The Tokachidake volcano group, central Hokkaido, is one of the most active volcanoes in Japan; three magmatic eruptions occurred from the crater area on the northwestern flank of Tokachidake in the 20th century. The Sandan-yama, Kamihorokamettokuyama, and Sampōzan edifices are on the southern flank of the volcano, and the first two bound the west-facing Nukkakushi crater. Although fumarolic activity and hydrothermal alteration are ongoing at Nukkakushi crater, its eruptive history remains unknown. Therefore, we performed a geological investigation of the Nukkakushi crater area. Based on topographical features, we inferred the following eruptive history. Sampōzan and Kamihorokamettokuyama formed during ca. 70-60 ka, after which the northern flank of Sampōzan collapsed and a new edifice (Nukkakushi volcano) was built within the collapse scarp. Finally, the collapse of the western flank of Nukkakushi formed Nukkakushi crater—perhaps during the Holocene, according to previous work. We identified eight Holocene eruptive products generated from the Nukkakushi crater area, the most recent of which was generated from a crater on the western flank of Sandan-yama sometime since the early 18th century. We also recognized three debris avalanche/landslide deposits that were generated within the last 750 years. Comparing the eruptive products of the northwestern crater area of Tokachidake with those of the Nukkakushi crater area revealed that magmatic eruptions from the two craters alternated until 1.8 ka. Their distinct magmatic compositions suggest the simultaneous existence of two isolated magma systems beneath Tokachidake and Nukkakushi, at least until that time. Since 1.8 ka, magmatic eruptions at the northwestern crater area of Tokachidake and phreatic eruptions at the Nukkakushi crater area have occurred in parallel. Moreover, around Nukkakushi crater, small-scale collapses/landslides have occurred. Previous studies recognized hydrothermal changes at Nukkakushi crater area, originating from the northwestern crater area of Tokachidake around the last two magmatic eruptions; it is therefore presumed that the Nukkakushi crater area was hydrothermally altered, even during periods of little eruptive activity. Such continuous and pervasive hydrothermal alteration explains the frequent collapses of edifices. The parallel yet contrasting eruptive activities in these adjacent areas are important for forecasting future eruptive activities and mitigating volcanic hazards.

Surface structures of magmatic dikes reflect dike emplacement processes. Excellent exposures of the Miocene Hashigui-iwa dike (Wakayama, SW Japan) exhibit well-preserved surface structures including drag folds, scour marks, extension fractures, and cusps. Scour marks are evident on the drag-folded surface, and are in turn cut by extension fractures. Inside the cusps, which are clefts formed by adjacent convex margin irregularities, the drag-folded margins enclose mudstone lenses. Based on our field observations, we infer an emplacement mechanism whereby magma fingers ascended through moderately consolidated host sediments, forming scour marks on chilled margins and causing repeated drag folding. Continued magma flux allowed the fingers to expand, generating extension fractures on the chilled margins. Ultimately, the fingers coalesced, forming the now-preserved cusp structures.

