Saturday, May 4, 2013

Root of a Volcanic Arc, Fidalgo Ophiolite

I got a good look at the root of an ancient volcanic arc on Fidalgo Island south of Anacortes. Plagio-granite intrusions injected into one another as feeder dikes to a long disappeared volcanic arc. 





This volcanic arc root is an estimated 160 million year old volcanic arc that is one part of the Fildalgo Ophiolite, a slab of ocean crust that accreted to North American approximately 100 million years ago. Being a Western Washington University geology grad student I am of the accretion to the south school with later lateral emplacement and further thrust stacking (brown-gsa-cord-07-san-juans.pdf). I made a modest contribution to this interpretation with my thesis in the North Cascades.

In addition to the igneous intrusions we noted a fair bit of slikenslides indicative of some localized slippage along plains of weakness in the rock likely from more recent folding.



And a substantial overhang. A good dry spot, but a little stinky.

And great views.

The purpose of this rock inspection was to assess a rock fall hazard. The rock exposures and views were all gravy.


As can be seen in last picture, there were rocks that had tumbled to the forest floor below sitting directly on undecayed sticks and needles indicating that rocks were reaching some areas of the slope below. Basically we were doing an assessment on a much smaller scale to avoid this:


 

3 comments:

Dave Wenning said...

Dan, where was this, up on Mt. Erie?

Dan McShane said...

Yes, near your back yard so to speak.

Geoffrey Middaugh said...

As a non-geologist, this was interesting. So much so that I reviewed the GSA Brown article, and created my own nappes on the kitchen table. So that's who it works. Plate tectonics wasn't on the syllabus in my Geology 101 course a few years ago.