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The “rocky” history of North America: Supercontinents and plate tectonics

by Timber Press on October 18, 2017

in Natural History

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.

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 of North America’s geology.

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, but it’s a good place to start on the tour of the continent’s most breathtaking landforms found in Aerial Geology.

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. 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.

Here, steam and gases escape from a post-eruption lava dome of Mount Saint Helens. Images courtesy of Planet Observer/UIG.

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. Th rust 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. Th e 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.

Mary Caperton Morton is a freelance science and travel writer. A regular contributor to EARTH magazine, where her favorite beat is the Travels in Geology column, Mary also inspires people to “See More of the World” with her blog Travels with the Blonde Coyote.

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