ALBERT

Description: Tūhua/Mayor Island is located approximately 45 km off the NE coast of the North Island of New Zealand. This island was formed by various explosive and effusive volcanic eruptions commonly influenced by magma–water interaction eruption events occurring since the Pleistocene. The wider area of theSWPacific contains numerous volcanic islands with a similar type of volcanic evolution. Tūhua/Mayor Island should be studied in more detail to understand the underlying volcanic mechanisms and apply this research to other volcanic islands in the SW Pacific. Mayor Island, also known by its indigenous Māori name Tūhua (obsidian in Māori), provides an ideal site for studying current volcanism. The present day island was formed around 150 ka ago and contains several rhyolitic lava-flows from different time periods, pyroclastic-flow deposits generated by small volume localized eruptions and ignimbrite deposited by large explosive eruptions. Our research utilized a qualitative–quantitative assessment of geodiversity estimates to highlight possible geosites for the collection of precise information about the geological evolution of this area, demonstrating the potential of geoeducational sites. The term geodiversity recognizes geological and geomorphological elements, which have shaped the Earth’s surface and underly our abiotic environment. Additionally, volcanic heritage was included in our equation, specifically tailored for Tūhua/Mayor Island and based on expert views (qualitative model). This model allows for a wider diversity for the area of research compared with the original method, which utilized only geological elements. The results show that areas with pyroclastic deposits exposed on the cliffs and in the centre of the collapse caldera should be considered for the further study for geosite planning.

Description: Published

Description: 2TM. Divulgazione Scientifica

Description: Highlights • Plinian eruptions linked to rheologically different mingling andesite magmas. • Magma decompression regimes producing variable degassing/crystallization kinetics. • Least explosive eruptions at slowest decompression, ascent and strain rates. • Plinian phases at rapid/intermittent magma decompression, ascent and strain rates. Abstract Estimating the kinetics of andesite magma vesiculation and crystallization inside volcanic plumbing systems is key for unraveling andesite Plinian eruption dynamics. The conduit kinetics provide the necessary input data for estimating the magma flow rates driving magma ascent and the fragmentation mechanisms controlling shifts in eruption explosivity and style. This information is crucial for increasing knowledge on expected hazards and for developing realistic eruption scenarios. In this work, we estimate conduit magma vesiculation and crystallization kinetics during the 3300 cal BP Upper Inglewood Plinian eruptive episode of Mount Taranaki, New Zealand. This episode comprised (i) low-intensity, conduit-opening phases of dome-collapse PDCs; (ii) pre-climactic, highly explosive phases of diverse PDCs, of up to violent 18-km-runout lateral blasts; (iii) climactic phases of steady 22-km-high Plinian eruption columns; and (iv) waning phases of column-collapse PDCs. By employing synchrotron microtomography, combined with mineral/glass chemistry and electron-microscopy, we quantified 3D vesicle and crystal size and shape distributions in juvenile pyroclasts over time, and corresponding number densities ranging from 1.1 × 105 to 2.5 × 106 mm−3 for vesicles, and from 8.0 × 104 to 5.1 × 106 mm−3 for crystals. Our results indicate that tapping of chemically alike yet rheologically contrasting magmas over a multi-phase andesite eruptive episode is linked to: (a) mafic magma recharge and differentiation in multiple storage reservoirs at distinct crustal levels, (b) stepwise to rapid magma decompression while mingling, producing variable pre- and syn-eruptive degassing and crystallization, and (c) syn-eruptive changes in melt viscosity, strain rate, localized shear deformation, and conduit geometry. The earliest and least explosive eruptive phases (≈ 2 × 106 kg s−1) were produced at the slowest rates of magma decompression (0.3–0.6 MPa s−1), ascent (0.01–0.02 m s−1) and strain (〈 0.002 s−1), driven by volatile diffusion and exsolution. All subsequent pre-climactic and Plinian phases (4 × 107–1 × 108 kg s−1) were produced at either rapid or intermittent rates of magma decompression (2.0–6.0 MPa s−1), ascent (0.06–0.2 m s−1) and strain (〉 0.003–0.010 s−1), powered by combined magma volatile supersaturation and delayed disequilibrium degassing, decompression-induced microlite crystallization and rapid heterogeneous vesiculation kinetics, shear deformation and magma mingling. These processes enabled complex fragmentation mechanisms of the rheologically most homogeneous magmas.