Precambrian Earth and Life History—The Archean Eon

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Title: Precambrian Earth and Life History—The Archean Eon

1
Chapter 8
Precambrian Earth and Life HistoryThe Archean Eon
2
Archean Rocks

The Twin Lakes
in Beartooth Plateau, MT and WY
is called one of the most scenic in the U.S.
Most of the rocks are Archean-aged gneiss.

3
Precambrian

The Precambrian lasted for more than 4 billion
years!
This large time span is difficult for humans to
comprehend
Suppose that a 24-hour clock represented
all 4.6 billion years of geologic time
then the Precambrian would be
slightly more than 21 hours long,
constituting about 88 of all geologic time

4
Precambrian Time Span

88 of geologic time

5
Precambrian

The term Precambrian is informal
but widely used, referring to both time and rocks
The Precambrian includes
time from Earths origin 4.6 billion years ago
to the beginning of the Phanerozoic Eon
542 million years ago
It encompasses
all rocks older than Cambrian-age rocks
No rocks are known for the first
640 million years of geologic time
The oldest known rocks on Earth
are 3.96 billion years old

6
Rocks Difficult to Interpret

The earliest record of geologic time
preserved in rocks is difficult to interpret
because many Precambrian rocks have been
altered by metamorphism
complexly deformed
buried deep beneath younger rocks
fossils are rare, and
the few fossils present are of little use in
stratigraphy
Subdivisions of the Precambrian
have been difficult to establish
Two eons for the Precambrian
are the Archean and Proterozoic

7
Eons of the Precambrian

The onset of the Archean Eon coincides
with the age of Earths oldest known rocks
approximately 4 billion years old
and lasted until 2.5 billion years ago
the beginning of the Proterozoic Eon
Eoarchean refers to all time
from Earths origin to the Archean
Precambrian eons have no stratotypes
unlike the Cambrian Period, for example,
which is based on the Cambrian System,
a time-stratigraphic unit with a stratotype in
Wales
Precambrian eons are strictly terms denoting time

8
What Happened During the Eoarchean?

Although no rocks of Eoarchean age are present on
Earth,
except for meteorites,
we do know some events that took place then
Earth accreted from planetesimals
and differentiated into a core and mantle
and at least some crust was present
Earth was bombarded by comets and meteorites
Volcanic activity was ubiquitous
An atmosphere formed, quite different from
todays
Oceans began to accumulate

9
Hot, Barren, Waterless Early Earth

about 4.6 billion years ago

Shortly after accretion, Earth was
a rapidly rotating, hot, barren, waterless planet
bombarded by comets and meteorites
with no continents, intense cosmic radiation
and widespread volcanism

10
Oldest Rocks

Judging from the oldest known rocks on Earth,
the 3.96-billion-year-old Acasta Gneiss in Canada
and other rocks in Montana and Greenland
some continental crust had evolved by Eoarchean
time
Sedimentary rocks in Australia contain detrital
zircons (ZrSiO4) dated at 4.4 billion years old
so source rocks at least that old existed
These rocks indicted that some kind
of Eoarchean crust was certainly present,
but its distribution is unknown

11
Eoarchean Crust

Early Eoarchean crust was probably thin
and made up of ultramafic rock
igneous rock with less than 45 silica
This ultramafic crust was disrupted
by upwelling mafic magma at ridges,
and the first island arcs formed at subduction
zones
Eoarchean continental crust may have formed
by collisions between island arcs
as silica-rich materials were metamorphosed.
Larger groups of merged island arcs
protocontinents
grew faster by accretion along their margins

12
Origin of Continental Crust

Andesitic island arcs
form by subduction
and partial melting of oceanic crust
The island arc collides with another

13
Dynamic Processes

During the Eoarchean, various dynamic systems
similar to ones we see today,
became operative,
but not all at the same time nor in their present
forms
Once Earth differentiated
into core, mantle and crust,
million of years after it formed,
internal heat caused interactions among plates
as they diverged, converged
and slid past each other in transform motion
Continents began to grow by
accretion along convergent plate boundaries

14
Continental Foundations

Continents consist of rocks
with composition similar to that of granite
Continental crust is thicker
and less dense than oceanic crust
which is made up of basalt and gabbro
Precambrian shields
consist of vast areas of exposed ancient rocks
and are found on all continents
Outward from the shields are broad platforms
of buried Precambrian rocks
that underlie much of each continent

