Download on Earth

Precambrian Time
• “Precambian” is the informal term for the interval
of time prior to the evolutionary radiation of
skeletonized animals at 543 mybp
• “Precambrian” is subdivided into:
– Archean Eon, from the origin of the Earth (4.6 bybp)
to the stabilization of Earth’s basic structure
(core/mantle/crust) (2.5 bybp)
– Proterozoic Eon, from 2.5 bybp to the beginning of
Cambrian time (543 mybp)
Earth History, Ch. 11
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Geologic time
Archean
43%
Phanerozoic
12%
Proterozoic
45%
Earth History, Ch. 11
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Precambrian rocks
• Although Precambrian time accounts for
88% of Earth’s history, Precambrian rock
exposures make up only about 20% of
Earth’s land surface
• Most Precambrian rocks have been
destroyed in the course of plate tectonic
cycles (and most remaining ones are buried
beneath the veneer of Phanerozoic rocks)
Earth History, Ch. 11
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Earth History, Ch. 11
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Precambrian rocks
• Cratons are the large, stable, interior regions of
continents that have not undergone major
deformation since Precambrian or early
Phanerozoic time
• Most Precambrian rocks are confined to cratons,
where they may be exposed in a “Precambrian
shield”
Earth History, Ch. 11
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Precambrian
shield area in
NW Canada
Earth History, Ch. 11
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Archean Time:
From the very beginning….
• Age of universe is estimated at ~15 billion
years (redshift evidence)
• Oldest radiometrically dated rocks on Earth
are ~4.1 billion years old
• But, meteorites and lunar rocks have been
dated at 4.6 billion years, suggesting that
our solar system is about that old
Earth History, Ch. 11
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Origin of our galaxy
and solar system
Solar nebula forms
(remains of supernova)
Rotation and
contraction to disk
Central concentration
of matter
Formation of discrete
rings of matter
Condensation of matter
into planets
Earth History, Ch. 11
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Origin of our galaxy and solar
system (cont.)
• Outer planets are composed largely of volatile compounds
• Denser, less volatile compounds make up the inner
planets
• Asteroid belt is a ring of debris that has not coalesced into
a planet
Earth History, Ch. 11
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Origin of Earth
• Primordial Earth accreted from successive
impacts of hot, giant asteroids (some the size of
Mars)
• Early Earth was molten because of heat from
energy of impacts and radioactive decay
• Dense materials sank to center of planet, with less
dense materials rising toward surface
• “Magma ocean” at surface eventually cooled to
form oceanic crust
Earth History, Ch. 11
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Origin of Earth (cont.)
Homogeneous
molten Earth
Segregation of
materials by density
Earth History, Ch. 11
Final differentiation
of core/mantle/crust
11
Earth’s early heat flow
• Earth had greater heat flow in
the Archean Eon than today,
because Earth’s radioactive
“furnace” was hotter
• “Hot spots” were numerous;
lithosphere was fragmented
into many small plates
• Felsic crust was partitioned
into small “protocontinents”
Earth History, Ch. 11
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Earth’s internal heat
Earth History, Ch. 11
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Origin of the Moon
• Moon originated when a large
(Mars-size) body collided
with Earth (“glancing blow”)
– Core of impacting body was
absorbed into Earth’s core
– Remaining mantle of impacting
body and was then captured in
Earth’s gravitational field
• Collision caused Earth’s rotation to
increase
• Moon has no water; a metallic core
and feldspar-rich outer layer;
relative abundance of iron and
magnesium differ from that in
Earth’s mantle
Earth History, Ch. 11
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Earth’s early atmosphere
• Earth did not inherit its atmosphere from the
initial asteroids that coalesced to form it
• Earliest atmosphere was generated by emission of
internal gases (similar to those emitted today
from volcanoes):
– Water vapor, hydrogen, hydrogen chloride, carbon
monoxide, carbon dioxide, nitrogen
• Note absence of oxygen, which was rare prior to
the advent of photosynthetic organisms!
