American Cordillera

The **American Cordillera** is a chain of mountain ranges. It is the western "backbone" of North America, Central America, South America and Antarctica. It is also the volcanic arc that forms the eastern half of the Pacific Ring of Fire.

The ranges begin with the Alaska Range and the Brooks Range in Alaska and run through the Yukon into British Columbia. The main belt of the Rocky Mountains along with the parallel Coast Ranges of mountains and islands continue through British Columbia and Vancouver Island.

In the United States, the cordillera branches to include the Rocky Mountains, the Sierra Nevada, and the Cascades and Coast ranges of Washington, Oregon, and California. In Mexico, the cordillera continues through the Sierra Madre Occidental and Sierra Madre Oriental, as well as the backbone mountains of the Baja California peninsula.

The ranges of the cordillera from Mexico northwards are called the North American Cordillera or Western Cordillera in the United States and Canada, and the Canadian Cordillera or Pacific Cordillera in Canada.

The cordillera goes on through the mountain ranges of Central America in Guatemala, Honduras, Nicaragua, Costa Rica, and Panama. It becomes the Andes Mountains of South America. The Andes with their parallel chains and the island chains off the coast of Chile continue through Colombia, Venezuela, Ecuador, Peru, Bolivia, Argentina, and Chile to the very tip of South America at Tierra del Fuego. Also, the range may be followed through the South Georgia Ridge across the Southern Ocean to the mountains of Graham Land on the Antarctic Peninsula.

## Causation

The cause of this remarkable geographical feature was the movement of the Americas away from the supercontinent of Pangaea. This began 200 million years ago.

The movement also explains the low-lying swamp features on the Eastern side of the two continents (most notably in Miami and Brazil).

This great feature is sometimes called the Continental Divide of the Americas. This refers to the drainage basins of the two continents.

## The Nazca plate

This oceanic tectonic plate sits east of South America and is moving under it. It is mainly responsible for the Andean mountain range. The subduction of the Nazca plate under southern Chile produced the largest earthquake  ever recorded on Earth, the moment magnitude 9.5 1960 Valdivia earthquake.

## California

California formed as a series of small island arcs, plus ocean sediments and oceanic crust. They were stuck to the western edge of North America, producing a series of deep basins and high mountain ranges. The oldest rocks date back to the Proterozoic era 1.8 billion years ago.

During the Cenozoic,  rock was stuck ("accreted") to the edge of California. Vast areas of marine sedimentary rocks were deposited. Examples of filled basins included the Los Angeles Basin, the Eel River Basin around Eureka and the 50,000 foot thick sedimentary sequences of the Ventura Basin. The San Andreas Fault became most active after the Miocene. It had up to 350 miles of offset in some places.

## Further reading

- Silberling N.J. _et al_ 1992. Lithotectonic terrane map of the North American Cordillera [Miscellaneous Investigations Series I-2176]. Reston, Va.:  U.S. Department of the Interior, U.S. Geological Survey.