This red dacite rock is displayed next
to the Devastated Area Interpretive Trail in
Lassen Volcanic National Park, California.
The exhibit panel is entitled
“New Rocks, Old Rocks.” Two of the rocks aligned in
front of the panel—including this red dacite—are
described as beeing 27,000 years old, having formed when
Lassen Peak first erupted. The “new rocks ” are
light dacite pumice,
banded pumice, which formed during eruptions of
Red dacite can be found in Lassen Volcanic at several sites; for example, in the Chaos Crater area at the end of Chaos Crags Trail. Also, along the Devasteted Area Trail you can touch a giant boulder of red dacit at the “Old Giants” interpretive exhibit. The photo above demonstrates the coarse-grained texture of red dacite with white grains of quartz phenocrysts embedded in a grayish pink matrix.
Dacite is a lava rock: intermediate in composition between andesite and rhyolite, it contains more silica (SiO2) than andesite and less than rhyolite. A total alkali-silica diagram illustrating the composition range of synthetic dacite as a function of SiO2 and Na2O + K2O content is available in the scientific literature (for example, see DOI: 10.1007/s00445-008-0217-y/Fig. 1).