Everything about Pre-cambrian totally explained
The
Precambrian (
Pre-Cambrian) is an informal name for the supereon comprising the
eons of the
geologic timescale that came before the current
Phanerozoic eon. It spans from the formation of
Earth around 4500
Ma (million years ago) to the evolution of abundant macroscopic hard-shelled animals, which marked the beginning of the
Cambrian, the first
period of the first
era of the
Phanerozoic eon, some 542 Ma. It is named after the Roman name for
Cymru/
Wales -
Cambria - where rocks from this age were first studied.
It isn't an "era" - it's known simply as "Precambrian"
Overview
Remarkably little is known about the Precambrian, despite it making up roughly seven-eighths of the
Earth's history, and what little is known has largely been discovered in the past four or five decades. The Precambrian fossil record is poor, and what fossils are present (such as those of
stromatolites) are of limited use for
biostratigraphic work. Many Precambrian rocks are heavily
metamorphosed, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath
Phanerozoic strata.
It is thought that the Earth itself coalesced from material in orbit around the sun roughly
4500 Ma and may have been struck by a very large (
Mars-sized)
planetesimal shortly after it formed, splitting off material that came together to form the
Moon (see
Giant impact theory). A stable crust was apparently in place by 4400 Ma, since
zircon crystals from Western Australia have been
dated at 4404 Ma.
The term
Precambrian is somewhat dated, but is still in common use among
geologists and
paleontologists. It was briefly also called the
Cryptozoic eon. It seems likely that it'll eventually be replaced by the preferred terms
Proterozoic,
Archaean, and
Hadean, and become a deprecated term. (See
geologic time scale.)
Life before the Cambrian
»
It isn't known when life originated, but carbon in 3800 million year old rocks from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3460 million years have been found in Western Australia. Probable fossils 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 contested reports of much older forms from Texas and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600 Ma. These are referred to as
Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that timespan.
A very diverse collection of forms appeared around 544 Ma, 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, the rapid radiation of forms called the
Cambrian explosion of life.
Planetary environment and the oxygen catastrophe
Details of
plate motions and such are only hazily known in the Precambrian. It is generally believed that small proto-continents existed prior to 3000 Ma, and that most of the Earth's landmasses collected into a single
supercontinent around 1000 Ma. The supercontinent, known as
Rodinia, broke up around 600 Ma. A number of
glacial periods have been identified going as far back as the
Huronian epoch, roughly 2200 Ma. The best studied is the
Sturtian-Varangian glaciation, around 600 Ma, which 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 it's thought to have been smothered in
reducing gases, containing very little free
oxygen. The young planet had a reddish tint, and its seas were thought to be olive green. Many materials with insoluble oxides appear to have been present in the oceans for hundreds of millions of years after the Earth's formation.
When evolving life forms developed
photosynthesis, oxygen began to be produced in large quantities, causing an ecological crisis sometimes called the
Oxygen Catastrophe. The oxygen was immediately tied up in chemical reactions, primarily with iron, until the supply of oxidizable surfaces 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 first combined.
Subdivisions
A diverse terminology has evolved covering the early years of the Earth's existence, but it's tending to settle out and come into greater use as
radiometric dating allows plausible real dates to be assigned to specific formations and features. The terms
Archean (older than about 2500 Ma),
Proterozoic (2500-600 Ma), and
Neoproterozoic (600-542 Ma) appear to have general currency. Some additional terms are included in the geological time line. See
Timetable of the Precambrian.
- Proterozoic : Modern use most often refers to the time from the lower Cambrian boundary, 542 Ma, back through 2500 Ma. The boundary has been placed at various times by various authors, but has now been settled at 542 Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary.
- Neoproterozoic : the upper (for example, youngest) geologic era of the Proterozoic eon, roughly from the Cambrian period lower boundary back to as far as 900 Ma, although modern use tends to represent a shorter interval : 542-600 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
- Mesoproterozoic : the middle division of the Proterozoic. Roughly from 900-1600 Ma. Corresponds to "Precambrian Y" rocks of older North American geology.
- Paleoproterozoic : The oldest subdivision of the Proterozoic. Roughly from 1600-2500 Ma. Corresponds to "Precambrian X" rocks of older North American geology.
- Archaean : Roughly from 2500-3800 Ma.
- Hadean : Prior to 3800 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although a very few old rock beds seem to be slightly older than 3800 Ma. Some zircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the moon and meteorites.
It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by
GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows.
Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.
Hadean: the Late Heavy Bombardment period.
Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due to increasing atmospheric oxygen content.
Transition: a period of continued iron banded formation until the first continental red beds.
Proterozoic: a period of modern plate tectonics until the first animals. Further Information
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