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# Precambrian

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Precambrian: A period of time prior to the evolution of abundant fossils. The Precambrian is presently thought to have lasted for about 3.6 billion years. Remarkably little is known about the Precambrian and what is known has largely been discovered in the past four or five decades.

It is thought that the Earth itself coalesced from material in orbit around the sun roughly 4.5 billion years before the present and may have been struck by a very large (Mars sized) object shortly after it was created resulting in the creation of the moon. It is believed that a stable crust probably was in place about 4.2 billion years ago although the oldest dated rocks are about 3.8 billion years old. It is not known when life originated, but Carbon in 3.8 billion year old rocks from islands off Western Greenland may be of organic origin. Well preserved bacteria older than 3.46 billion years have been found in Western Australia. Probable fossils a 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the precambrian. Excepting a few dubious reports of much older forms from Texas and India, the first complex multicelled lifeforms seem to have appeared roughly 600 million years before the present. A quite diverse collection of soft bodied forms is known from a variety of locations worldwide between 600 and 544 million years ago. These are referred to as Ediacaran or Vendian faunas. Hard shelled creatures appeared toward the end of that time span. A very diverse collection of forms appeared around 544 million years before the present starting in the latest preCambrian with a poorly understood "Small Shelly fauna" and ending in the very early Cambrian with a very diverse, and quite modern "Burgess fauna".

Details of plate motions and such are only hazily known in the Precambrian. It is generally believed that most of the Earth's landmasses collected into a single supercontinent around 1 billion years before the present. The supercontinent -- known as Rodinia -- broke up around 600 million years ago. A number of glacial periods have been identified going as far back as the Huronian roughly 2.2 billion years ago. The best studied is the Sturtian-Varanger glaciation that took place around 600 million years ago and may have brought glacial conditions all the way to the equator resulting in a "Snowball Earth".

The atmosphere of the early earth is poorly known, but is thought to have contained very little Oxygen. It appears that many materials whose oxides are insoluble were present in the oceans for several billion years after the Earth's creation. It is thought that life evolved, and developed the ability to create organic compounds from water and Carbon Dioxide. Oxygen was freed in the process. The Oxygen was immediately tied up in chemical reactions with various materials -- primarily Iron-- until the supply of Iron ran out. After that the modern high Oxygen atmosphere developed. Older rocks contain massive Banded Iron Formations that were apparently laid down as iron and oxygen combined.

A diverse terminology has evolved covering the early years of the Earth's existence, but it is tending to fall out of use as radiodating allows plausible real dates to be assigned to specific formations and features. The terms Archean (roughly -- older than 2.5 billion years), Proterozoic (600 million to 2.5 billion years), and Neoproterozoic (545-600 million years) still have some general currency. Some additional terms are included in the Geologic time line.

Proterozoic -- As originally used, everything prior to the Cambrian boundary which has been placed at various times by various authors. As of 2000 AD, the Cambrian boundary seems to be settling firmly around 544 Million years before present. Modern usage would often be as described in the previous paragraph 600ma-2500ma.

Neoproterozoic -- Roughly from the Cambrian boundary back to about 900 million years. Modern usage may tend to be a shorter interval -- 544ma-600ma. Coressponds to Precambrian Z rocks of older North American Geology.

Mesoproterozoic -- Roughly from 900ma to 1600ma. Corresponds to Precambrian Y rocks of older North American Geology.

Paleoproterozoic -- Roughly from 1600ma to 2500ma. Coressponds to Precambrian X rocks of older North American Geology.

Archean -- Roughly from 2500ma to 3800ma.

Hadean -- Prior to 3800ma. It was probably intended originally to cover the period before any preserved rocks were depositied. A very few old rock beds seem to be slightly older than 3800ma.

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Last edited July 31, 2001 4:05 pm (diff)
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