The Earth is estimated to be approximately 4.6 billion years old—an age obtained by dating meteorites. Rocks dating to around four billion years old are found on almost every continent, but they are not found at the Earth’s surface anywhere in the Southwest. The oldest rocks known on Earth are 4.3-billion-year-old rocks found along the eastern shore of Hudson Bay in northern Quebec. These are part of the Canadian Shield, the ancient core of the North American continental landmass, which has experienced very little tectonic activity (faulting and folding) for millions of years. Shields, or cratons, are the stable cores of all continents and are often covered by layers of younger sediments. They formed and grew during pulses of magmatic activity, as bodies of molten rock deep in the Earth’s crust contributed to form new crust. In the Southwestern US, the main cratonic elements are referred to as the Mojave Province (western Arizona and Utah), the Yavapi Province (Colorado, central Arizona, northwestern New Mexico, and southeastern Utah), and the Mazatzal Province (southeastern Arizona and New Mexico) (Figure 1.4). The composition of many of these rocks, suggests that they formed very deep in the Earth’s crust, perhaps 20–25 kilometers (12–15 miles) below the surface. Others originated in island arcs formed by subduction and associated volcanism and sedimentation. All were subsequently intruded by magma from deep in the mantle.

Figure 1.4: Cratonic elements of the Southwestern US.

In the Southwestern US, the oldest known rocks found at the surface are late Archean to early Proterozoic metamorphic rocks known as the Grouse Creek Block and the Farmington Canyon Complex, in what is now northwestern Utah (see Figure 1.4). These two sets of rocks formed between 2.5 and 2.6 billion years ago. Together with the other cratonic elements (frequently referred to as provinces or terranes), they make up the core of what is now the Southwest. As separate volcanic island arcs collided with each other in the early Proterozoic (between 1.8 and 1.6 billion years ago) (Figure 1.5), plate tectonics and the process of collision eventually formed a supercontinent known as either Nuna or Columbia. The collisional zones between these continental fragments are preserved as long belts of deformed metamorphic rock. Today, they form the subsurface basement rock that underlies much of the Colorado Plateau. Most of the Southwest’s cratonic rocks are deeply buried, but Yavapai Province rocks are visible at the bottom of the Grand Canyon in northern Arizona.

Figure 1.5: Plate tectonic model for the formation of crust by the addition of island arcs to the continent. Some of these terranes are now represented by the oldest rocks at the bottom of the Grand Canyon. The area between the trench and the arc is the fore-arc region, and the area behind the arc (i.e. on the side farthest from the trench) is the back-arc region.

Following its final assembly, the Nuna supercontinent continued to grow by volcanic and magmatic activity along its margins. It began to fragment about 1.6 billion years ago, in a process that lasted for several hundred million years. At this point, the continents began moving back toward each other, and the remainder of the Precambrian period saw the formation of a third supercontinent, called Rodinia (Figure 1.6). This landmass was fully formed by about 1.1 billion years ago. Preserved remnants of the continental collisions that formed this supercontinent are found widely across modern North America, including the Pikes Peak Granite in Colorado. Other remnants are found outside the Southwestern US in the Llano province and El Paso region of western Texas, and farther to the east in the Grenville rocks of the Adirondack and Appalachian mountains.

Figure 1.6: The supercontinent Rodinia, circa 1.1 billion years ago. Laurentia represents proto-North America.

The breakup of Rodinia, beginning nearly 800 million years ago, was associated with the formation of rifts throughout North America. Igneous activity occurred in rifted zones and continued slowly and irregularly until about 600 million years ago. North America’s rifted edges formed passive margins, where sediments were deposited on continental shelves into the early Paleozoic era. The rifted margin of western North America became a shallow continental shelf that extended from Canada through western Utah into southern California. Rivers brought sediment from the continent’s interior and deposited it on the shelf. In Arizona, these sediments form a thick sequence of sedimentary layers known as the Grand Canyon Supergroup, which constitutes one of the most complete middle to late Proterozoic geologic records in North America. These rocks record a long history—perhaps 200 million years—between the formation of continental crust by terrane collision and accretion to the more familiar events of the Cambrian period and later.

During the late Proterozoic, large areas of the continents were repeatedly covered with glaciers, including some that reached into low latitudes. Many geologists think that this interval may have included the most intense and widespread glacial development in Earth’s history. The term “Snowball Earth” is used to describe this proposed state, during which most of the Earth’s surface (pole-to-pole) was covered by glaciers. In the Southwest, geological evidence for this glacial expansion is represented by rocks formed from glacially derived sediments (tillites) found in western Utah.