lecture 4 history of earth
TRANSCRIPT
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The History of the Earth
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Origin of the UniverseThe universe began
about 14.4 billion years
ago
TheBig Bang Theory
states that, in the
beginning, the universe
was all in one place
All of its matter and
energy were squishedinto 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 eventuallyformed the stars and
galaxies
After about 10 billion
years, our solar systembegan to form
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We know how the Earth and Solar System are today
and this allows us to work backwards and determinehow the Earth and Solar System were formed
Plus we can out into the universe for clues on howstars and planets are currently being formed
Birth of the Solar System
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In cosmogony, the Nebular Hypothesisis the
currently accepted argument about how a SolarSystem can form
The Nebular Hypothesis
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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
Other Solar Systems
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A large gas cloud (nebula) begins to condense
Most of the mass is in the center, there is
turbulence in the outer parts
The Nebular Hypothesis
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The turbulenteddies collectmatter measuring
meters acrossSmall chunksgrow and collide,eventuallybecoming large
aggregates of gasand solid chunks
The Nebular Hypothesis
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Pictures from the Hubble Space Telescope show
newborn stars emerging from dense, compact pocketsof interstellar gas called evaporating gaseous globules
The Nebular Hypothesis
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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
The Nebular Hypothesis
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Protostar
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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
False Color Image of Protostar
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Birth of the Solar System
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Gravitational forces allow the inner planets toaccrue 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
Protoplanets
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Birth of the Solar System
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Size of the Planets
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Fig. 1.9
Venus, Earth and Mars
These maps are color coded to display different
elevations on the surface of each planet
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Earth is ~ 4,570,000,000 years old
The Age of the Earth
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 SpaceFor the first half billion years of its existence, the
surface of the Earth was repeatedly pulverized byasteroids and comets of all sizes
One of these collisions formed the Moon
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Formation of the MoonThe Giant Impact
Hypothesispredictsthat around 50 million
years after the initial
creation of Earth, a
planet about the size ofMars collided with Earth
This idea was first
proposed about 30
years ago, but it tookcalculations by modern
high-speed computers
to prove the feasibility
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Formation of the MoonThis 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 MoonPart 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
1. Collisions (Transfer ofkinetic energy intoheat)
2. Compression
3. Radioactivity ofelements (e.g. uranium,potassium, or thorium)
Three major factors that caused heating and meltingin the early Earths interior:
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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, theheavier elements, including
the melted iron, began to
sink down into the coreof
the Earth, while the lighter
elements such as oxygen
and silica floated up towards
the surface
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Global Chemical DifferentiationThis global chemical differential was completed by
about 4.3 billion years ago, and the Earth haddeveloped a innerand outer core, a mantleand crust
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Chemical Composition of Earth
Whole Earth:
Fe+O+Si+Mg = 93%
Crust:
Si+O+Al = 82%
Each of the major layers has a distinctive
chemical composition, with the crust being
quite different from the Earth as a whole
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The Evolving AtmosphereRight after its creation, the Earth is thought to have
had a thin atmosphere composed primarily ofhelium (He) and hydrogen (H) gases
The Earths gravity
could not hold theselight gases and they
easily escaped into
outer space
Today, H and He arevery rare in our
atmosphere
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The Evolving AtmosphereFor the next several hundred million years,
volcanic out-gassing began to create a thickeratmosphere 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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These would include:
Water vapor (H2O)
Sulfur dioxide (SO2)
Hydrogen sulfide (H2S)
Carbon dioxide (CO2)
Carbon Monoxide (CO)
Ammonia (NH3
)
Methane (CH4)
The Evolving Atmosphere
Note that oxygen (O2) gas is not created by
volcanic eruptions
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It is hypothesized that water vapor escaping from
the interior of the Earth via countless volcaniceruptions created the oceans (this took hundreds
of millions of years)
Creating the Oceans
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Creating the OceansThe 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 EarthBy 3.5 billion years ago, when the Earth was a
billion years old, it had a thick atmospherecomposed 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 EarthBy 3.5 billion years ago, the Earth also had
extensive oceans and seas of salt water, whichcontained 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 LifeThese 3.5 billion year old fossilized algae mats,
which are called stromatolites, are consideredto be the earliest known life on earth
They arefound in
Western
Australia
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A billion Year Old EarthThese stromatolite fossils, found in Glacier
National Park, half a planet away fromAustralia, also may be 3.5 billion years old
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A billion Year Old EarthStromatolites are
formed in shallow
seas or lagoons
when millions ofcyanobacteria(a
primitive type of
bacteria) live
together in acolony
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CyanobacteriaCyanobacteria, commonly called blue-green algae, is
a phylum of bacteria that obtain their energy throughphotosynthesis
This was the first life on Earth
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ProkaryotaBacteria 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 LifeThe composition of
the early atmosphere
and oceans were
conducive to the
creation of primitive
amino acids which are
the building blocks of
protein molecules, asdemonstrated in this
picture
How do you create cyanobacteria?
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MillerUrey ExperimentFifty years ago, Stanley Miller, a graduate student
working with cosmologist Harold Urey, was able tocreate amino acids by exposing a gas that simulated
the early Earth atmosphere to ultraviolet radiation
and water
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OxygenThe ability of cyanobacteria to perform oxygenic
photosynthesis is thought to have converted theearly Earth atmosphere into an oxidizing one,
which dramatically changed the life forms on
Earth and provoked an explosion of biodiversity
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OxygenBy around 2.2 to 2.4 billion years ago, the Earth
had developed an atmosphere that is very similarto todays atmosphere (nitrogen and oxygen)
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Banded iron formations (also known as BIFs) are a
distinctive type of rock often found in primordialsedimentary rocks
The structures
consist of
repeated thinlayers of iron
oxides, either
magnetite or
hematite,
alternating with
bands of iron-poor
shale and chert
Banded Iron Formations
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Banded Iron FormationsIt is hypothesized that the banded iron layers were
formed in sea water as the result of free oxygenreleased 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 ContinentsBy 2.5 billion years ago, thecontinents had been formed
The density of the continental
crust (2.8 gr/cm3) is lighter
that the crust found on oceanbottoms (3.2 gr/cm3), so the
continents rise above the
ocean floor
A question that remainsunanswered is, when did
plate tectonics start?
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Geologic Time
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Nucleus Bearing CellsNucleus-bearing cellsare called Eukaryotes
About 2.2 billion years
ago, the first primitive
Eukaryotes appearedon Earth
For 1.7 billion years,
Eukaryotes slowly
evolved and spread
across the Earth
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Nucleus Bearing CellsThen, at the start of the Cambrian, 570 million
years ago, there was an explosion in the diversityof life on Earth by Nucleus-bearing cells