Rhyolite tuff:
lithified ash.
This rock is displayed in
“Sibley's Geological
Treasures” collection on the roofed patio of the visitor
center in the Robert Sibley Volcanic Regional
Preserve. The above caption is the same that is given on-site
(June 11, 2012).
Rhyolite is a light-colored volcanic rock composed of
alkali feldspar,
silica and minor amounts of
mafic minerals
(magnesium- and iron-rich minerals).
Rhyolite tuff is consolidated volcanic ash, containing rhyolite
constituents and pumiceous, glassy fragments (glass shards).
