Commonly cited as an example of andesite (I’ve done it myself), today’s deskcrop is actually a porphyritic hornblende dacite. The sample is from Black Butte, which will be familiar to travelers of Interstate 5 in northern California. Black Butte is a dacitic dome immediately east of the interstate and just west of Mount Shasta. Black Butte can be seen on the left side of this GigaPan by Rich Gibson (misidentified in the title as a cinder cone) – Shasta is the snow capped peak on the right.
Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!