The History of the Earth

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The History of the Earth
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Origin of the Universe
The universe began about 14.4 billion years
ago The Big Bang Theory states that, in the
beginning, the universe was all in one place All
of its matter and energy were squished into an
infinitely small point, a singularity Then it
exploded
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Origin of the Universe
The tremendous amount of material blown out by
the explosion eventually formed the stars and
galaxies After about 10 billion years, our solar
system began to form
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Birth of the Solar System

• We know how the Earth and Solar System are today
  and this allows us to work backwards and
  determine how the Earth and Solar System were
  formed
• Plus we can out into the universe for clues on
  how stars and planets are currently being formed

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The Nebular Hypothesis

• In cosmogony, the Nebular Hypothesis is the
  currently accepted argument about how a Solar
  System can form

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Other Solar Systems

• We have now discovered over two hundred planets
  orbiting other stars
• The processes that created our solar system have
  also created an uncountable number of other solar
  systems

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The Nebular Hypothesis

• A large gas cloud (nebula) begins to condense
• Most of the mass is in the center, there is
  turbulence in the outer parts

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The Nebular Hypothesis

• The turbulent eddies collect matter measuring
  meters across
• Small chunks grow and collide, eventually
  becoming large aggregates of gas and solid chunks

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The Nebular Hypothesis
Pictures from the Hubble Space Telescope show
newborn stars emerging from dense, compact
pockets of interstellar gas called evaporating
gaseous globules
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The Nebular Hypothesis
Gravitational attraction causes the mass of gas
and dust to slowly contract and it begins to
rotate The dust and matter slowly falls towards
the center
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Protostar
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False Color Image of Protostar
The multi-colored area shows a dust disk
surrounding a newborn star The red-orange area at
the center represents the brightest region, which
contains the young star It is surrounded by the
cooler, dusty disk, which appears as yellow,
green and blue The diameter of the disk is about
20 times larger than our entire solar system
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The Sun

• After sufficient mass and density was achieved in
  the Sun, the temperature rose to one million C,
  resulting in thermonuclear fusion.
• H atom H atom He atom energy

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Birth of the Solar System
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Protoplanets

• Gravitational forces allow the inner planets to
  accrue and compact solid matter (including light
  and heavy atoms)
• Solar radiation blew gases (primarily hydrogen,
  helium) away from inner planets
• These gases were collected and condensed into the
  gas giants (Jupiter, Saturn, Uranus, Neptune)
• Beyond Neptune, ice and frozen gases form Pluto,
  Sedna and the Kuiper Belt Objects
• Left-over debris form comets and asteroids

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Birth of the Solar System
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Size of the Planets
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Venus, Earth and Mars
These maps are color coded to display different
elevations on the surface of each planet
Fig. 1.9
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The Age of the Earth
Earth is 4,570,000,000 years old
Meteorites give us access to debris left over
from the formation of the solar system We can
date meteorites using radioactive isotopes and
their decay products
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Geologic Time
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Bombardment From Space

• For the first half billion years of its
  existence, the surface of the Earth was
  repeatedly pulverized by asteroids and comets of
  all sizes
• One of these collisions formed the Moon

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Formation of the Moon
The Giant Impact Hypothesis predicts that around
50 million years after the initial creation of
Earth, a planet about the size of Mars collided
with Earth This idea was first proposed about 30
years ago, but it took calculations by modern
high-speed computers to prove the feasibility
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Formation of the Moon
This collision had to be very spectacular! A
considerable amount of material was blown off
into space, but most fell back onto the Earth
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Formation of the Moon
Part of the material from the collision remained
in orbit around the Earth By the process
collision and accretion, this orbiting material
coalesced into the Moon The early Moon orbited
very close to the Earth
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The Early Earth Heats Up
Three major factors that caused heating and
melting in the early Earths interior

• 1. Collisions (Transfer of kinetic energy into
  heat)
• 2. Compression
• 3. Radioactivity of elements (e.g. uranium,
  potassium, or thorium)

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The Core

• About 100 million years after initial accretion,
  temperatures at depths of 400 to 800 km below the
  Earths surface reach the melting point of iron

In a process called global chemical differential,
the heavier elements, including the melted iron,
began to sink down into the core of the Earth,
while the lighter elements such as oxygen and
silica floated up towards the surface
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Global Chemical Differentiation
This global chemical differential was completed
by about 4.3 billion years ago, and the Earth had
developed a inner and outer core, a mantle and
crust
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Chemical Composition of Earth
Each of the major layers has a distinctive
chemical composition, with the crust being quite
different from the Earth as a whole
Whole Earth FeOSiMg 93
Crust SiOAl 82
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Chemical Composition of Earth

• Lithosphere strong, rocky outer shell of the
  solid Earth including all the crust and the upper
  part of the mantle to a depth of 100 km (forms
  the plates)
• Asthenosphere weak,ductile layer of the mantle
  beneath the lithosphere deforms to accommodate
  the motions of the overlying plates
• Deep Mantle mantle beneath the asthenosphere
  (400 to 2900 km in depth)
• Outer core liquid shell composed of mostly iron
• Inner core innermost sphere composed primarily
  of solid iron

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Chemical Composition of Earth

