By popular demand, Thor explains the colorful variety of stones found in our Pacific Northwest.
Sara asked me to contribute a blog on our recent obsession with rock tumbling. First a little prologue about the local geologic context, and why we have such a variety of stones here in the Pacific Northwest:
A few hundred million years ago, there was no Washington, Oregon, or California. The ocean lapped up somewhere in what is now Idaho, Nevada, etc. There has been a subduction zone on the west coast of North America, which means that Pacific Ocean crust has been sliding under the North American continent all that time. The sliding is not smooth, however; the rocks get stressed and bend and then break in earthquakes, gradually lifting the crust and producing mountains. The descending crust melts when it gets deep enough, and the rising hot magma produces volcanoes and also cooks the surrounding rocks. In addition, the Pacific Ocean crust has lots of islands and seamounts. These bumps get scraped off when they hit the continent and gradually grow the continent westward in a process known as continental accretion.
All of this squeezing and heating changes rocks in a process called metamorphism. Metamorphism can turn a rather blah sedimentary rock (rocks formed from lake, ocean, or desert sediments) into a beautiful stone with garnets and mica. So, we have distant rocks coming from the west and being metamorphosed, magma and volcanoes rising, and the whole mess being squeezed and uplifted into mountains that form a rich source of varied rock types. Then we had glaciers cover the whole state to a depth of a mile or more. If you flew over Washington 15,000 years ago, all you would see would be ice and the tips of the Cascade Mountains peeking out. The glaciers carved out chunks of rock and carried them downslope toward the ocean. We’ve had numerous glacial advances and retreats, so these rocks were dumped pretty much all over, and their source may have been hundreds of miles away. This means when you browse through the pebbles on the beach you are looking at volcanic rocks, granites, sedimentary rocks (sandstone, shale, limestone), and metamorphic rocks that may have originally come from hundreds of miles away. That’s why I love polishing beach rocks; each one tells a story.
I started collecting fossils when I was a kid. Around age 10, I would load some tools and a lunch into an old army pack my father gave me and hike through the woods to a local fossil locality. That is what kids did then; leave the house to wander around and get in trouble. But you better be home for dinner! I pursued this passion into high school and college, eventually getting a degree in geology and becoming a college professor teaching paleontology and geology. Sara insisted I add this photo of my last on-campus teaching day after 40 years, holding a cast of an extinct American lion skull:
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2 thoughts on “A Rambling Writer Guest Post: Thor Talks Rocks”
This reminds me of the beach walks I had with my father on the east end of Long Island. we got fascinated by the pebbles and when mum gave him a tumbler for Christmas our fate was sealed. She used to get quite cross with us when we forgot to empty our pockets before the clothes went into the laundry!
I’ve still got a few jars of pretty polished stones. Lots of varieties of quartz including some luminous clear ones.
Pence, how fun for you! Thor is now “obsessed.” I’m loving it, too. (from Sara)