Precambrian Earth and Life History

1
Chapter 8
Precambrian Earth and Life HistoryThe Hadean and
Archean
-platforms and shield areas together make
craton -early planet volcanic, toxic gases
(CO2), -Hadean geology rare rocks
preserved!! -Archaen geology v little evidence,
greenstone belts-volcanics and sedimentary
rocks. -atmosphere and ocean evolve thru time
outgassing from volcanoes and development of
photosynthesis -early life forms single celled
bacteria, algae stromatolites - Archaen mineral
resources gold, chrome, platinum, iron
2
Precambrian Time Span

• 88 of geologic time

3
Archean Rocks

• The Teton Range
• is largely Archean
• gneiss, schist, and granite
• Younger rocks are also present
• but not visible

Grand Teton National Park, Wyoming
4
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
• 545 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

5
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
• the few fossils present are of little use in
  stratigraphy
• Subdivisions of the Precambrian
• have been difficult to establish
• eons for the Precambrian
• are the Hadean, Archean and Proterozoic

6
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
• Hadean is an informal designation
• for the time preceding the Archean Eon
• 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

7
US Geologic Survey Terms

• Archean and Proterozoic are used in our lecture
• following discussions of Precambrian history,
• but the U.S. Geological Survey (USGS) uses
  different terms
• Their Precambrian W begins within the Early
  Archean
• and ends at the end of the Archean
• Precambrian X corresponds to the Early
  Proterozoic,
• 2500 to 1600 million years ago
• Precambrian Y,
• from 1600 to 800 million years ago,
• overlaps with the Middle and part of the Late
  Proterozoic
• Precambrian Z is
• from 800 million years to the end of the
  Precambrian,
• within the Late Proterozoic

8
Archaen 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

9
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
• some continental crust had evolved by 4 billion
  years ago
• Sedimentary rocks in Australia contain detrital
  zircons (ZrSiO4) dated at 4.2 billion years old
• so source rocks at least that old existed
• These rocks indicted that some kind
• of Hadean crust was certainly present,
• but its distribution is unknown

10
Hadean Crust

• Early Hadean crust was probably thin, unstable
• and made up of ultramafic rock
• rock with comparatively little silica
• This ultramafic crust was disrupted
• by upwelling basaltic magma at ridges
• and consumed at subduction zones
• Hadean continental crust may have formed
• by evolution of sialic material (granite!)
• Sialic crust contains considerable silicon,
  oxygen
• and aluminum as in present day continental crust
• Only sialic-rich crust, because of its lower
  density,
• is immune to destruction by subduction

11
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

12
Cratons

• A shield and platform make up a craton,
• a continents ancient nucleus and its foundations
• 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

13
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

14
Canadian Shield

• The craton in North America includes 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

15
Canadian Shield Rocks

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

16
Archean Rocks Beyond the Shield

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

17
Archean Rocks

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

18
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
• greenish (chlorite)

19
Greenstone Belt Volcanics

• Abundant pillow lavas in greenstone belts
• indicate that much of the volcanism was
• under water, probably at or near a spreading ridge

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

Pillow lavas in Ispheming greenstone at
Marquette, Michigan
20
Ultramafic Lava Flows

• The most interesting rocks
• in greenstone belts cooled
• from ultramafic lava flows
• Ultramafic magma has less than 40 silica
• and requires near surface magma temperatures
• of more than 1600C250C hotter
• than any recent flows
• During Earths early history,
• radiogenic heating was higher
• and the mantle was as much as 300 C hotter
• than it is now
• This allowed ultramafic magma
• to reach the surface

21
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

22
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

23
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

24
Canadian Greenstone Belts

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

25
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

26
Archean Plate Tectonics

• Plate tectonic activity has operated
• since the Early Proterozoic or earlier
• Most geologists are convinced
• that some kind of plate tectonics
• 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

27
Archean Plate Tectonics

• As a result of the rapid movement of plates,
• continents, no doubt,
• 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
• were more common
• due to the higher temperatures

28
The Origin of Cratons

• Certainly several small cratons
• existed by the beginning of the Archean
• and grew by periodic continental accretion
• during the rest of that eon
• They amalgamated into a larger unit
• during the Early 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

29
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

30
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 Late Archean event in North
  America
• and resulted in the formation of several sizable
  cratons
• now in the older parts of the Canadian shield

