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In the hotspot-fed Galápagos Archipelago there are transitions between volcano morphology and composition, effective elastic thickness of the crust, and lithospheric thickness in the direction of plate motion from west to east. Through sampling on the island scale it is unclear whether these transitions are gradational or sharp and whether they result in a gradational or a sharp boundary in terms of the composition of erupted lavas. Clusters of interisland seamounts are prevalent on the Galápagos Platform, and occur in the transition zone in morphology between western and eastern volcanoes providing an opportunity to evaluate sharpness of the compositional boundary resulting from these physical transitions. Two of these seamounts, located east of Isabela Island and southwest of the island of Santiago, were sampled by remotely operated vehicle in 2015 during a telepresence-supported E/V Nautilus cruise, operated by the Ocean Exploration Trust. We compare the chemistries of these seamount lavas with samples erupted subaerially on the islands of Isabela and Santiago, to test whether seamounts are formed from melt generation and storage similar to that of the western or eastern volcanoes, or transitional between the two systems. There are no systematic variations between the two seamounts and variability in all samples can be related through < 10% fractional crystallization at 500–900 MPa. Both seamounts are interpreted to represent a single magmatic episode and eruptive event. Trace element compositions indicate they formed downstream of the hotspot center. The calculated extents of melting are consistent with generation of magmas sourcing the seamounts beneath lithosphere of intermediate thickness (~ 56 km). The seamount lavas have compositions that are nearly identical to a subset of lavas erupted subaerially on Santiago Island, suggesting lateral magma transport on the order of 10 km from their source region prior to eruption. The compositional characteristics and

Systematic seismic profiling and swath bathymetry of the seafloor around Santorini volcanic field, including the caldera, shed light to the structure and seismic stratigraphy of the accumulated volcanic deposits and the mechanisms which may have led to the generation of one or multiple tsunamis during the Minoan eruption. We propose that tsunamis may have been triggered by two distinct processes associated with the Minoan eruption: (i) Massive pyroclastic flows entering the ocean around the islands of Santorini. The total volume of the pyroclastic flow deposits on the seafloor around Santorini is about 54.5 km3. (ii) The collapse of the Minoan volcanic edifice below the sea-level created new space of minimum volume 4.5-5 km3 between the sea-level and the 200m depth contour. The total volume created may exceeded 10 km3. Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE).

Mount St. Helens has explosively erupted dacitic magma discontinuously over the last 40,000 years, and detailed stratigraphic data are available for the past 4,000 years. During this last time period the major-element composition of the dacites has ranged from mafic (62-64 wt% SiO2) to felsic (65-67 wt% SiO2), temperature has varied by about 150°C (770°-920°C), and crystallinity has ranged between 20% and 55%. Water content of these dacites has also fluctuated greatly. Although the source for the dacitic magmas is probably partial melting of lower crustal rocks, there is strong physical evidence, such as banded pumices, thermal heterogeneities in single pumices, phenocryst disequilibrium, contrasts between compositions of glass inclusions and host matrix glass, and amphibole reaction rims, that suggests that magma mixing has been prominent in the dacitic reservoir. Indeed, we suggest that the variations in major- and trace-element abundances in Mount St. Helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas. Magma mixing has also controlled the composition of andesites erupted at Mount St. Helens, and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite, or possibly rhyodacite, and basalt. The flux of the felsic endmember to the reservior appears to have been relatively constant, whereas the flux of basalt may have increased in the past 4,000 years, as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period. © 1995 Springer-Verlag.