The Earth
February 26, 2013
The Planets
2
How long ago did the solar
system form?
Definition: Cosmic Rays
High-energy particles that
constantly bombard objects in
space
Mostly they are hydrogen nuclei
(protons), helium nuclei and
electrons traveling at close to
the speed of light
Many produced by Supernovae
Interact with particles in Earth’s
atmosphere, cause a cascade
of other particles
This interaction produces
Carbon-14, 14C which will
decay (is unstable)
4
Radioactive Decay
• Radioactive decay - atomic nucleus
spontaneously loses energy
• Parent nuclei transforms to daughter nuclei
• We know the decay rate
Radiometric Dating
Calculating the absolute age of material by
measuring the abundance of a radioactive
isotope relative to a stable isotope
Example: Radiocarbon dating
Exponential Decay and
Radiometric Dating
Each type of radioactive nucleus decays at its
own characteristic rate, called its half-life
Half life is the time when 1/2 of the original amount
of material is left
Some Naturally Occurring
Radioactive Isotopes and Their Half-Lives
Radioactive Isotope
(Parent)
Radioactive Isotope
(daughter)
Half-Life (years)
Uranium - 238
Lead - 216
4.5 Billion
Potassium - 40
Argon - 40
1.26 Billion
Uranium - 235
Lead - 207
0.7 Billion
Carbon - 14
Nitrogen - 14
5730
Radiocarbon Dating
Assuming Cosmic Ray flux is
constant
Know how much 14C is produced
Compare with 12C (stable)
In the atmosphere: Ratio (14C/
12C)
atmosphere = constant
= 1 part per trillion
Measure (14C/12C)sample
The ratio of (sample/constant)
give age
Radiocarbon Dating at UA
Dead Sea Scrolls
200 B.C. to 68 A.D.
Shroud of Turin
1300 A.D.
Radiometric Dating of Rocks
Can tell when a rock solidified:
Uranium has a half life of 4.5 billion
years
Oldest rocks on Earth:
Acasta gneiss in Canada’s
Northwest Territories are
4.03 billion years old (Gyr)
Oldest mineral grains on Earth:
Zircon grains from Western
Australia 4.3 - 4.4 Gyr
Because plate tectonics are active Earth
Rocks are not a good indicator for Solar
System age
Radiometric Dating of Meteorites
Oldest rocks in the solar
system are meteorites
(pieces of asteroids that
land on Earth)
Radiometric dating of
meteorites reveals that
they are all nearly the
same age - 4.56 Gyr
4.5 Gyr taken to be the age of
the Solar System (and Earth)
Age of Earth:
4.5Gyr
how do we know?
radiometric dating of meteorites
(formed when solar systems was forming)
13
Earth’s Internal
Structure
• Temperature and pressure
increase with depth
• Inner core is solid and
made mostly of Iron
• Outer core is molten (liquid)
and made of Iron
• Mantle is solid and made of
iron, magnesium and other
minerals
Earth’s Internal Structure
• Inner Core -solid
– 80% iron, 20% lighter (Ni,Ox,S)
– Pressure keeps it solid
• Molten outer core -liquid
– Same composition as inner core
• Lower mantle - convective
• Upper mantle - solid but
weak
– Asthenosphere upper part of upper
mantle, more brittle
• Lithosphere - crust and
upper asthenosphere
– Floats on top of upper mantle
– Region of earthquakes, mountain
building, volcanoes, continental drift
Surface of Earth - A Jigsaw Puzzle?
• Alfred Wegener (1915)
• Observations
– Continents on either side of Atlantic
ocean fit together like pieces of a
puzzle
– Fossils of particular species found in
both South America and Africa
– Glacial sediments in South America,
Africa, Asia, India,
• Theory: Continental Drift - later Plate
Tectonics - not accepted until 1960s
Plate Tectonics
• Theory to explain movement of Earth’s lithosphere
• Lithosphere broken into 8 tectonic plates
• Boundaries of plates are regions of earthquakes, volcanoes,
mountain building, seafloor spreading
• Movement of plates few cm/year
• Three types of boundaries:
-Divergent
-Convergent
-Transform
Plate Boundaries
Divergent Boundaries regions are spreading
sea floor spreading
Convergent Boundaries boundaries are colliding
subduction zones
Transform - slipping
Plate Tectonics -Modern
Evidence
• Mid Ocean ridges
• continents do not just move through the
sea
• sea floor spreads carrying continents
with it
Plate Tectonics -Modern Evidence
Subduction Zones
Pacific Ring of FireRegion of large numbers
Of Earthquakes and
Volcanoes
75% of Worlds active
And dormant volcanoes
80% of World’s largest
earthquakes
Plate Tectonics - Modern Evidence
Earthquakes at plate boundaries San Andreas Fault - transform
Convection
22
What Drives Plate Tectonics?
• Convection in Earth’s mantle
• “Slab pull” as plates subduct
Media Reported Science
• Get in groups of 4-5, discuss each of your answers to the
MRS questions, discuss any differences, what you
thought of the study...
24