10:45 AM - 12:15 PM
[SEM15-P07] Eruption style of the Kusatsu-Shirane volcano, Gunma: A preliminary rock magnetic approach
Keywords:susceptibility, active volcano
The Kusatsu-Shirane volcano in Gunma Prefecture, Japan, consists of three pyroclastic cones and has phreatic eruptions repeatedly in recorded history (JMA, 2013). The eruption history was discussed by Hayakawa and Yui (1989) based on representative tephra layers. Based on petrological characteristics, Kametani (2021) clarified the eruption history over the past 10,000 years at the whole-volcanic scale.
In this study, our aim is to determine rock magnetic properties of sediment and to see if an eruption style can be estimated by high-temperature magnetic characteristics of volcanic sediments deposits surrounding the volcano.
Rock magnetic analyses were carried out on 73 Holocene sediment samples from 54 layers, collected at a "hair-pin turn" outcrop exposed along Route 292. Thermomagnetic curves, including strong- field magnetization versus temperature (Js-T) and the temperature dependence of magnetic susceptibility (k-T), determine high-temperature magnetic characteristics of each sample.
The Js-T results showed two Curie temperatures, 300-400 and around 550°C, for most samples regardless, different layers. Conversely, k-T behaviors during heating processes are divided into two patterns. Pattern (1) shows similar in trend of Js-T curves and similar Curie temperature to the Js-T measurement are observed. In contrast, two peaks are observed at around 300 and 400-500°C in pattern (2), unlike the heating curves observed on the Js-T. Both patterns are found at certain levels of depth in this outcrop.
In pattern (1), little alteration of the minerals has occurred during heating experiment, indicating high amount of magnetic minerals in the sample. Since these samples correspond to the ejecta of the magmatic eruptive presession (Kametani, 2021), pattern (1) suggests sediments from the magmatic eruption. In pattern (2), a clear alteration of minerals occurred in the furnace during heating measurement. A remnant sample after magnetic separation shows same k-T curve as the k -T curve of whole sample, while the magnetically separated sample shows similar k-T curve to the pattern (1). Therefore, the pattern (2) k-T curve is likely caused by “non-magnetic” minerals rather than magnetic minerals. These non-magnetic minerals would be hydrothermally altered minerals such as sulfides and iron-oxides. Because this volcano has a well-developed hydrothermal system (Terada, 2018), pattern (2) may refer sediments from a phreatic eruption.
These results indicate that the combination of Js-T curves and k-T curves can discriminate magmatic and phreatic eruptions. Thus, rock magnetic measurements can be an effective method to estimate the eruptive history of volcanoes that repeatedly erupt magmatically and phreatically, such as the Kusatsu-Shiranes volcano.
In this study, our aim is to determine rock magnetic properties of sediment and to see if an eruption style can be estimated by high-temperature magnetic characteristics of volcanic sediments deposits surrounding the volcano.
Rock magnetic analyses were carried out on 73 Holocene sediment samples from 54 layers, collected at a "hair-pin turn" outcrop exposed along Route 292. Thermomagnetic curves, including strong- field magnetization versus temperature (Js-T) and the temperature dependence of magnetic susceptibility (k-T), determine high-temperature magnetic characteristics of each sample.
The Js-T results showed two Curie temperatures, 300-400 and around 550°C, for most samples regardless, different layers. Conversely, k-T behaviors during heating processes are divided into two patterns. Pattern (1) shows similar in trend of Js-T curves and similar Curie temperature to the Js-T measurement are observed. In contrast, two peaks are observed at around 300 and 400-500°C in pattern (2), unlike the heating curves observed on the Js-T. Both patterns are found at certain levels of depth in this outcrop.
In pattern (1), little alteration of the minerals has occurred during heating experiment, indicating high amount of magnetic minerals in the sample. Since these samples correspond to the ejecta of the magmatic eruptive presession (Kametani, 2021), pattern (1) suggests sediments from the magmatic eruption. In pattern (2), a clear alteration of minerals occurred in the furnace during heating measurement. A remnant sample after magnetic separation shows same k-T curve as the k -T curve of whole sample, while the magnetically separated sample shows similar k-T curve to the pattern (1). Therefore, the pattern (2) k-T curve is likely caused by “non-magnetic” minerals rather than magnetic minerals. These non-magnetic minerals would be hydrothermally altered minerals such as sulfides and iron-oxides. Because this volcano has a well-developed hydrothermal system (Terada, 2018), pattern (2) may refer sediments from a phreatic eruption.
These results indicate that the combination of Js-T curves and k-T curves can discriminate magmatic and phreatic eruptions. Thus, rock magnetic measurements can be an effective method to estimate the eruptive history of volcanoes that repeatedly erupt magmatically and phreatically, such as the Kusatsu-Shiranes volcano.