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Universe > galaxy > solar
system
Our solar system has 1 star (our
sun); the galaxy has hundred’s of
millions; the universe
encompasses all the galaxies
Structure of Earth
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
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
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 Waal’s binding (planetesimals R=1-10 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
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
Composition of the early
Earth (prior to segregation)
Separation of the Fe/Ni core during melting
(mantle depleted in these elements)
Crustal formation
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
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.
The Oceans ( = 1 g/cm3)
~20-70 km
} ~2-5 km
Continental (granitic) crust
( = 2.7 g/cm3)
Mantle ( = 3.3 g/cm3)
Oceanic (basalt) crust
( = 2.8 g/cm3)
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
 Major accretion
– Once though to be 100
my
– Recent thought is planet
cooled quickly
– Water begins to
accumulate on Earth’s
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)
Importance of the moon
 Tides
 Gravitational attraction of moon & sun on earth’s
bulge causes precession of earth’s orbit
– Role in Milankovitch cycles (glacial cycles)
 Tends to stabilize tilt of the earth
–
–
–
–
Earth’s axis at an angle relative to plane of earth’s 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
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
Formation of the moon
 Early in Earth’s history (>4 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 earth’s rotations
– Caused the axial tilt?
– Therefore contributed to seasonality and glacial
cycles?
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 moon’s crust
– Some similarity to earth
Highlands
granite-like (anorthosite)
[> 4 bybp] – more like
continental crust
Maria (dark seas)
basalt-like
[3.1-3.9 bybp]
Moon structure
 Both maria and highlands are old
– Maria 3.1-3.9 by; lava flows into giant impact craters
– Highlands > 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
– Moon’s 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
Back to the Earth’s structure
 Earth is layered
 Heat did not escape
 Recycling, reheating, remelting,
recrystallization
 Density stratified
Most simply
1. Crust –
cold, rigid, thin
2. Mantle –
warmer, more
dense; outer part
rigid and inner
part plastic
(deformable)
The earth is layered & density stratified
3. Outer core –
transition zone
then thick liquid
zone
4. Inner core –
solid but warm,
very dense, rich
in magnetic
materials (Ni, Fe)
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 earth’s orbit – gave an idea of
mass
– Surface rocks predicted lower total mass if the
earth were homogeneous
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
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
Why are these
waves important?
•We can detect
these waves
independently
•They behave
differently passing
through different
media
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.
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 142o
shadow zone
Sharp increase in P-wave velocity at Moho
Seismology
 Changes in travel time and path tell us
about the earth’s structure
– Refraction of waves led to discovery of earth’s
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)
Earth’s 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 earth’s 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
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
TABLE I
Typical Densities of Earth Materials
Substance
Density*
Sea Water
1.02
Limestone
2.68-2.76**
Granite
2.64-2.76**
Sandstone
2.14-2.36**
Slate
2.6-3.3**
Basalt
2.4-3.1**
Average Density of Continents
2.7
Average Density of SiMa (Mantle Material)
3.3
* Actual densities vary slightly, depending
on chemical composition.
(** Source: Handbook of Chemistry and
Physics)
Physical responses
Lower mantle
Core
2900 – 6370 km
~3400
Dense, viscous liquid
Solid inner core
Classifying layers
By composition
Isostatic equilibrium and rebound
 This concept helps us understand the
“floating” of lithosphere on asthenosphere
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
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!
Next up
 Mantle convection
 Plate tectonics (Chapter 7)