Major subdivisions of the geologic time scale showing eras, eons,
and the Precambrian Supereon.
The phrase “eras tour” has entered our cultural lexicon following the outstanding success of Taylor Swift’s worldwide concert tour. The concept of her show was that it was based on the “eras” of her career. It explored her personal and musical journey through time. Likewise geologic eras can be used to reconnoiter the history of the planet and life.
In general, an era is a period of history characterized by certain events and attributes. Taylor’s eras coincide with the release of her albums. Each has a different theme and reflects events in a period of her life.
A geologic era is a type of geochronologic unit (e.g., an interval of geologic time) in the geologic time scale. The geologic time scale uses a hierarchy of geochronological units. From the largest to the smallest, they are:
Supereon
Eon
Era
Period
Epoch
Age
The development of the geologic time scale has a long and varied history that reflects the development of geology as a modern science. Prior to the existence of accurate numeric dating techniques, many of which utilize the radioactive decay of certain naturally occurring isotopes present in rocks, geologists determined the relative age of rocks largely using the fossils that they contain. Based on the fossils found in rocks of different age, geologists originally defined three eras of geologic time:
The boundaries between these divisions of geologic time are major events in the evolution of life. The Paleozoic Era began with the Cambrian Explosion when most major groups of animals, including those with hard parts (e.g., shells), rapidly appeared. A major extinction at the end of the Paleozoic saw the demise of 95% of all living species including the trilobites. Dinosaurs and many other groups became extinct at the end of the Mesozoic Era.
The Phanerozoic part of the geologic time scale showing its eras and periods. Most of the rocks exposed near Moab were deposited in the Mesozoic Era.
Together the Paleozoic, Mesozoic, and Cenozoic make up the Phanerozoic (“visible life) Eon. The rest of Earth’s history was designated the Precambrian because it came before the start of the Cambrian (the earliest period in the Paleozoic). However, it turns out that the Precambrian spans 87% of geologic time. It has subsequently been divided into three eons and seven eras.
The Proterozoic (“beginning life”) Eon was the time when simple organism that lacked hard parts thrived in the oceans before the Cambrian explosion. The Archean (from the Greek word for “early”) Eon reaches to the age of the oldest Earth rocks (4,031 million years old) and was a time when both the atmosphere and the Earth’s crust was very different than today. The Hadean (named for Greek god Hades) Eon represents the earliest interval of Earth’s history when conditions were hellish relative to today’s big blue marble teaming with life.
These older eons and eras of Earth’s history are not directly tied to the geology of southeastern Utah. Although it has a rich geologic history, Moab’s geologic story only covers the last 300 million years or so and spans a small part of the geologic time scale. Moab’s geologic eras are the Paleozoic, Mesozoic, and Cenozoic. A tour of these eras reveals the events that has led to the spectacular scenery of the canyon country of southeastern Utah. A geologic tour is strikingly different from a concert tour, but can be just as inspirational and is told through eras.
Moab’s Paleozoic Era
Like Taylor Swift, Moab also had a red era. Most of the oldest rocks exposed near Moab have a dramatic red hue and were deposited in the late Paleozoic. Sedimentary rocks from this era are exposed in the deepest canyons in southeastern Utah. The red color comes from the small amounts of iron oxide (hematite) that many of the rocks of this age contain. At the time, a high mountain range existed in western Colorado and sediments eroded from them were being deposited in southeastern Utah on continental flood plains.
Moab’s Mesozoic Era
The Mesozoic is the era of Earth’s history where Moab made its reputation. Most rock layers (formations to geologists) exposed near Moab were deposited during the Mesozoic. Almost all of the rock layers in Arches National Park and those that make up the upper layers of Canyonlands National Park are all Mesozoic in age. The Mesozoic may be better known in popular culture as the Age of Dinosaurs, and the Moab area also contains many important dinosaur fossils (both bones and tracks).
Moab’s Cenozoic Era
Relatively few rocks of Cenozoic age are found near Moab, although the La Sal Mountains formed during this time interval. The Cenozoic is important because that is when the current landscape with its dramatic cliffs and deep canyons came to be. Southeastern Utah became uplifted due to tectonic forces to high elevations during the Cenozoic. Later, during the last few million years, the Colorado River and its tributaries incised into the land, leaving cliffs, canyons, and rock domes that continue to erode to this day.
Late Paleozoic rocks exposed near the Colorado River below Dead Horse Point State Park. The canyons have been carved during the last few million years (during the late Cenozoic).
Mesozoic rocks exposed in Canyonlands National Park.
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.