EarthlifeHistorys-eonhadean_archean.pptx

Earth's History Hadean and Archean

1. h"p://www.arcadiastreet.com/cgvistas/earth/01_precambrian/earth_01_precambrian_2200b.htm:
The Hadean (Eoarchean) and Archean

2. The “Precambrian Eons”
The “Precambrian Eon” contains
three subdivisions:

Proterozoic Eon (2.5 Ga – 0.54 Ga)
Archean Eon (4.0 Ga – 2.5 Ga)
Hadean (4.5 Ga – 4.0 Ga)
The Hadean frequently doesn’t appear on geologic times scales because there are
no Earth rocks of this age. The beginning of the Archean is defined as the age of the
oldest known rock.

3. The Precambrian Eons
The Hadean, Archean
and Proterozoic Eons
represent ~88% of
Earth’s history.

There are no Hadean
rocks left on Earth, but
there are some detrital
grains found in Archean
rocks.

4. The Hadean
4,600,000,000 – 4,000,000,000
The Hadean is not an “official” geologic eon because no rocks of this age remain on Earth. A
combination of weathering, metamorphism, subduction and the massive bombardment of
Earth at ~4,100,000,000 have recycled all Earth’s Hadean rocks.
Major Events of the Hadean include:

Formation of the Earth
Several very large collisions (and many
smaller collisions)
Formation of Earth’s Moon
Differentiation of the Earth into core,
mantle and crust

5. The Hadean
4,600,000,000 – 4,000,000,000
The Earth formed in the cloud of material left over from the formation of the Sun. This disk
rotated around the young star and the larger pieces were drawn together by mutual
gravitational attraction.
Eventually, these protoplanets swept
up most of the debris in their orbital
path. The Earth collided at least once
(and perhaps half a dozen) with a very
large protoplanet.

One of these collisions is thought to
have resulted in the formation of
Earth’s Moon.

6. The “Big Whack” Theory of Lunar Formation
~4,500,000,000 years ago
Earth’s Moon has a very low density compared to Mercury, Venus, Earth and Mars. The “Big
Whack” theory of lunar formation was originally proposed to explain this oddity. Instead of
being formed from detritus in the early Solar System, the Moon was derived from the Earth
and a big impactor.
The theory assumes that the Earth
had already differentiated into crust
and mantle. An oblique collision
smeared mantle material into orbit
around Earth, which coalesced into
Earth’s Moon.

The Earth may have absorbed the iron
core of the collider.
Video:'h)p://www.pbs.org/wgbh/nova/tothemoon/origins2.html'

7. The constant bombardment prevented formation of solid crust until ~4,400,000,000
years ago. The first crust would have been ultramafic – basically a frozen crust of the
mantle. Differentiation into a thicker, more silica-rich crust would have begun as soon
as plate tectonics started.
The Hadean
4,600,000,000 – 4,000,000,000
Recent discoveries indicated that oceans
were present on this early frozen crust – so
the early Earth was much cooler than
previously suspected.'

8. A Cool Early Earth
4,400,000,000 – 4,000,000,000
h)p://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html'
Zircons also tend to incorporate metals like uranium that can be used for
radiometric dating and other chemical analyses.'
Zircon (ZrSiO
4
) crystals form in
various igneous environments and are
nearly impervious to most chemical
processes. They are also physically
tough and thus make very sturdy
sedimentary clasts. '

9. A Cool Early Earth
4,400,000,000 – 4,000,000,000
Evidence for this cool period in Earth’s early
history was found in detrital zircon crystals from
Australia’s Jack Hills metaconglomerate (a
metamorphosed sedimentary rock).

Radiometric dating indicates that the oldest part
of one zircon crystal is 4,400,000,000 years
old. The chemistry of the crystal indicates that
liquid water oceans must have been present on
Earth’s surface.'
h)p://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html'

10. A Cool Early Earth
4,400,000,000 – 4,000,000,000
Multiple samples were taken from this tiny crystal using a very small drill. Uranium-
lead dating gave an age of 4.4 Ga for the oldest part of the crystal'
h)p://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html'

11. A Cool Early Earth
4,400,000,000 – 4,000,000,000
The ratio of the two stable oxygen isotopes (which are sensitive to temperature) indicates
that the surface temperature during this early phase of Earth’s history was similar to
Archean temperatures – an eon for which we have direct evidence of rocks deposited in
oceans. '
h)p://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html'

12. A Cool Early Earth
4,400,000,000 – 4,000,000,000
Whatever differentiation of the crust occurred
at this time was at least partially destroyed
by the ~100,000,000 year late heavy
bombardment of the inner solar system by a
massive number of asteroids starting at ~4.0
Ga.

