The Western States: The Big Picture

The geologic history of the Western United States is a saga of moving continents and climate change that produced shifting coastlines, rising and eroding mountain chains, and ever-changing ecosystems. Though much of it is geologically young, its rocks and landscapes record over half a billion years of Earth history. Furthermore, it is a place where nearly all the processes that shaped it are still visible at some level.

The Western States are divided up into seven different geologic provinces (Figure 1.3): The Basin and Range (1), Columbia Plateau (2), Northern Rocky Mountains (3), Cascade-Sierra Mountains (4), Pacific Border (5), Alaska (6), and Hawai’i (7).

Figure 1.3: Geologic regions of the West.

With the exception of Hawai’i (which will be handled separately at the end of this chapter), the Western States are all on active plate margins (Figure 1.4). In the case of Alaska, Oregon, and Washington, thin oceanic crust is colliding with the thicker continental crust of the North American plate. As it does so, sediment, sedimentary rock, and even bits of the oceanic crust itself are scraped off the descending crustal plate and pushed onto the overlying plate. Just as a rug develops folds when pushed from the side, these rocks are wrinkled up into mountains like the Coast Range of Oregon and the Olympic Mountains of Washington in a process known as accretion (Figure 1.5).

Figure 1.4: Active plate margins of western North America.

Farther inland, as the oceanic crust descends deep into the upper mantle, the rock above the descending crust melts and forms a line of volcanoes on the surface. This process, called subduction, is responsible for creating the Cascades of Oregon and Washington as well as the Aleutian Islands and Wrangell Mountains of southern Alaska. Though they are also located on an active margin, the crustal plates in California are moving sideways past one another rather than colliding. This transform boundary, which includes the San Andreas Fault, is in reality a wide zone of north-south oriented faults with frequent and destructive earthquakes. Although volcanic activity is rare in this area, long, straight mountain ranges and troughs, such as Bodega Bay in northern California, are formed as blocks of crust are wrenched sideways.

In addition to tectonic activity, climate is a major player in the West’s geologic history. The past two million years, a time span called the Quaternary, have been a time of radical shifts in the Earth’s climate. Though we are currently in a period of rising global temperatures, much of the Quaternary has been characterized by ice ages in which glaciers repeatedly expanded to cover the northern half of North America, Europe, and Asia. During the most recent glacial advance, approximately 20,000 years ago, portions of the Cordilleran Ice Sheet buried southern Alaska and northern Washington under a mile of ice, carving deep fjords and glacial valleys. This ice sheet deposited huge quantities of glacial sediment in low-lying areas such as Puget Sound and also carved rugged mountain landscapes (Figure 1.6). Farther to the south, what are now modest mountain glaciers grew to become ice caps covering entire mountain ranges such as the Sierra Nevada and the Blue Mountains of Oregon. Even Hawai’i, which is now ice-free, saw small glaciers on the summit of Mauna Kea, its highest peak.

Figure 1.5: An idealized cross section of an active margin where an island arc (1) is added to the edge of a continent (2).

During each ice age, sea level dropped as more and more seawater became locked up in glacial ice. As the oceans fell, coastlines moved farther out to sea. Later, as the climate warmed and sea level rose, the former coastal lands were flooded, drowning river valleys, glacial valleys, and coastal plains.

Continental and Oceanic Crust

The lithosphere has two types of crust: continental and oceanic. Continental crust is less dense but significantly thicker than oceanic crust. The higher density of the oceanic crust means that when continental crust collides with oceanic crust, the more dense oceanic crust will be dragged (or subducted) under the buoyant continental crust. Although mountains are created at these oceanic/ continental crust collisions due to the compression of the two plates, much taller ranges are produced by continental/ continental collisions. When two buoyant continental crusts collide, there is nowhere for the crust to go but up! The modern Himalayas, at the collision site of the Asian and Indian plates, are a good example of very tall mountains formed by a collision between two continental crusts.

Figure 1.6: Continental glaciers originating in Canada spread across North America, including Alaska and Washington, during the Quaternary period.

Understanding Plate Boundaries

Active plate margins are the boundaries between two plates of the Earth’s crust that are colliding, pulling apart, or moving past each other as they move over the mantle. When one plate slides beneath another, it is called a convergent boundary or subduction zone. When two plates pull apart from each other, it is known as a divergent boundary or rift margin. When the plates slip past each other in opposite directions, it is called a transform boundary.