Ground displacement in a volcanic region constrained by precise geodetic measurements has motivated us to infer magmatic activity at depth, e.g., inflation/deflation of a magmatic deformation source. The inference was initially made by predicting the elastic response to a pressurized magma chamber. A prediction with an unrealistically high chamber pressure has then introduced other rheological models, including elastoplastic and viscoelastic rheologies. Viscoelasticity predicts transient behavior of deformation even though the source mechanism is constant with time, depending on the viscosity structure of the crust and the presence of an elastic layer in the uppermost crust, which requires volcano deformation to be considered a more complex phenomenon, but provides an excellent opportunity for inferring a magma chamber not only as a deformation source but also as a zone that has rheologically less strength. Indeed, geodetically measured rates of volcano deformation, often having constrained crustal viscosities in the range of ∼1017-1019 Pa s, are higher than those of postseismic deformation in regions where there is less volcanic activity. This suggests that crustal viscosities are lowered by higher geothermal gradients due to the presence of magma beneath volcanic regions. Understanding volcano deformation in terms of the interaction between magmatic activity and rheological properties of the crust also has the potential to offer a dynamical aspect to petrological and geophysical images of a magma chamber. The volcano deformation model is now required to introduce the magma plumbing system into the response of a viscoelastic crust, in which a magma chamber has excess pressure, which varies with time during conservation of the mass of magma, making it possible to predict throughout volcano dynamics from accumulation of magma to eruption in more self-consistent way.
北海道南西部に位置する洞爺カルデラは日本有数の陥没カルデラである。このカルデラの地下構造を解明するため，CSAMT法による比抵抗構造探査を行った。探査は洞爺カルデラを北東–南西方向に横断する測線上（測線長16 km，受信点17か所）で行い，カルデラ内の洞爺湖では小型ボートを用いた湖上測定を行った。データ解析は有限要素法を用いた2次元逆解析を用いた。その結果，洞爺カルデラの深度1500 mまでの比抵抗構造が得られた。洞爺カルデラの南西側には低比抵抗領域が存在し，新第三紀0第四紀の変質した安山岩であると推定される。カルデラの北東側には高比抵抗領域が存在し，新第三紀の珪化した流紋岩であると推定される。カルデラ内には均質な中比抵抗領域が存在し，軽石や石質岩片などの火山砕屑物からなるカルデラフィル堆積物であると推定される。カルデラ中央部の中島は高比抵抗領域からなり，中島がデイサイト質の溶岩ドーム群からなることと調和的である。中島とその周囲の隆起域の地下深部には低比抵抗領域（幅4500 m，厚さ1000 m）が存在する。この低比抵抗領域は，中島とその周囲の隆起域を形成した地下のマグマによりカルデラフィル堆積物が加熱され，熱水変質して形成されたと考えられる。
Kutcharo caldera is situated in the eastern part of Hokkaido, Japan. It is subcircular with a diameter of 20-26 km and was formed by repeated violent rhyolitic explosive eruptions from 340 to 35 ka. The caldera has three post-caldera volcanoes: Atosanupuri, Nakajima and Mashu. Our new tephrostratigraphical survey suggests that a pyroclastic fall deposit (Nakajima pumice), which was extruded from Nakajima, is exposed at the western slope of Atosanupuri. The deposit comprises dacitic pumice clasts (< 9 cm across) in a fine-grained matrix. The pumice clasts are coated with fine-grained ash, suggesting the deposit was produced by a phreatomagmatic eruption. The deposit directly overlies a scoria-fall deposit, which was ejected from Mashu volcano at 17-12 ka, and is overlain by Ma-k tephra, which was extruded from Mashu volcano at 10 ka. The stratigraphy of the deposit suggests that a phreatomagmatic eruption occurred at Nakajima volcano between 17 and 10 ka.
