Chapter 4: The Cenozoic Era

snow-capped mountains in the background, canyon with river in the foreground
The West Elk Mountains (in the background) are the remains of large, ancient volcanoes.

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The Cenozoic Era began about 70 million years ago and extends to the present. In our book of 1,000 pages, the Cenozoic represents only the last thirteen pages!

The beginning of this era coincides with the birth of the Rocky Mountains. The event is known as the Laramide Orogeny (orogeny means "mountain building"). The cause of the Laramide Orogeny reaches back more than 200 million years.

At the end of the Triassic period, the great supercontinent known as Pangea began to break apart, and North America began to separate from Europe. Far to the west, the North American crustal plate began colliding with and over-riding the Pacific-Farallon Plate. The collision between the two plates caused the crust to buckle and fold. This folding started in California and gradually moved its way eastward, finally reaching Colorado about 60 million years ago.

The stresses caused by the colliding plates to the west forced several Precambrian crustal "wedges" upwards, forming the Colorado Front Range and the Southern Rocky Mountains. In some areas, the mountain building was accompanied by volcanic eruptions and magma emplacement.

The West Elk Mountains were not the only volcanoes erupting during this time. About 28 million years ago, a series of volcanic ash flows that originated from the San Juan Mountains blanketed much of southern Colorado. The tremendous caldera eruptions of the San Juans were characterized by turbulent, flowing clouds of hot incadescent ash, gasses, and tiny shards of volcanic glass. Such plinian-type eruptions are sometimes referred to as nuée ardentes or "glowing avalanches". As the turbulent ash clouds settled out, the burning-hot ash and glass shards welded together to form a dense, erosion-resistant rock called welded tuff.


Last updated: August 3, 2020

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