Photo by Mike Fuhrmann/@PortraitOfAHiker. Rio Grande National Forest, Núu-agha-tʉvʉ-pʉ̱ (Ute) and Pueblos Native Lands.
By Callie Smith
What is the Continental Divide?
The Continental Divide Trail travels along the Great Divide which runs through both North and South America. The Great Divide separates the water that runs to the Atlantic and Arctic Oceans from the water that flows to the Pacific Ocean and expands all the way from Cape Prince of Wales in western Alaska, through the Rocky Mountains and all the way to the Andes Mountains of South America.
The Great Divide of the Americas is made up of a continuous series of mountain ranges which expand through Canada, the US, and Central and South America.
Geologic History of the CDT
The Rocky Mountains, which make up the Great Divide in the US, have a very interesting and complex geologic history. An ancestral mountain range, known as the Ancestral Rocky Mountains, once stood where the current Rocky Mountains now tower. These Ancestral Rocky Mountains existed in the late Paleozoic era between 320 million and 280 million years ago and were formed during the time when Africa and Northern America were coming together. These Rocky Mountains were eroded over time by rain, water, and glaciation, leaving layers of sedimentary rock behind, according to the University of Houston. This rock was later uplifted to form the current day Rocky Mountains.
According to the USGS, the Rocky Mountains are considered “weird” by geological standards. Most mountain ranges occur in areas where tectonic activity is present, however, this is not the case for the Rocky Mountains. Instead, the Rockies are located in the middle of a big and primarily inactive continental interior that isn’t close to a plate boundary. Normally, the Rockies would be expected to be only half the size of the mountains we see today.
Scientists were better able to understand this funky mountain range by studying tectonic plates off the coast of California. Around 70 million years ago a small tectonic plate made of dense ocean crust started sliding underneath the North American plate very shallowly. The shallow angle of this subduction (the sideways and downward movement of the edge of a plate of the earth’s crust into the mantle beneath another plate) meant that the farallon plate (an ancient oceanic plate that was eventually engrossed beneath the North American and Caribbean plates) could have reached farther east under the continental interior before plunging deep into the mantle, releasing water into the lithosphere (the rigid outer part of the earth, consisting of the crust and upper mantle) above. Water lowers the melting point of rock so scientists believe this newly melted magma migrated upward into the lithosphere above the sinking farallon plate. Sedimentary rocks from the western interior seaway and the pyroclastic material from this volcanism essentially buried the Rocky Mountains. Glaciers and rivers then eroded the less hardy sediment and revealed the tough core of the Rockies, which we see today.
Another puzzling thing about the Rockies is how they are able to stay upright at their size. Mountains require a crustal root underneath them that is thick enough to support their weight. There are no definitive answers as to what is keeping the Rockies afloat but one idea is that a combination of very dense crust underneath the mountains and hot underlying mantle is supporting the weight.
The spectacular Continental Divide Trail travels along the spine of these unique mountains for 3,100 miles from Mexico to Canada. We always knew the CDT was special but not THIS special!
Love learning more about the CDT? We’ll be sharing many more unique features and uses of the trail throughout 2022 in honor of our 10 year anniversary. Be sure to check out future blogs to learn more about this unique National Scenic Trail!
Callie Smith is the Community Engagement Coordinator at the CDTC. In her free time she enjoys whitewater rafting and cooking.
