Our bedrock consists of an exceptional diversity of rocks spanning over 100 million years of history. The diversity is due to our location at the convergent boundary between continental and oceanic plates. To enjoy this diversity you should visit Trinidad Beach, where the "Franciscan" subduction complex consists of a mix of rocks from both plates.
Blocks of hard rocks are set in a soft matrix of sheared shale and serpentinite, the latter representing metamorphosed sub-crustal mantle. The matrix is best seen at the base of sea cliffs in winter when big waves remove the sand.
The largest block is Trinidad Head, gabbro that was slowly crystallized from basaltic magma. The beach displays a variety of smaller blocks, wave-eroded into sea stacks*. Some consist of pillow greenstone, originally black basalt, exhibiting lumpy pillow structures which formed when red-hot lava intruded water and was rapidly chilled (see photo). Other blocks consist of chert — microcrystalline quartz — formed from countless microscopic silica skeletons that settled upon new oceanic crust. Each inch of chert, often interlayered with shale, represents a thousand years of accumulation. Sandstones were deposited rapidly from quake-triggered suspensions of coarse sediment coursing down the continental slope via submarine canyons. There are also a few metamorphic schists and gneisses that were pressure-cooked into foliated and coarsely recrystallized textures.
Scrutinize these rocks and you will have peeked into the depths of the Earth and aeons of time.
*Heidi Walters discussed off-shore sea stacks in the Journal's cover story of Oct. 13, 2005.
Prof. Ken Aalto is our local Franciscan expert.
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About The Author
Don Garlick
Don Garlick is a geology professor retired from Humboldt State University. He invites any questions relating to North Coast science, and if he cannot answer it he will find an expert who can. E-mail dorsgarlick@yahoo.com.
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