15
Cratons

A shield and platform make up a craton,
a continents ancient nucleus
Along the margins of cratons,
more continental crust was added
as the continents took their present sizes and
shapes
Both Archean and Proterozoic rocks
are present in cratons and show evidence of
episodes of deformation accompanied by
metamorphism, igneous activity,
and mountain building
Cratons have experienced little deformation
since the Precambrian

16
Distribution of Precambrian Rocks

Areas of exposed
Precam-brian rocks
constitute the shields
Platforms consist of
buried Pre-cambrian rocks

Shields and adjoining platforms make up cratons

17
Canadian Shield

The exposed part of the craton in North America
is the Canadian shield
which occupies most of northeastern Canada
a large part of Greenland
parts of the Lake Superior region
in Minnesota, Wisconsin, and Michigan
and the Adirondack Mountains of New York
Its topography is subdued,
with numerous lakes and exposed Archean
and Proterozoic rocks thinly covered
in places by Pleistocene glacial deposits

18
Canadian Shield Rocks

Outcrop of Archean gneiss in the Canadian Shield
in Ontario, Canada

19
Canadian Shield Rocks

Basalt (dark, volcanic) and granite (light,
plutonic) on the Chippewa River, Ontario

20
Amalgamated Cratons

Actually the Canadian shield and adjacent
platform
are made up of numerous units or smaller cratons
that amalgamated along deformation belts
during the Paleoproterozoic
Absolute ages and structural trends
help geologists differentiate
among these various smaller cratons
Drilling and geophysical evidence indicate
that Precambrian rocks underlie much
of North America,
exposed only in places by deep erosion or uplift

21
Archean Rocks Beyond the Shield

Archean rocks found
in areas of uplift in the Teton Range, WY

22
Archean Rocks Beyond the Shield

Archean Brahma Schist in the deeply eroded parts
of the Grand Canyon, Arizona

23
Archean Rocks

The most common Archean Rock associations
are granite-gneiss complexes
Other rocks range from peridotite
to various sedimentary rocks
all of which have been metamorphosed
Greenstone belts are subordinate in quantity
but are important in unraveling Archean tectonic
events

24
Greenstone Belts

An ideal greenstone belt has 3 major rock units

volcanic rocks are most common
in the lower and middle units
the upper units are mostly sedimentary
The belts typically have synclinal structure
Most were intruded by granitic magma
and cut by thrust faults
Low-grade metamorphism
makes many of the igneous rocks
green (chlorite, actinolite, epidote)

25
Greenstone Belt Volcanics

Pillow lavas in greenstone belts
indicate that much of the volcanism was
subaqueous

Pyroclastic materials probably erupted
where large volcanic centers built above sea
level

Pillow lavas in Ispheming greenstone belt at
Marquette, Michigan
26
Ultramafic Lava Flows

The most interesting rocks
in greenstone belts are komatiites,
cooled from ultramafic lava flows
Ultramafic magma (lt 45 silica)
requires near surface magma temperatures
of more than 1600C
250C hotter than any recent flows
During Earths early history,
radiogenic heating was greater
and the mantle was as much as 300 C hotter
than it is now
This allowed ultramafic magma
to reach the surface

27
Ultramafic Lava Flows

As Earths production
of radiogenic heat decreased,
the mantle cooled
and ultramafic flows no longer occurred
They are rare in rocks younger
than Archean and none occur now

28
Sedimentary Rocks of Greenstone Belts

Sedimentary rocks are found
throughout the greenstone belts
although they predominate
in the upper unit
Many of these rocks are successions of
graywacke
sandstone with abundant clay and rock fragments
and argillite
slightly metamorphosed mudrock

29
Sedimentary Rocks of Greenstone Belts

Small-scale cross-bedding and
graded bedding indicate an origin
as turbidity current deposits

Quartz sandstone and shale,
indicate delta, tidal-flat,
barrier-island and shallow marine deposition

30
Relationship of Greenstone Belts to
Granite-Gneiss Complexes

Two adjacent greenstone belts showing synclinal
structure

They are underlain by granite-gneiss complexes

and intruded by granite

31
Canadian Greenstone Belts

In North America,
most greenstone belts
(dark green)
occur in the Superior and Slave cratons
of the Canadian shield

