tuff breccia - spectacular pink tuff breccia with large clasts - Student unit of 5 specimens
Tuff is welded volcanic ash. Normally it contains fragments of rocks that were torn from the vent and ejected with the ash. These clasts (from the Greek klastos for broken in pieces) are usually angular and help students identify a tuff, separating it from rhyolite, a lava with the same general composition and color.
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 angular clasts, from the Greek klastos for broken in pieces, are characteristic of tuffs.
Silica rich rocks make up the continents. The extrusive volcanic rocks, tuff, tuff breccia, rhyolite (silica rich lava), pumice and obsidian have the same general composition as granitic rocks that have cooled far below the surface. They are generally light in color and not dense. Silica poor rocks make up the oceanic crust. Primarily basalt, they are dark, relatively heavy and rich in magnesium and iron.
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