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The Grey’s Landing ignimbrite is an exceptionally well-preserved and well-exposed lava-like ignimbrite in the Snake River Plain volcanic province of southern Idaho, USA. High-grade tuffs are typically intensely welded and rheomorphic, preserving evidence of syn- and post-depositional ductile flow. The Grey’s Landing ignimbrite underwent two distinct phases of rheomorphism: (1) syn-depositional welding and rheomorphism as the deposit aggraded, producing a penetrative, mylonite-like L > S fabric including sheath folds, a strong elongation lineation, and shear-sense indicators; and (2) post-depositional, en masse, gravity-driven flow of parts of the deposit down palaeo-slopes, producing a fold-dominated deformation characterised by large-scale folds of the upper surface of the ignimbrite. Thermal and rheological modelling suggest that the original magma was hot (~ 950 °C), volatile-rich (≤ 5 wt % fluorine), and low viscosity (≥ 106 Pa.s-1). Syn-depositional rheomorphism had a strain rate of ~ 10-5 s-1 and deformed ignimbrite with a viscosity similar to the original magma. Postdepositional rheomorphism was limited to 6 months at the upper surface and ≤ 16 years in the centre of the thickest ignimbrite, at a strain rate of ~ 10-6 s-1.

Low aspect-ratio ignimbrites are thought to be emplaced by particularly hazardous, radial, high-velocity pyroclastic density currents from caldera-forming eruptions. Their circular distribution has been inferred to record simultaneous flow in all directions from source, overtopping hills, rather than passively flowing down valleys. This study aims to understand how such currents behave and evolve during an eruption by mapping out the internal chemical-architecture of a zoned, low-aspect ratio ignimbrite sheet on the island of Pantelleria, Italy. The pristine, welded Green Tuff Formation (aspect ratio >1:1,000) was deposited during the most recent (c. 50 ka) explosive eruption on the island. The extensive flow-unit is zoned from pantellerite to trachyte, recording changes in the composition of the erupting magma with time. Detailed logging with very close-spaced sampling for chemical and petrographic analysis has distinguished an internal chemical stratigraphy that allows the brief history of the sustained current to be divided into successive time-periods. The compositional zones have been mapped around the island enabling the reconstruction of how the footprint of the sustained current shifted during the eruption as the current waxed and then waned. Furthermore, the mapped compositional zones can be used to assess how the current and its resultant deposit encountered and overtopped barriers, such as cone-shaped hills and transverse ridges. This study has revealed that the ignimbrite was not emplaced entirely radially: rather, it flowed into certain sectors before others, and that the leading edge of the current advanced and then retreated, and shifted laterally with time. Deposition was diachronous, and previously proposed lithofacies correlations within the ignimbrite are demonstrated to be incorrect. As the pyroclastic density current encroached upon topographic barriers, it was initially blocked, reflected, or deflected around the lower flanks of the barrier. As the mass-flux of the eruption increased, the current waxed and was progressively able to overtop topographic barriers.

Η Νίσυρος είναι ένα στρωματοηφαίστειο δομημένο από Πλειοκαινικά ηφαιστειακά προϊόντα που αποτελούνται κυρίως από ανδεσίτες και βασαλτικούς ανδεσίτες πάνω στους οποίους αποτέθηκαν ασβεσταλκαλικά ηφαιστειακά προϊόντα δακιτικής – ρυοδακιτικής σύστασης με την μορφή πυροκλαστικών αποθέσεων, ροών λάβας και δόμων λάβας. Οι νησίδες του Γυαλιού, της Στρογγυλής, της Παχειάς και της Περγούσας, αποτελούνται από Πλειοκαινικούς δόμους ρυολιθικής (Γυαλί), ανδεσιτικής (Στρογγυλή) και δακιτικής σύστασης (Παχειά και Περγούσα) οι οποίοι φιλοξενούν πυροκλαστικές αποθέσεις της ανώτερης κίσσηρης του Γυαλιού (Στρογγυλή) και ενότητες του τόφφου της Κω και της Παναγιάς Κυράς (Παχειά και Περγούσα). Στην εργασία παρουσιάζουμε την ιστορία γένεσης και φωτογραφίες από τη Νίσυρο, ακολουθώντας πέντε μόνο διαδρομές. / In the present dissertation we are recording the geological monuments of Nisyros island. We present the geological history of the island, providing photos of five routes we have followed.