tuff - copper-stained green welded tuff - teaching hand specimen
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This particular volcanic igneous rock is composed of volcanic ash and fragments of other igneous rocks (clasts) that were ejected from a volcanic vent and then welded together. Copper has stained it green.
Tuffs are light colored, usually shades of buff or gray, and since they are silica rich, they are not dense. A green tuff is unusual. This one has the typical angular rock fragments that were ripped from the walls of the vent welded into the ash. A student should know these fragments as clasts, from the Greek klastos for broken in pieces, and should know they are characteristic of tuffs.
If you are a science or earth science teacher purchasing this as a teaching specimen for your class, this should be compared with other silica rich volcanic igneous rocks, tuff breccia, pumice (which is so full of entrapped gases that it floats in water) and rhyolite (extruded as lava) and with silica rich plutonic igneous granitic rocks. A tuff breccia will have larger clasts of various igneous rocks than does this tuff.
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