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This thesis describes the volcanic deposits on Nisyros, Aegean sea, Greece and uses them to understand volcanic processes and the evolution of the island. Nisyros is a small (42 km<SUP>2</SUP> in area) symmetric stratovolcano which has evolved on the eastern end of the Aegean volcanic arc over the last 200 ka of the Quaternary. The first volcanic activity was submarine. This left a layer of pillow larvas of basaltic-andesite composition overlying the Tertiary sedimentary basement of the Aegean. Subsequent volcanic activity was subaerial, and over 20 lava and pyroclastic eruptions of basaltic-andesite to rhyolite composition led to the formation of a stratovolcano. The younger eruptions were rhyolitic and the magma emitted is estimated to be of the order of a few km<SUP>3</SUP> in volume. These eruptions led to the formation of a 10 km<SUP>2</SUP> in area caldera which now dominates the centre of the island. A suite of domes were extruded progressively on western side of the caldera floor with field appearances suggesting activity continuing into the Holocene. The activity documented in historical records and seen at the present day is entirely phreatic and is the expression of a geothermal reservoir at a shallow depth below the caldera floor. The aims of this project were to describe the volcanic stratigraphy, to use the xenolith suite to understand magmatic and vent erosion processes, and to use the deposits from the three rhyolite eruptions to understand the youngest volcanic history of the island and timing of the caldera formation. Fieldwork was used to deduce the stratigraphy of the island. The stratocone succession consists of 3 cycles that start with basaltic-andesite to andesite lavas (BA&AI, BA&AII. BA&AIII) and pyroclastic deposits (SPI, SPII, SPIII) and proceed to dacite to rhyolite lava extrusions (DI, DII and RI, DIII) and pyroclastics (MSP, PK, LP).

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The Taupe Volcanic Zone (TVZ) is a major Pliocene-Quaternary NNE-SSW orientated volcano-techonic complex, in central North Island, New Zealand. It is a region characterised by voluminous rhyolitic eruptions, high natural heat flow, intense shallow seismic activity and active NW-SE extension. The central portion of the TVZ is regarded as the most frequently active and productive silicic volcanic system on Earth, yet to date no direct evidence for the source for the magmatisim has been found. In February and December 2001, as part of the NIGHT (North Island GeopHysical Transect) experiment, a total of ten 500 kg land slots were fired into an NW-SE array that ran the width of central North Island, New Zealand. An additional passive array of broad-band and short-period instruments centred on the TVZ recorded local and teleseismic earthquakes for six and a half months. Forward and inverse modelling of this active and shallow (< 10 km) earthquake data shows low-velocity (2.0-3.5 km/s) volcanic sediments reaching a maximum thickness of 3 km beneath the central TVZ. Underlying these sediments to 16 km depth are velocities of 5.0-6.5 km/s, interpreted as quartzo-fieldspathic crust. East and west of the TVZ, these velocities are observed to depths of 30 and 23 km respectively. Beneath the TVZ, material with P-wave velocities of 6.9-7.3 km/s are observed to ~30 km depth and are interpreted as heavily intruded or underplated lower crust. Modelling of deep (> 40 km) earthquake events originating near the top of the subducting Pacific plate, reveals a low-velocity region (LVR) (Vp of 7.4-7.8 km/s) overlying a northwest dipping high-velocity structure that coincides with the Wadati-Benioff zone.

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