The fundamental structure of a collapse caldera is subsidence of a block(s) into a magma chamber with the evacuation of a massive volume of magma from the chamber. Decompression of a magma chamber caused by the extraction of magma from a magma chamber coinciding with pyroclastic eruption, effusive eruption, and lateral magma migration drives caldera collapse. Caldera-forming eruptions exhibit wide variations. Caldera-forming pyroclastic eruptions are characterized by a high eruption rate, whereas caldera-forming effusive eruptions have a much lower effusion rate. Caldera collapse caused by a pyroclastic eruption generally occurs within one day, whereas incremental collapses continue for up to one month in the case of some effusive eruptions. Collapse calderas also have wide structural variations. The aspect ratio of the roof of the magma chamber controls the development of caldera faults. The development of multiple caldera faults with a high aspect ratio causes piecemeal collapses, whereas a low aspect ratio results in the subsidence of coherent blocks detached by a simple ring fault. With the progress of caldera subsidence, the caldera structure develops from a flexural down sag to a double-ring fault system, and finally reaches an upward-flaring “funnel shaped” caldera following an intense collapse of the caldera wall. Calderas vary widely in size from 1 km to 100 km, and can also be divided into at least three classes by their internal structures. The largest group of more than 20 km across is characterized by flexural down-sag deformation. The intermediate-size group is characterized by well-developed caldera-border faults. The smallest calderas of less than 10 km across may have piecemeal structures. The existence of a pre-caldera volcanic edifice is also an indicator of differences in the magmatic system. Some calderas form at the summit of a pre-existing stratovolcano or shield volcano, whereas some large calderas form in a cluster of small volcanoes, involving non-volcanic basement rocks. A structural model of caldera development should involve these wide spectrums of collapse calderas. The development of collapse calderas is controlled by variations of magmatic activity, such as eruption style, eruption rate, and duration of eruption, as well as the architectures of their magma storage systems.
北海道洞爺カルデラは日本有数の陥没カルデラである。われわれは洞爺カルデラの形成過程を明らかにするため，洞爺火砕流堆積物（Toya Ignimbrite）の地質層序学的調査を行った。洞爺火砕流堆積物（層厚80～100 m）は，岩相の違いにより6つのユニット（ユニット1～6）に区分できる。最下位のユニット1は細粒な火山灰からなり，マグマ水蒸気噴火により形成された。ユニット2は火山豆石に富む厚い火砕流堆積物と火砕サージ堆積物からなり，大規模なマグマ水蒸気噴火により形成された。ユニット3は多数の薄いベースサージ堆積物とそれに伴う降下火砕堆積物からなる。ユニット4は小規模な火砕流堆積物からなり，粒径の大きな石質岩片（径 < 50 cm）を含む。ユニット5は軽石に富む厚い火砕流堆積物からなり，下部にラグブレッチャ（粒径 < 3 m）を伴う。ユニット6は軽石に富む火砕流堆積物からなる。ユニット1～6は土壌層や再堆積相を挟在しない。したがって，洞爺火砕流堆積物は時間間隙のない一連の噴火で形成されたと考えられる。各ユニットの組織，構成物，体積，石質岩片の岩石種などから，カルデラ陥没はユニット4の噴火で開始し，ユニット5の噴火でクライマックスに達した可能性が高い。ユニット1～6の軽石はすべて流紋岩質であり，一連の噴火はほぼ一定の組成のマグマ噴出により行われた。ユニット6はやや苦鉄質な軽石を含むことから，噴火の最末期にはやや苦鉄質なマグマも噴出したと考えられる。
Shikotsu volcano is a caldera volcano in southwestern Hokkaido. It is considered that a large-scale eruption started at ca. 60 ka (named as 60 ka Shadai eruption), mainly ejecting scoria fall and scoria flow deposits. The volcano repeated explosive eruptions every several thousand years, and after 10 ky of dormancy, a caldera-forming eruption took place at ca. 46 ka. Trench and boring surveys were carried out in the eastern part of Shikotsu caldera, and stratigraphy and changes in the components of the 60 ka Shadai eruption were reexamined. There are three types of juvenile materials in 60 ka eruption deposits: dacitic pumice, olivine-bearing andesitic scoria, and intermediate banded/gray pumice. As a result, tephra layers of the eruption are mainly classified into three units: Units A to C in ascending order. Soils and volcanic ash soils were not discovered among these layers, so these must derive from a single eruption sequence. Unit A consists of pumice fall deposits and ash fall deposits; Unit B of scoria fall deposits; and Unit C comprises a pyroclastic flow deposit and a following pyroclastic fall deposit. Each eruptive unit is subdivided into A1-A3, B1-B5 and C1-C2. Based on these eruptive units, the sequence of the 60 ka Shadai eruption is constructed as follows: Phase 1 was a pumiceous plinian eruption (A1, A2), and eruption rate abruptly decreased in A3. Phase 2 was a scoriaceous plinian eruption (B1-B5). Eruption rate was unstable in early Phase 2 (B1-B4); however, it gradually increased in late Phase 2 (intermittent eruption had repeated in B5). Consequently, a pyroclastic flow eruption occurred in Phase 3 (C1), followed by a plinian eruption (C2). Andesitic and mixed magmas were found to begin to erupt from Phase 2, while dacitic magma survived through the 60 ka Shadai eruption. These characteristics are difficult to explain based on the eruption from a zoned magma chamber or from a common eruptive vent. Dacitic and andesitic magmas are suggested to have existed in separated magma chambers, and dacitic magma at least was supplied through an independent vent system.
