File:Porphyritic andesite (Kate Peak Formation, Middle Miocene; Lyon County, western Nevada, USA) (15661069958).jpg

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Porphyritic andesite from the Miocene of Nevada, USA (4.6 cm across at its widest).

Andesite is an intermediate, extrusive igneous rock. Extrusive rocks form at or near the surface, or on the seafloor. Andesites are "supposed" to have an aphanitic texture (finely crystalline), but most are porphyritic. Porphyritic-textured igneous rocks have a mix of large and small crystals. The large crystals are called phenocrysts. Surrounding the phenocrysts is the groundmass, consisting of small crystals. Mineralogically, andesites are dominated by plagioclase feldspar, plus amphibole and/or pyroxene.

The andesite shown above is from an eroded Miocene stratovolvano in western Nevada. The whitish-gray crystals are plagioclase feldspar phenocrysts. Other phenocrysts in this material include biotite mica, hornblende amphibole, and pyroxene.

Stratigraphy: Kate Peak Formation, lower Middle Miocene, 14.53 Ma

Locality: Lyon County, western Nevada, USA
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Source Porphyritic andesite (Kate Peak Formation, Middle Miocene; Lyon County, western Nevada, USA)
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/15661069958 (archive). It was reviewed on 4 November 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

4 November 2019

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current14:41, 4 November 2019Thumbnail for version as of 14:41, 4 November 20192,126 × 1,756 (1.55 MB)Ainz Ooal Gown (talk | contribs)Transferred from Flickr via #flickr2commons

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