31
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
• Without a magnetic field,
• the solar wind would have swept away
• any atmospheric gases

32
Outgassing

• Once a core generated, the magnetic field
• protected the gases released during volcanism
• called outgassing
• they began to accumulate to form a new atmosphere
• Water vapor
• is the most common volcanic gas today
• but volcanoes also emit
• carbon dioxide, sulfur dioxide,

• carbon monoxide, sulfur, hydrogen, chlorine, and
  nitrogen

33
Hadean-Archean Atmosphere

• Hadean 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

34
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

35
Introduction of Free Oxygen

• Two processes account for
• introducing free oxygen into the atmosphere,
• one or both of which began during the Hadean
• 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 organism
  that practiced photosynthesis

36
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

37
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, some but probably not much
• of our surface water was derived from icy comets
• Probably at some time during the Hadean,
• the Earth had cooled sufficiently
• so that the abundant volcanic water vapor
• condensed and began to accumulate in oceans
• Oceans were present by Early Archean times

38
First Organisms

• Today, Earths biosphere consists
• of millions of species of bacteria, fungi,
• protistans, plants, and animals,
• whereas only bacteria are found in Archean rocks
• We have fossils from Archean rocks
• 3.3 to 3.5 billion years old
• Carbon isotope ratios in rocks in Greenland
• that are 3.85 billion years old
• convince some investigators that life was present
  then

39
How Did Life First Originate?

• To originate by natural processes,
• life must have passed through a pre-biotic stage
• in which it showed signs of living organisms
• but was not truly living
• In 1924, the great Russian biochemist,
• A.I. Oparin, postulated that life originated
• when Earths atmosphere had little or no free
  oxygen
• Oxygen is damaging to Earths
• most primitive living organisms
• Some types of bacteria must live
• where free oxygen is not present

40
How Did Life First Originate?

• With little or no oxygen in the early atmosphere
• and no ozone layer to block ultraviolet
  radiation,
• life could have come into existence from
  nonliving matter
• The origin of life has 2 requirements
• a source of appropriate elements for organic
  molecules
• energy sources to promote chemical reactions

41
Elements of Life

• All organisms are composed mostly of
• carbon (C)
• hydrogen (H)
• nitrogen (N)
• oxygen (O)
• all of which were present in Earths early
  atmosphere as
• Carbon dioxide (CO2)
• water vapor (H2O)
• nitrogen (N2)
• and possibly methane (CH4)
• and ammonia (NH3)

42
Proteinoid Microspheres

• Proteinoid microspheres produced in experiments

• Proteinoids grow and divide much as bacteria do

43
Next Critical Step

• Not much is known about the next critical step
• in the origin of life
• the development of a reproductive mechanism
• The microspheres divide
• and may represent a proto-living system
• but in todays cells nucleic acids,
• either RNA or DNA
• are necessary for reproduction
• The problem is that nucleic acids
• cannot replicate without protein enzymes,
• and the appropriate enzymes
• cannot be made without nucleic acids,
• or so it seemed until fairly recently

44
Much Remains to Be Learned

• The origin of life has not been fully solved
• but considering the complexity of the problem
• and the fact that scientists have been
  experimenting
• for only about 50 years
• remarkable progress has been made
• Debate continues
• Many researchers believe that
• the earliest organic molecules
• were synthesized from atmospheric gases
• but some scientists suggest that life arose
  instead near hydrothermal vents on the seafloor

45
Azoic (without life)

• Prior to the mid-1950s, scientists
• had little knowledge of Precambrian life
• They assumed that life of the Cambrian
• must have had a long early history
• but the fossil record offered little
• to support this idea
• A few enigmatic Precambrian fossils
• had been reported but most were dismissed
• as inorganic structures of one kind or another
• The Precambrian, once called Azoic
• (without life), seemed devoid of life

46
Stromatolites

• Present-day stromatolites form and grow
• as sediment grains are trapped
• on sticky mats
• of photosynthesizing blue-green algae
  (cyanobacteria)
• although now they are restricted
• to environments where snails cannot live
• The oldest known undisputed stromatolites
• are found in rocks in South Africa
• that are 3.0 billion years old
• but probable ones are also known
• from the Warrawoona Group in Australia
• which is 3.3 to 3.5 billion years old

