Fukashi Maeno

Heat flow in the Lesser Antilles island arc and adjacent back arc Grenada basin

Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimentation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to 15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are similar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present-day volcanism is confined to the region with the highest heat flow.

The outline of the 2011 eruption at Shinmoe-dake (Kirishima), Japan

The climactic phase of the 2011 eruption at Shinmoe-dake was a mixture of subplinian and vulcanian eruptive events, successive lava accumulation (lava dome) within the crater, and repetition of vulcanian events after the dome growth. It was preceded by inflation and elevated seismicity for about one year and by phreatomagmatic explosions of one week before. Small pyroclastic flows and ash-cloud surges formed during the subplinian events, when the eruption column reached the highest level and the vent was widened. A lava dome, which was extruded close to the vent of subplinian events, grew by swelling upward and filling the crater. After the vent was covered by the lava, an intense vulcanian event occurred from the base of the dome and the swelled dome became deflated. After that, vulcanian events were repeated for three months. Simultaneous eruption styles in the crater (vulcanian events, continuous ash emission and dome growth) and some phreatomagmatic events in the vulcanian stage probably are due to a complex upper-conduit system developed in water-saturated country rock.

Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340

IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of preexisting low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or microfaulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits composed of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution data set to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes.

Ballistic ejecta and eruption condition of the vulcanian explosion of Shinmoedake volcano, Kyushu, Japan on 1 February, 2011

The physical condition of the 1 February, 2011, vulcanian explosion at Shinmoedake volcano, Japan, is estimated based on the size of impact craters created by ballistic ejecta, using a ballistic trajectory model and a scaling law for impact crater formation. The initial velocity, impact velocity and mass of ejecta were estimated at 240–290 m/s, 140 ± 20 m/s and 1–3 ton, respectively. The gas mass fraction at the source was calculated to be 0.04–0.1, using the initial velocity and a theoretical model of vulcanian explosion. This gas mass fraction is higher than the petrologically estimated value for pre-eruptive magma. Low-angle jets from the explosion and the estimated depth and size of a pressurized gas region suggest a shallow source inside the lava dome. The observation and results imply that segregation and accumulation of gas in a shallow conduit played a role in an increase of excess pressure immediately below the dome surface, prior to the vulcanian explosion.

Precursory activity and evolution of the 2011 eruption of Shinmoe-dake in Kirishima volcano—insights from ash samples

After a precursory phreatic stage (2008 to 2010), the 2011 Shinmoe-dake eruption entered a phreatomagmatic stage on January 19, a sub-Plinian and lava accumulation stage at the end of January, a vulcanian stage in February–April, and a second phreatomagmatic stage in June–August. Component ratio, bulk composition, and particle size of the samples helped us define the eruptive stages. The juvenile particles were first found in the January 19 sample as pumice (8 vol%) and were consistently present as scoria and pumice particles thereafter (generally ~50 vol%, decreasing in weaker events). The January 19 pumice has water-quench texture. After the lava accumulation, particles of that lava origin came to account for 30~70 vol% of the ash. The second phreatomagmatic stage is proposed because of fine ash and long eruption period. The SiO2 contents of bulk ash are lower in post-January 19, 2011 eruptions, reflecting lower average SiO2 contents in 2011 ejecta than in past ejecta. The free-crystal assemblages were two pyroxenes + plagioclase + Fe-Ti oxides until 2010; olivine joined the assemblage in 2011, when juvenile ash was erupted. This change is consistent with the absence or smaller sizes of olivine phenocrysts in past ejecta.

Numerical simulation of tsunamis generated by caldera collapse during the 7.3 ka Kikai eruption, Kyushu, Japan

The relationship between tsunamis and scales of caldera collapse during a 7.3 ka eruption of the Kikai volcano were numerically investigated, and a hypothetical caldera collapse scale was established. Wave height, arrival time, and run-up height and distance were determined at some locations along the coastline around Kikai caldera, using non-linear long-wave equations and caldera collapse models using parameters showing the difference in geometry between pre- and post-collapse and the collapse duration. Whether tsunamis become large and inundations occur in coasts is estimated by the dimensionless collapse speed. Computed tsunamis were then compared with geological characteristics found in coasts. The lack of evidence of tsunami inundation at Nejime, 65 km from the caldera, suggests that any tsunamis were small; indicating that the upper limit of dimensionless caldera collapse speed was 0.01. On the other hand, on the coast of the Satsuma Peninsula, 50 km from the caldera, geological characteristics suggests that tsunamis did not inundate, or that even if tsunamis inundated the area, the traces of a tsunami have been eroded by a climactic pyroclastic flow or the tsunami itself and they have not been left. In numerical computations, when a dimensionless caldera collapse speed is more than 0.003, tsunami can inundate this area.

