The following text was modified from http://pubs.usgs.gov/gip/volc/text.html
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Explosively erupted rock is called tuff. You can recognize it because it usually feels sandy or grainy when you scratch it, and it often contains rock fragments, pumice, and mineral fragments. You have to look closely, with a hand-lens. Welded tuff is hard, unscratchable rock that melted a little and compressed on the ground. You can recognize it by its rock fragments, flattened pumice, and mineral fragments.
Note that the word "ash" means pulverized rock -- it is heavy and dense. It is not ashes, the light-weight remains of burned organic matter. Ash is essentially like sand -- heavy.