tuff breccia - hand specimen of a spectacular pink tuff breccia
This particular volcanic igneous rock is composed of volcanic ash and many fragments (clasts) of other igneous rocks that were ejected from a volcanic vent. Because the ash was very hot when it fell to earth, the ash and rock fragments were welded together.
A breccia (from the Italian breccia for rubble) is normally composed of angular broken rock fragments that have been cemented together, such as where faulting has shattered rocks. In this case, there are so many broken fragments in this tuff that it is called a tuff breccia. The angular fragments in a breccia show that the fragments were cemented where they were formed. If they had been transported by running water, the edges would have become rounded.
Tuffs are light colored, usually shades of buff or gray, and since they are silica rich, they are not dense. This pink tuff is spectacular, in part because of the varied clasts. A student should know that clasts, from the Greek klastos for broken in pieces, 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, 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 will have fewer clasts of various igneous rocks than does this tuff breccia, but all will be fairly light in color and not as dense as a silica poor rock.
This is an eye-popping rock. The white clasts are pumice. The dark ones are a variety of rocks ripped from the volcanic vent. The ash matrix is pink.
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