tuff - teaching hand specimen of a rhyolitic tuff with angular clasts of rocks ripped from the vent
This particular volcanic igneous rock, the Bishop tuff, is composed of volcanic ash and fragments of other igneous rocks and the mineral sanidine that were ejected from vents near Mammoth, California 700,000 years ago. Because the ash was very hot when it fell to earth, the ash and rock fragments were welded together.
Tuffs are light colored, usually shades of buff or gray, and since they are silica rich, they are not dense. The broken fragments or clasts (from the Greek klastos for broken in pieces) are characteristic of tuffs and make a tuff distinguishable from a rhyolite, the fine-grained lava that has the same composition as tuff.
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