The continental crust is made up mostly of
granitic rocks. It is thought that about 60% of
continental crust was formed by the end of the
Archaean (2.5 billion years ago). Since then, the
continental crust has grown primarily through the
addition material at subduction zones, both by
volcanoes and by the accretion of exotic terranes.
There are also several processes that modify the
continental crust after it is formed. For example,
the granitic bedrock could be eroded and
redeposited as a sedimentary rock (e.g.
conglomerates, sandstone or shale). When these
igneous (e.g. granite) or sedimentary rocks are
subjected to high pressures and temperatures they
may recrystallize to form metamorphic rocks. Some
examples of metamorphic rocks include schist,
gneiss and marble. You can also get basalts
erupted within continental crust, especially when
the crust is being extended and pulled apart (e.g.
Between 450-250 million years ago, several
continental plates collided to form the
super-continent Pangaea. This collision pushed up
the Appalachian Mountains, which would have been
comparable to modern Himalaya. The deformation and
metamorphism caused by the collision also created
gneisses and schists. The Northwestern part of
Connecticut was originally part of the proto-North
American plate, whereas the eastern part of
Connecticut was originally part of the pro-African
plate and much of the central part of the state
was originally part of the oceanic crust that
separated proto-North America from proto-Africa.
Starting around 200 million years ago, Pangaea
broke apart and the Atlantic Ocean formed. The
Connecticut valley formed as an aborted rift (a
place where the crust was extended and thinned but
did not break apart entirely) during this time;
sedimentary rocks were deposited in this basin
interbedded with basalt lava flows.
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