Transcript Origin of Earth - Acadia University

Origin of Earth
Nancy Van Wagoner, Acadia University
Perspective in Space
 We
live on planet Earth
 solar system
 Milky way galaxy
 universe
Nancy Van Wagoner, Acadia University
Origin of Universe
 Age?
perhaps 8-10 billion years
 Big Bang Theory
 At that time all matter that formed the sun,
formed the earth and us. Remember matter
changes form but is not destroyed.
Nancy Van Wagoner, Acadia University
 The
universe was mostly light elements
 Hydrogen
75% atomic weight 1
 Helium 25% atomic weight 2
Nancy Van Wagoner, Acadia University
Where did heavier elements
come from?
 thermonuclear
reactions in exploding stars
called supernova.
 For more information Supernova--Death of
a Star, National Geographic, v.173, n.5,
p.618, 1988.
Nancy Van Wagoner, Acadia University
 It
is postulated that in this expanding
universe about 6 billion years ago, a
supernova exploded. Gas and dust began to
spin which was caused by rotation of the
galaxy.
 Gravity concentrated most of the mass at
the centre which would become the sun and
producing a solar nebula.
sun
dust and gas
fig. 1.8, Thompson and Turk
 Planets
condensed from material in the
nebula but, because the nebula was
stratified with respect to temperature and
composition the planets are different in
composition.
Cold
Hot
Fe, Al, Ca
sun
Ice (H,C,N)
Mars
Earth
MercuryVenus
Jupiter
Asteroid Belt
Earth-like Planets
Jovian Planets
Look more closely at Earth.
 Earth
at one time was a homogeneous
spinning and contractng body
Fe Si K
Ca
Al
Mg
Si Al
Si
Nancy Van Wagoner, Acadia University
Ca
Na
 The
protoplanet grew hotter until it reached
the melting temperature of iron.
– Melting event
– The sources of heat were from
-
meteor impact
 - radioactive decay
 - gravitational contraction
Nancy Van Wagoner, Acadia University
Result of Melting event
-
lighter materials rose to the surface
 - heavier ones sank to the core
 The result: compositional and density
stratified the Earth.
Nancy Van Wagoner, Acadia University
Composition of Earth and
Crust
Crust
Fe
O
Mg
Si
S
Ni
Ca
Al
Co
Na
Mn
K
Ti
P
Cr
Nancy Van Wagoner, Acadia University
5.6
45
2
28
0.03
0.007
4.2
8. 2
0.002
2.4
0.09
2.1
0.57
0.10
0.01
Total Earth
35
28
17
13
2.7
2.7
0.61
0.44
0.20
0.14
0.09
0.07
0.04
0.03
0.01
Periodic Table Exercise
 notice
order of elements
 Elements expected near centre:
 Elements expected near surface:
 The Ur/Th surprise
– Ur and Th became concentrated in the crust
because they are about the same size and charge
as some of the lighter elements (K,Na) and
therefore can replace these elements in the
crystal structure of minerals.
Nancy Van Wagoner, Acadia University
Layers of Earth
Nancy Van Wagoner, Acadia University
Inner Core
 radius
= 1216 km
 composition =
Iron, Nickel +/silicon, carbon
1216 km
 state
= solid
Nancy Van Wagoner, Acadia University
Outer Core
 thickness
2270 km
 Composition = same as
inner core
 state = liquid
 density = 9-15 gm/cc
Nancy Van Wagoner, Acadia University
Mantle
 thickness
= 2885 km
 comp
=
iron and magnesium
silicates and oxides
 density = 4-7 gm/cc
Nancy Van Wagoner, Acadia University
Layers of the Mantle

100km
lithosphere = crust + upper mantle
– cool, rigid, brittle
 asthenosphere
= partially
molten
350 km
– hot, weak, plastic
 mesophere
= solid but hot
– due to high pressure
to base
of mantle
Nancy Van Wagoner, Acadia University
crust
continental
lithosphere
oceanic lithosphere
oceanic crust 5-10 km
rigid mantle
continental crust
30-70 km
Oceanic Crust
 dense
rocks = 3.2 gm/cc
 basalt (iron and magnesium silicates)
 relatively thin
Nancy Van Wagoner, Acadia University
Continental Crust
 less
dense = 2.8 gm/cc
 granitic Si, Al
 thicker
Nancy Van Wagoner, Acadia University
Why are there
Continents and
Ocean Basins ?
Nancy Van Wagoner, Acadia University
Plate Tectonics
 Lithosphere
broken into lithospheric plates
 Move with respect to each other
 rate = 1- 12 cm/yr.
Nancy Van Wagoner, Acadia University
Plate Margins
-
Divergent: spreading centre; new crust
created
 - Convergent: subduction; collision
 - Transform: earthquakes - San Andres fault
Nancy Van Wagoner, Acadia University