The early oceans and early atmosphere of
Earth were also lost in this event.'
h)p://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html'

13. h)p://www.geology.wisc.edu/zircon/cool_early/cool_early_home.html'

14. h"p://www.nmnh.si.edu/:
Acasta'Gneiss'
The beginning of the Archean Eon is
defined by the age of the world’s
oldest known rock formation ~4.0 Ga.

15. After the late heavy bombardment, the Earth’s
atmosphere was reformed primarily from out-
gassing from volcanoes.

Magma contains dissolved gases that would
have been liberated when the magma came to
the surface. Water (H
2
O), carbon dioxide (CO
2
),
nitrogen (N
2
) and hydrogen (H
2
) gas are the
major volcanic gases.

Methane (CH
4
) and ammonia (NH
3
) would form
from chemical reactions in the atmosphere.
Some free oxygen gas (O
2
) might be produced
when solar radiation blew apart water in the
upper atmosphere, but it remained a very minor
trace element until the onset of photosynthesis
at ~3.5 Ga by cyanobacteria.
Early Archean Eon

16. Archean Earth Conditions
 Atmosphere - CO
2
, H
2
O, N
2
, methane, ammonia
  Very little free free oxygen (O
2
)
  No stratospheric ozone layer (O
3
)
 Weather - storms dump acid rain, UV light shines down
 Surface - newly cooled Earth has thin crust, volcanic activity,
frequent bombardment by space junk, rapid weathering of
exposed rocks
 Temperature - drops below 100
o
C everywhere on surface,
allowing liquid oceans
 Oceans - filled with dissolved minerals

17. The Earth is approximately
4.5 billion years old, but
the oldest preserved rocks
are half a billion years
younger.
The oldest undoubted
fossils are more than 3
billion years old.
1,000,000,000 years is a
loooong time for a
biochemistry experiment
to run.

18. http://www.dc.peachnet.edu/~pgore/geology/geo102/precamb.htm
Miller-Urey Experiment, 1953
Mixture of methane, hydrogen, and
water sparked by electrical current
to simulate lightning.
Produced organic compounds
including amino acids.
Could the basic building
blocks of life form under
those conditions?

19. H
2
N
2
CO
2
H
2
O

NH
3
CH
4

20. H
2
N
2
CO
2
H
2
O

NH
3
CH
4
Cyanogen
Formaldehyde

21. Hydrogen cyanide
Cyanogen
Cyanoacetylene
Formaldehyde Acetaldehyde Propionaldehyde
Glycol acid
N-Methylalanine
Lactic acid
Aminobutyric acids
Formic acid
Acetic acid
Propionic acid
Urea
Aspartic acid
And others…
Other Compounds
Some organic and non-
organic chemicals produced
in Miller-Urey’s experiment.
Many of these molecules are
basic building blocks of more
complex organic molecules.

22. Glycine
Amino Acids
Alanine
Two amino acids (the building
blocks of proteins) were also
synthesized in this very simple
experiment.
Making the building blocks of
biochemistry is relatively easy.

23. M-U’s gases were not representative of
current theories of the Earth’s Hadean
atmosphere.
However, basic organic compounds form
under all hypothesized early Earth
chemical system as well as on other
planets and space bodies.
All that is required is the raw materials,
reducing conditions, and an energy
sources (e.g., sunlight, electricity,
radioactivity, and plain old heat).
Simple proteins have also been produced
in these experiments.
http://www.dc.peachnet.edu/~pgore/geology/geo102/precamb.htm

24. Earliest Traces of Life
The earliest of evidence of life on Earth primarily comes from stable isotope excursions
preserved in 3,800,000,000 sedimentary rocks. Organic chemistry tends to enriched in the
“light” carbon isotope (
12
C) compared to the “heavy” carbon isotope (
13
C).
Rosing, M.T. 1999.
13
C-depleted carbon microparticles in >3700-Ma
sea-floor sedimentary rocks from West Greenland. Science 283:
674-676.

