, a group of three small maars along the tip of the Oga Peninsula in NW Honshu, has been active during the Holocene (Ishikawa et al., 1957). Lakes 300-600 m wide fill the basaltic-to-dacitic maars, which are a noted locality for mantle-derived xenoliths. Ichinomegata, the easternmost, largest, and oldest of the three maars, is 600 m in diameter. Ninomegata and Sannomegata are located immediately east and south of Toga Bay (Togowan), which itself is a maar. The rim of the rhyolitic Toga (Togowan) maar, larger than the Megata maars, is breached by the sea on the western side and was formed about 450,000 years ago. The Megata maars are mostly late Pleistocene in age, but Murayama (1987) noted that pottery from roughly 4000 years before present (Jomon Period) has been found within tephra layers from the maars.
Summary of Holocene eruption dates and Volcanic Explosivity Indices (VEI).
|Start Date||Stop Date||Eruption Certainty||VEI||Evidence||Activity Area or Unit|
|2050 BCE (?)||Unknown||Confirmed||Tephrochronology|
|7050 BCE (?)||Unknown||Confirmed||Tephrochronology|
This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.
|Feature Name||Feature Type||Elevation||Latitude||Longitude|
|Maar||161 m||39° 57' 0" N||139° 45' 0" E|
|Ninomegata||Maar||199 m||39° 57' 0" N||139° 44' 0" E|
|Sannomegata||Maar||291 m||39° 56' 0" N||139° 43' 0" E|
|Maar||39° 57' 2" N||139° 43' 4" E|
|An aerial view from the NW with Toga Bay in the foreground shows Sannomegata (center), one of a group of the three small Megata maars along the tip of the Oga Peninsula. The names of the three maars, Ichinomegata, Ninomegata, and Sannomegata mean First, Second, and Third Megata, respectively. Lakes 300-700 m wide fill the maars, which are noted localities for mantle-derived xenoliths and are located immediately east and south of Toga Bay.
Copyrighted photo by Hiroshi Yagi (Japanese Quaternary Volcanoes database, RIODB, http://riodb02.ibase.aist.go.jp/strata/VOL_JP/EN/index.htm and Geol Surv Japan, AIST, http://www.gsj.jp/).
The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.
Hasenaka T, Ui T, Nakamura Y, Hayashi S, 1992. Traverse of Quaternary volcanoes in Japan. 29th Internatl Geol Cong, Kyoto, Field Trip A06, 74 p.
Hayakawa Y, 1994. A catalog of the volcanic eruptions during the last 2000 years in Japan. Sci Rpt Fac Education Gumma Univ, (in Japanese with English abs).
IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior..
Ishikawa T, Minato M, Kuno H, Matsumoto T, Yagi K, 1957. Welded tuffs and deposits of pumice flow and nuee ardente in Japan. 20th Internatl Geol Cong, Mexico City, Sec 1: 137-150.
Koyaguchi T, 1986. Life-time of a stratified magma chamber recorded in ultramafic xenoliths from Ichinomegata volcano, northeastern Japan. Bull Volc, 48: 313-323.
Nakano S, Yamamoto T, Iwaya T, Itoh J, Takada A, 2001-. Quaternary Volcanoes of Japan. Geol Surv Japan, AIST, http://www.aist.go.jp/RIODB/strata/VOL_JP/.
Ono K, Soya T, Mimura K, 1981. Volcanoes of Japan. Geol Surv Japan Map Ser, no 11, 2nd edition, 1:2,000,000.
Takahashi E, 1978. Petrologic model of the crust and upper mantle of the Japanese island arcs. Bull Volc, 41: 529-547.