Eruption styles and volcanic hazard in the Auckland volcanic field,New Zealand

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  • Eruption styles and volcanic hazard in the Auckland Volcanic Field, New Zealand(MEMORIAL VOLUME TO THE LATE PROFESSOR TERUHIKO SAMESHIMA)
  • Eruption styles and volcanic hazard in

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The Auckland field has been active for over 140,000 years, during this time eruptions have formed 49 small volcanoes. The magmas which have fed the field range from alkali basalt to nephelinite but the most recent eruption has been of transitional to tholeiitic basalt. Each eruption has involved a new batch of magma to produce volcanoes that are monogenetic in character. Detailed work on the erupted deposits show that the two main types of activity from the field have been magmatic and phreatomagmatic. The particular type of activity in an eruption is controlled by the water : magma ratio. In "dry" conditions, strombolian and hawaiian eruptions occur. At low water : magma ratios, phreatomagmatic eruptions are dominated by pyroclastic fall whereas at high water : magma ratios, phreatomagmatic activity is dominated by base surges. The water required for phreatomagmatic activity is ground-water or surface water. If water is excluded from the active vent and the magma supply is sufficient for continuation, then activity becomes wholly magmatic and can include the effusion of lava flows. The available ages of Auckland's volcanoes together with the volume of material produced in each, show a trend of increasing size with decreasing time. This pattern is interpreted to mean that the Auckland volcanic field is at an early stage in its evolution and that larger eruptions can be anticipated in the future.

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