Eutaxitic texture in ignimbrite of the Cat Mountain Rhyolite, Tucson Mountains, Arizona

This is a great example of eutaxitic structure that commonly typifies ignimbrites, i.e., glowing volcanic avalanches. Pyroclastic materials are hurled far into the air, and when the pyroclastic cloud collapses on itself, and descends back to earth, the contents become flattened and compacted under the enormous weight of the pile. For example, pumice globs become flattened (and stretched), and the glass rims of pyroclastic materials flatten out and break apart. What emerges is a foliation texture, but not one created by metamorphism. The foliation is a crude planar parallelism of rock fragments and volcanic objects.

I collected this near Gates Pass in the Tucson Mountains in 2008.  I like the fresh nature of this rock, the slabby (fracture-controlled) form, and the conspicuousness of the eutaxitic structure.  It is good to have an example of foliations that are “primary” in nature, and not created through metamorphism.  

Age & Formation
Cat Mountain Rhyolite, ~70 Ma.