Particle Size Analysis

1. Particle Size Analysis Theory The main size classes are: Boulder greater than 64mm Gravel 64 to 2mm Sand 2mm to 63 micron Silt 63 to 2 micron Clay less than 2 micron The size of particles affects the likelihood that they will be eroded from a surface and transported by water, wind, or ice. Sediments will accumulate where there is insufficient energy to transport them . Sediment particles can be a range of sizes from boulders of a metre in diameter to clay particles less than 2 microns (0.002 millimetres) in diameter. When we investigate these sediment deposits measuring the particle size distribution can indicate where the sediment came from, how it was transported and in what environment it was deposited. The sample is dispersed in a liquid through which the instrument sends a laser beam. The instrument detects the pattern in which the sample scatters the laser light and from this can calculate the size of the particles present. A sieve cannot measure fine particles less than 63 microns, (0.063mm). Specialist instruments like the Saturn digisizer made by Micromeritics, have been developed to measure these small particles. Methods Particle size can be investigated by sieving the sediment sample through a series of sieves to separate particles of different sizes. The weight of sediment retained in each sieve is then a measure of the proportion of the sample of each size class. Examples River sediments are sorted by the flow of the water, so gravel or sand is likely to be the dominant particle size. Lake sediments are formed in slow moving or still water so silt and clay are deposited. Coarser sand layers can indicate flood events entering the lake. Wind blown (aeolian) sediments tend to be well sorted sand or silt (wind blown silt is called loess). The particle size of aeolian deposits records the former wind direction as larger particles will be preferentially deposited nearer the sediment source area because they require more energy to transport. Glacial sediments are characterised by a lack of sorting so boulders and gravel will be present amongst clay, silt and/or sand . Each glacial sediment deposit (till) will often have a characteristic particle size distribution so tills formed in different ways or by different glaciers can be identified. In this study of a glacial sediments in Iceland, Professor Jane Hart used particle size analysis to differentiate between different types of till and to understand where the particles that compose the tills came from: The subglacial ash-rich till has a particle size distribution very similar to the ash it is derived from, but the particles have been abraded (rounded) beneath the glacier . The clay-rich till is derived from palagonite bedrock and has more fine particles and will behave very differently beneath the glacier. Above are particle size distributions of wind blown sediment deposits from sites in the USA, UK and Iceland studied by Dr Jo Nield. The particle size distributions of the samples are all similar and show most of the material is sand, so the deposits are likely to have been formed by the same process, in this case wind. Differences allow us to understand the effects of different environmental conditions in each location on sediment transport.