Key Words and Themes

Introduction

The structures (folds, reverse faults, and thrust faults) we observed in the Colorado Plateau and Rocky Mountains were the product of compressive forces transmitted inland from an ever-changing western convergent boundary.  For over 200 million years the subducting Farallon Plate transported island arcs and crustal fragments eastward into collision  with the west coast. With each addition the western margin of North America was extended further west. Compressive events, which we’ve come to know as orogenies (e.g. Sevier and Laramide orogenies), resulted from these collisions. Geologists call chunks of accreted debris exotic terranes—blocks of rocky real estate that are bound by faults, and distinctly different in history and lithology from their neighbors. The basement of the Cascades is an collage of exotic terranes, as are the Sierra Nevadas and the California Coastal Ranges.  The Olympic Peninsula, a conglomeration of oceanic basalts and deep-sea sediments, is the latest addition to the West coast. (See Olympic National Park.) To understand the complexity of terrane accretion along the west coast, page through the Mesozoic Paleogeography and Tectonic History, Western North America (Blakey and others). The details are far beyond the scope of this course.  However, you can at least develop an understanding of terrane accretion and its role in the building of the West.

The Cascades in the Pacific Northwest is the most volcanically active region in the conterminous United States.  Recently, major eruption have occurred on Mount Saint Helen, WA , 1980 and on Lassen Peak, CA, 1915.  While covering the Cascades you will learn a little about each of the following national parks and monuments:  North Cascades NP, WA; Mount Rainier, WA; Mount St. Helens National Volcanic Monument WA, Crater Lake NP, OR; and Lassen Volcanic NP, Ca. 

The Cascades

The Cascades are part of the Pacific Mountain System and extend approximately 500 miles from northern Washington State to northern California (figs. 1 and 2).  According to Kiver and Harris (1999) the Casacades are divided into the Northern, Middle and Southern Cascades (fig. 2).   Some include the Middle and Southern Provinces into a single Southern Province.  Only in the Northern Cascades of Washington can we see the complex assemblage of accreted terranes. In Oregon and Northern California the accreted collage is buried beneath a cover of Tertiary lava flows. The entire Cascade range supports a modern continental volcanic arc with several potentially deadly stratovolcanoes. (See fig. 2 for locations.) Excluding the towering volcanoes scattered throughout, the average peak elevation in the Southern and Middle Cascades is around 5,000 feet.  Elevations in the Northern Cascades average between 7000-9000 feet. 

The history of terrane accretion is best viewed in the exposed rocks in and around North Cascade National Park, Washington. (See Terranes of the North Cascades and Northern Cascade Geology).  As you can see, the geology is extremely complex.   The entire range of rocks from sedimentary-to-igneous-to-metamorphic are present. (Read WDNR description.) Each terrane is fault-bound and contains is own complex internal stratigraphy--often obscured by deformation and metamorphism.   Following accretion the region was subject to horizontal thrust faulting, and then sliced and dispersed along northwest trending strike-slip faults. Although the rocks are much younger than those in the core of the Grand Canyon and in the mountains of Glacier National Park they have been on the front line of a tectonic battle, and it shows.   Only where the large volcanic cones of Mount Baker and Glacier Peak rest is this complex foundation hidden in the North Cascades.
	Figure 1. Protracted subduction along the west coast, beginning in the Mesozoic, is responsible for the westward growth of Washington and Oregon.  Volcanism began around 43 ma (Late Eocene) and continues today.
	Figure  2. Subdivisions and active volcanoes in the Cascade Range. Boundaries between the Northern and Middle Cascades is the Olympic-Wallowa Lineament (1).  The Columbia River(2) separates the Middle and Southern Cascades.  Modified from Meyers, USGS/CVO. 2000

The Southern and Middle Cascades of southern Washington, Oregon, and northern California rest on a rugged platform of Tertiary volcanic rocks that flowed over and covered the accreted basement. Therefore, the exposed geology of the Middle and Southern Cascades lacks the complexity of the Northern Cascades. The Southern and Middle Cascades are noted for their dramatic stratovolcanos, such as Mt. Rainier, Mt. St. Helens, Mt Hood, and Mt Shasta, which tower like giants above the older volcanic platform. These tall volcanic mountains are refereed to as the High Cascades, or Eastern Cascades.

