The Paleozoic and Precambrian: Looking to the Distant Past

Over 600 million years ago, the entirety of what is now the Western States was either underwater or had not yet become part of North America. Seafloor sedimentary rocks found in the Klamath Mountains, the Sierra Nevada, the Basin and Range, and the central mountains of Alaska indicate that the entire region was underwater during the early Paleozoic and that the coastline of ancient North America was somewhere to the east, near Arizona, Utah, and Idaho (Figure 1.7).

Figure 1.7: The Western United States at 600 million years ago. The entire region is located in the southern hemisphere.

In the early Paleozoic, about 500 million years ago, what is now the west coast of North America was a passive margin—little or no volcanic activity, earthquakes, large mountain ranges, or plate boundaries were found in the Western States at that time. As the Earth’s continents began moving towards one another to eventually form the supercontinent Pangaea, the Western States became an active subduction zone, which began uplifting new mountains. Subduction also led to accretion, adding volcanic islands and seafloor sediment to the edge of the continent. One such episode created much of the present-day basement rock in Nevada and southern Oregon. Other periods of accretion created the Okanogan Highlands and sections of Alaska’s central mountains. By 400 million years ago, large mountain ranges had risen in parts of Alaska and the Basin and Range of Nevada and Oregon. As accretion continued over time, the coastline moved farther seaward. The landmass also began to rotate, moving the North American plate into a more modern orientation.