The volcanological significance of deep-sea ash layers associated with ignimbrites

1980 ◽  
Vol 117 (5) ◽  
pp. 425-436 ◽  
Author(s):  
R. S. J. Sparks ◽  
T. C. Huang
Keyword(s):  
Deep Sea ◽  
Eastern Mediterranean ◽  
Eruption Column ◽  
Size Distributions ◽  
Ash Fall ◽  
Column Collapse ◽  
Median Diameter ◽  
Grain Size Distributions ◽  
Bimodal Grain Size ◽  
Fall Deposits

SummaryMany volcanic ash layers preserved in deep-sea sediments are the products of large magnitude ignimbrite eruptions. The characteristics of such co-ignimbrite ash-fall deposits are illustrated by two layers from the Eastern Mediterranean: the Minoan ash, Santorini, and the Campanian ash, Italy. These layers are divisible into a coarse lower unit and a fine upper unit in proximal cores. Both layers also show striking bimodal grain size distributions in more distal cores. The coarser mode decreases in median diameter with distance from source whereas the finer mode shows no lateral variation. These features are interpreted in terms of a model for ignimbrite formation by eruption column collapse. Comparable volumes of ignimbrite and associated air-fall ejecta are produced.

scholarly journals Three-Dimensional in situ Reconstructions of Microstructures with Bimodal Grain Size Distributions

2019 ◽  
Vol 25 (S2) ◽  
pp. 370-371
Author(s):  
Ashley Bucsek ◽  
Lee Casalena ◽  
Darren C. Pagan ◽  
Partha P. Paul ◽  
Yuriy Chumlyakov ◽  
...  
Keyword(s):  
Grain Size ◽  
Three Dimensional ◽  
Size Distributions ◽  
Grain Size Distributions ◽  
Bimodal Grain Size

scholarly journals Grain Growth after Intercritical Rolling

2004 ◽  
Vol 467-470 ◽  
pp. 305-310 ◽  
Author(s):  
Roumen H. Petrov ◽  
Leo Kestens ◽  
Kim Verbeken ◽  
Yvan Houbaert
Keyword(s):  
Grain Size ◽  
Size Distributions ◽  
Orientation Microscopy ◽  
Final Rolling Temperature ◽  
Responsible Mechanism ◽  
Grain Size Distributions ◽  
Intercritical Region ◽  
Final Rolling ◽  
Parent Austenite ◽  
Bimodal Grain Size

The distribution of the characteristic texture components between the ferrite grains of different size classes has been studied in a steel with 0.082%C, 1.54% Mn, 0.35% Si, 0.055%Nb and 0.078%V after different rolling schedules with a final rolling temperature above or below Ar3. Microstructures and textures were characterized by means of optical microscopy and orientation microscopy. A strong grain refining effect together with a bimodal grain size distribution was observed in the steel both after final rolling in the intercritical region or in the austenite region, close to the Ar3 d temperature. The differences in grain size were interpreted on the basis of three potentially acting mechanisms: (i) transformation- induced recrystallization, (ii) increased mobility of specific grain boundaries and (iii) fast nucleation of ferrite grains on specific sites of the parent austenite microstructure. The experimental data clearly favoured the third of these assumptions as the responsible mechanism for the observed bimodal grain size distributions.

scholarly journals Transport and accumulation of plastic litter in submarine canyons—The role of gravity flows

Geology ◽  
2021 ◽  
Author(s):  
Guangfa Zhong ◽  
Xiaotong Peng
Keyword(s):  
Submarine Canyon ◽  
Turbidity Currents ◽  
Size Distributions ◽  
Submarine Canyons ◽  
Seabed Sediment ◽  
Gravity Flows ◽  
Grain Size Distributions ◽  
Manned Submersible ◽  
Bimodal Grain Size

Manned submersible dives discovered plastic litter accumulations in a submarine canyon located in the northwestern South China Sea, ~150 km from the nearest coast. These plastic-dominated litter accumulations were mostly concentrated in two large scours in the steeper middle reach of the canyon. Plastic particles and fragments generally occurred on the upstreamfacing sides of large boulders and other topographic obstacles, indicating obstruction during down-valley transportation. Most of the litter accumulations were distributed in the up-valley dipping slopes downstream of the scour centers. This pattern is tentatively linked to turbidity currents, which accelerated down the steep upstream slopes of the scours and underwent a hydraulic jump toward the scour centers before decelerating on the upstream-facing flank. Associated seabed sediment consisted of clayey and sandy silts, with unimodal or bimodal grain-size distributions, which are typical for turbidites. The focused distribution of the litter accumulations is therefore linked to turbidity currents that episodically flush the canyon. Our findings provide evidence that litter dispersion in the deep sea may initially be governed by gravity flows, and that turbidity currents efficiently transfer plastic litter to the deeper ocean floor.

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