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Home » FunFactFriday » #FunFactFriday – The Passive Margin

#FunFactFriday – The Passive Margin

Sweet Creek Falls in NE Washington

Most folks are aware of the basic, big-picture concepts behind Plate Tectonics, and that prior to their current configuration the continental land masses were assembled into a single supercontinent named Pangea, which existed roughly 250 million years ago. Pangea broke apart around 200 million years ago (approximately, the break-up wasn’t synchronous everywhere), and movement of tectonic plates transported the continents to where we have them today.

But this process of supercontinent break-up wasn’t a one-off! Prior to Pangea there was another supercontinent which existed between 1 billion and ~750 million years ago- named Rodinia. And when Rodinia began to break up is really when the geologic story of the Pacific Northwest begins!

So what’s up with this photo? This is Sweet Creek Falls, a short hike from WA 31 in the far northeast part of our state. I took this photo in early summer- and I bet that the fall colors are amazing right now… Though it’s not the waterfall we’re interested in, it’s the rocks the water is flowing over and in the plunge pool below!

The rocks that form the waterfall itself are slates- lightly metamorphosed shales and mudstones that were deposited in a deep ocean basin, these particular rocks initially deposited around 480- 450 million years ago. When supercontinents break apart, new ocean basins can form between land masses that were once adjacent to each other. The Atlantic Ocean is a perfect “modern” example of this, having formed by Pangea rifting apart. These particular slates at Sweet Creek falls were first rocks that formed in an ocean basin that was produced by Rodinia rifting apart! So in a sense, ~475 million years ago the Pacific Northwest didn’t exist, some rocks that we find today at the surface were under a few thousand meters of water…

And the rocks in the foreground plunge pool? These have a very different but related story! Some are slates that eroded from nearby, others have been transported here by the creek from some distance upstream. The tan-ish and speckled/ stripey rocks are a mixture of quartzite and gneiss; metamorphic rocks that formed when terrestrial (continental) sediments became buried in a very deep basin. The particular basin that these rocks originated in formed on the supercontinent of Rodinia prior to and during its break-up around 750- 500 million years ago!

So how did rocks that formed hundreds of kilometers apart (deep ocean basin vs. continental basin) and a couple hundred of million years from each other get to be in roughly the same place, and up at elevation in the Okanogan highlands? I’ll save that for another post! Plus I need to dig through a couple thousand photos to get a good example or 3 🙂


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