Transcript 22.2 – Formation of the Crust and Continents

22.2 – Formation of the
Crust and Continents
Formation of Crust
• Due to intense heat in early Earth,
scientists believe planet was covered with
hot, molten magma
• As Earth cooled, minerals and elements in
magma became concentrated by density
Differentiation
• The process by
which heavy
materials sink
towards center and
lighter materials stay
near the surface
Differentiation
• Explains relative densities of parts of Earth
• Larger amounts of dense elements are
found in the Earth as a whole rather than
in the crust
Earliest Crust
• Early crust formed as soon as upper layer
began to cool
• This crust was similar to the basaltic crust
that is found under the oceans today
– Pieces of early crust were recycled so that
none of the earliest crust exists today
Continental Crust
• Early crustal pieces carried water
• Introduction of water was essential for
formation of first continental crust
– Water reacted with mantle material and was
less dense than original crustal pieces
Continental Crust
• New less-dense material crystallized to
form small fragments of granite-containing
crust
• Due to volcanic activity, small fragments of
granite-rich crust continued to form as
microcontinents
Craton
• Continental core formed
from microcontinents
• Deepest and most
stable part of continent
• Archean cratons make up about 10% of
Earth’s continents
•Granite-rich cores extend into the mantle as
deep as 200 km
Craton
• Precambrian shield = exposed areas of
craton due to deep erosion
• In North American, we call it the Canadian
Shield
Growth of Continents
• Pangaea formed about 200 million years
ago
• The plate tectonic forces that formed
Pangaea began during the Archean Eon
Mountain Building
• During Proterozoic, the microcontinents
collided with each other
• They became larger but fewer in number
• As they collided they formed massive
mountains
Visualizing Continent Formation
• North America formed by many mountainbuilding episodes over billions of years
• By end of Precambrian, about 75% of
North America had formed
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