We have carried out paleomagnetic analyses of tephras from the Taupō eruption, one of the most violent eruptions on Earth in the past 5000 years. Pyroclastic deposits were collected with 7 cm³ cubes pushed into each unit of the Taupō eruption sequence, consisting of airfall units and overlying ignimbrite. Where possible, we targeted fine-ash layers and matrix, as lapilli sized material can significantly affect the quality of the analysis. The samples were oriented using a collection device specially designed to maximize accuracy. All samples were subjected to alternating field demagnetization, while samples from Taupō ignimbrite (Y7)—the only unit deposited hot were also subjected to thermal demagnetization. The characteristic remanent magnetizations (ChRMs) for specimens from unit Y1, the lower and upper parts of unit Y4, and unit Y7 are well determined (α₉₅ < 3.3°). The declinations and inclinations of site-mean ChRMs range from 3.0° to 7.1° and − 53.4° to − 56.2°, respectively, in close agreement with published results from lithic fragments of the Taupō ignimbrite (Y7). The mean ChRM of unit Y3 does not fit within 95% confidence limits of the ChRM of other units. We presume this is a consequence of unit Y3 samples containing relatively coarse grains and of probable secondary process of the deposit. This outlier aside, our results show consistency between thermoremanent magnetizations of ignimbrite and detrital remanences of co-eval ashfalls, thus validating our method for further tephra research. Both geological observations and paleomagnetic estimation using angular difference suggest that the duration of the Taupō eruption sequence was less than a few tens of years. Furthermore, matching the overall mean ChRM direction (Dec = 4.3°, Inc = − 55.3°, α₉₅ = 1.3°, N = 38 specimens) to the New Zealand paleosecular variation record using the MATLAB dating tool, most likely supports a younger age (ca. 310 CE) than the reported wiggle match eruption age of 232 ± 10 CE. Graphical Abstract

Some calderas are geometrically complex that may be related not to a single eruption, magma body, or structure. In order to reveal their forming processes, multidisciplinary methods should be applied. Akan volcano has E-W elongated and irregular-shaped caldera (24 × 13 km), implying a complex mechanism of formation. Akan caldera results from successive explosive eruptions from 1.4 to 0.1 Ma. On the basis of duration of dormancy and petrological features (mainly whole-rock major element compositions) of juvenile materials, these eruptions have been grouped into 17 eruptive groups (Ak1–Ak17), each of which consists of a single or a sequential phase. In order to investigate the processes of caldera formation, we focus on the younger eruptive groups (Ak1–Ak7: 0.8 to 0.2 Ma) that have relatively large magnitudes (>10 km³) and likely control the present caldera shape. We performed component analysis of lithic fragments from pyroclastic fallout deposits, whole-rock trace element analysis of juveniles, and gravitational survey of the caldera. We grouped Ak1–Ak7 into three types, namely, type A (Ak1, Ak2), type B (Ak3–Ak5), and type C (Ak6, Ak7), based on the lithic componentry, most of which are accessary and accidental fragments from vent and conduit areas. The characteristic lithic component in each type is as follows: altered rock in type A, aphyric dacite in type B, and pyroxene andesite in type C. These data suggest that explosive eruptions of each type are derived from distinct sources. The whole-rock composition of juvenile materials of each type also shows distinct trends on Harker diagrams. These three chemical trends are nearly parallel, suggesting that three different magma systems were active. This is consistent with the lithic componentry showing three spatially distinct vent sources. The geological and petrological evidence is supported by a Bouguer anomaly map. Akan caldera is characterized by three circular closed minima, indicative of three depressed segments that correspond to the source regions, each of which possibly discharged the three types of magma. Caldera-forming eruptions of Akan caldera occurred from at least three distinct sources with distinct magma systems. In conclusion, Akan caldera is a composite caldera, and its shape reflects the distribution of multiple source regions. The case study of Akan caldera shows a possible time-space evolutionary pathway for a caldera complex where several smaller calderas are nested.