Arcs and Subduction

The remnants of an older volcanic arc, The Western Cascades, lies approximate 50 east of the present arc.  The proximity of a volcanic arc to the subduction zone, and even an arc's very existence is dependent on the angle of the subducting plate (fig. 3).  This is because melting won't occur until the plate has reached a depth of 60 miles or more.   The angle of subduction is in turn related to the proximity of the oceanic ridge where the sea-floor crust was created.  For the past 30 million years, the westward-moving North American Plate has been overtaking the oceanic-rise west of the Farallon Plate (now Juan De Fuca Plate).  The descending crustal slab is younger, more buoyant, and harder to subduct.   During the Laramide Orogeny, which uplifted the Colorado Plateau and Rockies, volcanism was shut off for a time, supporting the augment that the event was related to a period of very shallow subduction (c.f. fig. 3c). When the older Western Cascades formed 37-17 million years ago the oceanic rise was farther from the trench and subduction was steeper (fig. 3a).  Around 10-5 million years ago plate reorganization and shallowing of subduction shifted the arc to it's present location in the Eastern Cascades. From Mt Baker in the North Cascades to Mt Lassen(fig. 2) at the tip of the southern Cascades the activity along modern arc is related to a subducting remnant of the Farallon Plate known as the Juan de Fuca Plate.

Figure 3. Relationship of arc formation to angle of subduction.  The eroded Western Cascades were created during a time (37-17ma) of steeper subduction (a).  Shallowing of subduction shifted the arc to its present location in the Eastern Cascades (b) around 10-5 million years ago.  Geologist relate the older Laramide Orogeny (75-35 ma), which was largely unaccompanied by volcanic activity, to a period of flat-slab subduction (c).

Magmas and Volcanic features

Rocks created from the solidification of molten rock (magma) are igneous.   The Silver Plume Granite (Longs Peak) in the Colorado Rockies and the Purcell sill in Glacier National Park are examples of intrusive igneous features.  The volcanoes of the modern Cascades are extrusive landforms, created by magma that has reached the surface.

The extrusion of lava onto the surface can create a variety of volcanic landforms.  The type and size of the landform depends on the magma viscosity, gas content, and volume of ejected material (fig. 4).  The geometry and distribution of vents can also be important. Magmas are classified by their silica content.  Silica, made of the elements silicon(Si) and oxygen(O), is the basic building block of the most abundant class of minerals on earth, the silicate minerals.  The important point to understand is that high-silica lavas are viscous (fig. 4). They don't flow easily causing gasses to build up to violently explosive levels.  Eruptions are largely pyroclastic (video), which means that the eruptive products are spewed into the air.  Once the gas is removed the lava forms steep-walled, pasty flows that hug the vent forming domed masses of glassy rubble.  Nearly all of the prominent  volcanic peaks of the Cascade Range are stratovolcanoes--steep-sided mountains composed of alternating accumulations of andesitic-to-rhyolitc tuffs and flows accumulated over thousands of years. 

Low silica (basaltic) magmas behave quite differently.  They are more fluid and travel far from the area of eruption, often under their own cooled crust.  These basaltic magmas build broad platforms or plains (e.g. Snake River Plain) or gently sloping volcanoes known as shield volcanoes (e.g. Hawaiian Volcanoes NP). If gas is present, magma will be propelled into a glowing fountain. Falling debris from the fountain forms a steep cone of cinder surrounding the vent. Figure  4 illustrates these concepts and includes some of the features found in the parks covered below.

Figure 4. Diagram relating magma type to eruptive style and the volcanic landforms produced.