No abstract

Abundant white to light grey coloured pumice without ferromanganese oxide coating occurs within the Quaternary sediments of the Central Indian Ocean Basin. Two distinct groups of pumice are identified from their geochemical composition, which allow one to define two different origins linked to two separate eruptions. One group of pumice is a dacitic type characterized by high Fe, Ti, Mg, Al and Ca with comparatively low contents of Si, rare earth elements (Σ REE-69 ppm), Rb, Sr, U, Th, Ba, V, Nb, Sc, Mo and Co which strongly suggest an origin from the 1883 Krakatau eruption. The other group is rhyolitic and is characterized by low contents of Fe, Ti, Mg and Ca and high Si , ΣREE content (121 ppm), Rb, Sr, U, Th, Ba, V, Nb, Mo, Co, and Sc and correlates well with the composition of the Youngest Toba Tuff (YTT) eruption of ~ 74 ka from Northern Sumatra and is being reported for the first time. Therefore, correlation of the pumice to the 1883 Krakatau and YTT eruptions indicates that the pumice drifted to the Central Indian Ocean Basin and eventually sank when it became waterlogged. However, physical properties such as density, specific gravity, porosity and degree of saturation required for sinking of pumice for both 1883 Krakatau and YTT are almost similar.

One of the major requirements of planning coastal protection works is to understand the processes of erosion, deposition, sediment transport, flooding and sea level changes which continuously alter the shoreline. Significant erosion can affect the stability and productivity of aquatic environment which may have severe implications for coastal inhabitants. The middle coastal plains of Odisha State on the east coast of India were investigated for morphological assessment of shoreline. Accurate demarcations of shorelines were carried out at parts of Odisha coast specifically along Gahirmatha, Paradip and coast above Devi River to quantify erosion and accretion at annual to decadal scale for the years 1990, 2000 and 2012. Satellite-derived remote sensing data (Landsat and IRS P6) were used in the study. Digital shoreline analysis system discovered the eroded and accreted parts of the study area. Gahirmatha and coast above Devi River experienced heavy erosion during 2000-2012 compared with 1990-2000, whereas Paradip coast has comparatively undergone accretion during 2000-2012. Some accreted spots are identified nearby river mouths, which are attributed to heavy accretion of eroded materials by the action of sediment transport. The detailed analysis reveals a maximum erosion of 124, 33 and 154 m in a decade at Gahirmatha, Paradip and coast above Devi River, respectively. Southern parts of Gahirmatha coast showed highly dynamic behavior near Hukitola Bay and Barrier Island and are acting as a natural breakwater to conserve the shoreline. This region had undergone severe geomorphologic changes due to natural as well as human interventions and poses a threat. This coast exhibits unique reasons for erosion with various degrees of combinations of sediment depletion, human activities, high frequency of cyclones and floods, sea level rise, etc. This study concludes that the shoreline of Odisha coast is under heavy erosion and needs scientific and management attention.

About 400 pumice clasts collected from the Central Indian Ocean Basin (CIOB) were studied for their morphology and were classified based on their shape and size. A majority of the samples range between <1 cm and 36 cm and in the Zinggs shape diagram plot in the equant and oblate fields. The Corey Shape Factor for most of the samples is close to 0.7, which is common for volcaniclastic material. The physical properties such as density, specific gravity, void ratio, porosity, moisture content and degree of saturation, were determined for 30 pumice samples. Density varies from 0.21 to 0.74 g/cm3 specific gravity 1.84 to 3.27, void ratio 2.21 to 10.67, porosity 67% to 91%, moisture content during sinking 0.44 to 2.35 and degree of saturation varies from 26.5% to 86%. Binocular and electron microscopy studies reveal that 60% of the vesicles are elongated, 30% are spherical and 10% are fibrous. Petrography of the pumices exhibits vitrophyric texture with phenocrysts of feldspars and clinopyroxenes. X‐ray diffractrogram and mineral analyses confirm plagioclase to be a major phase, while quartz and orthoclase are not uncommon. Todorokite is commonly present in the ferromanganese oxide coating present over some of the pumices. This paper also delves into some details concerning the controversial origin of the pumices and glass shards in the CIOB.