2:45 PM - 3:00 PM
[SVC29-05] Phreatomagmatic stratigraphy and eruptive history of maar cluster at the southern coastal area, Miyakejima Volcano, Japan
Keywords:eruptive stratigraphy, phreatomagmatic eruption, maar, tephra ring, Miyakejima Volcano
Phreatomagmatic eruptions induced by the interaction of magma with external water are one of the most violent eruption styles, particularly in basaltic volcanoes which are generally characterized by non-explosive lava effusion1. In those volcanic islands, explosive phreatomagmatic activity can be caused by intrusion of magma into the water-saturated subsurface deposits in coastal areas. Thus, it is necessary to reveal the eruptive history of phreatomagmatic explosions in the coastal areas of the volcanic island, like Miyakejima, to evaluate volcanic risks because coastal areas of volcanic islands are important residential and touristic environments. Miyakejima volcano has a circular shape of ~ 8 km in diameter and is one of the most active basaltic volcanoes in the northern part of Izu–Bonin arc. Many maar craters distribute along the coastal area of the volcano particularly in the southern area where at least 5 maars are found in an area of 4 km2. We carried out geomorphological analysis, geological survey, radiocarbon dating and petrological analysis of juvenile materials of eruption deposits to reveal the phreatomagmatic maar-forming eruption history at the southern area of Miyakejima volcano.
Five maars (Furumio, Yamamio, Kokoma, Mizutamari, Tsurune) are recognized in the survey area. Each of the maars consists of a circular crater (maar) and its surrounding skirts of pyroclastic deposits (tephra ring)(Fig. 1). Tsurune maars is composed of at least 1 km long N–S aligned fissure vents from the southern flank to the coast. The eastern rims of Tsurune maars and large tephra rings (> 400–500 m in width) are well preserved, while the western side is not recognized. Mizutamari maar (~ 1 km in diameter) has a tephra ring (~ 300–400 m in width) that is moderately preserved. Furumio (~ 1 km), Yamamio (~ 300 m), and Kokoma (~ 500 m) maars are N–S aligned inside Tsurune maars and have relatively narrow tephra rings (~ 200–300 m in width). The geomorphology of their maars is relatively fresh, and they are not widely overlaid by thick younger eruptive deposits.
Tephrostratigraphic survey reveals at least three maar-forming eruption units separated by paleosols and/or erosion surfaces which suggest significant hiatuses. We named the three eruption units as Tsurune tephra (Tr: 3.4 ka), Mizutamari tephra (Mz: 3.0 ka), and Furumio tephra (Fr: 1.9 ka2), respectively, based on the names of the inferred source maars.
All the eruption units consist of a scoria fall deposit at the base and overlying layers of pyroclastic density currents (PDC) deposits and ash-fall deposits. The lower scoriae are well vesiculated, suggesting a magmatic fragmentation, while juvenile materials in upper PDC deposits show poorly vesiculated, blocky shaped with glassy quench margins, suggesting that magma–water interaction. The thickness of Tr, Mz and Fr are thicker toward Tsurune maars, Mizutamari maar and Furumio–Yamamio–Kokoma maars, respectively, suggesting their source maars. For example, the proximal (~ 150 m from maar rim) facies of PDC Tr deposit, is more than 15 m thick and contains large blocks (< 2 m in diameter), while the distal facies (~ 750 m from maar rim), is 5 m in thickness with blocks of < 1 m.
The eruption history of maar cluster in the southern part of Miyakejima volcano can be summarized as follows: at 3.4 ka, the Tsurune maars formed at least 1 km from the southern flank towards the coast. Then the Mizutamari maar formed at 500 m east of Tsurune maars at 3.0 ka, then at 1.9 ka, the Furumio–Yamamio–Kokoma maars erupted in a near fissure system to Tsurune maars. These maars distribute only in the coastal area below ~ 200 m above sea level. Aquifers in the coastal area of Miyakejima are situated at shallow level below the ground3. The three eruption units indicate a shift of eruption style from a dry magmatic eruption that formed vesiculated at the base of each tephra layer, to a phreatomagmatic one that produced poorly-vesiculated and blocky-shaped juvenile materials in the PDC deposits. This eruption sequence is also recognized in the younger eruptions of Miyakejima such as the Oyama eruption (9th century)2 and the AD 1983 eruption4. These eruptions started with effusive magmatic activity such as a mild lava fountain at flank. Then, phreatomagmatic explosions occurred when the eruption fissures propagated and encountered external (shallow ground) water in coastal areas. It should be emphasized that this is a common eruption sequence of Miyakejima volcano which can be used for eruption prediction and hazard assessment and management.