• Continents Formed from solidified magma that
  floated up from the Mantle

Oceans and Atmosphere Fluid and gaseous outer
layers believed to have been created by
out-gassing of gases and fluids from volcanic
eruptions (in a process called volatile transfer)
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The Evolving Atmosphere
Right after its creation, the Earth is thought to
have had a thin atmosphere composed primarily of
helium (He) and hydrogen (H) gases
The Earths gravity could not hold these light
gases and they easily escaped into outer
space Today, H and He are very rare in our
atmosphere
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The Evolving Atmosphere
For the next several hundred million years,
volcanic out-gassing began to create a thicker
atmosphere composed of a wide variety of
gases The gases that were released were probably
similar to those created by modern volcanic
eruptions
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The Evolving Atmosphere
These would include Water vapor (H2O) Sulfur
dioxide (SO2) Hydrogen sulfide (H2S) Carbon
dioxide (CO2) Carbon Monoxide (CO) Ammonia
(NH3) Methane (CH4)
Note that oxygen (O2) gas is not created by
volcanic eruptions
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Creating the Oceans
It is hypothesized that water vapor escaping from
the interior of the Earth via countless volcanic
eruptions created the oceans (this took hundreds
of millions of years)
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Creating the Oceans
Astronomers also hypothesize that comets
impacting the Earth were a major source of water
that contributed to creation of the
oceans Remember, that comets are best described
as dirty ice balls
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Creating the Oceans
The earliest evidence of surface water on Earth
dates back about 3.8 billion years
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Geologic Time
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A billion Year Old Earth
By 3.5 billion years ago, when the Earth was a
billion years old, it had a thick atmosphere
composed of CO2, methane, water vapor and other
volcanic gases
By human standards this early atmosphere was very
poisonous It contained almost no oxygen
Remember, today our atmosphere is 21 oxygen
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A billion Year Old Earth
By 3.5 billion years ago, the Earth also had
extensive oceans and seas of salt water, which
contained many dissolved elements, such as iron
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A billion Year Old Earth
But most important, by 3.5 billion years ago,
there was life on Earth
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The Creation Of Life
These 3.5 billion year old fossilized algae mats,
which are called stromatolites, are considered to
be the earliest known life on earth
They are found in Western Australia
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A billion Year Old Earth
These stromatolite fossils, found in Glacier
National Park, half a planet away from Australia,
also may be 3.5 billion years old
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A billion Year Old Earth
Stromatolites are formed in shallow seas or
lagoons when millions of cyanobacteria (a
primitive type of bacteria) live together in a
colony
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Cyanobacteria
Cyanobacteria, commonly called blue-green algae,
is a phylum of bacteria that obtain their energy
through photosynthesis
This was the first life on Earth
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Prokaryota
Bacteria are cells without nuclei and are called
Prokaryota While prokaryotes are nearly always
unicellular, some are capable of forming groups
of cells called colonies
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The Creation Of Life
How do you create cyanobacteria?
The composition of the early atmosphere and
oceans were conducive to the creation of
primitive amino acids which are the building
blocks of protein molecules, as demonstrated in
this picture
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Miller Urey Experiment
Fifty years ago, Stanley Miller, a graduate
student working with cosmologist Harold Urey, was
able to create amino acids by exposing a gas that
simulated the early Earth atmosphere to
ultraviolet radiation and water
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Oxygen
The ability of cyanobacteria to perform oxygenic
photosynthesis is thought to have converted the
early Earth atmosphere into an oxidizing one,
which dramatically changed the life forms on
Earth and provoked an explosion of biodiversity
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Oxygen
By around 2.2 to 2.4 billion years ago, the Earth
had developed an atmosphere that is very similar
to todays atmosphere (nitrogen and oxygen)
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Banded Iron Formations
How do we know that there was no oxygen in the
early Earth atmosphere? Oxygen oxides native iron
and created minerals such as hematite (iron oxide
which is F2O3)
Simply put, water and oxygen creates rusts out of
iron
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Banded Iron Formations
Banded iron formations (also known as BIFs) are a
distinctive type of rock often found in
primordial sedimentary rocks
The structures consist of repeated thin layers of
iron oxides, either magnetite or hematite,
alternating with bands of iron-poor shale and
chert
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Banded Iron Formations
BIFs are primarily found in very old sedimentary
rocks, ranging from over 3 to 1.8 billion years
in age
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Banded Iron Formations
It is hypothesized that the banded iron layers
were formed in sea water as the result of free
oxygen released by photosynthetic cyanobacteria
combining with dissolved iron in the oceans to
form insoluble iron oxides, which precipitated
out, forming a thin layer on the seafloor
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The Continents
By 2.5 billion years ago, the continents had been
formed The density of the continental crust (2.8
gr/cm3) is lighter that the crust found on ocean
bottoms (3.2 gr/cm3), so the continents rise
above the ocean floor A question that remains
unanswered is, when did plate tectonics start?
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Geologic Time
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Nucleus Bearing Cells
Nucleus-bearing cells are called Eukaryotes
About 2.2 billion years ago, the first primitive
Eukaryotes appeared on Earth For 1.7 billion
years, Eukaryotes slowly evolved and spread
across the Earth
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Nucleus Bearing Cells
Then, at the start of the Cambrian, 570 million
years ago, there was an explosion in the
diversity of life on Earth by Nucleus-bearing
cells