Rock-magnetic evidence for the low-temperature emplacement of the Habushiura pyroclastic density current, Niijima Island, Japan

Abstract

The emplacement temperatures of accidental and juvenile fragments in the Habushiura pyroclastic density current (PDC) deposit were estimated using analysis of rock magnetism. The Habushiura PDC was generated in a costal or shallow offshore area during the early stage of the AD 886 phreatomagmatic eruption on Niijima Island, Japan. We collected 160 samples from 11 beds, which include three lithofacies types: a massive lapilli tuff (Facies A); a graded and/or diffusely stratified lapilli tuff (Facies B); and a swarm of large pumice (Facies C). Juvenile specimens from Facies A and C show a wide range of emplacement temperatures, from less than 150 °C up to 300 °C, in contrast with the emplacement temperatures lower than 150 °C of Facies B. Morphological features of the ash components and random changes in palaeomagnetic direction by stepwise thermal demagnetization indicate that low-temperature emplacement occurred by phreatomagmatic eruptions and ingestion of ambient air by the turbulent current. The emplacement temperatures of accidental fragments were up to 380 °C, which is higher than juvenile fragments. They are thought to have cooled more slowly than juvenile fragments due to the smaller surface area with respect to their volume.

Future Risks of Tsunami‐Induced Seawater Intrusion Into Unconfined Coastal Aquifers: Insights From Numerical Simulations at Niijima Island, Japan

“…Setoyama, and Mt. Marujimamine were above sea level, and others were below sea level (Nakaoka & Suzuki‐Kamata, ). In 886 CE, the latest eruption of the Mukaiyama Volcano occurred with a phreatomagmatic explosion of rhyolite magma at the southern part of Niijima Island (Koyaguchi, ).…”

“…in the central part of the island (Figure b). The seafloor depth within a horizontal distance of 1,000 m from the coastline of the island is less than 30 m (Nakaoka & Suzuki‐Kamata, ).…”

“…Figure 1. Niijima Island, Japan: (a) location map; (b) topographic map showing land surface elevations (Ministry of Land, Infrastructure, Transport and Tourism, 2015), bedrock surface elevations (Construction Section of Niijima Village, 2001;Isobe & Nakashima, 2001;Shindo, 1980), sea bottom elevations(Nakaoka & Suzuki-Kamata, 2015), anticipated tsunami hazard zone(Tokyo Disaster Management Council, 2013), and the line of cross section of the 2-D model (see Figure2); (c) geologic map modified afterIsshiki (1987). The observation wells shown in (c) are the ones where water table elevations were measured (see Figure3b).…”

Paleomagnetism and rock magnetism as tools for volcanology

“…Studies have refined these emplacement temperature estimates in recent years with application to a wider variety of volcanic materials, such as millimeter-sized lithic fragments, ignimbrites of different degrees of welding, and unconsolidated matrix of PDC deposits (e.g., Cioni et al 2004;McClelland et al 2004;Lerner et al 2019b). New studies have also addressed temporal and spatial heterogeneity of peak temperature within a hot PDC deposit (Bowles et al 2018) and the relation between juvenile and lithic clast and matrix temperatures within PDC deposits (Nakaoka and Suzuki-Kamata 2015). Paleomagnetic techniques have also been combined with other emplacement temperature techniques, like charcoal reflectance, to produce more precise temperature estimates (Pensa et al 2018).…”