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Most of Earth’s volcanoes are under water. As a result of their relative inaccessibility, little is known of the structure and evolution of submarine volcanoes. Advances in navigation and sonar imaging techniques have made it possible to map submarine volcanoes in detail, and repeat surveys allow the identification of regions where the depth of the sea floor is actively changing. Here we report the results of a bathymetric survey of Monowai submarine volcano in the Tonga–Kermadec Arc, which we mapped twice within 14 days. We found marked differences in bathymetry between the two surveys, including an increase in seafloor depth up to 18.8 m and a decrease in depth up to 71.9 m. We attribute the depth increase to collapse of the volcano summit region and the decrease to growth of new lava cones and debris flows. Hydroacoustic T-wave data reveal a 5-day-long swarm of seismic events with unusually high amplitude between the surveys, which directly link the depth changes to explosive activity at the volcano. The collapse and growth rates implied by our data are extremely high, compared with measured long-term growth rates of the volcano, demonstrating the pulsating nature of submarine volcanism and highlighting the dynamic nature of the sea floor.

Geophysical investigations of the northern Hikurangi subduction zone northeast of New Zealand, image fore‐arc and surrounding upper lithospheric structures. A seismic velocity (Vp) field is determined from seismic wide‐angle data, and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling. We found that the subducting Hikurangi Plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics, limestone, and chert. The upper plateau crust is characterized by Vp = 4.9–6.7 km/s overlying the lower crust with Vp > 7.1 km/s. Gravity modeling yields a plateau thickness around 10 km. The reactivated Raukumara fore‐arc basin is >10 km deep, deposited on 5–10 km thick Australian crust. The fore‐arc mantle of Vp > 8 km/s appears unaffected by subduction hydration processes. The East Cape Ridge fore‐arc high is underlain by a 3.5 km deep strongly magnetic (3.3 A/m) high‐velocity zone, interpreted as part of the onshore Matakaoa volcanic allochthon and/or uplifted Raukumara Basin basement of probable oceanic crustal origin. Beneath the trench slope, we interpret low‐seismic‐velocity, high‐attenuation, low‐density fore‐arc material as accreted and recycled, suggesting that underplating and uplift destabilizes East Cape Ridge, triggering two‐sided mass wasting. Mass balance calculations indicate that the proposed accreted and recycled material represents 25–100% of all incoming sediment, and any remainder could be accounted for through erosion of older accreted material into surrounding basins. We suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore‐arc highs have not properly been accounted for.

OBJECTIVES: We sought to determine the magnitude, direction, and statistical significance of the relationship between active travel and rates of physical activity, obesity, and diabetes. METHODS: We examined aggregate cross-sectional health and travel data for 14 countries, all 50 US states, and 47 of the 50 largest US cities through graphical, correlation, and bivariate regression analysis on the country, state, and city levels. RESULTS: At all 3 geographic levels, we found statistically significant negative relationships between active travel and self-reported obesity. At the state and city levels, we found statistically significant positive relationships between active travel and physical activity and statistically significant negative relationships between active travel and diabetes. CONCLUSIONS: Together with many other studies, our analysis provides evidence of the population-level health benefits of active travel. Policies on transport, land-use, and urban development should be designed to encourage walking and cycling for daily travel.

A study of microhardness at the wear surface of polyethylene implants before and after use is reported. The microhardness values supportprevious electron microscopic observations of an embrittled region extending into used implants by showing that, after use, the hardness ofpolyethylene implants increases above the initial value the nearer the wear surface is approached. It is shown that these changes accompany ahardening of the surface layer and are related to an increase in the crystallinity of the polymer. The increasing hardness towards the edge ofthe concave surface of the implant is discussed in terms both of the local variation of crystallinity and changes in the interlamellar regions.

The correlation between microhardness, H, yield stress, Y, and Young's modulus, E, has been explored on various chain-extended polyethylene (PE) samples and compared to chain-folded PE. Mechanical properties have been derived from tensile and compressive experiments. The tensile yield stress, Y(t), is shown to correlate with hardness following H ~ 3Y(t) while H ~ E(t)/10 (E(t) is Young's modulus derived from tensile experiments). On the other hand, the compressive yield stress, Y(c), is shown to correlate to H following the mechanical models of elastoplastic indentation. Hence, H/Y(c) increases with decreasing elastic strain tending towards H ~ 3Y(c) for a fully plastic deformation. The atmospheric pressure compression-moulded samples show the lowest H/Y(c) ratios as a consequence of the largest fraction of compliant phase. Moreover, the H/E(c) ratio (E(c) being the elastic modulus under compressive force) is shown to diminish with the decrease of Y(c)/E(c). Chain extended PE provides the lowest values for the H/E(c) ratio from all the PE samples investigated. (C) 2000 Elsevier Science Ltd. All rights reserved. | The correlation between microhardness, H, yield stress, Y, and Young's modulus, E, has been explored on various chain-extended polyethylene (PE) samples and compared to chain-folded PE. Mechanical properties have been derived from tensile and compressive experiments. The tensile yield stress, Yt, is shown to correlate with hardness following H to approximately 3Yt while H to approximately Et/10 (Et is Young's modulus derived from tensile experiments). On the other hand, the compressive yield stress, Yc, is shown to correlate to H following the mechanical models of elastoplastic indentation. Hence, H/Yc increases with decreasing elastic strain tending towards H to approximately 3Yc for a fully plastic deformation. The atmospheric pressure compression-moulded samples show the lowest H/Yc ratios as a consequence of the largest fraction of compliant phase. More

The variation of hardness with indentation time has been investigated for chain-extended polyethylene (PE), other PE samples crystallized under different conditions and paraffins. Hardness is shown to decrease with indentation time for all the samples investigated according to a power-law. Chain-extended PE, produced by high pressure crystallization or annealing, flows at the lowest rate under the indenter of all the PE samples considered. On the other hand, paraffins creep at the highest rate. Creep behaviour depends markedly on the crystal thickness of the material. The mechanical properties at long indentation times seem to be determined primarily by the deformation modes of the crystals. The temperature dependence of hardness and that of the creep behaviour has also been investigated. In chain-extended PE, the softening of the sample and the higher rate of creep with increasing temperature are discussed in terms of the thermal expansion of the unit cell.