Geological Formation of the Canyons
From the geological point of view, the canyons are relatively young. They began forming about 90 million years ago. Between 80-40 million years ago, the greater part of the mountains of the western part of North and South America were built as a result of great crustal movements of the Earth and currents existing in the Earths Upper Mantle. (We call this activity Plate Tectonics).
These forces caused the intrusion in the central part of Mexico of granites, where they are found on the surface. These were emplaced 90 million years ago. Some volcanism accompanied their intrusion.
The eruptions were not necessarily as violent as those that occurred 28-18 million years ago. They caused thousands of meters of thickness of ash, dust and lava to be deposited. The ash became indurated until almost cement-like. With millions of years of volcanism, eruptions and coolings, a dense, hard cap formed (to make what we call the Rhyolite Plateau).
For the following 7 million years, the Barrancas were shocked by violent earthquakes, resulting from the faulting and tectonic activity that created the Sea of Cortez (Gulf of California.)
The land literally was pushed up and separated slowly and gradually what is now the Baja California Peninsula from the rest of Mexico.
Millions of years of erosion formed the Barrancas del Cobre. For the last 5 million years the Sea of Cortez has been filling in, and the Barrancas continue to deepen and straighten by the erosion and wearing away of the volcanic debris to the Sea of Cortez.
These Barrancas are the result of violent eruptions millions of times more powerful than occurred at Mt. St. Helens (1980).
In comparison with the Grand Canyon, the Barrancas are 25 million years younger but are four times as large. In some parts, 1000 meters deeper.
There are all three types of rocks present in the Barrancas: igneous, sedimentary and metamorphic. The igneous rocks are of volcanic origin, but not necessarily of eruption. Some cooled slowly some kilometers below the surface, others closer to the surface, resulting in different varieties of rocks.
The sedimentary rocks of volcanic origin cooled slowly near the surface, and show the effects of the entrance of water over millions of years. Some of these rocks can be seen at Divisadero Station.
The metamorphic rocks formed by slow cooling and high temperatures (and pressures) over millions of years.
Some interesting rocks which you may encounter are the granite "pomes" (left as erosion remnants), arsenic, calcite, quartz or gneiss. (Many minerals and semi-precious stones are also found in the area).
Note: Translated from Spanish by unknown author. Mary McNeil, translator. Comments in parentheses by translator. June 29, 1994