32
Evolution of Greenstone Belts

Models for the formation of greenstone belts
involve Archean plate movement
In one model, plates formed volcanic arcs
by subduction

and the greenstone belts formed
in back-arc marginal basins

33
Evolution of Greenstone Belts

According to this model,
volcanism and sediment deposition
took place as the basins opened

34
Evolution of Greenstone Belts

Then during closure,
the rocks were compressed,
metamorphosed,
and intruded by granitic magma

The Sea of Japan
is a modern example
of a back-arc basin

35
Another Model

In another model
although not nearly as widely accepted,
greenstone belts formed
over rising mantle plumes in intracontinental
rifts
The plume serves as the source
of the volcanic rocks in the lower and middle
units
of the developing greenstone belt
and erosion of volcanic rocks and flanks for the
rift
supply the sediment to the upper unit
An episode of subsidence, deformation,
metamorphism and plutonism followed

36
Greenstone BeltsIntracontinental Rift Model

Ascending mantle plume
causes rifting
and volcanism

37
Greenstone BeltsIntracontinental Rift Model

Erosion of the rift flanks
accounts for sediments

38
Greenstone BeltsIntracontinental Rift Model

Closure of rift
causes compression
and deformation

39
Archean Plate Tectonics

Plate tectonic activity has operated
since the Paleoproterozoic or earlier
Most geologists are convinced
that some kind of plate tectonic activity
took place during the Archean as well
but it differed in detail from today
Plates must have moved faster
with more residual heat from Earths origin
and more radiogenic heat,
and magma was generated more rapidly

40
Archean Plate Tectonics

As a result of the rapid movement of plates,
continents grew more rapidly along their margins
a process called continental accretion
as plates collided with island arcs and other
plates
Also, ultramafic extrusive igneous rocks,
komitiites,
were more common

41
Archean World Differences

The Archean world was markedly different than
later

but associations of passive continental margin
sediments
are widespread in Proterozoic terrains

We have little evidence of Archean rocks
deposited on broad, passive continental margins

but the ophiolites so typical of younger
convergent plate boundaries are rare,
although Neoarchean ophiolites are known

Deformation belts between colliding cratons
indicate that Archean plate tectonics was active

42
The Origin of Cratons

Certainly several small cratons
existed during the Archean
and grew by periodic continental accretion
during the rest of that eon
They amalgamated into a larger unit
during the Proterozoic
By the end of the Archean,
30-40 of the present volume
of continental crust existed
Archean crust probably evolved similarly
to the evolution of the southern Superior craton
of Canada

43
Southern Superior Craton Evolution

Greenstone belts (dark green)
Granite-gneiss complexes (light green

Geologic map

Plate tectonic model for evolution of the
southern Superior craton
North-south cross section

44
Canadian Shield

Deformation of the southern Superior craton
was part of a more extensive orogenic episode
that formed the Superior and Slave cratons
and some Archean rocks in Wyoming, Montana,
and the Mississippi River Valley
This deformation was
the last major Archean event in North America
and resulted in the formation of several sizable
cratons
now in the older parts of the Canadian shield

45
Atmosphere and Hydrosphere

Earths early atmosphere and hydrosphere
were quite different than they are now
They also played an important role
in the development of the biosphere
Todays atmosphere is mostly
nitrogen (N2)
abundant free oxygen (O2),
or oxygen not combined with other elements
such as in carbon dioxide (CO2)
water vapor (H2O)
small amounts of other gases, like ozone (O3)
which is common enough in the upper atmosphere
to block most of the Suns ultraviolet radiation

46
Present-day Atmosphere Composition

Nonvariable gases
Nitrogen N2 78.08
Oxygen O2 20.95
Argon Ar 0.93
Neon Ne 0.002
Others 0.001
in percentage by volume

Variable gases
Water vapor H2O 0.1 to 4.0
Carbon dioxide CO2 0.038
Ozone O3 0.000006
Other gases Trace
Particulates normally trace

47
Earths Very Early Atmosphere

Earths very early atmosphere was probably
composed of
hydrogen and helium,
the most abundant gases in the universe
If so, it would have quickly been lost into space
because Earths gravity is insufficient to retain
them
because Earth had no magnetic field until its
core formed (magnetosphere)
Without a magnetic field,
the solar wind would have swept away
any atmospheric gases

