Geology of the Pacific Northwest with Instructor Eric Bard - Notes from the thrid class

By Keith Douville, Geology Class Student and Scholarship Volunteer

On September 24 we had our third class meeting of Geology of the Pacific Northwest at the Methow Valley interpretive center to discuss the Cascade Episode, which covered the early establishment of the roots of the Cascade Mountains that occurred to the west of the Methow Valley, and the great basalt flows to the Southeast. 

The Cascades in Winter.  Photo by John Scurlock

The Cascade Episode begins at the end of the Challis Episode and still exists today.  It is important to note right away that the uplifting and rise of the modern Cascade Mountains (and the Olympics which remained a shallow marine coast) did not occur until about 5-7 million years ago, and that for much of the Cascade Episode only slight elevation rise was occurring.  The Cascade Episode is marked by a northward subduction of the Kula plate, causing its eventual breakup and the installation of the Farallon or Juan de Fuca plate on the coast of North America, roughly about where the I-5 corridor lies today.  The Farallon plate was originally further south, near California, but moved northward and began a shallow subduction under the North American Plate.  This subduction of the Farallon plate produced the magmas of the Cascade arc. 

For long periods of time (36 million years), this magmatism along an Andes-type margin produced volcanic centers up and down the Pacific coast.  Here in the North Cascades, greater uplift has allowed much of the volcanic rocks to erode away leaving only the plutonic roots (such as Dome Peak and parts of the Pickets).  Further to the south, generally south of Snoqualmie Pass (such as in the Ellensburg formation, the Ohanapecosh, and Stevens Ridge) many of these volcanic rocks remain in place at the surface and cover earlier material.   In the Ohanapecosh, andesites, rhyolites, and ash/mudflow material is nearly 10,000 ft thick in places and hides evidence of earlier rocks effectively from view.  Not all of the early volcanic centers are known, but some ancient calderas identified include Fife’s Peak which shows an explosive history of volcanism.

 

Mount St. Helens

Today we have many stratovolcanoes in the Cascades such as Rainer, Adams, Baker, and Glacier Peak.   These large and steep volcanoes have magma filled chambers and can spew ash, pyroclastic flows, and lava.  These differ from calderas left behind because calderas are the collapsed remains after magma chambers have been expended and can cover even larger areas.  Some volcanoes are more gentle sloped and have lava which flows more easily, with less explosive power, and are known as shield volcanoes.  Many of our volcanoes are active, others are dormant, and few are dead.  These volcanoes provide beauty and can provide rich soils, but are also hazardous to live near, especially when covered in glaciers which can liquefy into lahars during eruption events.  Ashfall and earthquakes are a more likely hazard here in the Methow today.  Monitoring of seismic activity helps us manage this risk today and hopefully we will not experience major eruptions in our lifetime. 

Magmas evolve and this evolution can be seen in the rocks produces at different times in the life of a volcano as well.  Early on, basalts and andesites are found from activity.  Later, silicas found in rock indicate a more explosive nature from the disgorging of more root rocks.  Less mafic mineral gradually become present and we begin to see more dacites and rhyolites.  The Methow volcanoes show high silica content and andesites, indicating an explosive past.  The silica rich felsic rocks are formed from a tight network of bonds under high pressure. 

Despite the large amount of volcanism in the Cascade episode, it pales in comparison to earlier ones.  Essentially it has been a chain of volcanoes up and down the coast, occasionally rising above the coastal plain.  Volcanic intrusions primarily are located at fault zones.  Magmatic activity seems to be correlated with a steepening of the subduction zone.  Uplift only recently changed the landscape 5-7 million years ago, giving birth to the rain shadow effects that we see today.  The uplifting that occurred also created a “fold” with the Olympic Mountains to the west, the Puget Trough forming the low spot, and The Cascades to the east.  This folding that occurred relatively recently is probably caused by resistance to the western movement of the North American plate at the subduction zone, and this explains the uplifting. 

Basalt columns and terraces near Dry Falls.

The great basalt floods for which Eastern Washington is known also occurs during this period.  The Columbia River basalts in Washington and Oregon and the Chilcotin Plateau basalts of British Columbia are over 5000 feet thick in places and cover nearly all of the older geology with some 200,000 km3 of material.  Immense amounts of lava flowed through hundreds of dikes and spread out over the landscape all the way to the ocean.  This occurred many times, and the area originally covered is probably even more extensive than we realize due to erosion and uplifting of the Cascades, which occurred later.  Lava basalt flows such as these are usually seen at spreading centers between plates, or at hot spots where a mantle plume of magma extends through the crust.  The Yellowstone hot spot has left its mark across the North American plate as it moved to the west, and the timing seems right to attribute the Columbia basalt floods to it.  It also seems the North American plate covered a spreading center in California, and the consequent Nevada-Oregon rift zone lies in line with the Chief Joseph dike swarms where much of the lava appears to have originated.  So while still debated, the hot spot combined with a rifting zone provided the conditions for these floods.  The Ginkgo Petrified Forest near Vantage, WA contains logs that were once buried in mud and encased in the lava flows.  The species there tell of a time before the Cascades were uplifted, a time when arid conditions did not prevail.  

The Cascade episode is one I find very interesting due to its more recent activity than the other earlier episodes.  The basalt floods that are visible as you travel Eastern Washington are intriguing to me, and are indicative of what I thought of when I thought of the Cascades originally-lava, and lots of it.  I also like the idea of living among some of the youngest mountains in the world, only recently uplifted.  Makes you want to keep studying geology so you can keep unlocking mysteries, doesn’t it?