47
Oldest Known Organisms

• Charles Walcott (early 1900s) described
  structures
• from the Early Proterozoic Gunflint Iron
  Formation of Ontario, Canada
• that he proposed represented reefs constructed by
  algae

• Now called stromatolites,
• not until 1954 were they shown
• to be products of organic activity

Present-day stromatolites Shark Bay, Australia
48
Stromatolites

• Different types of stromatolites include
• irregular mats, columns, and columns linked by
  mats

49
Other Evidence of Early Life

• Carbon isotopes in rocks 3.85 billion years old
• in Greenland indicate life was perhaps present
  then
• The oldest known cyanobacteria
• were photosynthesizing organisms
• but photosynthesis is a complex metabolic process
  
• A simpler type of metabolism
• must have preceded it
• No fossils are known of these earliest organisms

50
Earliest Organisms

• The earliest organisms must have resembled
• tiny anaerobic bacteria
• meaning they required no oxygen
• They must have totally depended
• on an external source of nutrients
• that is, they were heterotrophic
• as opposed to autotrophic organisms
• that make their own nutrients, as in
  photosynthesis
• They all had prokaryotic cells
• meaning they lacked a cell nucleus
• and lacked other internal cell structures typical
  of eukaryotic cells (to be discussed later in the
  term)

51
Photosynthesis

• A very important biological event
• occurring in the Archean
• was the development of
• the autotrophic process of photosynthesis
• This may have happened
• as much as 3.5 billion years ago
• These prokaryotic cells were still anaerobic,
• but as autotrophs they were no longer dependent
• on preformed organic molecules
• as a source of nutrients
• These anaerobic, autotrophic prokaryotes
• belong to the Kingdom Monera,
• represented today by bacteria and cyanobacteria

52
Fossil Prokaryotes

• Photomicrographs from western Australias
• 3.3- to 3.5-billion-year-old Warrawoona Group,
• with schematic restoration shown at the right of
  each

53
Archean Mineral Resources

• A variety of mineral deposits are Archean
• but gold is the most notably Archean,
• although it is also found
• in Proterozoic and Phanerozoic rocks
• This soft yellow metal is prized for jewelry,
• but it is or has been used as a monetary
  standard,
• in glass making, electric circuitry, and chemical
  industry
• About half the worlds gold since 1886
• has come from Archean and Proterozoic rocks
• in South Africa
• Gold mines also exist in Archean rocks
• of the Superior craton in Canada

54
Archean Sulfide Deposits

• Archean sulfide deposits of
• zinc,
• copper
• and nickel
• occur in Australia, Zimbabwe,
• and in the Abitibi greenstone belt
• in Ontario, Canada
• Some, at least, formed as mineral deposits
• next to hydrothermal vents on the seafloor,
• much as they do now around black smokers

55
Chrome

• About 1/4 of Earths chrome reserves
• are in Archean rocks, especially in Zimbabwe
• These ore deposits are found in
• the volcanic units of greenstone belts
• where they appear to have formed
• when crystals settled and became concentrated
• in the lower parts of various plutons
• such as mafic and ultramafic sills
• Chrome is needed in the steel industry
• The United states has very few chrome deposits
• so must import most of what it uses

56
Chrome and Platinum

• One chrome deposit in the United States
• is in the Stillwater Complex in Montana
• Low-grade ores were mined there during war time,
• but they were simply stockpiled
• and never refined for chrome
• These rocks also contain platinum,
• a precious metal, that is used
• in the automotive industry in catalytic
  converters
• in the chemical industry
• for cancer chemotherapy

57
Iron

• Banded Iron formations are sedimentary rocks
• consisting of alternating layers
• of silica (chert) and iron minerals
• About 6 of the worlds
• banded iron formations were deposited
• during the Archean Eon
• Although Archean iron ores
• are mined in some areas
• they are neither as thick
• nor as extensive as those of the Proterozoic Eon,
  
• which constitute the worlds major source of iron

58
Pegmatites

• Pegmatites are very coarsely crystalline igneous
  rocks,
• commonly associated with granite plutons,
• composed of quartz and feldspars
• Some Archean pegmatites,
• such in the Herb Lake district in Manitoba,
  Canada,
• and Zambia in Africa, contain valuable minerals
• In addition to minerals of gem quality,
• Archean pegmatites contain minerals mined
• for lithium, beryllium, rubidium, and cesium

