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Volcanic Ash 

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Volcanic ash USGSVolcanic ash is volcanic rock whch has been violently disentegrated in a volcanic eruption. Ash consists of mineral,rock and volcanic glass fragments smaller than 2mm. It can resemble wheat flour is highly abrasive. A violent volcanic eruption can blanket an area in this "grey snow" however it does not melt, can collapse roofs,block aeroplane engines and if when wet can set like concrete. If Ash mixes with flowing water a lahar or volcanic mud flow can form.Volcanic ash fall from Mt St Helens USGS