Proximal pyroclastic deposits of the 46 ka caldera-forming eruption of Shikotsu volcano are investigated at new outcrops along the Opoppu river, south of the volcano. The deposits can be divided into 6 units, from A to F in ascending order, according to lithofacies, components, and time intervals. Based on the stratigraphy of the deposits, the sequence of the 46 ka Shikotsu eruption is revealed. Activity started with phreatomagmatic and phreatoplinian eruptions (Phase 1: Unit A). The eruption style changed to magmatic without a time interval before the plinian eruption (Phase 2: Unit B). The eruption column was intermittently unstable, producing pyroclastic surge and flow deposits during the latter period of Phase 2. The lithic breccia content also increased in the latter period. After a possible erosional interval, explosive eruptions occurred, producing voluminous pyroclastic flows and ended with the effusion of lag breccia (Phase 3: Unit C). After a dormant period, pyroclastic flows effused intermittently (Phase 4: Units D and E). At the final stage, small plinian eruption occurred, associated with a pyroclastic surge (Phase 5: Unit F). Juvenile materials of the 46 ka Shikotsu eruption are mainly crystal-poor (CP type) rhyolite pumice from Phases 1 to 3. Small amounts of crystal-rich (CR type) dacite and andesite pumice occurred from the final stage of the Phase 3. Two types of silicic material coexisted during Phases 4 and 5. According to temporal changes of lithic contents, the caldera collapse was almost completed at the end of Phase 3 with the formation of lag breccia. Therefore, it could be considered that activity in Phases 4 and 5 was either the terminal phase of caldera-forming activity or the initial activity of post-Shikotsu caldera volcanoes.
The durations of caldera-forming eruptions that produce large volumes (> 100 km3) of pyroclastic ejecta are poorly understood due to the absence of direct observations. However, clarifying the timescale of these catastrophic hazardous events is essential for understanding associated eruption dynamics and links with the eruptible portions of the underlying magma system. Case studies addressing the time scale of caldera-forming eruptions are reviewed. Three Quaternary large volume caldera-forming eruption deposits from Yellowstone (US), Taupo (New Zealand) and Kutcharo (Japan) volcanoes are inferred from deposits to have lasted for periods of at least for months to years. However, these estimations are generally based on geological evidence, such as re-worked deposits between eruption units, and not quantitative evidence. Proposed here is a new method of timescale estimation based on paleomagnetic secular variation. A sampling procedure was developed for accurate oriented samples of pyroclastic deposits including volcanic ash. This procedure makes it possible to obtain the mean remanent magnetization of a tephra layer with a 95% confidence limit of about 2°, which is comparable to those of well-determined directions for lava. Based on this procedure, the 7.3 ka Kikai caldera-forming eruption was investigated as a trial. Samples for paleomagnetic measurement were collected from the basal ash-rich part of the lowermost plinian pumice fall (Koya pumice fall) at Satsuma Iwo-jima, Kyushu. The difference of 6.9 degrees in the remanent magnetizations between Koya pumice and reported data of the uppermost co-ignimbrite ash-fall (Kikai–Akahoya Ash) suggests the caldera-forming eruption of a considerable duration (> 50 years) on the basis of average rate of secular variation. Paleomagnetic directions from pyroclastic deposits could be a powerful tool for estimating timescales of large explosive eruptions.
Toya caldera, eastern Hokkaido, was formed approximately 110 ka, and has two post-caldera volcanoes, Nakajima and Usu volcano. Caldera-forming eruptions ejected a large-scale Toya pyroclastic flow, and related co-ignimbrite ash which covered a wide area in northern Japan. Usu volcano is one of the most active volcanoes in Japan, and has had repeated magmatic eruptions during historical time since AD 1663. The latest major eruption occurred in AD 2000. General geology and representative outcrops of eruptive deposits from Toya caldera and its post-caldera volcanoes are introduced, on the basis of a training field course at the 6th IAVCEI Collapse Caldera Workshop held in September 2016.