Transcript Geologic Time and Origins of the Earth

Earth Origins
• AFTER “Big Bang”
• Earth Origins are described by the Nebular
Hypothesis
Nebular Hypothesis 1
• Solar system did not exist 5 billion years ago
• A giant cloud of dust and gas occupied this
area of space
– Many times larger than the present solar system
– A lot of space between individual atoms of gas,
metal and ions
Nebular Hypothesis 2
• Something caused this cloud to condense
• Maybe a nearby star exploded, sending its mass
out into space
• This mass collided with our dust cloud
N.H. 2, continued
• Nearby supernova caused cloud to condense
• Accretion began
Nebular Hypothesis 3
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Cloud continues to condense
Center of cloud attracts the most matter
Cloud flattens into a disk and begins to rotate
Incredible amount of heat generated in the center
– So much heat that atoms are fused together, and the
sun “turns on”
As the nebula collapses, it heats up,
spins faster, and flattens.
Nebular Hypothesis 4
• A new sun in the center of a disk of dust
• More volatile elements move away from the sun
– H, He, Methane, Ammonia
• Accretion continues in the disk
– Planets form by collision
Nebular Hypothesis 5
• Accretion continued until most of the matter
accreted into planets
• Accretion occurred in stages, with the “Late
Heavy Bombardment” happening ~3 b.y.a.
Why is the N.H. a hypothesis
and not a theory?
• Observations are indirect
• We were not there to see it
• What we can see is very slow
Making the Earth 1
• Accretion acts over an extended area (the disk)
and for a extended period of time
• Solid grains condense out of the nebula’s gas
– This is a chemistry process
• Grains accrete into larger bodies (planetesimals)
– This is a dynamic, collisional process
• Planetesimals collide to produce protoplanets
• Protoplanets accrete more material and become
genuine planets
Making the Earth 2
• Earth cools a bit:
– Elements, minerals condense/crystallize
• Iron, nickel, then rocky material
• Layered Earth forms:
– Stays molten due to heat of radioactive elements
and L.H.B.
• Iron and nickel sink to center, rocky material
floats outward (density sort).
• This is called differentiation
Layered
Earth