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Texture and Structure of Fluvial Deposits
Fluvial Transport and Downstream Sediment Modifications in an Active Volcanic Region
River systems draining the active volcano Fuego are dominantly braided, and characterized by a high sediment transport rate. In a single year (1976) approximately 6 million tonnes of sediment are estimated to have been eroded from the cone by two river systems, with some 4 million tonnes reaching the sea.
Downstream modifications in sediment texture and composition are closely interrelated with fluvial transport mechanics. Grain size decreases exponentially downstream from 398 mm to 1.0 mm during some 90 km of fluvial transport. This results from decreasing flood-flow competence, and does not reflect in situ abrasion or abrasion or abrasion during transport. Abrasion results only in a 6% reduction of the volume of individual boulders, and causes surface pitting and loss of sharp edges. Concomitantly, boulder roundness increases downstream from 0.4 to 0.8. Sediment composition is also modified during fluvial transport. Grains in transport are derived largely from the products of the 1974 eruption of Fuego. They consist of fresh, unweathered feldspar rock fragments, pyroxene rock fragments, and olivine rock fragments together with free crystals of feldspar, pyroxene, and olivine. Rock fragment abundance decreases some 45% during 90 km of transport, while free crystal abundance increases 48%. The rate of rock fragment breakdown is different for different species of fragment, feldspathic rock fragments being apparently the most susceptible to physical breakdown. The physical breakdown of rock fragments results from fracturing in the glass groundmass which binds component crystals.
The most rapid rates of breakdown of sand-size rock fragments occurs in areas of high slope (the Volcanic Highlands), during the first 30 km of transport. Physical breakdown of rock fragments is probably directly related to the mode of sand grain transport, and results from impact shattering during transportation in intermittent suspension (saltation).
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