10:00 〜 10:15
[SVC28-23] 北海道アトサヌプリカルデラおよび赤井川カルデラの構造とその形成過程―砂箱実験とフィールドデータの比較―
キーワード：カルデラ、アナログ実験、アトサヌプリカルデラ、赤井川カルデラ
Calderas are volcanic depressions that are induced by inflation or deflation of magma chambers beneath volcanic edifices and are common features in volcanic areas. Various field data have been used to investigate caldera structure and to understand processes of its formation. Calderas are generally several km or more in diameter and the depth of magma chambers is often estimated over 10 km from the earth’s surface, whereas only a few caldera–forming eruptions have been observed scientifically because they are infrequent phenomena. Therefore, it is still controversial how the deeper structure of calderas is and how this is formed, because of these spatiotemporal problems. To overcome previously mentioned spatiotemporal obstacles, various analogue sandbox experiments have been carried out. The knowledge gained from sandbox experiments help us infer the inner structure of calderas and its processes of development. Analogue sandbox experiments generally attempt to simplify phenomena and structures in nature. Yet, natural calderas are under complex conditions (e.g., topography, geological features, stress field). Hence, this study verifies whether sandbox experiments can simulate caldera structures and their development processes within different conditions or not, by comparing field data and results from sandbox experiments. Atosanupuri and Akaigawa that are under different condition each other are chosen as subject areas.
Atosanupuri Caldera is located in the center of Kutcharo Caldera, eastern Hokkaido, northern Japan. According to previous studies, Kutcharo Caldera was formed between 40 and 30 ka, whereas Atosanupuri Caldera was developed approximately 20 ka. Atosanupuri Caldera has L–shaped somma, and its diameter is about 3 km. Lacustrine terraces are developed around Kutcharo Lake, which is the caldera lake of Kutcharo, and remarkable uplift of these terraces is detected on the near part of Atosanupuri Caldera. On the other hand, the significant uplift of the basement and lacustrine deposits around Atosanupuri Caldera is also inferred by drilling surveys. Based on these field data, inflation of a magma chamber beneath Atosanupuri Caldera accompanied by uplift of earth’s surface is assumed. Accordingly, calderas induced by inflation of magma chambers are experimentally simulated in this section. The mixture of dry quartz sand and wheat flour is used as analogue upper crust, and columnar foamed polystyrene is employed as analogue magma chamber. The scale of models is 10-5. When the initial depth of the magma chamber analogue is between 17.5 and 21 cm, a caldera whose diameters is about 3 cm is formed. Both the depth of magma chamber analogue and the size of experimental calderas and those of Atosanupuri Caldera are similar to each other. Thus Atosanupuri Caldera is quantitatively reproduced by sandbox experiments. In other words, analogue sandbox experiments are applicable to natural calderas within the conditions on Atosanupuri Caldera.
Akaigawa caldera is located at the southwestern Hokkaido, and is surrounded by the circular mountain range, which is called Yoichigawa Caldera. Akaigawa caldera is thought to have been formed between 2.0 and 1.6 Ma. The caldera is bounded by the subcircular lineament which is 4 km in diameter, and an uplift of basement is not confirmed by field data. They suggest that Akaigawa Caldera was induced by deflation of the magma chamber underlying the caldera. Thereupon a sandbox experiment that simulate deflation of magma chambers is carried out in this section. The scale, apparatus, and material are same as the previous section. Subcircular or circular calderas are reproduced by deflation of the magma chamber analogue. However, all calderas reproduced in this section are smaller in diameter than scaled Akaigawa Caldera. The caldera may have been influenced by the pre–existing structure, namely, Yoichigawa Caldera. Further field data and analogue experiments are required to verify this hypothesis.
Atosanupuri Caldera is located in the center of Kutcharo Caldera, eastern Hokkaido, northern Japan. According to previous studies, Kutcharo Caldera was formed between 40 and 30 ka, whereas Atosanupuri Caldera was developed approximately 20 ka. Atosanupuri Caldera has L–shaped somma, and its diameter is about 3 km. Lacustrine terraces are developed around Kutcharo Lake, which is the caldera lake of Kutcharo, and remarkable uplift of these terraces is detected on the near part of Atosanupuri Caldera. On the other hand, the significant uplift of the basement and lacustrine deposits around Atosanupuri Caldera is also inferred by drilling surveys. Based on these field data, inflation of a magma chamber beneath Atosanupuri Caldera accompanied by uplift of earth’s surface is assumed. Accordingly, calderas induced by inflation of magma chambers are experimentally simulated in this section. The mixture of dry quartz sand and wheat flour is used as analogue upper crust, and columnar foamed polystyrene is employed as analogue magma chamber. The scale of models is 10-5. When the initial depth of the magma chamber analogue is between 17.5 and 21 cm, a caldera whose diameters is about 3 cm is formed. Both the depth of magma chamber analogue and the size of experimental calderas and those of Atosanupuri Caldera are similar to each other. Thus Atosanupuri Caldera is quantitatively reproduced by sandbox experiments. In other words, analogue sandbox experiments are applicable to natural calderas within the conditions on Atosanupuri Caldera.
Akaigawa caldera is located at the southwestern Hokkaido, and is surrounded by the circular mountain range, which is called Yoichigawa Caldera. Akaigawa caldera is thought to have been formed between 2.0 and 1.6 Ma. The caldera is bounded by the subcircular lineament which is 4 km in diameter, and an uplift of basement is not confirmed by field data. They suggest that Akaigawa Caldera was induced by deflation of the magma chamber underlying the caldera. Thereupon a sandbox experiment that simulate deflation of magma chambers is carried out in this section. The scale, apparatus, and material are same as the previous section. Subcircular or circular calderas are reproduced by deflation of the magma chamber analogue. However, all calderas reproduced in this section are smaller in diameter than scaled Akaigawa Caldera. The caldera may have been influenced by the pre–existing structure, namely, Yoichigawa Caldera. Further field data and analogue experiments are required to verify this hypothesis.