Adakite near Chimacum, Washington

Tamanowas Rock rising above the access gate. Peninsula Daily News.

Dan McShane has posted another neat geological location on his website, Reading the Washington Landscape. He visits an exposure of an oddball type of felsic lava and pyroclastics, adakite, at Tamonowas Rock, near Chimacum, a little south of Port Townsend. It’s not really a field trip, because his post doesn’t give much in the way of directions, or describe the outcrops. Use the Google Earth map on Dan’s post to find the trailhead; the east facing rock cliff rises just to the west of the road. Dan’s photos look pretty neat. I’ll definitelyvisit this place, probably when I investigate the basalt dike nearby at Nodule Point, which both Dan and I have written about previously.

Until I know more, I can expand only slightly on Dan’s post about Tamanowas Rock. This is a cliff of Eocene subaerial adakitic lava and lava breccia just west of the town of Chimacum. Adakite is a type of lava similar to dacite, and is interpreted to be derived from the melting of subducted ocean floor basalt. Generally, lavas (be they basaltic, andesite, dacite, or rhyolite) that erupt in convergence zones, such as Cascadia, are believed to result from partial melting of the wedge of the mantle (not the descending ocean plate) that lies between the subducted ocean slab and the overiding continental plate. However, there is evidence that small amounts of the subducted ocean floor basalt itself may also melt and add a recognizable trace element signature to some rare erupted products. The pyroclastic rocks exposure at Tamanowas consists of angular clasts of hornblende dacite up to one meter in diameter in a fine-grained matrix. Dan has a link in his blog’s post to a 2004 GSA abstract written by undergraduate geology students in Jeff Tepper’s petrology class at University of Puget Sound. Jeff regularly involves many of his students in petrologic research, pretty cool for an undergrad program! A number of their projects have been published. Another link on Dan’s post will take you to more than you could ever want to read about adakites. A news article on the S’Kallam tribe’s effort to protect the rock, sacred to them, appeared in the North Kitsap Herald in April 2010. A search using ‘Tamanowas Rock’ will net a number of websites dealing with this land issue. A Peninsula Daily News article with photos is here. I’ve borrowed some of theirs for this little post, with credit.

So, stay tuned for an eventual field trip guide to Tamanowas Rock to be published on this website.

Dave

The gated road at the access point. Photo Peninsula Daily News.

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