Earth’s rigid outer shell is broken into dozens of major and minor tectonic plates. Most of North America sits atop the North American Plate.
The geologic story of North America is a fascinating one. It’s also more than 4 billion years long—much more than we could ever hope to cover here. However, if you’re curious about the world around you, enjoy the big-picture perspective, and are interested in some of the processes that are constantly reshaping our planet, here’s a crash course on the basics. It’s a good place to start on your aerial tour of the continent’s most breathtaking landforms.
The most powerful geologic force on Earth is plate tectonics, which governs the formation and breakup of continents (such as North America) and supercontinents (such as ancient Pangaea). The planet’s outer shell, called the lithosphere, is broken into eight major tectonic plates and many smaller ones, which slide around the planet, driven by the convective forces (motion created by heat) produced by the planet’s hot inner mantle and core. Plate tectonics move continents, build mountains, fuel volcanoes, and set off earthquakes.
North America as we know it was formed around 200 million years ago, after numerous journeys to the equator and back to its more mid-latitude location. Prior to that, it was a fragment of the supercontinent Pangaea, known as Laurentia. Pangaea amassed around 300 million years ago and began to break up around 200 million years ago, when the Mid-Atlantic Ridge (a spreading ridge that runs under Iceland) began opening, forcing apart the conjoined landmasses that would become North and South America, Eurasia, and Africa—and creating the Atlantic Ocean.
The majority of North America is situated on the North American Plate: a massive tectonic puzzle piece that runs from Mexico and the Caribbean to the Arctic, and from the edge of the Pacific Ocean to the spreading Mid-Atlantic Ridge under the Atlantic Ocean. As the Atlantic Ocean continues to widen at a rate of about an inch per year, the North American Plate is pushed toward the southwest, where it collides with the Pacific Plate—the major plate underlying the Pacific Ocean—and several smaller oceanic plates.
Earth’s rigid outer shell is broken into dozens of major and minor tectonic plates. Most of North America sits atop the North American Plate.
Continental crust is less dense than waterlogged oceanic crust, so the more buoyant North American Plate rides over the top of the Pacific Plate, forcing the oceanic plate downward, forming a subduction zone. The tremendous forces generated at these plate boundaries, or subduction zones, fuel earthquakes and volcanism along the active western margin of the continent, such as the magnitude 9 megaquake unleashed by the Cascadia Subduction Zone on January 26, 1700, and the eruption of Mount Saint Helens on May 18, 1980.
Tectonic plates are bounded and fractured by faults, where two adjacent masses of rock meet and move relative to each other. Major types of faults include transform faults, thrust faults, and normal faults. Transform faults, also known as strike-slip faults, occur when bordering volumes of rock slide past one another in a lateral motion, with little or no vertical movement. The San Andreas Fault on the coast of California is an example of this kind of fault, where the North American Plate is moving laterally to the Pacific Plate, at a rate of one to two inches per year. This movement sometimes occurs smoothly—aseismically—and other times makes violent jumps that unleash earthquakes.
Earthquakes can also be generated in the interiors of tectonic plates, where tectonic forces are squeezing the lithosphere together or pulling it apart in compressional and extensional landscapes. Thrust faults occur when compressive forces move blocks of rock over adjacent blocks, often creating mountains, sometimes with older layers of rock forced on top of younger layers. This reverses the usual order of geology, in which younger rocks sit on older rocks. Thrust faults can be found in the Rocky Mountains of Glacier National Park, where rocks more than a billion years old have been forced on top of rocks that are merely 100 million years old.
Normal faults occur in extensional environments, where Earth’s lithosphere is being stretched across a wide region. As the crust thins and weakens, blocks of rock drop downward, forming valleys and basins. The most famous example of this kind of tectonic setting is the basin and range area that stretches for hundreds of miles across Nevada into eastern California. Here, extensional stresses in the interior of the continent have created a rippled pattern of elongated mountain chains separated by broad valleys and basins.