59
Summary

• Precambrian encompasses all geologic time
• from Earths origin
• to the beginning of the Phanerozoic Eon
• The term also refers to all rocks
• that lie stratigraphically below Cambrian rocks
• Terms for Precambrian time include
• an informal one, the Hadean,
• followed by two eons, the Archean and Proterozoic
• Some Hadean crust must have existed,
• but none of it has been preserved
• By the beginning of the Archean Eon,
• several small continental nuclei were present

60
Summary

• All continents have an ancient stable nucleus
• or craton made up of
• an exposed shield
• and a buried platform
• The exposed part of the North American craton
• is the Canadian shield,
• and is make up of smaller units
• delineated by their ages and structural trends
• Archean greenstone belts are linear,
• syncline-like bodies found within
• much more extensive granite-gneiss complexes

61
Summary

• Greenstone belts typically consist of
• two lower units dominated by igneous rocks
• and an upper unit of mostly sedimentary rocks
• They probably formed by plate movements
• responsible for opening
• and then closing back-arc marginal basins
• Widespread deformation took place
• during the Late Archean
• as parts of the Canadian shield evolved

62
Summary

• Many geologists are convinced
• some type of Archean plate tectonics occurred,
• but it probably differed
• from the tectonic style of the present
• For one thing, Earth had more heat
• and for another, plates probably moved faster
• The early atmosphere and hydrosphere
• formed as a result of outgassing,
• but this atmosphere lacked free oxygen and
• contained abundant water vapor and carbon dioxide

63
Summary

• Models for the origin of life
• by natural processes require
• an oxygen deficient atmosphere,
• the appropriate elements for organic molecules,
• and energy to promote the synthesis
• of organic molecules
• The first naturally formed organic molecules
• were probably monomers,
• such as amino acids,
• that linked together to form
• more complex polymers such as proteins

64
Summary

• RNA molecules may have been
• the first molecules capable of self-replication
• However, how a reproductive mechanism evolved is
  not known
• The only known Archean fossils
• are of single-celled, prokaryotic bacteria
• or blue-green algae (cyanobacteria)
• Stromatolites formed by photosynthesizing
  bacteria
• are found in rocks as much as 3.5 billion years
  old
• Carbon isotopes indicate
• life was present even earlier

65
Summary

• The most important
• Archean mineral resources are
• gold, chrome, zinc, copper, and nickel

66
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 4 to 6
  times as great as it is now

• With less heat outgassing decreased

67
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

68
Ocean water

• The volume and geographic extent
• of the Early Archean 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

69
Earliest Organisms

• The earliest organisms, then,
• were anaerobic, heterotrophic prokaryotes
• Their nutrient source was most likely
• adenosine triphosphate (ATP)
• from their environment
• which was used to drive
• the energy-requiring reactions in cells
• ATP can easily be synthesized
• from simple gases and phosphate
• so it was doubtless available
• in the early Earth environment

70
Fermentation

• Obtaining ATP from the surroundings
• could not have persisted for long
• because more and more cells competed
• for the same resources
• The first organisms to develop
• a more sophisticated metabolism
• which is used by most living prokaryotic cells
• probably used fermentation
• to meet their energy needs
• Fermentation is an anaerobic process
• in which molecules such as sugars are split
• releasing carbon dioxide, alcohol, and energy

71
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)
• oxygen not combined with other elements
• such as in carbon dioxide (CO2)
• water vapor (H2O)
• ozone (O3)
• which is common enough in the upper atmosphere
• to block most of the Suns ultraviolet radiation

72
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 0.034
• Ozone O3 0.0006
• Other gases Trace
• Particulates normally trace

73
Experiment on the Origin of Life

• Is it plausible that monomers
• originated in the manner postulated?
• Experimental evidence indicates that it is

• During the late 1950s
• Stanley Miller
• synthesized several amino acids
• by circulating gases approximating
• the early atmosphere
• in a closed glass vessel

74
Experiment on the Origin of Life

• This mixture was subjected to an electric spark
• to simulate lightning

• In a few days
• it became cloudy
• Analysis showed that
• several amino acids
• typical of organisms
• had formed
• Since then,
• scientists have synthesized
• all 20 amino acids
• found in organisms

75
Second Crustal Evolution Stage

• Several sialic continental nuclei
• had formed by the beginning of Archean time
• by subduction and collisions
• between island arcs

