Phreatic (non-juvenile) eruptions are the most common type of magmatic activity on Earth. Here we review the characteristics of phreatic eruptions, which occur when overheated water is rapidly vaporized. Tephra layers produced by phreatic eruptions are composed mainly of clay-rich volcanic ash with variably altered lapilli and volcanic blocks. A single phreatic eruption can last between one hour and one day; however, eruptions may occur successively over a period of years to decades. The total volume of tephra produced by a phreatic eruption is typically 10^4-6m³, maximum <10⁸ m³. Phreatic eruptions may be accompanied by diverse phenomena, including: tephra fallout, ejected rock fragments, low-temperature pyroclastic flows, and syneruptive-spouted type lahars. There are few detailed descriptions of low-temperature (~100°C) pyroclastic flows and syneruptive-spouted type lahars associated with phreatic eruptions. Detailed studies of phreatic phenomena are required, as it remains difficult to identify and reconstruct these processes based on the characteristics of the deposits.

This paper reviews the hydrothermal systems of Kusatsu-Shirane volcano, Japan, which are associated with phreatic eruptions. The existence of hydrothermal systems at this volcano is easily explained: hot springs are derived from unique thermal water that results from condensation of magmatic gas. Kusatsu-Shirane also exhibits fumaroles characterized by high H₂S and CO₂ contents, which are separate from the condensation of magmatic gas. Clay layers composed of smectites control the subsurface flow of thermal water. Hypocenter distributions of microearthquakes approach from depth to a bell-shaped impermeable clay layer underlying the Shirane pyroclastic cone, indicating the clay layer’s role in storing thermal water supplied from depth. Sources of low-frequency earthquakes, ground deformation, and demagnetization/magnetization are located around the bell-shaped impermeable clay. These observations indicate that a hydrothermal reservoir exists under the clay layer. Phreatic eruptions seem to result from the growth of cracks connecting the reservoir to the surface. Precursory changes in volcanic activity precede phreatic eruptions at Kusatsu-Shirane in most cases; however, the contents of such precursors do not correlate with the ejecta mass, locations, and lead times of eruptions. Kusatsu-Shirane has been continuously monitored since the 1970s. The phreatic eruption of 1976 was predicted based on geochemical observations, but no precursor warning was detected before the onset of a series of phreatic eruptions in 1982-1983. Microearthquake swarms that occurred in 1989-91 and 2014 were followed by demagnetization and changes in the chemical composition of the water in Yugama Crater Lake and the fumaroles. These changes were similar to precursors of past phreatic eruptions at Kusatsu-Shirane, but no phreatic eruption occurred at Yugama Crater within 2-3 years of either set of changes. Multiparameteric monitoring, including geophysical and geochemical observations, is a powerful tool for detecting changes in volcanic activity, but it is difficult to identify precursors of phreatic eruptions.

The active Iwate Volcano is located on the volcanic front of the Northeast Japan Arc. Yakushidake (2038 m), which is the youngest stratocone of Iwate Volcano, formed after a large-scale sector collapse associated with the Hirakasa debris avalanche deposit. This study re-examines the explosive eruptive history of the Yakushidake stratocone via tephra-stratigraphic study and radiocarbon dating.

New ¹⁴C ages indicate that the Hirakasa debris avalanche occurred at 8.5–9.9 cal ka BP. Twelve phreatic eruption units (labeled here Iw-ph12 to Iw-ph1, from oldest to youngest) and one argillaceous pyroclastic density flow deposit (the Yunosawa pyroclastic deposit, YPD) were identified by detailed ¹⁴C age dating and X-ray diffraction mineralogical analyses. The average recurrence interval of phreatic eruptions is 500–1000 years. Phreatic tephra deposits crop out around the Yakushidake summit crater, the Ojigokudani fumarolic area, and the Amihariyumoto geothermal area. The thickest phreatic tephra deposit, Iw-ph7, was erupted from Amihariyumoto at 4.0–4.5 cal ka BP. An ashfall volume of ca. 2.3×10⁷ m³ for the eruption was estimated using the calculation method of Hayakawa (1985).

Explosive magmatic eruptions deposited 15 tephra units. Three of the more voluminous explosive units (Iw-SuS, Iw-OdS, and Iw-KS) eruptions have approximate magnitudes of VEI = 3 and were associated with the pyroclastic surge deposits. The repose interval between voluminous explosions is c. 2000 years. The volcanic history of Yakushidake Volcano is dominated by two vigorous eruptive phases: YV1 (associated with tephra deposits Iw-W6d to Iw-OkS) and YV2 (tephra layers Iw-SS to Iw-KS and KSr). In both phases, small-scale magmatic eruptions preceded the more voluminous (VEI = 3) events. There is evidence for recurrent phreatic activity during the vigorous eruptive phases; however, individual magmatic activities and phreatic eruptions do not always coincide.

The eruptive history and eruption style of Asahidake volcano, the youngest volcanic edifice of the Taisetsu Volcano Group, central Hokkaido, Japan, are investigated to evaluate its long-term volcanic hazards. The Asahidake edifice consists of a pyroclastic cone whose growth is associated with lava effusions since the late Pleistocene. The eruption rate was relatively high, with an estimated 1.0 km³ dense rock equivalent (DRE)/ky from 15 ka to 9 ka that rapidly decreased to 0.03 km³ DRE/ky from 9 ka to present. After the latest magmatic eruption ca. 5 ka, there was a 2 ky dormant period that was followed by a large-scale phreatic eruption ca. 2.8 ka, which possessed the following eruption sequence. The sequence began with edifice collapse that produced a debris avalanche and formed the Jigokudani horseshoe-shaped crater, followed by phreatic explosions. Lahar flows then effused from many small craters and fissures that had formed in and at the opening part of the Jigokudani crater. This eruptive activity decreased after the eruption sequence. We recognize that the most recent small-scale phreatic eruption occurred ca. 0.7 ka. Although the current fumarole activity is remarkable, it appears that the eruptive activity has declined considerably since the 2.8 ka eruption. Considering the temporal change in eruptive activity, it is possible that the activity of Asahidake volcano has monotonically decreased over the past 10,000 years. However, in the context of volcanic hazard mitigation, it should be noted that small-scale phreatic explosions and/or effusions of lahar similar to the 0.7 ka eruption might potentially occur and endanger tourism activities.