tuff - tan rhyolitic tuff with angular clasts of rocks ripped from the vent - teaching hand/display specimen
This particular volcanic igneous rock is composed of volcanic ash and fragments of other igneous rocks that were ejected from vents during the eruption of the Long Valley Caldera 760,000 years ago. The ash and rock fragments were welded together, forming a tuff sheet over 300 feet thick. Over 600 cubic kilometers of material was ejected, with an ash fall blanketing much of the western United States from the Pacific Northwest to Nebraska for a period of roughly six days.
The Long Valley Caldera eruption was the culmination of approximately 4 million years of volcanic activity related to stretching associated with Basin and Range extensional tectonics. Evolved magma began to accumulate and differentiate about 1 million years ago in the mid-to-upper crust and by around 850 thousand years ago had become compositionally zoned. Magma shifted in composition from basaltic to rhyolitic. The Long Valley eruption was likely caused by a recharge event that injected additional magma into the Bishop magma chamber, with the extra heat and gas leading to an overpressure and eventual violent Plinian eruption with collapse of the overlying rock into the magma chamber. This was one of the largest Quaternary eruptions in the world.
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 aphanitic lava that has the same composition as tuff. Aphanitic, from the Greek "a" for not, and "phaneros," for visible.
The first field photo shows the dark weathered surface that develops on this tuff. The second field photo shows boulders that have broken off the cliff more recently and which have not had as much time to develop the dark weathered surface. Does the weathered tuff look anything like the freshly-trimmed hand specimens? That's why a geologist usually has a hammer in their hand.
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