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Precambrian Earth History
Summary
I. Introduction
Precambrian makes up over
88% of all geologic time.
Ranges from 4.6 billion -545
million years ago.
Difficult to interpret because
of:
•Complex deformation of rocks
•Intense metamorphism
•Rocks deeply buried
•A lack of fossils
Precambrian is divided into three parts:
•Hadean (Pre-Archean)
•Archean
•Proterozoic
Paleozoic Era
Mesozoic Era
Cenozoic Era
Precambrian Era
88% of all history
The Precambrian divisions are defined
broadly by atmospheric changes
• Hadean: Lots of carbon dioxide, water vapor and
methane
• Archean: Water vapor forms oceans, oxygen starts
to be made by photosynthetic organisms
• Proterozoic: Significant oxygen in atmosphere,
massive drop in carbon dioxide
Some boundaries
coincide with
other events
Present-day plate
tectonics “begins”
Period of heavy
bombardment
{
Period of major accretion
(~ 10-30 my)
II. Hadean 4.5 to 3.8 billion years ago
No rocks exist from the early Hadean,
Except for meteorites.
Earth and other planets would have
been molten at the beginning of
their histories.
Solidification of the molten material into rocks
happened as the Earth cooled.
Once solid rocks formed Earth’s geological history
began.
III. Archean 3.8-2.5 billion years ago
Crust began to form 3.96 billion years ago.
Archean 1st true geological period:
Abundant granites & gneiss’
1st fossils (3.5 b.y.o)
Large amount of radioactive material led to:
•Volcanism yields1st atmosphere & seas
Bacterial Micofossil
•Subduction-Collisions of islands formed
the 1st small continents
•Metamorphism
Archean age rocks are known
as Greenstone belts.
Greenstone belt
IV. Archean Continental Crust
1st Continents and their
characteristics begin to
form during the Archean.
Shield -Large region of exposed
Precambrian rock.
Platform - Precambrian rocks
covered with younger rock.
Craton - Shield and
platform together form a
continental core.
Mobile Belt – Regions of
deformation.
Shield
Craton
Platform
Archaen-Proterozoic transition
To modern plate tectonics
Present-day plate
tectonics “begins”
period of
rapid crustal
growth
Period of heavy
bombardment
{
1. Early plates became bigger and thicker
2. Continued recycling of oceanic crust
formed large amounts of buoyant
continental crust
•
Continued partial melting/distillation
•
Separation of Si and other elements from
Mg and Fe
•
Conversion of mafic material to felsic
material through rock cycle
3. Decrease in heat production slowed mantle
convection
•
Drove system to larger convection cells
•
Allowed larger plates to travel farther
on the Earth’s surface and cool more
•
Led to subduction rather than collision of
plates
•
Modern plate tectonics
Period of major accretion
(~ 10-30 my)
V. Proterozoic 2.5 billion to 543 million years ago
Proterozoic period differed because
of cooler crustal conditions
The cooler crust caused:
•Erosion to form sedimentary rocks
•Less tectonic activity and metamorphism.
•Less extrusive ultramafic rocks.
Sedimentary rocks and colliding islands accreted
around stable cratons (Laurentia - ”North America”)
Middle
Early
Archean
Archean
Rocks
Rocks
Late
Precambrian
Archean
Rocks
Proterozoic Rocks
3.3
4billion
to
billion
3.3tobillion
to2.5
2.9
years
billionago
yearsago
ago
42.9
billion
to
540
million
billion
years
years
ago
2.5 billion to 540 million years ago
VI. Crustal Evolution
Proterozoic crustal evolution began 2 b.y.a (billion
years ago).
First evidence of a
Wilson Cycle occurring.
VI. Crustal Evolution
The two most significant Proterozoic rock :
1. Ophiolites- large pieces of oceanic crust
thrusted onto the continent.
“Tiger-eye”
2. Banded Iron Formations (BIF’S)Thick sequences of iron oxide and
quartz.
O2
Si
Fe
O2
O2
O2
Fe
Chert Layer
Si
Fe Fe Fe
O2
Fe
O2
Si
Fe
Si
Fe
Si
Oceans were filled with Fe and Si ions from eroding continents
Blue-green algae produce O2 which begins to oxidize the Fe
Fe ions run out and the blue green algae create too much O2 and die off.
Remaining O2 combines with the Si to form chert.
Algae come back and produce more O2 and Iron
again layer
more hematite.
This process repeats for 500 million years.
Fe2O3 (hematite)
SiO4 (chert)
Fe2O3 (hematite)
VII. Proterozoic Supercontinent
Late Proterozoic: first supercontintent called Rodinia
Rodinia :
•Laurentia (N. America)
•Australia
•Antarctica
•S. America
IX. Atmosphere and Oceans Origins
The differentiated core formed a
magnetosphere allowing an
atmosphere to develop.
Water vapor from volcanoes formed the
1st seas as Earth’s temperature cooled.
No free oxygen= No ozone (O3 ) layer.
Proterozoic blue green algae
begin producing O2.
By the late Proterozoic O2
levels increased to 10%.
X. Early Life
No one knows for certain when life began.
The first organisms probably microbes:
•Single-celled
•Anaerobic (required no O2 )
•Mostly heterotrophic (require external food sources)
Prokaryotes - lack a cell nucleus and internal cell structures.
Oldest fossil in the world are 3.5 b.y.o Cyanobacteria.
Blue green algae is a photosynthetic bacteria and is
considered to be one of the oldest life forms.
X. Early Life
Colonies of cyanobacteria form stromatolites.
Stromatolites- Layers of cyanobacteria and sediment.
reproduced asexually, so genetic diversity was low.
X. Early Life
Lack of organic diversity in the
Precambrian is due to asexual
reproduction of prokaryotes.
2.1 billion years ago more complex
Eukaryotic cell design began.
Eukaryotes - is an organism with a complex nucleic
cell
Reproduce sexually which allows genetic information
to be exchanged.
Sexual reproduction increased the rate of evolution dramatically.