Universe galaxy solar system

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Universe gt galaxy gt solar system

• Our solar system has 1 star (our sun) the galaxy
  has hundreds of millions the universe
  encompasses all the galaxies

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Structure of Earth
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Planets derived from material circling early sun
(star)

• Particles in solar nebula clumped to form
  planetesimals
• Planetesimals collided to form larger planets by
  accretions
• Fractionation of material among inner rocky and
  outer gassy planets

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Age of planets

• Oldest rocks on earth 4.4 by old
• Dating zircon
• Planet is older 4.6 by
• Time since crystallization age of planets
• Solar system formed 10 100 my earlier

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Composition of Early Earth

• Earth is layered
• Liquid outer core and solid inner core Fe Ni
• Mantle silicate
• Crust continental oceanic
• Planetary formation
• Initial accretion homogeneous
  non-gravitational weak van der Waals binding
  (planetesimals R1-10 km)
• Then gravitational attraction accretions
  (protoplanets)
• Major and large collisions major accretions
• Really large collisions melting (allowing Ni
  and Fe to separate) magma formation
• Collisions also brought water and other volatiles

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Chemical composition of Earth

• Evidence of melting, chemical fractionation and
  separation
• Assume composition of early Earth composition
  of chondritic meteorite
• Mantle depleted in Fe
• Separation of Ni/Fe core during melting

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Crustal formation
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Formation of the crust

• Continental versus oceanic crust
• Continental crust
• Repeated recycling and partial melting of mantle
  material and oceanic crust
• Repeated heating, cooling, subsidence, burial,
  and melting leads to distillation/segregation of
  lighter granitic material from heavier oceanic
  crust mantle further chemical separation of
  elements
• As old as 3.8 by
• Oceanic crust
• Young (100 my) controlled by plate tectonics
• More dense

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Bowen's reaction series demonstrates how the
cooling and crystallization of a primary magma of
basaltic composition can change from basaltic to
andesitic to rhyolitic, through reactions between
mineral grains and magma followed by separation
of mineral grains and magma.
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The Oceans (? 1 g/cm3)

2-5 km
Oceanic (basalt) crust (? 2.8 g/cm3)
Continental (granitic) crust (? 2.7 g/cm3)
20-70 km
Mantle (? 3.3 g/cm3)
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Segregation

• Separation of Ni/Fe core during melting
• Crust formed from partial melting of the mantle
• Crustal material enriched in Na, Si, and Al
• Depleted in Mg
• Further fractionation formed continental
  (granite) and oceanic (basalt) crust

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• Major accretion
• Once though to be 100 my
• Recent thought is planet cooled quickly
• Water begins to accumulate on Earths surface
• Began forming crustal material
• Heavy bombardment
• 500-700 my
• Continued to bring material and volatiles (and
  water) to earth

Period of heavy bombardment
Period of major accretion (first 10-30 my)
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Importance of the moon

• Tides
• Gravitational attraction of moon sun on earths
  bulge causes precession of earths orbit
• Role in Milankovitch cycles (glacial cycles)
• Tends to stabilize tilt of the earth
• Earths axis at an angle relative to plane of
  earths orbit
• Causes seasonality
• Tilt of axis varies between 21.8o and 24.4o
• Without moon, tilt would vary by a greater amount
• Up to 85o
• Wreak havoc with climate due to extreme
  seasonality

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Formation of the moon

• Lots of theories implausible or statistically
  unlikely
• Capture
• Fission spinning of earth ejected moon
• Binary accretion Earth and moon formed side by
  side
• Likely a collision (unlikely, but plausible)
• Debris reassembled in orbit around earth
• Analysis of moon rocks compared with earth rocks

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Formation of the moon

• Early in Earths history (gt4 bybp)
• Moon formed 30-50 my after solar system
• Formed during accretion
• Impact with a nearly fully-formed Earth?
• Impact led to termination of accretion?
• Impact may have affected earths rotations
• Caused the axial tilt?
• Therefore contributed to seasonality and glacial
  cycles?

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History of the moon

• Before 4 bybp
• Moon formed from hot debris after collision then
  solidified
• Formation of small core
• The next billion years
• Volcanic activity formed the moons crust
• Some similarity to earth

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Highlands granite-like (anorthosite) gt 4 bybp
more like continental crust
Maria (dark seas) basalt-like 3.1-3.9 bybp
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Moon structure

• Both maria and highlands are old
• Maria 3.1-3.9 by lava flows into giant impact
  craters
• Highlands gt 4 by
• Little or no evidence of tectonic activity in the
  last 3 by
• Small size allowed internal heat to escape w/o
  mantle convection
• Moons surface pock-marked by comet and asteroid
  impacts
• No evidence of plate tectonics or other landscape
  forming processes
• Moon has no atmosphere or oceans
• Size to small to gravitationally retain gas and
  volatiles

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Back to the Earths structure

• Earth is layered
• Heat did not escape
• Recycling, reheating, remelting,
  recrystallization
• Density stratified

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Most simply

• Crust
• cold, rigid, thin

• Mantle
• warmer, more
• dense outer part
• rigid and inner
• part plastic
• (deformable)

• Outer core
• transition zone
• then thick liquid
• zone

4. Inner core solid but warm, very dense, rich
in magnetic materials (Ni, Fe)
The earth is layered density stratified
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How do we know this?

