The Daily Bucket is a regular feature of the Backyard Science group. It is a place to note of any observations you have made of the world around you. Rain, sun, wind...insects, birds, flowers...meteorites, rocks...seasonal changes...all are worthy additions to the bucket. Please let us know what is going on around you in a comment. Include, as close as is comfortable for you, where you are located. Each note is a record that we can refer to in the future as we try to understand the patterns that are quietly unwinding around us.
San Juan Islands, Pacific Northwest
Jurassic era to present day
(All photos by me. In Lightbox...click to enlarge)
I like rocks. Some are very beautiful, and each tells a story much older than our short, ephemeral human lives. These rocks will still be here long after our species is extinct. The rocks on the beaches where I live are particularly various in color and texture and pattern...there are many stories here, hundreds of millions of years long.
One of my favorite kinds of rocks are the local greenstones. Here's a bit of their story.
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The San Juan Islands are composed of a series of overlapping layers of rock types, called terranes, brought along on an oceanic plate that is colliding with North America. Each terrane crashed onto the continent, piling over the last, kind of like the cop cars piling up in the Blues Brothers movie. Imagine mountain-sized, and in very sloowww motion. ...
Like that. OK,
so...
These geologic terranes are now diagonally angled so the edges are exposed to the air, with the cross section being the surface of the ground (where it isn’t covered by Ice Age deposits). These different chunks of oceanic rock that were carried onto the edge of the continent were formed in a variety of different geologic settings, far away, so we have quite a mix of bedrocks, cheek by jowl. Most were formed in the Jurassic, 150 million years ago, and accreted onto North America later on, in the Cretaceous. Some are plutonic mantle material, some are from underwater volcanoes, some are sedimentary rock from fragments big and small, and almost all these rocks have been at least partially metamorphized by the tectonic pressure cramming them together here at the edge of the continent. A more detailed account of our local geology can be found here.
Weathered and eroded from bedrock, fragments rest momentarily in jumbles on the beaches, rolling around in waves and currents when the tide comes in. Some are more eroded than others. This beach has boulder-sized rocks, so I generally take photos rather than collecting them. Other beaches have cobbles or sand, more collectible. Here's a view of this beach.
Come see my bucket of rocks below, and some more of the greenstone story...
Viewed from above in a geologic map of the San Juans, the different colors differentiate the various exposed terranes. The places with no color are covered with very recent Ice Age deposits covering the older bedrock. The glacial sediment was dropped here only thousands of years ago, but the rocks of the sediment are much older than that - millions of years - transported from their bedrock origins in Canada.
WASHINGTON DIVISION OF GEOLOGY AND EARTH RESOURCES GEOLOGIC MAP GM-53
Part of the island bedrock I live on is the Fidalgo Igneous Complex, much of which is greenstone, composed of an assortment of rock types like peridotite, gabbro, quartz diorite, tonalite, pyroxenite, andesite, basalt and chert, all of which contain a wide range of minerals. The stress of the accretion (car pileup) has partially metamorphosed the rock as well as cracking it, the cracks then filling with veins of quartz. It is quite a mix, as you might expect! The cobbles and boulders along the beaches are bits of these rocks, plus the broken-down bits from the other terranes, plus stones dropped by the Ice Age glaciers.
Walking the beach looking at stones is a lot of fun, with new discoveries each time as the water action rolls the rocks around every day bringing "new" ones up to the beach surface.
Many of them contain various proportions of olivine, a green mineral, hence: greenstone. These photos are all beach stones on Aleck Bay. Mid to low tide level is a good time because the stones are partially out of the water...when wet by the sea their colors are more beautiful. Or you can go out in the rain. That's when the beach photo above was taken, today.
Do you like rocks too?
Enjoy the greenstones...
(any you'd like to take home?)
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Finding any interesting things underfoot or elsewhere in your neighborhood? The Bucket is open for your nature observations.
And -
"Green Diary Rescue" is Back!
"Green Diary Rescue" will be posted every Saturday at 1:00 pm Pacific Time on the Daily Kos front page. Be sure to recommend and comment in the diary.