Download History of Geology The way the earth changes: Fossils, erosion

History of Geology
The way the earth changes: Fossils,
erosion & modification in the landscape
1. Shells on the top of mountains
2. Changing landscape after rain, snow, tempest,
volcanic eruption, etc.
3. Presence of strata with clear boundaries and
superposition
4. Change in the climate
Earth Changing : fossils & erosion
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Fossils has been
used as jewels for
thousands of years
They were found as
decoration in Neanderthal man tombs
Earth Changing : fossils & erosion
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Fossil: From Latin fossus meaning “having
been dug”
Fossils were discovered and described during
the Greek and Roman periods
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How to explain that rocks
can have the shape of
shells?
How to explain the presence
of shells far from ocean?
Earth Changing : fossils & erosion
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Xenophanes (540
BCE) concluded from
his examination of
fossils that water once
must have covered all
of the Earth's surface
He supposed a world
with alternating two
extremes: earth and
water
Earth Changing : fossils & erosion
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The ancient alchemy
tried to transform
things into others
(metal into gold)
Aristotle though that
living thing could
arise spontaneously
from rocks or mud
Fossils would be the
failure of this process
Earth Changing : fossils & erosion
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Theophrastus said
that fossils had never
been living organisms
They would be only
unusually shaped
rocks
A “shaping force”
within the earth would
form stones in various
shapes
Earth Changing : fossils & erosion
Eratosthenes
(276 BCE)
though that the
strait of
Gibraltar was
closed and the
level of the
Mediterranean
sea higher than
now
Earth Changing : fossils & erosion
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The opening of
the “pillar of
Hercules” would
have lower the
sea level
The presence of
fossil is explained
by ancient high
sea level
Earth Changing : fossils & erosion
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Strabo (24 CE) said
that the level of land
would change according to earthquakes,
eruptions, tsumanis,
landslips, etc.
Fossils would have
been deposited after
such disasters
Earth Changing : fossils & erosion
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Big fossil bones
were attributed to an
ancient races of
giant
Giants were
supposed to
populate the world
before some
disaster that would
have been their end
Earth Changing : fossils & erosion
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Greeks and Romans
could explain the
fossilisation of still
existing and complete
organisms
They could not
imagine that
nummulites,
Glossopetræ,
ammonites could be
also fossils
Earth Changing : fossils & erosion
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Pliny supposed that
the glossopetræ fall
from the sky during
lunar eclipse
Earth Changing : fossils & erosion
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Pliny supposed that
the glossopetræ fall
from the sky during
lunar eclipse
Nummulites are
abundant in some
limestones, especially
in egypt
Pyramids are built with
nummulite limestones
Earth Changing : fossils & erosion
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Nummulites are small
lenticular fossils (1-10
cm) that was
generated by
unicellular organism
They are common in
tertiary limestones
They are often found
around the
Mediterranean sea
Earth Changing : fossils & erosion
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Guides in the Egyptian
pyramids used to say
that nummulites in the
stones were the
fossilised food of the
slaves employed for
building
Most of the Greek and
Roman philosophers
did not believe this
story but had no
explanation
Earth Changing : fossils & erosion
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Ammonites are an extinct
group of mollusc
They were numerous
during the Secondary
Era
Some of them may be
more than two metre
wide
Ammonites are good
index fossil and allow
precise dating
Earth Changing : fossils & erosion
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The name “ammonite”
comes from the
common name that was
given to these fossil until
th
the 18 century: horns of
Ammon
They were not
supposed to be “fossils”
but some natural curious
rocks.
No explanation for them
Earth Changing : fossils & erosion
Amon (Râ) was a god in Egyptian mythology who
in the form of Amun-Ra became the focus of the
most complex system of theology in Ancient
Egypt
He created
himself,
without father
nor mother
and is the
god of sun
Earth Changing : fossils & erosion
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Amon was often
represented with the
head or as a ram
The spiral shape of
the ammonites
resembles ram's horn
Even now, names of
ammonites end with
-ceras, which mean
“horn” in greek
Earth Changing : fossils & erosion
In China, fossil bones
and teeth were
considered as
“dragon bones”. They
were used, as
powder, as medicine
to cure anything.
In some areas of
China, they are still
sold as such today.
Earth Changing : fossils & erosion
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Greeks and Roman noticed the existence of
fossils
They tried several hypothesis to explain their
presence:
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The sea once covered the whole earth
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Increase of the sea level
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Shaping force of the Earth
Nummulites, ammonites and glossopetræ were
interpreted not as fossils but “precious stones”
with magical properties
Earth Changing : fossils & erosion
What was the significance of fossil shells for Greek
and Roman philosophers?
Earth Changing : fossils & erosion
What did they think about ammonites?
Earth Changing : fossils & erosion
What did they think about ammonites?
And bout nummulites?
Earth Changing : fossils & erosion
How did they explain giant fossil bones?
Earth Changing : fossils & erosion
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The silt carried along
by rivers to the sea is
the sign of the
erosion of the
continents
Earth Changing : fossils & erosion
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The silt carried along
by rivers to the sea is
the sign of the
erosion of the
continents
Floods, especially the
Nile floods, were
known to erode river
banks and to leave
more or less thick silt
deposit
Earth Changing : fossils & erosion
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Representations of
floods are ancient
Greeks and Romans
supposed that
continents should
become progressively
flat under the action
of erosion
Earth Changing : fossils & erosion
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Representations of
floods are ancient
Greeks and Romans
supposed that
continents should
become progressively
flat under the action
of erosion
The sea should, in
contrast, fill up with
silt
Earth Changing : fossils & erosion
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Erosion happen at
human time scale
near to the seashore
Roman and Greek
were navigator
people who noticed
this phenomenon and
tried to explain it
Earth Changing : fossils & erosion
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Landslides and their
relations with rain and
water were known
and described by
early philosophers
Earth Changing : fossils & erosion
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Landslides and their
relations with rain and
water were known
and described by
early philosophers
The phenomenon of
erosion, landslide,
sedimentation were
integrated in some
global vision of the
world
Earth Changing : fossils & erosion
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Ancient text about
erosion, landslide,
floods, sedimentation
are missing
Description of these
phenomenon are
rather precise in the
Greek and Roman
world
Explanations and
interpretation are
missing
Earth Changing : fossils & erosion
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Theophrastus and Aristotle though that the
world were eternal, without end or beginning
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They had to explain how the earth is not flat
Their theory of the world invoked a flatteningrenewing cycle with disaster like eruption and
earthquake to make new mountains
Zeno of Citium (~300 BCE) argue that with no
beginning the earth should be flat: for him the
world was created recently with valley and
mountains