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The activity below replaces this week’s WarmUp Quiz and ConcepTest. In order to receive credit, this activity must be completed before lab. After reading the pre-lab material, list the rock(s) commonly found in each geologic region. Rodinia: __________________________________________________________________________________ Kootenay Arc: _____________________________________________________________________________ Intermontane Superterrane: ___________________________________________________________________ North Cascades Superterrane: _________________________________________________________________ Successor Basins: ___________________________________________________________________________ Olympic Subduction Complex & Crescent Terrane: ________________________________________________ Columbia River Region: ______________________________________________________________________ Cascade Volcanic & Plutonic Arc: ______________________________________________________________ Region formerly covered by Cordilleran Ice Sheet: _________________________________________________ Lab partners:__________________ ________________________ _____________________________ ________________________ _____________________________ ________________________ Name: __________________________ TA: ____________________________ Day: ___________ Time: __________
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Figure 1. World map showing one possible reconstruction of the Rodinia Supercontinent (750 Ma). Pacific Northwest Farallon Plate North American Plate Volcanic arc Fore arc Subduction zone Figure 2. An illustration of a subduction zone. Lab #8: Geology of Washington 108 Geologic History of Washington State Region 1: Rodinia Only the northeastern part of Washington State has been around long enough to be called a native part of North America, all of the rest of Washington consists of foreign imports that formed some place else and were added to North America during the past 200 million years. The eastern part of Washington State is a remnant of a supercontinent called Rodinia that formed ~1.1 billion years, and began to split up by ~750 million years ago. The separate fragments of Rodinia are now dispersed throughout the world. Reconstructions are a bit uncertain but during the Late Precambrian it seems that Washington State was connected to either Siberia or Antarctica (Fig. 1). The remnants of Rodinia that make up northeastern Washington is continental crust, which is generally composed of granite. Region 2: Kootenay Arc The Kootenay Arc is a thick wedge of sedimentary and volcanic rocks that accumulated near the continental margin of North America between ~550-250 million years ago. It consists of sandstones, conglomerates, lava flows, and pyroclastic rocks that are typically formed along subduction zones (Fig. 2). The arc formed very close to the margin of North America, and was swept up to become part of the continent by the Intermontane Superterrane (Region 3) as it collided with North America about 160 million years ago.
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Figure 3. A map showing the different terranes that make up the North Cascades Superterrane. Image source: Geology of the Noth Cascades, A Mountain Mosaic by Tabor and Haugerud. Lab #8: Geology of Washington 109 Region 3: Intermontane Superterrane The Intermontane Superterrane is a huge block of rock that accreted to (“docked with”) the western margin of North America ~160 million years ago. In Washington, these rocks include the Kootenay Arc and extend to about the middle of the state, which would have been the Pacific coastline at that time. This superterrane consisted of a volcanic island arc (with andesitic magma) and marine sedimentary rocks (limestone) with fossils from the Tethyan Ocean. The Tethyan Ocean is an ancient ocean near Asia, which means that some terranes found in the Pacific Northwest travelled great distances. The Intermontane Superterrane also includes serpentinized ultramafic rocks brought up from the mantle (serpentine), as well as granitic plutons. After the various terranes joined to
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