Thursday, January 2, 2014

The Stacks are Odded Against Us! Subduction's Gift in Southern Oregon and Northern California

Sea stacks on Highway 1 near Elk, in Northern California
Sea stacks are isolated rocky islands found along rugged coasts where wave erosion is intense. They form for a variety of reasons (from glaciers in places like Maine, for instance, or from lava flows in Hawaii). If the rocks vary in composition over short distances, some outcrops will be more resistant to erosion and may last longer against the onslaught of the breakers. This is the gift of subduction in areas like Oregon and Northern California. Rocks exposed along the coast tend to include a hodge-podge of ocean-related rocks, including graywacke sandstone, shale, basaltic ocean crust, limestone, chert, and the occasional seamount (seafloor volcano). Some are really tough, and others are easily eroded.
Sea stacks at Point Orford in Oregon
Subduction happens when a thick slab of oceanic crust and upper mantle (the lithosphere) sinks beneath another slab of lithosphere. The crustal material is drawn into the mantle where some of the rock may melt, producing volcanic activity at the surface above. In Oregon and Northern California the oceanic lithosphere is sinking beneath the edge of the North American continent along what is known as the Cascadia Subduction Zone. The volcanoes of the Cascades Range are the very visible evidence of this intense geologic activity.

Subduction zones produce some of the worst of geologic disasters, including massive earthquakes and devastating tsunamis. But the deformation of the Earth's crust also causes the formation of mountains and rugged coast lines that make some of the world's most beautiful scenery. And that is where today's post originates. I had the privilege of spending a few days making my way south along Oregon 101 and Highway 1 in Northern California on my way home from Christmas visits, and we saw a stunning variety of shoreline features including the sea stacks that I am emphasizing today.
Sea stacks near Brookings, Oregon, I think. Anyone recognize them?
The Cascadia Subduction Zone has also collected bits and pieces of continents from elsewhere (called exotic terranes) and mashed them into the edge of the continent. The rocks of the Klamath Mountains in southern Oregon and Northern California originated in this fashion. These metamorphic rocks contribute to the extreme variety of rocks found along the coast.
Restless seas in the morning at Fort Bragg in Northern California
The weather was supremely perfect for viewing the scenery (if not an utter disaster in the making for water-planners; the ongoing drought is the worst ever). There are storms out there somewhere because the waves were quite large at times.
At the mouth of the Navarro River just south of Mendocino in Northern California
This isn't the first time I've used the dumb pun above; I put up a series of sea stack pictures last year when I made my first journey down the Oregon coast in many years. The thing is, these stacks are completely different. There is a lot of very spectacular scenery, and every trip along that route will be a different experience.
Sea arch in a stack at Navarro Beach in Northern California near Mendocino.
Navarro Beach was our last long stop. We actually had to get home at some point after two weeks on the road. It was a truly beautiful day. Happy New Year to all!
Sea stacks at Navarro Beach on the Northern California coast.

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