File:Hornblende andesite (Mount Shasta, California, USA) 3.jpg

English: Andesite from the Cenozoic of California, USA.

This is lava from Mt. Shasta, a prominent volcano in northern California. It is part of the Cascade Range, a north-south linear chain of active and potentially active volcanoes in America's Pacific Northwest. It extends from northern California to Oregon, Washington State, and into British Columbia, Canada. The Cascade Range formed as a result of tectonic subduction - the offshore Juan de Fuca Plate is diving below the North American Plate. The diving plate causes melting in the mantle. The melt rises and emerges at the surface at volcanic centers. Famous Cascade Range volcanoes include Mt. St. Helens, which had a large eruption in May 1980, Mt. Rainier near Seattle, Mt. Hood, which is the highest peak in Oregon, and Mt. Mazama, which destroyed itself 7,700 years ago in an enormous eruption that produced the modern-day Crater Lake Caldera.

The rock itself is andesite, an intermediate, extrusive igneous rock. It can be aphanitic (finely crystalline from relatively quick cooling of lava), but it is usually porphyritic, as is this example. The moderately small, black-colored phenocrysts are hornblende amphibole.

Locality: unrecorded/undisclosed site at or near Mount Shasta, northern California, USA

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Info. at:

en.wikipedia.org/wiki/Mount_Shasta