48
Outgassing

Once a magnetosphere was present
Atmosphere began accumulating as a result of
outgassing
released during volcanism
Water vapor
is the most common volcanic gas today
but volcanoes also emit
carbon dioxide, sulfur dioxide,

carbon monoxide, sulfur,
hydrogen, chlorine, and nitrogen

49
Archean Atmosphere

Archean volcanoes probably
emitted the same gases,
and thus an atmosphere developed
but one lacking free oxygen and an ozone layer
It was rich in carbon dioxide,
and gases reacting in this early atmosphere
probably formed
ammonia (NH3)
methane (CH4)
This early atmosphere persisted
throughout the Archean

50
Evidence for an Oxygen-Free Atmosphere

The atmosphere was chemically reducing
rather than an oxidizing one
Some of the evidence for this conclusion
comes from detrital deposits
containing minerals that oxidize rapidly
in the presence of oxygen
pyrite (FeS2)
uraninite (UO2)
But oxidized iron becomes
increasingly common in Proterozoic rocks
indicating that at least some free oxygen
was present then

51
Introduction of Free Oxygen

Two processes account for
introducing free oxygen into the atmosphere,
one or both of which began during the Eoarchean.
1. Photochemical dissociation involves
ultraviolet radiation in the upper atmosphere
The radiation disrupts water molecules and
releases their oxygen and hydrogen
This could account for 2 of present-day oxygen
but with 2 oxygen, ozone forms, creating a
barrier against ultraviolet radiation
2. More important were the activities of
organisms that practiced photosynthesis

52
Photosynthesis

Photosynthesis is a metabolic process
in which carbon dioxide and water
combine into organic molecules
and oxygen is released as a waste product
CO2 H2O gt organic compounds O2
Even with photochemical dissociation
and photosynthesis,
probably no more than 1 of the free oxygen level
of today was present by the end of the Archean

53
Oxygen Forming Processes

Photochemical dissociation and photosynthesis
added free oxygen to the atmosphere

Once free oxygen was present
an ozone layer formed
and blocked incoming ultraviolet radiation

54
Earths Surface Waters

Outgassing was responsible
for the early atmosphere
and also for Earths surface water
the hydrosphere
most of which is in the oceans
more than 97
However, somebut probably not much
of our surface water was derived from icy comets
Once Earth had cooled sufficiently,
the abundant volcanic water vapor
condensed and began to accumulate in oceans
Oceans were present during Eoarchean times

55
Ocean water

The volume and geographic extent
of the Eoarchean oceans cannot be determined
Nevertheless, we can envision an early Earth
with considerable volcanism
and a rapid accumulation of surface waters
Volcanoes still erupt and release water vapor
Is the volume of ocean water still increasing?
Perhaps it is, but if so, the rate
has decreased considerably
because the amount of heat needed
to generate magma has diminished
Much of volcanic water vapor today
is recycled surface water

56
Decreasing Heat

Ratio of radiogenic heat production in the past
to the present

The width of the colored band indicates
variations in ratios from different models

Heat production 4 billion years ago was 3 to 6
times as great as it is now

With less heat outgassing decreased

57
First Organisms

Today, Earths biosphere consists
of millions of species of monera, fungi,
protistans, plants, and animals,
whereas only bacteria and archea are found in
Archean rocks
We have fossils from Archean rocks
3.3 to 3.5 billion years old
Chemical evidence in rocks in Greenland
that are 3.85 billion years old
convince some investigators that organisms were
present then

58
What Is Life?

Minimally, a living organism must reproduce
and practice some kind of metabolism
Reproduction ensures
the long-term survival of a group of organisms
whereas metabolism
such as photosynthesis, for instance
ensures the short-term survival of an individual
The distinction between
living and nonliving things is not always easy
Are viruses living?
When in a host cell they behave like living
organisms
but outside they neither reproduce nor metabolize

59
What Is Life?

Comparatively simple organic (carbon based)
molecules known as microspheres

form spontaneously
can even grow and divide in a somewhat
organism-like fashion
but their processes are more like random chemical
reactions, so they are not living

60
How Did Life First Originate?

To originate by natural processes,
from non-living matter (abiogenesis), life must
have passed through a prebiotic stages
in which it showed signs of living organisms
but was not truly living
The origin of life has 2 requirements
a source of appropriate elements for organic
molecules
energy sources to promote chemical reactions

61
Elements of Life