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HISTORICAL GEOLOGY
PRECAMBRIAN GEOLOGY.
Introduction
Precambrian time = 87% of
Earth history and yet much
less is known about
Precambrian history than
Phanerozoic history (13%),
because there are few
fossils and the rocks are
highly deformed.
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Formation of the Solar System and Earth – Solar Nebula Hypothesis
Clues: 1. All planets orbit the same direction and in the same plane around
the sun.
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2. The inner planets
are dense and rocky;
the outer planets are
less dense and
gaseous.
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The Solar Nebula Hypothesis: whole solar system
formed from a cloud of rotating dust and gas
(explains why all planets rotate the same way).
As the cloud contracted under gravity, it flattened
into a disk (explains the orbital plane).
Most material collected at the center (proto-sun), the
rest collected in masses orbiting the sun (protoplanets).
Compression by gravity caused heating in the protosun until nuclear fusion started (hydrogen combined
into helium). The resulting Solar Wind (charged
atomic particles) drove lighter gases from the inner
planets, leaving them rocky and denser (explains
contrast in composition between inner and outer
planets).
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When did all this occur? Based on dating the moon, meteorites and earth
rocks, the earth formed about 4.6 billion years ago.
Are any other planets like the earth? No – only earth seems to have liquid
water (and lots of it). Mars probably used to have liquid water (seas) a long
time ago (about 3.5 billion), but it has been stripped away by the solar
wind.
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THE ARCHEAN (4.6-2.5 BILLION)
The earth is about 4.6 billion years old.
Either during or soon after its formation
it was very hot and differentiated into a
core and mantle. This was during the
Archean Eon, which lasted about 2.1
billion years.
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The crust had not formed at this
time – the surface was probably
molten ultramafic rock, an
extension of the mantle.
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As the surface cooled (over millions of years) the atmosphere was
produced by outgassing and was probably similar to volcanic gases,
modified by chemical processes e.g. H2O rained out -> oceans; H2 lost
to space; CO reacted with H2O to form CO2 and H2; CO2 combined
with Ca and Mg to form carbonate rocks (late in this period). The
atmosphere was rich in carbon dioxide and water vapor, but had little
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to no oxygen.
komatiite
The mantle is made of ultramafic rock – dense rock made of
ferromagnesium minerals such as olivine and pyroxenes (Silicate of
Mg and Fe; and Silicates of Al, Ca, Mg and Fe). An intrusive
example is peridotite; the extrusive form is komatiite – these solidify
at higher temperatures than basalt. It is likely that patches of
komatiite crust formed during the Archean, but being denser, sunk
and were “recycled”.
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The Archean Crust
Important question –
“how did the modern crust
originate? (basalt oceanic crust
(started forming about 4.5
billion years ago; granitic
continental crust – started
forming about 4.4 billion years
ago).
Basalt formed at a mid
oceanic ridge due to partial
melting of peridotite.
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The answer involves cooler temperatures, the development of midoceanic ridges, subduction zones and partial melting*. As the earth
cooled (around the beginning of the archean) and temperatures
fluctuated, masses of mantle magma may have gone through cycles of
solidifying and melting. The resulting magma may have been more
basaltic (forms at a lower temperature and by partial melting* of
peridotite). Modern Plate tectonics had begun, forming basaltic
oceanic crust at mid-oceanic ridges. This resulted in cooler, larger,
longer-lasting crustal rock slabs. These collided and were subducted
and also underwent partial melting* – felsic minerals melt first and
produce a more felsic magma and more felsic rocks (e.g. tonalite – a
quartz-rich intrusive igneous rock) – recycling of these rocks made
them more and more felsic until granitic rocks formed (still in the early
Archean). When these early volcanic rocks grew above sea level
(became land) they were eroded and formed the first sedimentary rock
(greywackes, conglomerates) possibly as early as 4.4 billion years ago.
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To recap: basaltic oceanic crust formed first at mid-oceanic ridges
and was probably wide-spread (all over the world); continental crust
formed later as oceanic crust was subducted and partially-melted,
releasing more and more felsic magma. This magma formed small
patches of continental crust near subduction zones. When these
patches grew above sea-level, they were eroded and produced the
first sedimentary rocks.
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Greenstone Belts
Evidence for these processes is found in GREENSTONE BELTS –
which record increasingly felsic volcanic rocks, metamorphosed and
topped by sedimentary rocks (also metamorphosed) (the
metamorphic mineral chlorite is green – hence greenstone belts).
The sedimentary rocks in the upper part of the sequence often contain
conglomerates and greywackes, suggesting steep, rugged coastlines
and the absence of shallow seas. All are intruded by later granites.
The entire sequence records the changing conditions – cooling
temperatures, more felsic rocks and the first appearance of
sedimentary rocks.
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Continental crusts grew larger by much the same way as they
do today:
•Small crustal slabs collided and became sutured together
•Continental growth by accretion occurred at subduction zones
•Sedimentation accumulated around the coastlines
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Hence, the early contiental crust consisted of greenstone belts,
accretionary sediment wedges and suture zones where two crustal
slabs had welded together.
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The Proterozoic (2.5 - 0.54 billion).
These processes
continued into the
Proterozoic, building
the North American
craton. So the craton is
actually separate
provinces, welded
together - ages of the
provinces get younger
further out from the
center. The Proterozoic
is distinguished by more
modern Tectonics and
the first shallow marine
rocks (formed on wide
continental shelves).
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Therefore, it is probable that cratons developed in much the
same way as more recent parts of continents i.e.
1. orogenesis forming mountains, folded and metamorphosed rocks
(at subduction zones and suture zones).
2. continental growth by accretion (many cratons consist of smaller
continental masses, or provinces, joined together).
3. Failed intracratonic rifting (e.g. mid continental rift – volcanic
and sedimentary rocks)
The Proterozoic rocks, being younger, formed around the margins,
and inbetween, the older archean cratons.
In contrast to the archean, prolonged sedimentation built wide
continental shelves and coastal plains allowing sandstones and
carbonates to form (the first appearance of shallow marine/lowland
environments). Most proterozoic rocks have been subject to
varying degrees of folding, metamorphism, orogenesis and erosion.
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The Wopmay Belt – example of a proterozoic orogenic belt.
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The shields (or cratons) of all major land masses have a similar history.
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In the late
(Neo)proterozoic,
nearly all land
masses were brought
together by plate
movements to form a
single continent –
RODINIA,
surrounded by an
ocean MIROVIA.
Rodinia included
LAURENTIA – the
Precambrian craton
of North America – it
was sutured to
Rodinia by the
Grenville Orogeny.
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