• All we see is the crust!
• Deepest drill-hole 12,063 m (7.5 miles)
• Still crustal
• Deepest ocean drilling 2 km (1.2 miles)
• Still crustal
• Studies of the earths orbit gave an idea of
  mass
• Surface rocks predicted lower total mass if the
  earth were homogeneous

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Mohorovicic Moho discontinuity

• Density discontinuity P waves arrived at
  seismic station before they should have in an
  homogeneous earth
• Boundary between the crust and mantle
• Discovered by Croatian geophysicist based on
  observations of seismic waves generated by
  earthquakes.
• Fun fact there was an effort to drill a
  Mohole but failed due to lack of and
  technology

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Evidence for layering

• Mainly we know depend on seismology
• Seismic waves generated from earthquakes
• Primary P-waves (compression waves
  longitudnally propagated waves oscillate in same
  direction as movement like sound waves)
• Secondary S-waves (transverse waves
  horizontally propagated oscillate perpendicular
  to movement like water waves)
• 1900 identified P S waves on a seismograph
  (Oldham)
• Waves were passing through the earth faster than
  predicted
• Wave speed increases with increasing density!
• Waves were being refracted (bent so they changed
  direction)
• Hypothesized that there were areas of Earth with
  different densities
• 1906 no S-waves passed through the earth
• Shadow zone no S-waves
• P-waves took longer than expected

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Why are these waves important?

• We can detect
• these waves
• independently

• They behave
• differently passing
• through different
• media

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Point of origin of seismic source.
Prediction of earthquake waves passing through a
planet of regularly changing density.
Prediction of earthquake waves passing through a
homogeneous planet.
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What P waves do in around liquid outer core
(bend)
What S waves do around liquid outer core (do not
penetrate).
P-wave shadow zone
142o
P-wave shadow zone
142o
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Sharp increase in P-wave velocity at Moho
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Seismology

• Changes in travel time and path tell us about the
  earths structure
• Refraction of waves led to discovery of earths
  core and Moho
• Travel time of waves led to discovery of layers
• Now we use changes in travel time and path tell
  us about location of disturbances (earthquakes or
  bombs)

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Earths functional layers

• Crust we know most about it continental crust
  is less dense
• Moho a density discontinuity that separates
  crust from the mantle
• Depth varies under continents and oceans
• First thought that this was layer where crust
  moved relative to earths interior BUT, outer
  layer of mantle moves with crust!
• Lithosphere crust plus rigid mantle (not
  totally rigid but, movements cause things like
  earthquakes and volcanoes
• Asthenosphere plastic layer of mantle
  lithosphere floats on asthenosphere
• Mantle includes part of lithosphere,
  asthenosphere and solid mesosphere

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• Chemical composition
• of layers
• Crust lightweight (0.4 mass/1 volume of
  earth) ocean crust (basalt O, Si, Mg Fe) is
  denser than continental crust (granite O, Si,
  Al)
• Mantle denser (68 mass/83 volume of earth) -
  Si, O, Fe Mg
• Core densest (31.5 mass/16 volume of earth) -
  mainly Fe Ni with some Si, S and heavy elements

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Physical responses
Lower mantle
3400
Core
2900 6370 km
Dense, viscous liquid Solid inner core
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Classifying layers By composition
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Isostatic equilibrium and rebound

• This concept helps us understand the floating
  of lithosphere on asthenosphere

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Isostacy

• Ocean basins and continents float on
  asthenosphere at equilibrium so that total
  pressure at depth in mantle is everywhere the
  same.
• Depending on density, things will float at a
  certain height and displace a different amount of
  water
• Most mass is below the surface, what sticks out
  of the fluid is supported by bouyancy of
  displaced fluid below the surface
• Examples icebergs, ships, blocks of wood of
  different densities in water

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What does this mean?

• Mountains have roots that are deeper than surface
  expression
• As erosion removes mass from the top of a
  mountain, the roots shrink upward or the
  asthenosphere rebounds
• Example younger (higher) Rockies have deeper
  roots than older Appalacians
• Example continental rebound from glaciers (Great
  Lakes Long Island Sound examples) sea level
  decreases even though more water!

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Next up

• Mantle convection
• Plate tectonics (Chapter 7)