The Precambrian
Learning Objectives
The Precambrian makes up the longest period of geologic time (87%).
It is divided into three eons based not on fossils but rather radiometric dates. The Precambrian was originally delineated because no visible fossils exist in Precambrian rocks so early geologists thought life did not exist during the Precambrian. Modern scientific techniques have revealed a wealth of microscopic fossils in some Precambrian rocks so the Precambrian time scale was revised.
Hadean: 4.6 - 3.8 billion years
Archean: 3.8 - 2.5 billion years
Proterozoic: 2.5 billion - 544 million years
Shield: the geologically stable interior of the continent that contains the oldest rocks on the continent.
Platform: the portion of the shield that is overlain by sediments.
Craton: both the shield and the platform combined.
Hadean Eon:
The Hadean Eon begins with the formation of the Earth 4.6 bya and ends with the oldest known rocks 3.8 bya. It is delineated by the fact that no rocks exist from this time. The end of the Hadean 3.8 bya corresponds with the oldest rocks known. (Recently rocks 3.96 bya were discovered in Canada) There is no direct evidence about the Hadean because no rocks exist from this period of Geologic Time. Geologists assume that during this period the surface was very hot and it continually melted and resolidified. With each cooling/melting cycle differentiation occurred to bring more and more of the lighter silicates to the surface.
Archean Eon:
The Archean Eon begins with the oldest rocks 3.8 bya and lasts until 2.5 bya. During this time the Earth was hotter than today and heat flow would have been faster. Most geologists agree that plate tectonics moved at a faster rate during the Archean. As the Earth cooled crustal rocks formed, collided, subducted and rifted. Throughout this time the Earth's crust continued to differentiate between lighter sialic rocks and more mafic rocks.
Two main rock associations exist from the Archean:
Greenstone belts: these rocks consist of sediments and pillow basalts with other volcanic material. They loosely represent the back arc basin of a subduction zone.
Granulite Associations: these consist of metamorphosed material and loosely represent the metamorphosed fore arc basin material from accretion and welding of microcontinents to form the shield.
By three billion years ago both Australia and South Africa had large stable continental interiors. In some places ancient environments are recognizable.
Archean Life:
Life began in the Archean. The atmosphere was much different than today's. There was no oxygen, so there was no ozone. Because of this most geologists agree that life began in the oceans. (Refer to class notes)
The exact mechanism for the beginnings of life may never be known but the Miller-Urey experiment illustrated how the building blocks of life, complex organic molecules and amino acids, could have formed from the chemical mixture present in the earliest oceans.
The oldest fossils date to 3.5 billion years ago and are the fossils of primitive bacteria.
During the Archean photosynthetic cyanobacteria developed. This was an important step in the evolution of life because of the impact it had on the planet. Cyanobacteria produce oxygen as a by-product. Oxygen was a poison to some early bacteria but eventually life evolved to live in an oxygenated environment. As cyanobacteria flourished in the early oceans they produced great quantities of oxygen. The oxygen saturated the oceans and then bubbled out into the atmosphere. Once oxygen became present in the atmosphere iron containing rocks began to oxidize forming the red beds or banded iron formations. After millions of years the oxygen produced by cyanobacteria saturated the atmosphere and the composition of the atmosphere changed. It is believed from geochemical studies that atmosphere seen on Earth today developed around 2.5 billion years ago.
Oxygen Revolution: this term refers to the oxygenation of the oceans and atmosphere. The banded iron formations are the geologic evidence of this event.