The 2.5-kilometer-wide summit crater of Pinatubo volcano (1486 m asl), located about 90 km northwest of Metro Manila, central Luzon, was formed by a climactic eruption on June 15, 1991. In 2014, we can climb to the summit along the O'Donnell River on the northern slope within one day. This photo was taken in April 2006 from northern crater rim where this route terminates. The lake extends behind the cliff to the left. After the climactic eruption, a lava dome formed within the crater, which has been submerged in the lake. The lake water is an impressive cobalt blue. Hot springs are discharging from the bottom of the lake and the shore. The significant fracture rock mass in the background is the remaining part of the lava dome formed before the eruption. The people on the lakeshore in the foreground are tourists with a guide enjoying a beautiful crater-lake. (Photograph and explanation: Mitsuru OKUNO and Tetsuo KOBAYASHI)
This paper examines changes to the river system and faulting in the Ikachi Basin and surrounding area in the southwestern part of the Chugoku Mountains since the Middle Pleistocene, based of an investigation of the fluvial terrace and tectonic landforms. Fluvial terrace surfaces in the study area are classified into five levels: H, M1, M2, L1, and L2, in descending order. The M1 terrace surface is widely observed in the Ikachi Basin, and there is a narrow band of Sanbe-Kisuki tephra on the top layer of the terrace deposit, suggesting that the surface was formed around 110-115 ka. Aira-Tn tephra is observed in the L2 terrace deposit, indicating that it was formed around 30 ka. The distribution of terrace and deposit indicates the existence of the Paleo-Shiwari River, which differed from the river system existing today. The Paleo-Shiwari River flowed northwestward from the southeastern margin of the Ikachi Basin, and from near Hizumi, westward through the basin. There is a possibility that the upper reaches of the Paleo-Shiwari River reached Yashiro Island. The Paleo-Shiwari River lost its upper reaches as a result of river capture around the current Obatake-Seto in Middle Pleistocene. Furthermore, as a result of continued large-scale uplifting in the downstream area of the Paleo-Shiwari River basin, accompanied by activities of the Hizumi and Oguni faults since the Middle Pleistocene, the height of the riverbed of the Paleo-Shiwari River increased and its riverbed slope became gentle. At the same time, continued large-scale subsidence with faulting from the downstream basin of the Yuu River to Aki-Nada led to a gradual steepening of the riverbed of the Yuu River, and the valley head of the Yuu River along the fracture zone expanded due to erosion. Subsequently, the Paleo-Shiwari River was captured by the Yuu River at the Hizumi depression around 110-115 ka during the formation period of the M1 surface. It is concluded that river capture between the Yuu River and the Shiwari River occurred due to the influence of crustal movements.
Tephras interbedded with Holocene sediments in coastal lowlands along the Sanriku Coast, northeast Japan are described, and well-known widespread and local tephras are correlated based on morphology, refractive index, and chemical composition of volcanic glass shards, stratigraphy, and radiocarbon age. As a result, tephras that are correlated with Towada-a (To-a), Towada-Chuseri (To-Cu), Kikai-Akahoya (K-Ah), Oguni pumice, Towada-Nambu (To-Nb), and Hijiori-Obanazawa (Hj-O) are identified. In particular, To-Cu is distributed throughout the study area and is recognized to be a useful key tephra. In addition, To-a and Oguni pumice were discovered for the first time on the Sanriku Coast. The results suggest that the morphologies and the refractive indexes of volcanic glasses are useful and important for distinguishing tephras and correlating with widespread and local tephras on the Sanriku Coast.
The process of terrace formation in Japan is discussed in the context of advances in tephrochronology. In particular, the ages of fluvial and marine terraces correlate with climate and glacio-eustatic sea-level changes. However, previous studies do not distinguish between the influences of climate changes and base-level changes (glacio-eustatic sea-level changes) for terrace formations, and mainly target northeast Japan due to advantages related to tephrochronology and terrace development. Therefore, this study focuses on the Ohmi Basin, Shiga Prefecture, southwest Japan, to reveal the process of terrace formation under conditions with no base-level changes, and compares them to those in northeast Japan. To achieve this aim, a drilling survey at terrace surfaces and a cryptotephra analysis are conducted. Widespread tephras, Kikai-Akahoya (K-Ah) tephra, Aira-Tn (AT) tephra, and Kikai-Tozurahara (K-Tz) tephra, are used to correlate fluvial terraces and establish the chronology of fluvial terraces in the eastern and western parts (Koto and Takashima regions, respectively) of the Ohmi Basin. Terrace correlation shows that terraces formed during Marine Isotope Stage (MIS) 2 are distributed in both regions under different tectonic settings. This indicates that climate change is the main factor of terrace formation in the Ohmi Basin. Therefore, river conditions during MIS 1, 2, and 5 are compared, and influences of climate changes and crustal movements for terrace formation are estimated. As a result, terrace formation in the Takashima region is explained by climate changes and fault movements. On the other hand, terrace formation in the Koto region is explained by climate changes and tectonic tilting. Consequently, these results suggest that the fluvial terraces in the Ohmi Basin are climatic terraces and that older to younger terrace steps could be the result of a combination of climate changes and crustal movements without base-level changes.
