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Quantification of the grain size distribution of sediments allows interpretation of processes of transport and deposition. Jutzeler et al. (2012) developed a technique to determine grain size distribution of consolidated clastic rocks using functional stereology, allowing direct comparison between unconsolidated sediments and rocks. Here, we develop this technique to characterize hydraulic sorting and infer transport and deposition processes. We compare computed grain size and sorting of volcaniclastic rocks with field-based characteristics of volcaniclastic facies for which transport and depositional mechanisms have been inferred. We studied pumice-rich, subaqueous facies of volcaniclastic rocks from the Oligocene Ohanapecosh Formation (Ancestral Cascades, Washington, USA), Pliocene Dogashima Formation (Izu Peninsula, Honshu, Japan), Miocene Manukau Subgroup (Northland, New Zealand) and the Quaternary Sierra La Primavera caldera (Jalisco State, Mexico). These sequences differ in bed thickness, grading and abundance of matrix. We propose to evaluate grain size and sorting of volcaniclastic deposits by values of their modes, matrix proportion (< 2 mm; F-1) and D16, instead of median diameter (D50) and standard deviation parameters. F-1 and D16 can be uniformly used to characterize and compare sieving and functional stereology data. Volcaniclastic deposits typically consist of mixtures of particles that vary greatly in density and porosity. Hydraulic sorting ratios can be used to test whether inferred density of mixed clast populations of pumice and dense clasts are hydraulically sorted with each other, considering various types of transport under water. Evaluation of this ratio for our samples shows that most studied volcaniclastic facies are deposited by settling from density currents, and that basal dense clast breccia are emplaced by shear rolling. These hydraulic sorting ratios can be applied to any type of clastic rocks, and indifferently on consolidated and unc

In the Izu Peninsula (Japan), the Pliocene pumice-rich Dogashima Formation (4.55 ± 0.87 Ma) displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave-base marine setting. It includes high-concentration density current deposits that contain clasts that were emplaced hot, indicating an eruption-fed origin. The lower part of the Dogashima 2 unit consists of a very thick sequence (<12 m) of massive grey andesite breccia restricted to the base of a submarine channel, gradationally overlain by pumice breccia, which is widespread but much thinner and finer in the overbank setting. These two breccias share similar mineralogy and crystal composition and are considered to be co-magmatic and derived from the destruction of a submarine dome by an explosive, pumice-forming eruption. The two breccias were deposited from a single, explosive eruption-fed, sustained, sea floor-hugging, water-supported, high-concentration density current in which the clasts were sorted according to their density. At the rim of the channel, localised good hydraulic sorting of clasts and stratification in the pumice breccia are interpreted to reflect local current expansion and unsteadiness rather than to be the result of hydraulic sorting of clasts during fall from a submarine eruption column and/or umbrella plume. A bimodal coarse (>1 m) pumice- and ash-rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension. In Dogashima 1 and 2, thick cross- and planar-bedded facies composed of sub-rounded pumice clasts are intercalated with eruption-fed facies, implying inter-eruptive mass-wasting on the flank of a submarine volcano, and reworking and resedimentation by high-energy tractional currents in a below wave-base environment.

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., publisher