Vulcanian activity is one of the most common eruption styles of arc andesitic volcanism on Earth. It ejects and deposits volcanic bombs around the source crater. Although paleomagnetic studies of volcanic bombs are limited, such studies can potentially provide more opportunities for high-resolution paleomagnetic dating of volcanic activity. In this study, paleomagnetic dating was applied to large (> 1 m) volcanic bombs around active craters in the Azuma volcano group, NE Japan. Oriented samples were collected from the interior parts of five large volcanic bombs situated on gentle slopes, a few hundred meters from the source crater. More than six core samples were collected from each bomb and all samples were subjected to a range of rock magnetic experiments, including anisotropy of magnetic susceptibility (AMS) and thermal/alternating field demagnetization (THD/AFD) analyses. The Characteristic Remanent Magnetization (ChRM) directions for specimens from all bombs were well-defined, have small α₉₅ (< 2.5º), and are in close agreement with each other. Comparing our measured overall mean direction (D_m = 355.5º, I_m = 49.8º, α₉₅ = 1.6º) with modeled geomagnetic field estimates and a reference secular variation curve for this area (using MATLAB-based archaeomagnetic dating tool), we suggest that the volcanic bombs were produced in the historical Meiji period (1893–1895 CE) eruption. In addition, a combination of the data of ChRM, AMS, thermomagnetic analyses, hysteresis measurement, and XRF analysis indicates that the volcanic bombs were derived from a plug of lava in the conduit under the solidification point (ca. 800 °C), but above the Curie point of the titanomagnetite remanence carrier (around 300 °C). We show that volcanic bombs can be powerful for paleomagnetic dating if certain sampling conditions, such as quantity, situation, size and portion are satisfied. Graphical Abstract

To constrain the age and duration of large-scale caldera-forming eruptions, we measured paleomagnetic directions of pyroclastic deposits from the 30 ka Aira caldera eruption sequence, and the 60–45 ka Iwato pyroclastic flow deposits around Aira caldera in southern Kyushu, Japan. The 30 ka Aira caldera eruption sequence consists of Osumi pumice fall (OS), Tarumizu pyroclastic flow deposit (TM), Ito ignimbrite (IT) and co-ignimbrite ash (AT), in ascending order. Oriented samples were collected by drilling for OS, TM and welded ignimbrites, and by cubing for non-welded (unconsolidated) pyroclastic flow deposit. We systematically sampled Ito ignimbrite with the degree of welding as: non-welded (IT1), moderately welded (IT2), and strongly welded (IT3) facies that is limitedly distributed in north of Aira caldera. Mean paleomagnetic directions of OS, TM, IT1 and IT2 are consistent with those previously reported for AT and welded facies of IT. Although OS samples were collected from multiple pumice clasts at proximal locations, we obtained well-defined mean paleomagnetic directions corresponding to those of co-eruptive pyroclastic flow (TM). This suggests that even clast-supported and non-welded pumice fall can retain thermoremanent magnetization at the time of deposition. Significance tests for our mean paleomagnetic directions showed that of all sequential units of the Aira caldera-forming eruption deposits, only IT3 has a different paleomagnetic direction. Based on reported paleosecular variation changing rates in Japan, and on the angular difference associated with error of the two paleosecular directions, we estimate the time gap between IT3 and IT2, to be 24.3 ± 16.3 years. A conductive cooling model explains this time difference as due to differences in cooling time between quickly, and slowly cooled parts of a thick single ignimbrite unit. Although the possibility that multiple flow units occurred within tens of years of each other cannot be excluded, there are no observations of clear flow unit boundaries in the Ito ignimbrite to support the suggestion that the Aira caldera-forming eruption sequence was deposited within a shorter time than years. In addition, the age of Iwato pyroclastic flow deposit, which has been situated between 60 and 45 ka, was estimated by calculating the angular distance between the mean paleomagnetic direction and the modeled secular variation curve from the GGF100 ka. As the result, we determine the most probable age to be around 56 ka. Graphical abstract

Atosanupuri Volcano is one of the active volcanoes, located in the Akan-Shiretoko Volcanic Chain, eastern Hokkaido. Tephro-stratigraphic and tephro-chronologic studies were conducted on two drill cores and new outcrops around the volcano to reveal a recent history of the explosive eruptions. Widespread tephra layers, such as the 10th century B-Tm tephra, and the 2.7 cal. ka BP Ta-c2 tephra were distinguished by the geochemical composition of volcanic glass. In addition, 14 C ages of organic samples underlying the eruption deposits were obtained. Using these data, we identified seven phreatic eruption deposits, At-ph1 to At-ph7, overlying the T-c2 tephra; these include the previously described At-a and At-b tephras. At-ph7 yields the oldest 14 C calibrated age (2.5-2.7 cal. ka), and the deposition ages of At-ph6 to At-ph1 range from cal. AD 554 to 1678. At-ph4 (At-b) is the thickest phreatic ejecta, with a volume of ~3 × 10 6 m 3. Five phreatic eruptions occurred from Atosanupuri Volcano between 1,500 and 1,000 cal. BP. The youngest eruption, At-ph1 (At-a) occurred at 300 ~ 400 cal. BP, which might be documented in historical records.