Transcript chapter8_ARCHEAN

Chapter 8
Precambrian Earth and Life History
The Archean
Introduction
 Our reference of time, in geology, spans
billions of years
The Earth is 4.6 billion years old
The largest unit of time on Earth is
the Precambrian.
It lasted more than 4 billion years
Fig. 19.2, p. 495
Fig. 19.1, p. 494
Introduction
Geologists divide the Precambrian into
two eons
 Eons
Archean
Proterozoic
 If all geologic time were
represented by a 24-hour
clock, the Precambrian would
be more than 21 hours long
and constitute more than 88%
of all geologic time.
Fig. 19.1, p. 494
What Happened During the Eoarchean?
The earliest part of the Archean history is the Eoarchean
(4.6 bya – 3600 bya).
 Volcanism was widespread. Earth was a hot, rapidly rotating, barren
waterless planet bombarded constantly by meteorites and comets.
Fig. 19.3a, p. 495
What Happened During the Eoarchean?
The earth was still accreting from the planetesimals and
differentiating. There are few rocks from this time
period. Some crust had formed, possibly by 4.4 bya.
The process of continental accretion was slowly building
the continents from small island arcs.
Island arc
Sea level
Oceanic
lithosphere
Asthenosphere
Island arc
Continental Foundations-Shields,
Platforms, and Cratons
 Each continent today has an ancient, stable craton
made up of a Precambrian shield and platform.
 Areas of exposed
Precambrian rocks
constitute the shields.
 Platforms consist of
buried Precambrian
rocks.
 A shield and its adjoining
platform make up a
craton.
Fig. 19.5, p. 498
The Geologic Evolution Of North America
Continental Foundations-Shields,
Platforms, and Cratons
Direct evidence of continental accretion as a
model for the formation of continents comes from
the Canadian Shield in North America
The Canadian Shield is made up of several
smaller cratons that accreted.
Fig. 19.8b, p. 500
Archean Rocks
Archean Earth History
The Archean 4.6 bya – 2.5 bya
Archean Rocks
Fig. 19.6, p. 498
Another Archean Rock in Greenstone belt
Komatiites
The most interesting rocks in Greenstone belts
They are ultramafic
Have high amount of Iron  Means that magma
was hot enough to melt-iron rich minerals.
Magmas flowing were a lot hotter that are today.
Komatiites must have cooled from molten rock
that was at least 1600°C at the surface of the
earth. Today’s highest recorded surface flowis
1350°C
Archean Earth History
Archean Plate Tectonics and the
Origin of Cratons
Many geologists think that Archean plates moved
faster than plates do now because Earth
possessed more radiogenic heat.
Small cratons would have grown more rapidly to
become larger continents.
Several small cratons existed, 30-40% of present
continental crust existed.
We did not however, have significant amounts of
sedimentary rocks which are products of
weathering in an oxygen rich atmosphere with
abundant water.
Archean Earth History
Archean Plate Tectonics and
the Origin of Cratons
 Origin of a Greenstone Belt in a Back-Arc Basin
Fig. 8.10, p. 1160
Another Hypothesis for Greenbelt
Archean Plate Tectonics
The Archean 4.6 bya – 2.5 bya
Archean Rocks
Archean rocks are mostly granite-gneiss
complexes and subordinate greenstone belts.
One model for the origin of greenstone belts holds
that they formed in back-arc marginal basins.
Fig. 19.8, p. 500
Origin and Evolution of the
Atmosphere and Hydrosphere
The Atmosphere
Earth’s earliest atmosphere was derived by the release
of gases during outgassing (from volcanoes).
 The Archean atmosphere
 Lacked water and free oxygen
 Rich in carbon dioxide
 The Proterozoic atmosphere
 21% free oxygen
 Some carbon dioxide
 The oceans now covered 71%
of the Earth’s surface,
close to today’s level.
Fig. 19.17a, p. 512
Origin and Evolution of the
Atmosphere and Hydrosphere
The Atmosphere - The Proterozoic atmosphere had
some oxygen. Two processes account for the introduction of
free oxygen.
 Photochemical dissociation
 Water vapor in the atmosphere
is broken down by UV rays to
oxygen and free hydrogen.
Results in an ozone barrier but
no free oxygen.
 Photosynthesis accounts for
most of the free oxygen.
 Cyanobacteria, present since the
Archean, did not become
common until about 2.3 bya
 These photosynthesizing
organisms added free oxygen to
the evolving atmosphere
 They form structures known as
stromatolites.
Fig. 19.19a, p. 514
Origin and Evolution of the
Atmosphere and Hydrosphere
The Hydrosphere
All water on Earth is part of the hydrosphere.
Most of it (more than 97%) is in the oceans
 Meteorite and comet
impacts and outgassing
yielded the hydrosphere
Escapes
Hydrogen
H
Water
H2O
Carbon
dioxide
CO2
To atmosphere
Nitrogen
N
Water
H2O
To oceans
Volcano
Erosional debris
Stepped Art
Fig. 19-17a, p. 512
Photochemical of the
Atmosphere
Life-Its Origin and Early History
Abiogenesis
How life originated from nonliving matter
One point of clarification on abiogenesis—it
does not hold that a living organism such as
a bacterium, or even a complex organic
molecule, sprang fully formed from nonliving matter. Rather than one huge step from
nonliving to living, the origin of life involved
several small steps, each leading to an
increase in organization and complexity.
Life-Its Origin and Early History
 Fossil as as old as 3.5 bya have been found on Earth.
Today we have five kingdoms with millions of species.
How did life originate from non-living matter?
The Origin of Life
Energy such as lightning and ultraviolet radiation
acting on chemical elements present on Earth may
have yielded the first living things.
Fig. 19.18, p. 513
Another Hypothesis
That first life was near black smokers
 Animation 1
 Animation 2
Life-Its Origin and Early History
Archean Organisms
All known Archean fossils represent
prokaryotic bacteria.
Prokaryotes reproduce
asexually,(without the
recombination of genes) and are
single-celled, no internal organs
Not many life forms existed, mostly
bacteria and stromatolites
Stromatolites formed by
photosynthesizing bacteria
may date from 3.5 bya ago.
Prokaryotic organism is also called
Cyanobacteria
 Cyanobacteria is a single organism also known as BlueGreen Algae. When large colonies of cyanobacteria
flourish they create a single green film on the surface of a
lake or pond,  pond scum
Stromatolites
Resources in Precambrian Rocks
Archean Resources
More than 50% of the world’s gold has come from
Archean rocks in South Africa.
Massive sulfide deposits containing zinc, copper,
and nickel are known from Australia, Zimbabwe
(Africa) and Canada. These are similar to the black
smokers on the seafloor.
About 25% of the world’s chromium reserves are
also in Archean rocks, along with platinum deposits.
Archean pegmatites in the African Shield and the
Canadian Shield have yielded valuable gem-quality
minerals.
End of
Chapter 8