tuff - rhyolitic tuff with pumice clasts - a classic ignimbrite or pumice-dominated pyroclastic deposit - teaching hand specimen
This particular volcanic igneous rock, the Bishop Tuff, is composed of volcanic ash and fragments of pumice that were ejected as a pyroclastic flow from vents near Mammoth, California 700,000 years ago. It is a classic example of an ignimbrite, where the ash was ~600˚C , causing the ash and pumice fragments to weld together. The Bishop tuff is variable. These specimens were collected in one area where it contains considerable numbers of darker pumice clasts (from klastos, Greek = broken in pieces). In other areas, the clasts are angular fragments of various igneous rocks ripped violently from the walls of the vent.
The term “ignimbrite” was coined by the New Zealand geologist Peter Marshall in 1935. This term was originally used only to refer to welded tuffs. These are pyroclastic rocks that were so hot right after the deposition from the pyroclastic cloud that individual clasts adhered to each other. However, this restriction no longer applies.
This is a self-teaching tuff. It looks ashy and is gritty to the touch. Students will get the picture. The pumice fragments are a light purple and the ashy matrix has a slight purplish cast. Tuff and rhyolite often look similar, since they have a similar silica rich composition and are both generally light in color. The angular clasts in a tuff separate it from a rhyolite.
Tuffs are light colored, usually shades of buff or gray, and since they are silica rich, they are not dense. These specimens are fairly light weight for their size, reinforcing the volcanic ash origin.
Ash from this particular eruption was carried eastward by the prevailing westerlies and forms an identifiable thin layer in Europe. Near Bishop, California, the tuff sheet is over 300 feet thick.
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