Where This Rock Is From (place, type of locality, etc.)
Oregon. Found in a pile of aggregate that was dredged from the Willamette River somewhere south of Portand. I'm in the process of locating what drainage area the rock came from.
What This Rock Is
A flow-banded rhyolite, with phenocrysts of quartz, covered with a volcaniclastic ash with plagioclase and small amounts of olivine.
What I Like About This Rock
What I like is the flow banding and the parallel grooves on one of the partly covered sides of the rhyolite. I'm not sure what caused the parallel grooving. Also, although not observable without a hand lens, there are the micron sized bits of olivine in the ash. My guess on formation of this rock combination is a nonexplosive flow of rhyolite later covered with ash fall that came from deeper in the magma chamber, since olivine and rhyolite are widely seperated on the mafic to felsic range. I would appreciate any input on the causes of the parallel grooves.
Advice
- I look for differences in rocks from their surroundings.
- Gravel, aggregate and other materials used as commercial ground coverings can be serendipitous.
- First impressions are subject to recall.
