Monday, May 06, 2013

Rosario Head

My previous posts from here have been about the beach (September 2009, March 2012).  But Rosario Head, which defines the southern side of this pocket beach, is a classic destination for geologists trying to figure out how Fidalgo Island was actually built.


Rosario Head is a spectacular exposure of deep-ocean sediments - very old ones.  Pillow basalts, erupted beneath a Jurassic sea, cherts deposited from a rain of radiolaria in deep water away from any significant sediment sources, and black argillites. These seabed rocks are typical of the upper portion of ophiolites, which are basically preserved sections of oceanic crust.

A vast majority of the world's past ocean bottoms have been dragged by subduction back into the mantle and recycled, but on complicated plate margins like ours here in the Pacific Northwest small slices of these oceanic rocks can get thrust up onto the edge of the continent and preserved.  Preservation is a messy exercise, however, and more often than not the rocks are sliced and diced at many different scales -- like these.

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