76
Dynamic Processes

• During the Hadean, 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

77
Basic Building Blocks of Life

• Energy from
• lightning
• and ultraviolet radiation
• probably promoted chemical reactions
• during which C, H, N and O combined
• to form monomers
• comparatively simple organic molecules
• such as amino acids
• Monomers are the basic building blocks
• of more complex organic molecules

78
Polymerization

• The molecules of organisms are polymers
• such as proteins
• and nucleic acids
• RNA-ribonucleic acid and DNA-deoxyribonucleic
  acid
• consisting of monomers linked together in a
  specific sequence
• How did polymerization take place?
• Water usually causes depolymerization,
• however, researchers synthesized molecules
• known as proteinoids
• some of which consist of
• more than 200 linked amino acids
• when heating dehydrated concentrated amino acids

79
Proteinoids

• The heated dehydrated concentrated
• amino acids spontaneously polymerized
• to form proteinoids
• Perhaps similar conditions
• for polymerization existed on early Earth,
• but the proteinoids needed to be protected
• by an outer membrane or they would break down
• Experiments show that proteinoids
• spontaneously aggregate into microspheres
• which are bounded by cell-like membranes
• and grow and divide much as bacteria do

80
What Happened During the Hadean?

• Although no rocks of Hadean 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
• Earth was bombarded by meteorites
• Volcanic activity was ubiquitous
• Atmosphere formed, quite different from todays
• Oceans began to accumulate

81
Protobionts

• Protobionts are intermediate between
• inorganic chemical compounds
• and living organisms
• Because of their life-like properties
• the proteinoid molecules can be referred to
• as protobionts

82
Precambrian 4 Billion Years

• The Precambrian lasted for more than 4 billion
  years!
• Such a 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

83
Amalgamated Cratons

• Actually the Canadian shield and adjacent
  platform
• is made up of numerous units or smaller cratons
• that amalgamated along deformation belts
• during the Early Proterozoic
• 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,
• in places exposed by deep erosion or uplift

84
Archean Rocks Beyond the Shield
Rocky Mountains, Colorado

• Archean metamorphic rocks found
• in areas of uplift in the Rocky Mtns

85
RNA World?

• Now we know that small RNA molecules
• can replicate without the aid of protein enzymes
• Thus, the first replicating systems
• may have been RNA molecules
• Some researchers propose
• an early RNA world
• in which these molecules were intermediate
  between
• inorganic chemical compounds
• and the DNA-based molecules of organisms
• How RNA was naturally synthesized
• remains an unsolved problem

86
Second Crustal Evolution Stage

• This second stage in crustal evolution
• began as Earths production
• of radiogenic heat decreased
• Subduction and partial melting
• of earlier-formed basaltic crust
• resulted in the origin of andesitic island arcs
• Partial melting of lower crustal andesites,
• in turn, yielded silica-rich granitic magmas
• that were emplaced in the andesitic arcs

87
Canadian Shield Rocks

• Gneiss, a metamorphic rock, Georgian Bay Ontario,
  Canada

88
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 the rift margins supplied
• the sediment to the upper unit
• An episode of subsidence, deformation,
• metamorphism and plutonism followed

89
What Is Life?

• Minimally, a living organism must reproduce
• and practice some kind of metabolism
• Reproduction insures
• the long-term survival of a group of organisms
• whereas metabolism
• such as photosynthesis, for instance
• insures 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

90
What Is Life?

• Comparatively simple organic (carbon based)
  molecules known as microspheres

• form spontaneously
• show greater organizational complexity
• than inorganic objects such as rocks
• 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

91
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 Late Archean ophiolites are known

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

92
Evolution of Greenstone Belts

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

93
Evolution of Greenstone Belts

• Then during closure,
• the rocks were compressed, deformed,
• cut by faults,
• and intruded by rising magma

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

94
Monomer and Proteinoid Soup

• The origin-of-life experiments are interesting,
• but what is their relationship to early Earth?
• Monomers likely formed continuously and by the
  billions
• and accumulated in the early oceans into a hot,
  dilute soup (J.B.S. Haldane, British biochemist)
• The amino acids in the soup
• might have washed up onto a beach or perhaps
  cinder cones
• where they were concentrated by evaporation
• and polymerized by heat
• The polymers then washed back into the ocean
• where they reacted further

95
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
• a sandstone with abundant clay and rock fragments
• and argillite
• a slightly metamorphosed mudrock