Being located at the junction of four states, Four Corners Monument might just as well be considered a political park. However, geology is everywhere and therefore this is a perfect example of a geological park as well. The Four Corners Monument is a Navajo Tribal Park that designates the boundaries between the states of Utah, Arizona, Colorado, and New Mexico. It also designates the boundaries between the Navajo Nation and the Ute Mountain Ute Tribe Reservation. We visited the park on my birthday back in March of 2019.
|Me, laying across all four states at the monument.
"... the Four Corners monument was established at the point he [surveyor Chandler Robbins] determined, to the very best of his ability and using the available technology, to be the prescribed location of 109 degrees 03 minutes West longitude and 37 degrees North latitude."
|A closer look at the geodetic survey marker, designating the actual Four Corners point.
The Four Corners Monument sits within the Colorado Plateau. An area that is being forced upwards by the subducted Farallon Plate.
|Location of the Colorado Plateau. Image courtesy of Woodward, 1973.
Starting ~100 million years ago, along the west coast of North America was a subduction zone. This is where one plate goes beneath another plate. In this instance the Farallon Plate subducted, or went beneath, the North American Plate.
|Graphic of the Farallon Subduction. Image courtesy of the NPS.
Over time the majority of the Farallon Plate was completely subducted, including the mid-ocean ridge (aka spreading center), leaving behind a new type of plate boundary along the coast of California. Instead of a subduction zone, there now was left a transform plate boundary, where one plate slides passed another one. This plate boundary is better known as the San Andreas Fault. Remnants of the Farallon Subduction Zone still exist along the coasts of northern California, Oregon, and Washington.
|The geological features of the Colorado Plateau surrounding the Four Corners Monument. Image courtesy of Woodward, 1973.
|Geological Map of the Four Corners Monument. Image courtesy of NGMDB.