The Paleozoic: Formation of a Continent
At the beginning of the Paleozoic, during the Cambrian, the area that is now the states of California, Oregon, Washington, Idaho, and Nevada did not yet exist as part of the North American continent. The edge of North America’s continental shelf was located at approximately the Arizona-Utah-Nevada-Idaho line (Figure 1.6). In the late Devonian (370 million years ago), a portion of the continental shelf adjacent to present-day Idaho and Nevada transitioned from quiet passive margin to an active subduction zone, where oceanic crust plunged beneath the continent. Here, as oceanic crust descended deep into the upper mantle, the rock above the descending crust melted to form a line of volcanoes on the surface. Subduction also led to accretion—sediment, sedimentary rock, and even bits of the oceanic crust itself were scraped off the descending crustal plate and pushed onto the overlying plate (Figure 1.7). Just as a rug develops folds when pushed from the side, these rocks were wrinkled up into mountains. Volcanic islands carried along by the subducting plate also accreted to the edge of the continent. The landmass began to rotate, moving the North American plate into a more modern orientation.
During the Carboniferous, plate tectonics led to the initial stages of Pangaea’s assembly. As North America began to collide with Gondwana, forces from the collision began to affect the continent’s topography. During the Mississippian (340 million years ago), most of the West Coast had transformed into a subduction zone. A series of exotic terranes, consisting of sedimentary rock made from former seafloor sediment, slabs of volcanic and granitic rock, and the remains of volcanic islands, collided with and accreted to western North America. These collisions deformed and elevated the continent’s topography, generating two major mountain-building events: the Antler Orogeny (340 million years ago) and the Sonoman Orogeny (245 million years ago).
Figure 1.6: The Northwest Central US during the late Cambrian, approximately 500 million years ago. The entire region is located in the southern hemisphere—note the position of the equator.
Continental and Oceanic Crust
The lithosphere includes 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 denser 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.
During the Pennsylvanian (300 million years ago), compressional forces from the collision and tension from coastal subduction combined to deform the continent’s interior, buckling the crust and creating deep basins between uplifted blocks. Shallow inland seas spread across the interior of the continent, covering parts of North America’s Precambrian shield (Figure 1.8). Uplift formed a mountain range, known as the Ancestral Rocky Mountains, in Wyoming, Colorado, and New Mexico, and land was also raised above sea level in Canada, Montana, and the Dakotas. Sediments that eroded from this range and other uplifted areas were transported to the inland sea and the continental margins, forming deposits of conglomerates, sandstones, shales, limestones, and evaporite minerals. Although the Ancestral Rocky Mountains has long since eroded away, remnants of its core remain, and can be seen today in Colorado and Utah. As accretion continued over time, the coastline moved farther seaward (Figure 1.9). Sea level fell in the late Paleozoic, during the Pennsylvanian and Permian, as continental collisions progressed to form the supercontinent Pangaea.
Figure 1.9: The Northwest Central US during the early Triassic, approximately 245 million years ago.
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.