Akihiko Fujinawa

Generation of block and ash flows during the 1990-1995 eruption of Unzen Volcano, Japan

Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990–1995 eruption of Unzen volcano, Japan. Two different types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991–1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows.

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.

Endogenous growth of dacite dome at Unzen volcano (Japan), 1993–1994

A dacite dome at Unzen volcano grew mainly exogenously when it was small and the effusion rate was high, but endogenously when the dome became large and the effusion rate declined. The endogenous dome that has grown since late 1993 shows a shape classified as “Pelean,” whereas the earlier stage had a “low lava dome” shape. The carapace of the endogenous dome moved like the crust of a basaltic lava pillow, although the dome is several hundred times larger than such pillows. The surface carapace was carried from the inside of the dome where it had been produced, and it thickened as it cooled. The crater floor was strongly deformed by the advancing endogenous dome. The movement and crater floor deformation can be compared to that of a tractor tread moving on unconsolidated ground.

Tholeiitic and calc-alkaline magma series at Adatara volcano, northeast Japan: 1. Geochemical constraints on their origin

Abstract Low-alkali [“island arc” by Jakes and Gill (1970) or “low-K” by Gill (1978)] tholeiitic rocks are closely associated with calc-alkaline rocks in the outer volcanic zone (called the Nasu volcanic zone) of northeast Japan. Major-element, trace-element (Ni, Cr, Co, Sc, V, Zr, Nb, Zn, Rb, Cs, Ba, Sr, Ce and Y) and Sr isotope compositions are presented for 9 low-alkali tholeiitic and 16 calc-alkaline samples of Adatara volcano belonging to the Nasu volcanic zone. In the trace elements vs. SiO 2 diagram the tholeiitic series and calc-alkaline series show different trends to each other for Co, Sc, V, Zr, Rb, Sc, Ba and Sr. As expected from K 2 O contents, the calc-alkaline samples are enriched in incompatible elements such as Rb, Cs, Ba, Ce and Zr compared with the tholeiitic samples. On the other hand, contents of Nb in the calc-alkaline samples are somewhat lower than those in the tholeiitic samples. Plots of Ni and Cr contents vs. SiO 2 for the tholeiitic samples come near smooth trend lines whereas those for the calc-alkaline samples are scattered. Sr isotopic ratios in the tholeiitic series lie within the range between 0.7056 and 0.7058 and between 0.7048 and 0.7052 in the calc-alkaline series. Smooth major- and trace-element compositional trends in the tholeiitic series are successfully modelled by simple fractional crystallization of plagioclase, hypersthene, augite, titanomagnetite and minor olivine. Similarly, crystal fractionation models can closely reproduce major- and most of the trace-element variations in the calc-alkaline series, but failed to account for irregular variations of Ni and Cr. The scattering of Ni and Cr probably reflects simultaneous crystal fractionation and (subordinate) magma mixing in the subvolcanic chamber. The inferred fractionated phases for the calc-alkaline series are richer in augite and titanomagnetite and slightly poorer in plagioclase than those for the tholeiitic series. The tholeiitic magma and calc-alkaline magma are not derived from a common parental (tholeiitic) magma and the calc-alkaline parental magma is richer in SiO 2 and incompatible elements (in particular Rb and Zr), poorer in radiogenic Sr and possibly more oxidized than the tholeiitic parental magma.

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.