Earth History, Ch. 11
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Earth’s early oceans
• Ocean water originated partly from emitted
water vapor and partly from icy comets as they
melted upon entry into Earth’s atmosphere
– 15 million small comets (~12 meters in diameter) enter
Earth’s atmosphere every year!
• Salts were added to the oceans from rivers
carrying by-products of chemically weathered
rocks
– Salinity stabilized very early in Archean time because
salt is removed from the oceans by precipitation of salt
minerals
Earth History, Ch. 11
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Origin of continents
• Earth’s early crust was entirely oceanic
crust of mafic composition
• Earliest continental (felsic) crust must have
originated from a mafic parent, but how?
– When mafic crust is subducted and melted, the
resulting extrusive volcanics still possess a
mafic or intermediate composition
– Igneous activity associated with hot spots can
produce felsic volcanics!!
Earth History, Ch. 11
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Origin of continents:
Iceland example
• Iceland is a volcanic island situated over a hot spot along
the mid-Atlantic ridge
• Here, lower oceanic crust contains isolated “pods” of felsic
material that have segregated from igneous material in the
mantle
• Mafic magma flows to the surface along faults; in doing so
it melts felsic bodies along the way  felsic volcanics
• As volcanics pile up, isostatic sinking of Iceland causes
partial melting and further segregation of felsics  more
felsic volcanics
Earth History, Ch. 11
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Origin of continents:
Iceland example
About 10% of Iceland’s crust is felsic in composition
Earth History, Ch. 11
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Origin of continents:
Iceland example
• Iceland’s crust is 8–10 km thick, about twice the
average thickness of oceanic crust
• Iceland is only about 16 million years old and still
growing—it’s a protocontinent!
• Archean continents remained small: lithospheric
plates were all small because of Earth’s high heat
flow
• In Proterozoic time, once the pace of plate
tectonics slowed, protocontinents were sutured
together to form larger continents
Earth History, Ch. 11
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Archean continental crust
• Oldest dated continental crust minerals are ~4.4
billion years old
• Oldest large area of continental crust is ~3.8–4.0
billion years old (NWT Canada)
• Geologists believe that by ~3.5 bybp, total volume
of continental crust reached its present level
– No net gain or loss since then, because as new felsic
material is added by igneous activity, old felsic material
is consumed at subduction zones
Earth History, Ch. 11
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Archean rocks
• Archean sedimentary rocks are mostly of
deep-water origin
– Sandstones, cherts, shales, banded-iron
formations
– Very few, if any, limestones or evaporites
– No well developed continental shelves for
accumulation of shallow-water deposits
Earth History, Ch. 11
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Archean rocks
Earth History, Ch. 11
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Archean rocks (cont.)
• Banded iron formations
– Alternating bands of iron-rich layers and chert layers
– Thought to have precipitated from hot marine water
associated with igneous activity
– Iron is weakly oxidized (looks like iron), suggesting
little or no exposure to oxygen
• Very few banded iron formations younger than 1.9
billion years old (when atmospheric O2 increased)
• Most iron deposits younger than 1.9 billion are
highly oxidized (red beds)
– Principal source of world’s iron ore
Earth History, Ch. 11
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Banded iron formations
Iron layers
Chert layers
(red)
Earth History, Ch. 11
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Archean rocks (cont.)
• Greenstone belts
– Make up large portions of Archean terranes
– Age of most greenstone belts is ~2.5–3.0 billion years
– Elongate belts of weakly metamorphosed rock
separating larger masses of felsic protocontinents
– Include metamorphosed mixtures of mafic and felsic
volcanics, volcanic sediments, turbidites
• Assemblage of precursor rocks is characteristic of forearc
basins and subduction zones
– Probably formed along subduction zones where
protocontinents were sutured together
Earth History, Ch. 11
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Formation of greenstone belts
Time 1
Time 2
Earth History, Ch. 11
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Greenstone belts
Satellite view of
Archean greenstone belts
and felsic protocontinents
in western Australia
25 mi
Earth History, Ch. 11
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Life on Earth
• Why Earth is well suited for harboring life:
– Right size
• Gravitational pull of larger planets creates an
atmosphere too dense for penetration of sunlight
• Gravitational pull of smaller planets is too weak to
retain an atmosphere
– Right temperature
• Most H2O is in the form of liquid water, not water
vapor
Earth History, Ch. 11
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The Archean fossil record
• All Archean fossils are prokaryotes
– Archeobacteria and Eubacteria
• The oldest known forms are bacterial
filaments like modern cyanobacteria
– 3.2 to 3.5 billion years old, from Western
Australia
• Stromatolites known in rocks 3.4 billion
years old and younger
Earth History, Ch. 11
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The Archean fossil record (cont.)
