File:Pillow basalts & jasper (Franciscan Complex, Jurassic; Point Bonita, California, USA) 1 (40298109333).jpg

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Pillow basalts in the Jurassic of California, USA.

The bulbous, rounded masses of volcanic rock seen here are pillow basalts. The morphology is diagnostic of underwater lava eruptions. These rocks are part of the Franciscan Complex - they represent the top portion of Jurassic-aged oceanic crust. This basaltic crust originally formed at an ancient mid-ocean ridge and got plastered onto the edge of the North American continent along a now-inactive subduction zone. The upper portions of the subducting oceanic slab (the Farallon Plate) got scraped off to form an accretionary wedge.

The dark red areas are red chert, also known as jasper.

The basalts have been metamorphosed into greenstone, but much of the outcrop is still dark gray to black in color.

Stratigraphy: Franciscan Complex, Jurassic

Locality: Point Bonita, southwestern tip of the Marin Peninsula, Golden Gate National Recreation Area, north of San Francisco, coastal-western California, USA
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Source Pillow basalts & jasper (Franciscan Complex, Jurassic; Point Bonita, California, USA) 1
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@N05/40298109333 (archive). It was reviewed on 10 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

10 October 2019

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current00:22, 10 October 2019Thumbnail for version as of 00:22, 10 October 20193,008 × 2,000 (4.86 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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