References:
1. Lorenz, 2007, JVGR; 2. Geshi et al., 2022, EPS; 3. Gresse et al., 2021, JGR, 4. Aramaki and Hayakawa, 1984, Bul. Volcanol. Soc. Japan.
Five maars (Furumio, Yamamio, Kokoma, Mizutamari, Tsurune) are recognized in the survey area. Each of the maars consists of a circular crater (maar) and its surrounding skirts of pyroclastic deposits (tephra ring)(Fig. 1). Tsurune maars is composed of at least 1 km long N–S aligned fissure vents from the southern flank to the coast. The eastern rims of Tsurune maars and large tephra rings (> 400–500 m in width) are well preserved, while the western side is not recognized. Mizutamari maar (~ 1 km in diameter) has a tephra ring (~ 300–400 m in width) that is moderately preserved. Furumio (~ 1 km), Yamamio (~ 300 m), and Kokoma (~ 500 m) maars are N–S aligned inside Tsurune maars and have relatively narrow tephra rings (~ 200–300 m in width). The geomorphology of their maars is relatively fresh, and they are not widely overlaid by thick younger eruptive deposits.
Tephrostratigraphic survey reveals at least three maar-forming eruption units separated by paleosols and/or erosion surfaces which suggest significant hiatuses. We named the three eruption units as Tsurune tephra (Tr: 3.4 ka), Mizutamari tephra (Mz: 3.0 ka), and Furumio tephra (Fr: 1.9 ka2), respectively, based on the names of the inferred source maars.
All the eruption units consist of a scoria fall deposit at the base and overlying layers of pyroclastic density currents (PDC) deposits and ash-fall deposits. The lower scoriae are well vesiculated, suggesting a magmatic fragmentation, while juvenile materials in upper PDC deposits show poorly vesiculated, blocky shaped with glassy quench margins, suggesting that magma–water interaction. The thickness of Tr, Mz and Fr are thicker toward Tsurune maars, Mizutamari maar and Furumio–Yamamio–Kokoma maars, respectively, suggesting their source maars. For example, the proximal (~ 150 m from maar rim) facies of PDC Tr deposit, is more than 15 m thick and contains large blocks (< 2 m in diameter), while the distal facies (~ 750 m from maar rim), is 5 m in thickness with blocks of < 1 m.
The eruption history of maar cluster in the southern part of Miyakejima volcano can be summarized as follows: at 3.4 ka, the Tsurune maars formed at least 1 km from the southern flank towards the coast. Then the Mizutamari maar formed at 500 m east of Tsurune maars at 3.0 ka, then at 1.9 ka, the Furumio–Yamamio–Kokoma maars erupted in a near fissure system to Tsurune maars. These maars distribute only in the coastal area below ~ 200 m above sea level. Aquifers in the coastal area of Miyakejima are situated at shallow level below the ground3. The three eruption units indicate a shift of eruption style from a dry magmatic eruption that formed vesiculated at the base of each tephra layer, to a phreatomagmatic one that produced poorly-vesiculated and blocky-shaped juvenile materials in the PDC deposits. This eruption sequence is also recognized in the younger eruptions of Miyakejima such as the Oyama eruption (9th century)2 and the AD 1983 eruption4. These eruptions started with effusive magmatic activity such as a mild lava fountain at flank. Then, phreatomagmatic explosions occurred when the eruption fissures propagated and encountered external (shallow ground) water in coastal areas. It should be emphasized that this is a common eruption sequence of Miyakejima volcano which can be used for eruption prediction and hazard assessment and management.
References:
1. Lorenz, 2007, JVGR; 2. Geshi et al., 2022, EPS; 3. Gresse et al., 2021, JGR, 4. Aramaki and Hayakawa, 1984, Bul. Volcanol. Soc. Japan.