The kaleidoscope of colors on this U.S. Geological Survey map represents the dominant—and diverse—rock types exposed at the surface throughout North America. Sedimentary rocks are represented in green, blue, and yellow; volcanic in orange and purple; and metamorphic in textured shades of the parent rock. For a closer look at the map, visit http://ngmdb.usgs.gov/gmna/.
The oldest rocks on the planet are more than 4 billion years old and some are found in North America. They exist as part of the Canadian Shield—a region of pre-Cambrian volcanic and metamorphic rock that forms the geologic core of the continent. The Canadian Shield stretches from the Great Lakes region north to the Arctic, underlying more than half of Canada. In turn, this shield is part of the North American Craton, a much larger, somewhat younger, mass of bedrock that also underlies the continent. These ancient forms are the foundation of North America. Over the past 4 billion years, the continent has taken many shapes, but these basement rocks have always made up the root of North America.
If you look at a geologic map of the continent, the swirling maze of colors representing the different kinds of exposed rock is downright psychedelic. North America’s eons-long habit of wandering down to the equator and up toward the poles has left behind a trail of myriad rock types that were deposited in wildly varied environments, ranging from hot, steamy tropics to arctic, ice age conditions.
Rocks are made up of minerals. Some rocks, like quartz, are composed of just one kind of mineral. Other rocks, such as gneiss, consist of several types of minerals—for example, quartz, feldspar, and mica. Rocks can generally be classified in one of three categories: igneous, sedimentary, and metamorphic. Igneous rocks can be erupted above ground at volcanoes, either in the form of oozing lava or explosive ash. Sometimes, volcanic eruptions take place underwater, through vents or fissures on the seafloor. When lava hits air or water, it cools quickly, forming dark, monochromatic rocks, often with lots of escaped air bubbles. Igneous rocks that form underground, such as granite, tend to cool slowly, allowing for large crystals to form. This makes these intrusive rocks especially crystalline, colorful, and hard.
Sedimentary rocks are formed when sediments accumulate in thick layers and are cemented into rock through a process called lithification. Sedimentary rocks often form with distinctive layers, representing varying conditions at the time the sediment was deposited. The most common sedimentary rocks in North America are sandstone (formed from many layers of sand and other fine-grained sediments) and limestone (made up of the calcium carbonate–rich skeletons of marine organisms such as corals, mollusks, and shelled protozoa).
Metamorphic rocks are transformed when existing rocks—igneous, sedimentary, or previously metamorphosed—are cooked by extreme heat and pressure, often deep within the planet or during episodes of mountain building. Metamorphism often significantly alters the chemical composition of the parent rock and changes its physical form into a completely different type of rock with a new set of properties. For instance, relatively soft sandstone can be metamorphosed into quartzite, one of the planet’s hardest rocks. Rocks that are already metamorphic may be cooked again and again, changing their properties each time.
Eons of uplift and erosion, mountain building and glacial carving, volcanic outbursts and super rifts have shaped the North American continent into one of the most geologically diverse landmasses on Earth. A wide variety of rocks have been laid down over many periods, with some stretches of time famous for producing types of rock that are many feet—even miles—thick. For example, during the Mesozoic Era, at a time when dinosaurs ruled the planet (roughly 250 to 65 million years ago), an immense inland sea covered much of what is now the desert Southwest. Sedimentary layers accumulated at the bottom of this sea would later be lithified into sandstone and sculpted by wind, rain, and freeze-thaw cycles into the sinuous canyons, soaring arches, and spooky towers that make southern Utah’s Red Rock Country world famous—there’s no other place quite like North America’s southwestern desert anywhere on the planet.
The rocks and landforms we see on the surface today are the result of differential erosion; various kinds of rock erode in different ways at different rates. Softer rocks weather readily, sometimes disappearing altogether from the topographic record, while harder rocks may resist erosion for billions of years.
Perhaps the most important thing to remember about North America’s—and every continent’s—geology is that all these processes are ongoing. As anybody who has witnessed a rockfall or felt an earthquake knows: geologic time includes now. And one of the best ways to wrap your mind around the incomprehensibly grand concept of geologic time is to see the landscape on a grand scale—from high above.
