11:00 AM - 1:00 PM
[SVC31-P13] Igneous history and petrological characteristics of the Ryuohsaki area, southeastern Izu-Oshima Island
Keywords:Izu-Oshima
Izu-Oshima is an active volcano located at the northernmost part of the Izu-Ogasawara arc. It is based on three paleo volcanic bodies and is divided into the Senzu Group, the older Oshima Group, and the younger Oshima Group. Magmatic steam explosions, which are caused by the contact between water and magma, have frequently occurred in the coastal area of Izu Oshima Is. In Izu Oshima Is., the houses are concentrated in the coastal area, so it is very important to understand explosive eruptions in the coastal area.
In this study, the distribution of volcanic ejecta in the Ryuozaki area of southeastern Izu-Oshima Is. was examined to understand the volcanic activity pattern of the area. The sea cliffs distributed in the Ryuozaki area are composed of Ryuozaki lava, Ryuozaki volcaniclastic rocks and Habu volcaniclastic rocks. The Ryuozaki and Habu breccias were subdivided into lower, middle and upper parts based on their lithology.
The Ryuozaki lava is located at the bottom of the Ryuozaki area and forms aa clinker at the surface with dark red or black in color. This lava is equivalent to the older Oshima Group in the southeastern part of Izu-Oshima Is., but has a higher Mg value (43-46) than the Shimobara Lava, which is estimated to have been active at the same time.
The Ryuozaki volcaniclastic rocks (maximum thickness about 20 m) is an angular shaped breccia covered on top of the Ryuozaki lava, and its matrix is composed of fine-grained gray volcanic ash. The lower layer is composed of many fine parallel laminae a few centimeters thick. The middle part of the layer contains angular gravels of about 20 cm in diameter, and some bomb sag structures are observed. The upper layer is accompanied by cross laminae (period of several to ten meters), and is inferred to be a violent explosion because it is deposited by cutting the lower layer. The direction of flight inferred from the bog-sag structure suggests that the origin of the eruption is off the coast of Ryuozaki. In addition, the characteristics of the lithology suggest a transition from moderate to intense of explosive activity. Petrological characteristics and whole rock chemical analysis revealed that the rock fragments in this formation originated from two lava sources, Fudeshima volcano and Ryuozaki lava.
The Habu volcaniclastic rocks (maximum thickness: 25 m) was deposited above the Ryuozaki volcaniclastic rocks and is thought to have been deposited by a magmatic steam explosion originating in Habu in the 9th century. The cross lamina in the lower part suggests that the pyroclastic surge was caused by magmatic steam explosion. The central layer this formation contains gravels up to 40 cm in diameter, and the upper layer contains gravels up to 1 m in diameter (subrounded to angular). The shape of the gravel suggests that the environment at the time of the eruption was close to the coast. As a result of petrological characterization and chemical analysis, all the rock fragments in this formation were observed, including those of Fudeshima volcano, Ryuozaki lava, older Oshima group lava, and Habu lava origin. These results indicate that magmatic phreatomagmatic explosions with violent explosions such as pyroclastic surges and huge cinder cones have occurred several times around Ryuozaki in the southeastern part of Izu-Oshima Island.
In this study, the distribution of volcanic ejecta in the Ryuozaki area of southeastern Izu-Oshima Is. was examined to understand the volcanic activity pattern of the area. The sea cliffs distributed in the Ryuozaki area are composed of Ryuozaki lava, Ryuozaki volcaniclastic rocks and Habu volcaniclastic rocks. The Ryuozaki and Habu breccias were subdivided into lower, middle and upper parts based on their lithology.
The Ryuozaki lava is located at the bottom of the Ryuozaki area and forms aa clinker at the surface with dark red or black in color. This lava is equivalent to the older Oshima Group in the southeastern part of Izu-Oshima Is., but has a higher Mg value (43-46) than the Shimobara Lava, which is estimated to have been active at the same time.
The Ryuozaki volcaniclastic rocks (maximum thickness about 20 m) is an angular shaped breccia covered on top of the Ryuozaki lava, and its matrix is composed of fine-grained gray volcanic ash. The lower layer is composed of many fine parallel laminae a few centimeters thick. The middle part of the layer contains angular gravels of about 20 cm in diameter, and some bomb sag structures are observed. The upper layer is accompanied by cross laminae (period of several to ten meters), and is inferred to be a violent explosion because it is deposited by cutting the lower layer. The direction of flight inferred from the bog-sag structure suggests that the origin of the eruption is off the coast of Ryuozaki. In addition, the characteristics of the lithology suggest a transition from moderate to intense of explosive activity. Petrological characteristics and whole rock chemical analysis revealed that the rock fragments in this formation originated from two lava sources, Fudeshima volcano and Ryuozaki lava.
The Habu volcaniclastic rocks (maximum thickness: 25 m) was deposited above the Ryuozaki volcaniclastic rocks and is thought to have been deposited by a magmatic steam explosion originating in Habu in the 9th century. The cross lamina in the lower part suggests that the pyroclastic surge was caused by magmatic steam explosion. The central layer this formation contains gravels up to 40 cm in diameter, and the upper layer contains gravels up to 1 m in diameter (subrounded to angular). The shape of the gravel suggests that the environment at the time of the eruption was close to the coast. As a result of petrological characterization and chemical analysis, all the rock fragments in this formation were observed, including those of Fudeshima volcano, Ryuozaki lava, older Oshima group lava, and Habu lava origin. These results indicate that magmatic phreatomagmatic explosions with violent explosions such as pyroclastic surges and huge cinder cones have occurred several times around Ryuozaki in the southeastern part of Izu-Oshima Island.