3.5 billion year old bacteria
preserved in chert from
Western Australia
Modern cyanobacterial
filaments
Earth History, Ch. 11
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Fossilized bacterial filaments:
3.2 billion years old, NW Australia
diameter of
filaments = 2 µm
Earth History, Ch. 11
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Oldest known stromatolites:
3.45 billion years old, W Australia
Earth History, Ch. 11
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The Archean fossil record (cont.)
3.2 billion year old stromatolite
from South Africa
Growth of cyanobacterial mats
Earth History, Ch. 11
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Origin of life
• Basic attributes of life:
– Ability to reproduce
– Self-regulation (ability to sustain orderly
internal chemical reactions)
• Proteins are among the compounds
required for reproduction and regulation
• Amino acids are the building blocks of
proteins
Earth History, Ch. 11
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Origin of life
• Laboratory synthesis
of amino acids from
simulated early
atmosphere
• Stanley Miller Soup
(1953)
–
–
–
–
–
–
Hydrogen (H)
Ammonia (NH3)
Methane (CH4)
Water vapor (H2O)
Electrical spark
No O2
Amino acids
collected here
Earth History, Ch. 11
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Stanley Miller
Earth History, Ch. 11
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Origin of life
• Miller’s assumption was that no O2 existed in
Earth’s early atmosphere
– Incorrect: at least some was there (but not much)
• Experiment did produce many types of amino
acids that combined to form simple protein-like
compounds
• Amino acids later discovered in Murchison
meteorite (1969) in the same relative proportions
as in Miller’s soup
– Thus, amino acids could have been delivered to the
Archean Earth from space
Earth History, Ch. 11
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Origin of life
• Nucleic acids
DNA and RNA—
also essential for
life
• DNA carries genetic
code and has ability
to replicate itself
nucleotide
bases
sugar
phosphate group
Earth History, Ch. 11
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Origin of life
• RNA also can replicate itself
– Messenger RNA carries information from DNA to sites
where proteins are formed
– Transfer RNA ferries amino acids to sites where
proteins are formed, and serves as a catalyst in protein
growth
• RNA probably was the nucleic acid in the earliest
true form of life, with DNA evolving later
• Once RNA and DNA had originated,
semipermeable cell membranes evolved that
could protect the chemical system of the primitive
organism while allowing certain compounds to
pass in and out
Earth History, Ch. 11
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Origin of life
• Where did life form?
– Probably not at the Earth’s surface in shallow
pools, as once believed
• Presence of oxygen would have inhibited the
“cooking” of “Stanley Miller soup”
– Most likely in the deep sea, away from O2, and
probably near a “vent” of hot water
• Modern chemosynthetic bacteria are abundant near
vents on mid-ocean ridges
• They derive energy by consuming chemical
compounds and allowing reactions to occur within
their cell membranes
Earth History, Ch. 11
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Mid-ocean ridge “vents”
Earth History, Ch. 11
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Origin of life
• Mid-ocean ridges are the most likely sites
for origin of life and early bacterial
evolution
– Enormous size  many opportunities for key
events to take place
– Anoxic (no O2) water with necessary amino
acid building blocks present
– Other key materials present
• Phosphorus, nickel, zinc, clays
– Modern “vent” bacteria are genetically the
most primitive archeobacteria known
Earth History, Ch. 11
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