Transcript Formation of the Crust and Continents

Zircon (Zirconium Silicate)
 Extremely stable and resistant to change
 Occurs in small amounts in granite and felsic rock.
 Australian zircon may be residue from crust that no longer
exists.
Earth`s Early Heat Sources
1.
Asteroid activity
1.
During the
Proterozoic and
Archean Eons
asteroids (silica and
metal rich rocks)
were hypothesized
to have been
consistently hitting
Earth.These impacts
transferred energy in
the form of heat.
Earth’s Early Heat Sources
2. Gravitational contraction
1.
As more and more
asteroids strike Earth,
its overall mass starts
to change and
therefore Earth`s
gravitational constant
changed. This increase
of compressional
energy resulted in
excessive thermal
energy.
Earth`s Early Heat Sources
3. Abundance of radioactive isotopes
1.
2.
There were more radioactive isotopes during the Proterozoic
and Archean Eons than there are now
Radioactive decay produces heat as a by-product
Try these questions
Read section 22.1 and
answer questions 2, 5 and 6
The Early Earth
 Earth’s Differentiation and formation of crust
 Earth`s asthenosphere differentiated over time into layers
based on density
 Iron sank; oxygen and silicon floated up to the surface
*THIS IS CALLED DIFFERENTIATION
The Early Earth
 Early igneous crust was most probably basaltic in nature (low
silica, low in dissolved gases)
 Crust was rapidly recycled during this time
 Upon the first crusts’ subduction, continental crust was
created
 Primary basaltic crust brought water into the mantle during
subduction and created low density continental crust.
 Early continental crust were called microcontinents
The Early Earth
 The Precambrian Shield is
an example of persistent
Precambrian rock
(craton: oldest and most
stable part of a
continent).
 In North America, most
of this craton now exists
in Canada, hence
Canadian Shield
The Early Earth
 At the end of the Proterozoic, (~1 bya) Earth’s first supercontinent
formed (RODINIA)
 Try these questions:
 P. 627 Q: 1-6