Transcript Precambrian - E. R. Greenman

Precambrian Earth and
Life History
Precambrian Time Span
The
Precambrian
lasted for more
than 4 billion
years!

88% of
geologic time
Precambrian

The term Precambrian is informal
but widely used, referring to both time and rocks



The Precambrian: 4.6 bya to 542 mya
Oldest known rocks are 3.96 by old
Other information about pC is inferred using
what we know about planet formation
Key Events of Precambrian time
Acasta Gneiss is dated at
3.96 bya. It is near Yellowknife Lake , NWT Canada
Zircons possibly a bit older in Australia
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
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 early Archean
time (3.8 bya)
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 indicte that some kind
of Eoarchean crust was certainly present,
but its distribution is unknown
Early Archean Crust

Early Archean 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

Early Archean 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
Origin of Continental Crust

Andesitic island
arcs


form by
subduction
and partial
melting of
oceanic crust
The island arc
collides with
another
(accretion)
• Each continent has an ancient, relatively flat interior with very
little tectonic or mountain-building activity. These large “tracts”
of exposed metamorphic rocks in continental interiors are called
Precambrian shields, and are some of the oldest crustal rocks.
M&W, Fig. 7.1
Continental Foundations




Continents consist of rocks with composition
similar to that of granite
Continental crust is thicker and less dense than
oceanic crust
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
Cratons




A shield and platform make up a craton, a
continent’s 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 show
evidence of episodes of deformation
accompanied by metamorphism, igneous activity,
and mountain building
Cratons have experienced little deformation since
the Precambrian
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
Canadian Shield Rocks

Outcrop of Archean gneiss in the Canadian
Shield in Ontario, Canada
Archean Rocks Beyond the Shield

Archean Brahma Schist in the deeply eroded
parts of the Grand Canyon, Arizona
Early Continents (Cratons) Archean
•Archean cratons consist of regions of light-colored felsic rock (granulite gneisses)
• surrounded by pods of dark-colored greenstone (chlorite-rich metamorphic rocks
–Pilbara Shield, Australia
–Canadian Shield
–South African Shield
Mafic Greenstone
Belts
Felsic Islands
40km
Archean Plate Tectonics



Plate tectonic activity has operated since the
early Proterozoic (or perhaps late Archean)
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 Earth’s origin and more
radiogenic heat, and magma was generated more
rapidly
Formation of Rodinia



Grenville Orogeny 1.3-1.0 bya
Collisions between N. Am, S. Am, Africa and
Antartica creates supercontinent
Climate change!
Snowball Earth

Rodinia: abundant basalts with easily weathered Ca
feldspars. Ocean gets Ca+ + . CO2 tied up in extensive
limestones. Less greenhouse effect. Atmosphere can’t
trap heat – Earth gets colder

Grenville Orogeny left extensive highlands from high
latitudes to equator

About 635 mya glacial deposits found in low latitudes
and elevations

Huge Ice sheet reflects solar radiation “Albedo”

Some workers believe oceans froze
Break up of Rodinia

Hypothesis: Ice an insulator, heat builds up

Heavy volcanic activity poured CO2 into
atmosphere – greenhouse effect

Warming melted snowball earth
Earth’s Very Early Atmosphere



Earth’s very early (Hadean-Archean) atmosphere
was probably composed of hydrogen and helium
If so, it would have quickly been lost into space
because Earth’s gravity is insufficient to retain
them and 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
Outgassing

Once a magnetosphere was
present atmosphere began
accumulating as a result of
outgassing
 Water vapor is the most
common volcanic gas today but
volcanoes also emit carbon
dioxide, sulfur dioxide, carbon
monoxide, sulfur compounds,
hydrogen, chlorine and nitrogen
Archean Atmosphere


Archean volcanoes probably emitted the same
gases, thus an atmosphere developed
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
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)
But oxidized iron becomes increasingly common
in Proterozoic rocks indicating that at least some
free oxygen was present then
Introduction of Free Oxygen

Two processes account for introducing free
oxygen into the atmosphere, one or both of
which began during the early Archean.
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
Photosynthesis


Photosynthesis is a process in which carbon
dioxide and water combine into organic
molecules and oxygen is released as a waste
product
6CO2 + 6H2O + sunlight + chlorophyll 
C6H12O6 + 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
Earth’s Surface Waters




Outgassing was responsible for the early
atmosphere and also for Earth’s surface water
the hydrosphere
However, some—but 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 early Archean times
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
With less heat
outgassing
decreased

Heat production
4 billion years
ago was 3 to
6 times as great
as it is now
First Organisms



Today, Earth’s biosphere consists of millions of species
of bacteria, archea, 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
Oldest Known Organisms



The first organisms were members of bacteria
and/or archaea, both of which consist of
prokaryotic cells, cells that lack an internal,
membrane-bounded nucleus and other structures
Prior to the 1950s, scientists assumed that life
must have had a long early historybut the fossil
record offered little to support this idea
The Precambrian, once called Azoic (“without
life”), seemed devoid of life
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)
Stromatolites


Present-day stromatolites form and grow as
sediment grains (calcium carbonate) are trapped
on sticky mats of photosynthesizing cyanobacteria.
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
At right is a layered
stromatolite, produced by the
activity of ancient
cyanobacteria. The layers were
produced as calcium
carbonate precipitated over the
growing mat of bacterial
filaments; photosynthesis in the
bacteria depleted carbon
dioxide in the surrounding
water, initiating the precipitation.
The minerals, along with grains
of sediment precipitating from
the water, were then trapped
within the sticky layer of
mucilage that surrounds the
bacterial colonies, which then
continued to grow upwards
through the sediment to form a
new layer. As this process
occured over and over again,
the layers of sediment were
created.
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
They all had prokaryotic cells
The earliest organisms, then, were anaerobic,
heterotrophic prokaryotes
Fossil Prokaryotes

Photomicrographs from western Australia’s 3.3- to
3.5-billion-year-old Warrawoona Group, with
schematic restoration shown at the right of each