The Asama-Maekake volcano has been active during the last 10,000 years. Large-scale eruptions that occurred in the 18^th and 12^th centuries have been well studied, whereas little information is available for older eruptions. In this paper, we aim to reconstruct the history of this volcano in detail through a combination of extensive geological survey and ¹⁴C dating. The observation and description of twenty-one trench excavations, two drilling core samples, and many outcrops enabled us to build a stratigraphy of the eruptive products in much greater detail than ever before. The trench excavation sites cover an area of nearly 180 degrees around the volcano. These sites were selected mainly in the medial area at distances between 5 and 10 km from the summit crater. Many older deposits buried by thick younger deposits were found. The pyroclastic fall deposits of this volcano vary from a thick pumice layer to pumice grains scattered in the black soil, indicating a variation in the scale of sub-plinian eruptions. More than 120 samples for ¹⁴C dating were taken from the black soil immediately beneath the pyroclastic fall deposits. Some charcoals contained in the pyroclastic flow deposits were also subjected to dating. The ¹⁴C dating results were used for the correlation of deposits of different localities and distributions of some pyroclastic fall deposits older than 2000 years were revealed. The qualitative evaluation of the number and scale of eruptions throughout history might be possible using these data. Four classes of eruptive scales are recognized in the pyroclastic fall deposits in this study. Class 1: Defined by the isopach line for 128 cm thickness being able to be drawn on the map and the area enclosed by the 64 cm isopach line being more than 25 km². The deposits are recognized at distant locations more than 50 km from the summit crater. Class 2: Defined by that the isopach line for 64 cm thickness being able to be drawn on the map and the area enclosed by the 16 cm isopach line being more than 15 km². Class 3: The deposit of this class is recognized as a distinct layer that continues horizontally at each locality. In most cases, the observed thickness is less than several tens of centimeters and generally no structure can be observed. Class 4: This class comprises scattered pumice grains in the soil, for which the measurement of thickness is impossible. The deposits of classes 3 and 4 are seldom found at distances farther than 15 km from the crater. Most of the pyroclastic fall deposits of the period between 9400 and 3100 cal BP are of Classes 3 and 4. On the other hand, a large-scale eruption (Class 1) occurred about 2000 years ago, generating pyroclastic fall deposits in distant areas of more than 50 km from the crater. The recurrence interval of large-scale eruptions during the last 2000 years is estimated to be about 700 years. This is less frequent than in the period prior to 2000 years ago. Therefore, a change in eruption mode occurred about 2000 years ago when eruptions became infrequent but large in scale.

Nikko-Shirane Volcano located on the border of Gunma and Tochigi prefectures had the largest eruption on the historic records in AD 1649. We reconstructed the eruption event based on the geological mapping of the pyroclastic fall deposit and craters at the summit, ¹⁴C dating of soil underlying the pyroclastic fall deposit and interpretation of historic records. The pyroclastic fall deposit is observed in a 10×6 km area around Nikko-Shirane Volcano and thickens to the summit of Mt. Shirane. The pyroclastic fall deposit is preserved at＞4 km east from the summit and observed 5-8 cm thick around Lake Yunoko and 20 cm thick in maximum around the southern part of Senjogahara. Based on the historic records of the 1649 eruption, the craters with about 220 m in long axis diameter and 30 m deep located next to a small shrine at the summit were opened. Thus, the 1649 eruption is considered to occur at the summit of Mt. Shirane and pyroclastic materials fell east to southeast ward. The total mass of pyroclastic fall deposit is estimated at 2×10⁷-3×10⁷ m³ which is a digit larger than the previous report, and it is comparable to Volcanic Explosive Index＝3 and Magnitude＝3.4-3.6. The pyroclastic material contains essential vesicular vitreous particles consisting 1-48 % (mean 19 %) of component in 250-2000 μm fraction. Combination of the essential particles in the 1649 pyroclastic materials suggests that a magmatic eruption was occurred during the 1649 eruption. The essential particles are concentrated in three principal distribution axes of the pyroclastic fall deposit extending to the east, southeast and west. However, the pyroclastic fall deposit is composed of a lot of fine particles, indicating that the 1649 eruption would be possible of a phreatomagmatic eruption triggered by magma intrusion to an aquifer below the volcanic body. Around the time of the eruption, lahar occurred at the western valley of Mt. Shirane and flowed through Ohirogawara to the Nigamatazawa River.

Volcanic gases are high temperature gases degassed from a magma at depths, emitting to the surface. The volcanic gases give us important clues for understanding of eruptive phenomena as their emissions are closely related to the amount of degassed magma within the volcano. The quantification of the volcanic gases is also important for environmental problems and disaster preventions because they contain toxic species. The main components of the volcanic gases are water, carbon dioxide, and sulfur dioxide (SO₂). The SO₂ gas has been used as an index of volcanic gas flux because SO₂ is readily quantified using remote-sensing techniques based on ultraviolet (UV) spectroscopy and the atmospheric air is SO₂ free. In this article, the importance of the SO₂ flux and overview of current and future remote-sensing approaches from ground are discoursed. Benefits for practical operations given from the recent developments are highlighted, stressing how these brand-new techniques could be applied to help monitoring of volcanoes.