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tuff
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Yellowstone National Park: welded tuffs A chart categorizes common textural terms for rocks by grain size in millimeters, dividing them into igneous and metamorphic, sedimentary, and pyroclastic types, with terms like phaneritic, sand, silt, breccia, and ash shown in relation to size.
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Yellowstone National Park: welded tuffs
Yellowstone National Park: welded tuffs Welded tuffs in Yellowstone National Park.

tuff

geology
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tuff, a relatively soft, porous rock that is usually formed by the compaction and cementation of volcanic ash or dust. (The Italian term tufa is sometimes restricted to the soft, porous, sedimentary rock formed by the chemical deposition of calcite, or calcium carbonate, or silica from water as sinter.) Tuffs may be grouped as vitric, crystal, or lithic when they are composed principally of glass, crystal chips, or the debris of pre-existing rocks, respectively. Some...

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