Small-scale tectonic landforms are identified from detailed aerial photograph interpretations, in order to clarify evidence of faulting since the late Pleistocene period along the marginal fault zone at the western foot of the Suzuka Mountains, central Japan. Surface fault traces at least 9-km long are recognized in the central part of this fault zone. Along the Uso River, small tectonic scarps are recognized on young fan terraces where assumed faults were identified previously by seismic reflection profiling. These scarps suggest that the most recent movement at the fault occurred during or after the late Pleistocene period. Progressive vertical displacement is recognized in the subsurface structure of Kobiwako Group, indicating that the fault zone has been active since the Plio-Pleistocene period. This fault may extend further to the north and south in the western foot area of the Suzuka Mountains.
The Irosin caldera and Bulusan volcano in the Bulusan Volcanic Complex (BVC) are located at the southeastern end of Luzon Island, the Philippines. This is an overview of all of the papers in the special issue “Geology and Recent Eruptions of Irosin Caldera and Bulusan Volcano, Southern Luzon, Philippines: Parts I and II” and related studies. Moriya (2014) tentatively outlines the evolution of 84 volcanoes on the Philippine Islands. Kobayashi et al. (2014a, b) establish the sequence of caldera-forming eruptions. Kobayashi (2014) compares this sequence with that of the Aira caldera in Japan. Danhara et al. (2014) describe the petrography of Irosin ignimbrite and related deposits. Komazawa et al. (2014) reveal a semi-circular feature with a steep gravity gradient in the Bouguer anomalies. Takashima and Kobayashi (2014) obtain 4 thermoluminescence (TL) ages of 36 ± 8, 38 ± 10, 33 ± 8, and 45 ± 10 ka for ignimbrite and co-ignimbrite ash falls. Mirabueno et al. (2014) identify 1 pyroclastic flow and 12 fallout deposits, including 5 possible fallout deposits, intercalated with lahars and fluvial sediments within the caldera. Kinoshita and Laguerta (2014) introduce image recording of volcanic clouds for managing volcanic hazards. Delos Reyes et al. (2014) present the distribution and petrography of fallout tephra from the 2006-2007 eruptions. Taguchi et al. (2014a) report hot and cold springs around Bulusan volcano.
The eruption sequences of two caldera volcanoes, Aira in Japan and Irosin in the Philippines, were examined, and an eruption sequence for caldera volcanoes is constructed as follows: first, eruptions of felsic magma as a precursory event; second, a plinian eruption associated with intra-plinian flows followed; third, a fine-grained ignimbrite eruption occurred, which is followed by a catastrophic caldera-forming eruption; fourth, voluminous co-ignimbrite ash was generated and dispersed over a wide area; and, finally, post-caldera volcanoes were formed. In addition, intense earthquakes occurred either at the late stage of the plinian phase of the Irosin caldera or shortly before the final caldera-forming eruption of the Aira caldera. A similar eruption sequence was observed for the 7.3 cal ka BP caldera-forming eruption at Kikai caldera in Japan. Therefore, the eruption sequences observed at caldera volcanoes in Japan and the Philippines are considered to form the most fundamental processes of a caldera-forming eruption.
Core drilling at Site IrBH-2 within the Irosin caldera in Sorsogon Province, southern Luzon reached a depth of 50 m. Systematic logging and documentation were carried out to describe and interpret the sediments. The accelerator mass spectrometer (AMS) radiocarbon dates obtained from plant fragments at 7.02-10.40-m depth were 1000 to 1800 BP. Lahars and fluvial deposits were the predominant deposits in the core sequence. The upper 12 m consisted mostly of andesitic fluvial and minor lahar deposits. These deposits may be correlated with the reworking of eruptive products from resurgent andesitic volcanism. One pyroclastic flow and 12 fallout deposits, including five possible fallout deposits, were intercalated with reworked sediments at depths of 12-50 m. The refractive index of representative samples indicated that post-caldera eruptions involved mainly andesite to dacite, with minor rhyolite magmas. The rhyolite fallout in the core had similar petrographic characteristics to the 41 cal kBP Irosin ignimbrite, suggesting that the fallout and the ignimbrite were sourced from the same magma.
The Bulusan volcano is one of the active volcanoes in the Philippines, and is located in southern Luzon. In 2006 and 2007, 19 and 7 distinct phreatic eruptions of the volcano occurred, respectively. Each discrete event produced tephra that was dispersed by prevailing winds mainly to the west or southwest of the summit vent, blanketing portions of the western Sorsogon Peninsula. Some events were recorded as explosion-type earthquakes. Whenever possible, fallout tephra deposits were mapped and sampled to determine the volume and composition of material produced from each eruptive event. Based on the dispersal maps, the average volume was estimated to be 105 m3. No juvenile magma were detected in ash and lithic samples by petrographic and X-ray fluorescence analysis. Therefore, all analyzed samples were considered products of phreatic eruptions.
The methods, results, and prospects of image recording of volcanic clouds are discussed by considering two volcanoes in the Philippines, Mayon and Bulusan. At Mayon volcano, video and network cameras are utilized for automatic time-interval long-term recordings. Near-infrared and night-shot modes in addition to the conventional visible mode are adopted there. Since the inception of recording in June 2003, the daily activity of the volcano was the almost continuous ejection of white vaporous plumes. Explosive eruptions with lava ejections that occurred in July–August 2006 and December 2009 were recorded during both daytime and nighttime. Near-infrared and night-shot modes were very effective for observing flows and falls of hot lava and also the foreboding indication of hot lava glow at the summit crater some months earlier at nighttime. At Bulusan volcano, video recording using a digital high-vision video camera with smooth interval mode began in November 2010. Records of explosive eruptions in late 2010 were obtained during daytime at a fixed point located 23 km from the crater.