Late Pleistocene stratigraphy of IODP Site U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles

Marine sediments around volcanic islands contain an archive of volcaniclastic deposits, which can be used to reconstruct the volcanic history of an area. Such records hold many advantages over often incomplete terrestrial data sets. This includes the potential for precise and continuous dating of intervening sediment packages, which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores. Here we discuss a marine record of eruptive and mass-wasting events spanning ∼250 ka offshore of Montserrat, using new data from IODP Expedition 340, as well as previously collected cores. By using a combination of high-resolution oxygen isotope stratigraphy, AMS radiocarbon dating, biostratigraphy of foraminifera and calcareous nannofossils, and clast componentry, we identify five major events at Soufriere Hills volcano since 250 ka. Lateral correlations of these events across sediment cores collected offshore of the south and south west of Montserrat have improved our understanding of the timing, extent and associations between events in this area. Correlations reveal that powerful and potentially erosive density-currents traveled at least 33 km offshore and demonstrate that marine deposits, produced by eruption-fed and mass-wasting events on volcanic islands, are heterogeneous in their spatial distribution. Thus, multiple drilling/coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands. This multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during Expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands.

The relationship between eruptive activity, flank collapse, and sea level at volcanic islands: a long-term (>1 Ma) record offshore Montserrat, Lesser Antilles

Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass-wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (?930 ka to ?900 ka, ?810 ka to ?760 ka, and ?190 ka to ?120 ka) that coincide with periods of increased volcano instability and mass-wasting. The youngest of these periods marks the peak in activity at the Soufriere Hills volcano. The largest flank collapse of this volcano (?130 ka) occurred towards the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufriere Hills edifice, and their timing also coincides with periods of rapid sea-level rise (>5 m/ka). Available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea-level rise, but this is not observed for volcanoes in intra-plate ocean settings. We thus infer that rapid sea-level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean-island settings.

Morphological evolution of a new volcanic islet sustained by compound lava flows

We investigated the creation of a volcanic islet and emplacement of lava flows in the sea by analyzing data from the island-forming eruption at Nishinoshima, Japan, that has been continuing since November 2013. Aerial observations and satellite images were used to perform a quantitative analysis of the eruption processes. The most intriguing characteristic of the lava flows is the development of lobes and tubes from breakouts and bifurcations of andesitic ‘a’ā-type lava flows. Internal pathways that fed lava to the active flow front were eventually developed by crust solidification and dominated the lava transport. The average discharge was ~2 × 10 5 m 3 /day, and the total volume of erupted material reached ~0.1 km 3 at the end of February 2015. Fractal analysis of the lava-flow margins suggests that the growth pattern is self-similar, with a fractal dimension (D) of ~1.08–1.18, which is within the range of subaerial basaltic lava flows. The morphological evolution of Nishinoshima is controlled primarily by effusion of lava with an apparent viscosity of 10 4 –10 6 Pa∙s, average discharge of ~2.3 m 3 /s, and eruption duration lasting ~2 yr. Our data and analyses suggest that the effect of lava coming in contact with seawater, as well as the variations in the lava discharge rate on local and overall scales, are important factors affecting the development of crust and the lava transport system.

Unconfined slumping of a granular mass on a slope

This study investigates the gravitationally driven dynamics of dense granular materials, released from rest and allowed to flow down a slope until they stop moving. Laboratory experiments were performed in which a measured volume of material was released from rest in a cylindrical tube and spread across an unconfined rigid plane inclined at angles less than the angle of repose. Upon release, the particles initially spread outward radially. However, up-slope motion is rapidly suppressed while down-slope motion is promoted, which leads to an approximately ellipsoidally shaped deposit once the flow has been fully arrested. The flows were modeled under the shallow layer approximation and integrated numerically to capture the motion from initiation to final arrest. In modeling, two types of Coulomb-type friction models were employed. One had a constant friction coefficient, and another had a friction coefficient that depends upon the dimensionless inertial number of the motion. When the initial aspect ratio of a granular mass is small and the slope angle is low (<5°), the model with a constant friction coefficient can capture the shape of the deposit. However, when the slope angle is increased, the friction model that is dependent on inertial number becomes more important. For granular columns of initially high aspect ratios, the shallow water model fails to reproduce some aspects of the experimental observations. Finally, the dependence of the shape and depth of the deposit upon dimensionless parameters that characterize the system is examined under the constant friction coefficient model, demonstrating that the deduced scaling arguments are borne out by the numerical simulations and laboratory data.