“Turbiditic and pelagic sedimentary rocks from the Isua
supracrustal belt in west Greenland [more than 3700
million years ago (Ma)] contain reduced carbon that is
likely biogenic. The carbon is present as 2- to 5-
micrometer graphite globules and has an isotopic
composition of δ
13
C that is about –19 per mil (Pee Dee
belemnite standard). These data and the mode of
occurrence indicate that the reduced carbon represents
biogenic detritus, which was perhaps derived from
planktonic organisms.”

25. These microfossils are
preserved in chert of the
Swartkoppie Formation. Figure
from Knoll and Barghoorn
(1977).

The have been caught in the act
of dividing just like living
bacteria.

Age: 3,500,000,000 years
http://www.syslab.ceu.hu/corliss/4-HadArchEarth.html
Earliest Body Fossils

26. http://www.ucmp.berkeley.edu/
Cyanobacteria
In addition to archaebacteria, some eubacteria (“true bacteria”)
also evolved early in the history of life on Earth. Cyanobacteria
(blue-green algae) fossils are found in 3.0 billion year old rocks.
Phototrophs

27. Large bloom due to pollution of Bedetti Lake, Argentina
http://www-cyanosite.bio.purdue.edu/
enlarged 2500X
Modern Cyanobacteria

28. http://www.rz.uni-frankfurt.de/~schauder/mats/microbial_mats.html
Cross section of a microbial mat in a salt marsh. Approximately 5 cm deep.
copyright 1997 Rolf Schauder
Green Layer - filamentous
blue-green algae
(cyanobacteria)
Pink Layer - Purple sulfur
bacteria
Black Layer - Iron sulfide,
produced by abundant
sulfur-reducing bacteria
Grey Layer - color change
due to formation of mineral
pyrite, bacteria less
abundant

29. http://www.wf.carleton.ca/
Stromatolites are sedimentary
structures formed by the activity of
microbial mats. They are found in
rocks over 3.0 billion years old.
Stromatolites

30. http://www.ldeo.columbia.edu/~small/personal/Images/Stromatolites.jpg
Hyper-saline bay exposed at low tide, Shark’s Bay, Australia
Modern Stromatolites

31. http://www-cyanosite.bio.purdue.edu/
Hakati Shale, Middle Proterozoic
Grand Canyon NP, Arizona
Ancient Stromatolites
Characteristic laminated
structure

32. http://www.nature.com/nature/journal/v405/n6787/fig_tab/405625a0_F1.html
Archean Ecology
Communities of bacteria and archaebacteria
Photosynthesis by blue-green algae
(cyanobacteria) in shallow water
releases oxygen into the water
O
2
O
2

33. 33
Archean plate tectonic differed from modern plate tectonics:
Plates likely moved faster
Magma was likely generated more rapidly
Continents likely grew more rapidly along their margins as plates
collided with island arcs and other plates
Continental crust was thinner than it is today
Archean Plate Tectonics

34. 34
Craton: Precambrian core of continental crust

Shield: exposed craton

Platform: A Precambrian sedimentary sequence
covering part of a craton

Both Archean and Proterozoic rocks are present in
the cratons. All the Archean rocks record
evidence of episodes of mountain building. Most
cratons have experienced very little deformation
since the Precambrian
Cratons, Shields, and Platform Terminology

35. 35'

36. The Canadian shield comprises most of northeastern
Canada a large part of Greenland parts of
Minnesota, Wisconsin, Michigan, and New York

Topography is subdued and thinly covered in places by
Pleistocene glacial deposits.

Unmetamorphosed Proterozoic sedimentary and
volcanic rocks overlie Archean metamorphic
rocks.

Greenstone belts in Archean shields in the other
continents have the same stratigraphic relationship
as those within the Canadian Shield
The Canadian Shield

37. 37
Greenstone Belts and Granulites

Greenstones are metamorphosed ultramafic and mafic volcanic rocks and
associated clastic sedimentary rocks. Archean-age greenstone belts are
present in all cratons

Granulites are strongly metamorphosed granites and gneisses that intruded
and underlie the greenstones

38. 38
Subduction of oceanic crust formed volcanic
island arcs
Island Arc Model for Formation of Greenstone Belts
Greenstone belt began
forming in the back-arc
basin
Magma underplated the
back-arc basin
Compression caused the basin to be
folded and intruded by magma

39. 39
Intracratonic Model for Formation of Greenstone Belts
Greenstone belts (dark green)
Granite-gneiss complexes (light green
The intracratonic model for evolution of
part of the Superior craton