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Continental
Crust
Continents are built of blocks of crust
varying in age, size, rock composition, and structure. Typically, most
continents have stable, older interiors called cratons. Cratons are
typically composed of very old igneous and metamorphic rocks, some of which have
undergone several episodes of metamorphism and deformation. Most cratons
are billions of years old. The oldest cratonic rocks date back to 3.96
billions years and contain detrital zircons that are approximately 4.3 billion
years old.
The continent surrounded the cratons is
composed of younger, structurally more complicated rocks covered with layers of
younger sediments and sedimentary rocks. Some continental margins are composed
of the remnants of ancient oceanic lithosphere, volcanic arcs, or mountain
ranges and parts of continental masses that were attached or
"accreted" to the cratons. These margins are called active convergent
margins. The active convergent margins of continentals are often a mosaic
or crazy quilt of lithosphere fragments that have been accreted as a result of
the collision of tectonic plates plates. The accreted fragments can be
either continental or oceanic in origin. An example of this type of margin is
found in the Pacific northwest of North America.
The other type of continental margin is
that of a passive margin. Passive margins occur on the edge of a
continental mass that is fused to adjacent oceanic crust typically formed
through continental rifting. Passive margins consists of accumulations of
relatively thick sedimentary sequences deposited in shallow seas over submerged
continental crust. The sediments of the Atlantic coastal plains were
deposited on a passive margin. The sedimentary rocks and sediments deposited on
the passive margins typically become younger as distance from the craton
increases. Collision of a passive margin with another tectonic plate
transforms the passive margin into an active convergent margin. The
sediments and sedimentary rocks that now are part of the metamorphosed, folded,
and faulted Appalachian Mountains were once a passive margin before the
collision with western Africa during the Late Pennsylvanian and Late Triassic.
Many scientists believe
that plate tectonics has been operating since very early in the Earth's history,
perhaps as early as 3.8 billion years ago. It is generally accepted that
200 to 300 million years ago during the Permian, most if not all of the
continental masses on Earth were assembled into one giant supercontinent called
Pangea, that later broke up into smaller continents.
It is possible that there
have been several cycles of supercontinent formation, and subsequent break-up
and scattering of the fragments through geologic time. The history and
future of the Earth consists of a repeating cycle of plate collisions and
break-ups.
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