Microgravity and height changes caused by volcanic activity: Four

Microgravity measurements and levelling surveys on volcanoes are not always easy to make, but are useful for studying volcanic processes quantitatively. Gravity changes associated with volcanic activity are not always significant. Precision of microgravity measurements depend critically on the procedures adopted, and those applied in the present paper are described. Levelling technique is now orthodox, and some empirical laws relating ground deformation to volcanic activity are deduced from the accumulated data. Gravity changes occur at the same time and places as ground deformations. The relationship between microgravity and height changes are discussed from the standpoint of analyzing the data obtained on volcanoes. The observational results obtained on four volcanoes in Japan are separately analyzed because each volcano exhibits different patterns of gravity changes and deformations. During the 1977–1982 activity of Usu volcano, deformation was accompanied by microgravity changes frequently observed at a particular benchmark at the base of the volcano for about five years. The gravity changes prove to be not a direct effect of magma movements but to be caused by the deformations of ground strata and aquifers around the benchmark. The 1983 eruption of Miyakejima volcano was associated with local gravity changes around the eruptive fissures due to magma intrusion which was approximately modelled. Similarly the 1986 eruption of Ooshima volcano caused gravity changes on the volcano, but these were poorly correlated with elevation changes and their origins were not uniquely interpreted. To detect gravity changes associated with the activity of Sakurajima volcano, an equigravity point was selected at the north of the volcano besides the gravity points on and around the volcano itself. The probable gradual accumulation of magmas beneath the volcano for eight years is substantiated by observed microgravity and elevation changes.     

The subsurface structure of the AIRA caldera and its vicinity in Southern Kyushu, Japan

Southern Kyushu, Japan, includes a chain of large and small calderas and active volcanoes, and the greatest part of it is covered with thick pyroclastic ejecta. The regional and local structures of this area are discussed from the standpoint of physical volcanology, with consideration of all available data.The regional structure of this area is examined in the light of gravity and geomagnetic anomalies. Two layers of the earth's uppermost crust are defined by spectrum analysis of the gravity anomalies. These two layers are identical with the two identified by seismicwave velocities. The Bouguer gravity anomalies are relatively high and rather monotonous over outcrops of the Mesozoic basement and the granite, but are relatively low and perturbed over calderas and caldera-like structures. Two low-gravity anomalies in Kagoshima Bay are remarkable. One is circular, with its center on the Aira caldera. The other is elongated between the Satsuma and Oosumi peninsulas. The southern end of the latter anomaly is occupied by the Ata caldera. Discussion of the gravity anomalies of the Aira caldera suggests that the subsurface basement has a funnel shape and is overlain by ‘fallback’. The sub bottom geology of the caldera suggests that it is formed by a few smaller depressions, though the distribution of the overall gravity anomalies is parallel with its shape.The southern part of Kagoshima Bay is characterized by a graben-like topography and low-gravity anomalies and, moreover, by several calderas. The middle part, between the Aira and Ata calderas, may have a graben-like structure. A profile crossing the bay through Sakurajima volcano is modeled on the basis of results from drilling and gravity surveys. The basement has a graben-like structure and is filled with coarse and low-density deposits, and the structure continues northwards to the Aira caldera with a funnel shape.A comparison of this area with the Taupo-Rotorua depression in New Zealand and Lake Toba in Indonesia, leads the authors to the conclusion that such major volcanic depressions may have been formed by amalgamation of a series of caldera-like structures which were formed by multiple violent explosions accompanied by ejection of a tremendous amount of pyroclastic material.     

Crustal deformation caused by the 1914 eruption of Sakurajima volcano, Japan and its secular changes

Analysis of ground-deformation data obtained at and around Sakurajima volcano during the 1914 eruption indicates that the deformation may be interpreted by assuming a model with two pressure sources, one shallow (about 2 km deep) and vertically directive and the other deep (about 8 km deep) and obliquely, directly beneath the volcano. This model is reasonable from the viewpoint of the volcanic processes.The local upheaval near the centers of eruption has scarcely recovered because it surpassed the elastic limit. The recovery of the regional depression after the eruption can be interpreted as pressure accumulation beneath the volcano. It may be concluded that the center of pressure would remain at the deeper source beneath the volcano, but that the pressure would change, resulting in surface deformation. The depression and its recovery suggest the presence of a pressure focus or a ‘magma reservoir’ beneath the volcano.     