Tholeiitic and calc-alkaline magma series at Adatara volcano, Northeast Japan: 2. Mineralogy and phase relations

Abstract Low alkali [low-K by Gill (1978) or island arc by Jakes and Gill (1970)] tholeiitic magma suites are closely associated with [medium-K by Gill (1978)] calc-alkaline suites in the outer volcanic zone (called Nasu volcanic zone; NVZ) of Northeast Japan. Chemistry and phase relations of phenocrysts are studied for representative 5 low-alkali tholeiitic and 8 calc-alkaline rock samples of Adatara volcano in NVZ, in order to examine the mineralogical evolutionary mechanisms of the two series of magmas. Also clarified are differences in the magmatic temperature and oxygen fugacity between the two magma series. The correlations between whole rock chemical compositions and mineralogy in the tholeiitic suite at this volcano are well-documented, supporting the argument that the members of this suite are related through fractional crystallization of observed phenycrystic phases (Part 1; Fujinawa, 1988). Mineralogical observations in the calc-alkaline suite are compatible with the views based on their whole rock chemical compositions; although fractional crystallization process is dominant in the evolution of the calc-alkaline magma, intermittent mixing of (small amounts of) magnesian high-temperature magmas operates concurrently. Temperature estimates with the two-pyroxenes geothermometer range from 1080°C to 970°C in the tholeiitic suite, whereas from 990° to 850°C in the calc-alkaline suite. Temperature-oxygen fugacity trends indicate that the low-alkali tholeiitic magma evolves under conditions of higher temperature and lower oxygen fugacity than the calc-alkaline magma. In view of the fact that the hyperstene and augite phenocryst cores in the calc-alkaline samples are compositionally distinguishable from the relevant phases in the tholeiitic samples, it is unlikely that magma mixing played an essential role in the generation of the calc-alkaline suite at Adatara volcano.

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.

High-Na dacite from the Jean Charcot trough (Vanuatu), Southwest Pacific

Abstract Rifting that produced the Jean Charcot Trough (JCT) in the Vanuatu back-arc region commenced 2–3 Ma, leading to SiO 2 -bimodal magmatism in the northern half of the trough. Dacite with high Na 2 O (>6.0%) and low K 2 O ( Zr Rb >10 , Nb Rb >0.5 , Y Rb >2.0 , Nb La >0.5 , and Ba Zr ), but are quite different to lavas from the active Vanuatu volcanic arc. The dacites are compositionally close to oceanic plagiogranite produced during fractional crystallization of an N-MORB basaltic magma, and mass balance calculations show that they may be similarly derived from the associated basalts. We show that JCT basalts are derived from a mantle source compositionally very close to that beneath the central spreading axis of the NFB, with minimal contamination by a subduction component. Westward injection of diapiritic source mantle similar to that of the NFB back-arc basin basalts may have caused rifting of the JCT. During incipient back-arc rifting, high Na/K felsic magmas may have evolved from parental low-K basalts via side-wall crystallization within a large lower crustal magma chamber.

Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure

Recent studies hypothesize that some submarine slides fail via pressure-driven slow-slip deformation. To test this hypothesis, this study derives pore pressures in failed and adjacent unfailed deep marine sediments by integrating rock physics models, physical property measurements on recovered sediment core, and wireline logs. Two drill sites (U1394 and U1399) drilled through interpreted slide debris; a third (U1395) drilled into normal marine sediment. Near-hydrostatic fluid pressure exists in sediments at site U1395. In contrast, results at both sites U1394 and U1399 indicate elevated pore fluid pressures in some sediment. We suggest that high pore pressure at the base of a submarine slide deposit at site U1394 results from slide shearing. High pore pressure exists throughout much of site U1399, and Mohr circle analysis suggests that only slight changes in the stress regime will trigger motion. Consolidation tests and permeability measurements indicate moderately low (~10−16–10−17 m2) permeability and overconsolidation in fine-grained slide debris, implying that these sediments act as seals. Three mechanisms, in isolation or in combination, may produce the observed elevated pore fluid pressures at site U1399: (1) rapid sedimentation, (2) lateral fluid flow, and (3) shearing that causes sediments to contract, increasing pore pressure. Our preferred hypothesis is this third mechanism because it explains both elevated fluid pressure and sediment overconsolidation without requiring high sedimentation rates. Our combined analysis of subsurface pore pressures, drilling data, and regional seismic images indicates that slope failure offshore Martinique is perhaps an ongoing, creep-like process where small stress changes trigger motion.