This particular volcanic igneous rock, the Bishop tuff, is composed of volcanic ash and fragments of pumice that were violently ejected at hurricane speed as a pyroclastic flow from vents near Mammoth, California 700,000 years ago. It is a classic ignimbrite. Because the ash was ~ 600˚ C, the ash and pumice fragments were welded together. The Bishop tuff is variable. These specimens were collected in one area where it contains darker pumice clasts (clast from klastos, Greek for "broken in pieces"). 

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. There are enough large purplish pumice clasts in the tuff to get the idea across that it is a pumice dominated ignimbrite.

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 fragments (clasts) of various igneous rocks than does this tuff. 

These hand specimens fit in a 3 3/4” x 5” specimen tray. We have a number of thicker or larger examples as hand/display specimens, same price, but more expensive to ship. These exhibit more of the pumice clasts and would be economical if purchased with other specimens, as they could ride in the same box with no additional shipping charge.

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