The size of the auroral belt during magnetic storms

Using the auroral boundary index derived from DMSP electron precipitation data and the Dst index, changes in the size of the auroral belt during magnetic storms are studied. It is found that the equatorward boundary of the belt at midnight expands equatorward, reaching its lowest latitude about one hour before Dst peaks. This time lag depends very little on storm intensity. It is also shown that during magnetic storms, the energy of the ring current quantified with Dst increases in proportion to L_e^–3, where L_e is the L-value corresponding to the equatorward boundary of the auroral belt designated by the auroral boundary index. This means that the ring current energy is proportional to the ion energy obtained from the earthward shift of the plasma sheet under the conservation of the first adiabatic invariant. The ring current energy is also pronortional to E_mag, the total magnetic field energy contained in the spherical shell bounded by L_e and L_eq, where L_eq corresponds to the quiet-time location of the auroral precipitation boundary. The ratio of the ring current energy E_R to the dipole energy E_mag is typically 10%. The ring current leads to magnetosphere inflation as a result of an increase in the equivalent dipole moment.     

The 1989 submarine eruption off eastern Izu Peninsula,Japan: ejecta and eruption mechanisms

The submarine eruption of a new small knoll, which was named Teishi knoll, off eastern Izu Peninsula behind the Izu-Mariana arc occurred in the evening of 13 July 1989. This is the first historic eruption of the Higashi-Izu monogenetic volcano group. The eruption of 13 July followed an earthquake swarm near Ito city starting on 30 June. There were subsequent volcanic tremors on 11 and 12 July, and the formation of the Teishi knoll on the 100 m deep insular shelf 4 km northeast of Ito city. There were five submarine explosions, which were characterized by intermittent domelike bulges of water and black tephra-jets, which occurred within 10 min on 13 July. Ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria, highly vesiculated pumice, and lithic material. Petrographical features suggest that the pumice was produced by vesiculation of reheated wet felsic tuff of an older formation. The Teishi knoll, before the eruption, was a circular dome, 450 m across and 25 m high, with steep sides and a flat summit. Considerations of submarine topographic change indicate the knoll was raised by sill-like intrusion of 10⁶ m³ of magma beneath a 30 m thick sediment blanket. This shallow intrusion is assumed to have started on 11 July when volcanic tremors were observed for the first time, but there was no indications of violent interaction between wet host sediments and intruding magma. The submarine eruption of 13 July appears to have been Friggered by a major lowering of the magma-column. The basalt scoria, having crystal-contents of more than 60%, is assumed to be derived from the cooled plastic margin of the shallow intrusive body. However, glassy scoria, which would indicate the interaction between hot fluidal magma and external water, was not observed. A scenario for the 1989 submarine eruption is as follows. When rapid subsidence of the hot interior of the intrusive magma occurred, reduced pressure caused the implosion of cooled plastic magma, adjacent pressurized, hot host material, and wet sediment. The mixing of these materials triggered the vigorous vapor explosions.     

Incorporation rate measurements of <Superscript>10</Superscript>Be, <Superscript>230</Superscript>Th, <Superscript>231</Superscript>Pa,and <Superscript>239,240</Superscript>Pu radionuclides in manganese crust in the Pacific Ocean: A search for extraterrestrial material

In order to estimate the deposition rate of extraterrestrial material onto a manganese crust in a search for supernova debris, we analyzed the contents of ¹⁰Be, ²³⁰Th, ²³¹Pa, and ^239,240Pu in a sample of manganese crust collected from the North Pacific Ocean. On the basis of the depth profile of ¹⁰Be, the growth rate of the manganese crust was determined to be 2.3 mm Myr⁻¹. The uptake rates of ¹⁰Be, ²³⁰Th, and ²³¹Pa onto the manganese crust were estimated to be 0.22–0.44%, 0.11–0.73%, and 1.4–4.5%, respectively, as compared to the deposition rates onto the deep-sea sediments near the sampling station, while that for ^239,240Pu was 0.14% as compared to the total inventory of seawater and sediment column. Assuming that sinking particles represent 0.11–4.5% of the uptake rates, the deposition rate of extraterrestrial material onto the manganese crust was estimated to be 2–800 μg cm⁻²Myr⁻¹ according to the uptake of ¹⁰Be onto the manganese crust. Further, our estimate is similar to the value of 9–90 μg cm^{− 2}Myr⁻¹ obtained using the integrated global production rate of ¹⁰Be and the deposition rate of ¹⁰Be onto the manganese crust.     

