Geology of the Pacific Northwest with Eric Bard -- Class 1 Notes

By Keith Douville, student in the class
 

Geology of the Pacific Northwest is the latest course offered by the Methow Conservancy.   Hosted at the new Methow Valley Interpretive Center in Twisp, this class is very popular and 25 eager students excitedly filled the center this Monday to learn about the mysteries of our area that lie hidden beneath our feet.  Our instructor, Eric Bard, began our geologic time travel adventure with a big picture look at some of the forces and their effects on this region. 

Our class, rocking on!

In my first attempt at a blog, I will document parts of this class in the hopes of allowing you to take part in it with us.  I will do my best to discuss concepts presented but encourage you to continue this work with further reading and inquiry as I am somewhat of a novice to the physical sciences.  I will include links for you to explore at the end of this blog.

The Pacific Northwest is a very young place, geologically speaking.   The land around us did not form until Jurassic Time, and since that period a great number of very complex changes have taken place.  This makes understanding of our local geology a complex task.  While this may make some cringe at the daunting task of making sense of it all, it is really a blessing for geologists both professional and amateur to have opportunities to unlock mysteries as theories develop and change with each new breakthrough in scientific understanding.  Part of the challenge (and fun!) for beginners like me is to think of things on a geologic time scale based on millions of years. 

One of the first concepts to understand as we discuss geologic history is the theory of plate tectonics.  Our earth has a crust of solid land which floats upon a plastic-like liquid mantle of molten rock.  The molten rock of the mantle moves as heat is dissipated into the cold of space; much like boiling water in a pan circulates.  This movement of the mantle causes shifts in the earth’s crust, and rifts form in the crust causing tectonic plates to form.  When plates move, they collide with other plates in predictable ways.   Convergence occurs where plates collide, causing buckling or subduction when heavier, denser, older plates under the ocean sink below lighter plates of the continent.  In other situations plates slide and slip laterally against each other, known as a transform boundary.  Plates which are converging at one point are diverging at others, allowing extrusion of the mantle and formation of new crust material.  The movement of plates is slow and constant, at about the same speed that our fingernails grow.  Occasionally plates get stuck and pressure builds, until a breaking point is reached and sudden movement (an earthquake) occurs.  The deepest and strongest earthquakes occur at subduction zones.  Convergent plate plate margins are considered to be active margins as is the Pacific NW due to subduction off the west coast. 

Prior to the formation of the Pacific Northwest, supercontinents made up of the large land masses of the world existed.  We do have some ancestral rocks from this time such as dark gray argillites found in the Colville area and east into Montana, and isolated outcrops of gneiss in the Pacific Northwest - especially along the eastern fringes of Washington.  The supercontinent Rodinia rifted near the modern Spokane area, and that is where the west coast of the ancient continent of Laurentia (North America) met the oceans at a passive margin.  The supercontinent of Pangaea eventually followed, and the Atlantic Ocean began to form as North America pushed to the west.  The early Rocky Mountains formed to the east due to active subduction of what was known as the Farallon Plate during the Mesozoic Era.  This marks the beginning accretion phases of Pacific Northwest geology, a very important concept.   During these accretion events, geologic terranes collide and form, adding to the west coast of North America.  The Kootenay arc was a deformation event with rocks folding and buckling under the pressure.  Because this was a coastline, many of these rocks are made up of marine sediments that had eroded from the landform of Laurentia.  Eventually this now active margin of western “North America” became magmatic and this true volcanic arc further fragmented earlier evidence of prior time periods. 

Now into the Jurassic time period with an active margin on the west coast of North America, it is time to begin adding land – and lots of it – to create much of the Pacific Northwest as we know it today.  The Omenica episode came first with the Aleutian-like Intermontane Belt of stuck together islands crashing into the westward-driving North American continent.  This collision ended the Kootenay arc and created a new subduction zone again on the new west coast with an associated volcanic arc.   In our immediate area, our piece of the puzzle is known as the Quesnella-Oakanogen subcontinent or geologic terrane.  The volcanic arc associated with this collision allowed intrusive granite plutons (pockets of slowly cooling magma underground) east of the Pasayten fault such as the Loomis pluton, Toats coulee, the green rocks of Knob hill above Palmer Lake (these are volcanic metamorphosed basalts), and later Oval peak and the great Goat Wall.  Deformation of rocks associated with the collision occurred as well, and uplifting of lighter limestones is evident in the landscape, with marine fossils in pockets as well.  As volcanic activity increased along the new islands forming the western margin of the continent (which was the trailing margin of the Intermontane Belt), erosion from mountains both east and west caused thick sediments up to 4000 meters thick to fill what became the Methow basin. 

This pattern continues again and again, adding new lands to the west of the Methow, in each case with subduction, volcanic arcs and magmatic intrusions, erosion and sedimentation, uplifting, and faulting causing an immense variety and complexity to the landscape.  The geological landscape changes rapidly in relatively short distances, and this landscape is further changed by glaciations.  Because loose rocks have been moved far and wide from their original positions by ice, it is important to look for bedrock outcroppings as you interpret the geology of an area, and recognize that anything smaller may have travelled from far to your site to confuse you. 

In a very general summary, the lands in the Pacific Northwest west of Spokane area formed very recently from our North American plate crashing into the oceanic plates again and again.  This highly dynamic process has given us a varied and interesting and complex landscape of unique geology, but one that is fascinating to interpret.  Next week we will take a deeper look at the rocks of the Methow area and see what kind of story they can tell us. 

Further Reading:

Evolution of the Pacific Northwest, an excellent free text!  http://www.northwestgeology.com/

The Rock Cycle  http://www.windows2universe.org/earth/climate/geosphere_rockcycle.html
Burke Museum site:  http://www.burkemuseum.org/static/geo_history_wa/

Paleomap Project:  http://www.scotese.com/
UC-Berkley Geologic Time Scale: http://www.ucmp.berkeley.edu/help/timeform.php

Keith has spent his first summer working on the Beaver Project crew here in the Methow Valley and he's eager to learn all he can about this amazing place.