The Precambrian Supereon
The Longest Geologic Time Division
Aug 2, 2009 Alexandra Matiella Novak
The Precambrian Supereon makes up the largest amount of geologic time. The Precambrian stretches from the beginning of Earth’s formation – about 4.5 billion years ago – to about 0.543 billion, or 543 million, years ago. This is roughly 88 percent of Earth’s history and the most important events in Earth’s geological, atmospheric and biological evolutions occurred during this time.
This supereon is composed of three eons:
- Hadean
- Archean
- Proterozoic
The Hadean Eon
The Hadean Eon refers to the time immediately following the formation of planet Earth 4.5 billion years ago to 3.9 million years ago. During the Hadean the environment of the Earth was much different than it is today, an extremely hot molten surface with an atmosphere composed mostly of carbon dioxide unable to support life. In fact, the name Hadean is derived from the Greek name “Hades”, meaning “underworld". Very few rock formations are exposed today that date back to the Hadean Eon.
The Archean Eon
The Archean Eon is divided into four eras : the Eoarchan, the Paleoarchean, the Mesoarchean and the Neoarchean. It is during these eras that Earth began to evolve into the planet it is today, hence the name “Archean” which is derived from a Greek word meaning “beginning”. The earliest known fossils are found in rock formations deposited during the Paleoarchean about 3.5 billion years ago. This tells geologists and paleontologists that it was during this time that life on planet Earth first began to evolve. These first forms of life in the geologic record are seen as bacteria microfossils, such as stromatolites. This form of life became abundant throughout the Archean, but began to decline during the Proterozoic.
The Proterozoic Eon
The Proterozoic Eon marks a period of great changes on the surface and in the atmosphere of the Earth, leading to an environment much like what exists today. This last eon of the Precambrian spans from 2.5 billion to 543 million years ago. Eras included in the Proterozoic include the Paleoproterozoic, the Mesoproterozoic and the Neoproterozoic. The geologic record of the early Proterozoic is marked with rapid continental accretion and supercontinent cycles, which played an important role in the evolution of today’s plate tectonic system. Many rock formations in the stratigraphic record deposited at this time are exposed today in many areas of the world and tell geologists a lot about the changes in the environment during this time.
The Evolution of Modern Life
Besides evidence of the shaping and shifting of land masses, evidence of a changing atmosphere is also locked into Proterozoic rock formations. The presence of sedimentary layers stained red with oxidized iron minerals provides clues that about 2 billion years ago the Earth’s air changed to an oxygen-rich atmosphere. This would have been detrimental to the existing bacteria life and explains why the abundance of bacteria fossils is not present in rock formations from the early Proterozoic. However, this atmosphere would have now forced the adaptation and evolution of oxygen-dependent organisms, which is evident in the fossil record with the presence of eukaryotic forms of life.
From here, more complex species began to evolve and by the end of the last era in the Proterozoic – the Neoproterozoic – the earliest animals were present. Rock formations from the Neoproterozoic Era contain fossils of trilobites and archeocyathids, which were both soft-bodied marine animals that thrived in tropical environments. Rock formations from this era also include evidence of glacial deposits and four different periods of glaciation.
Precambrian rock formations are the oldest rock formations exposed today. For example, the Vishnu basement rocks of the Grand Canyon are the oldest rocks in the canyon and were deposited during the Paleoproterozoic Era of the Precambrian.
Source:
The Divisions of Precambrian Time
Vishnu Basement Rocks of the Grand Canyon
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