Geochemical composition of dykes along the Cameroon Line (CL): Petrogenesis and similarities with the Central Atlantic Magmatic Province

We report whole-rock geochemistry and Sr–Nd–Pb isotopic compositions of mafic dykes intruded in the Precambrian granito-gneissic basement complex, exposed at Nyos, Batibo, Dschang and Foumban on the Cameroon Line. The dykes are alkaline (Batibo), transitional (Foumban), and subalkaline (Nyos, Batibo and Dschang) with SiO₂ of 45–54 wt% and MgO of 2–9 wt%, similar to dykes reported in other areas of the Cameroon Line (CL) and the Central Atlantic Magmatic Province (CAMP). The abundances of rare earth elements (REE) and the Primitive Mantle normalised patterns for the Nyos, Batibo and Dschang dykes are similar to those of MORB, indicating that the dykes formed at shallower depths by a higher degree of partial melting relative to the Foumban dykes and the alkaline lavas of the CL. The transitional basaltic dykes with steeper REE patterns have their sources at deeper levels in the lithospheric mantle, possibly the garnet-spinel transition zone and were generated by a lower degree partial melting of the lithospheric and plume components. The Nyos and Batibo subalkaline dykes show similar isotopic compositions with a spectrum extending from depleted (DMM-like) to enriched (EM1-like) mantle, indicating the similarity in their source components. The Dschang dykes show distinct isotopic characteristics with relatively unradiogenic Nd-Pb isotope compositions compared to the Batibo and Nyos dykes. The Foumban transitional dykes with characteristic wide ranges in Sr-Nd-Pb isotopic compositions reveal varying contributions from enriched mantle components (EM1 and EM2) in addition to its plume signature similar to those of CL lavas. The Nyos and Batibo dykes alongside other dykes on the CL have low TiO₂ abundances (<2 wt%), negative PM-normalised Nb-anomalies, and moderately to strongly enriched REE patterns, and isotopic composition that overlaps with those of CAMP, suggesting a similar lithospheric origin.     

Incorporation of U,Pb and Rare Earth Elements in Calcite through Crystallisation from Amorphous Calcium Carbonate: Simple Preparation of Reference Materials for Microanalysis

Uncertainty for elemental and isotopic measurements in calcite by LA‐ICP‐MS is largely controlled by the homogeneity of the reference materials (RMs) used for calibration and validation. In order to produce calcite RMs with homogeneous elemental and isotopic compositions, we incorporated elements including U, Pb and rare earth elements into calcite through heat‐ and pressure‐induced crystallisation from amorphous calcium carbonate that was precipitated from element‐doped reagent solution. X‐ray absorption spectra showed that U was present as U(VI) in the synthesised calcite, probably with a different local structure from that of aqueous uranyl ions. The uptake rate of U by our calcite was higher in comparison with synthetic calcite of previous studies. Variations of element mass fractions in the calcite were better than 12% 2RSD, mostly within 7%. The ²⁰⁷Pb/²⁰⁶Pb ratio in the calcite showed < 1% variations, while the ²³⁸U/²⁰⁶Pb ratio showed 3–24% variations depending on element mass fractions. Using the synthetic calcite as primary RMs, we could date a natural calcite RM, WC‐1, with analytical uncertainty as low as < 3%. The method presented can be useful to produce calcite with controlled and homogeneous element mass fractions and is a promising alternative to natural calcite RMs for U‐Pb geochronology.     

Issues in radiocarbon and U-series dating of corals from the last glacial period

Radiocarbon calibration beyond the extent of tree-ring records depends on U-series dating of fossil corals or speleothem, both of which can provide independent calendar ages. Less direct methods rely on layer counting and comparison with other well-dated records. In spite of considerable effort to provide a reliable radiocarbon calibration curve beyond 25,000 years, the majority of the data show large atmospheric radiocarbon peaks which are inconsistent both in magnitude and timing between different determinations. The results of the most recent work [Chiu, T.-C., Fairbanks, R.G., Mortlock, R.A., Bloom, A.L., 2005. Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals. Quaternary Science Reviews 24 (16–17), 1797–1808], from Araki Island fossil corals, indicate a monotonic variation from about 33 to 49 ka, with no radiocarbon peaks, but with some gaps in the data. The difference between this and previous results, from fossil corals, has been attributed to selection of better-quality samples and rigorous analytical methods. However, previous results from Huon Peninsula [Yokoyama, Y., Esat, T.M., Lambeck, K., Fifield, L.K., 2000. Last ice age millennial scale climate changes recorded in Huon Peninsula corals. Radiocarbon 42 (3), 383–401; Cutler, K.B., Gray, S.C., Burr, G.S., Edwards, R.L., Taylor, F.W., Cabioch, G., Beck, J.W., Cheng, H., Moore, J., 2004. Radiocarbon calibration and comparison to 50 kyr BP with paired ¹⁴C and ²³⁰Th dating of corals from Vanuatu and Papua New Guinea. Radiocarbon 46 (3), 1127–1160] show radiocarbon peaks exclusively located within the gaps in the Araki data. The timing of the gaps are not random, but appear to be related to severe climate and sea-level variations associated with Heinrich events initiated in the North Atlantic. We propose that the Huon and Araki data sets are complementary rather than exclusive and that the absence of coral growth at Araki Island during Heinrich events presumably reflect local adverse conditions for coral growth.