Tectonics

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Lecture 30

• Tectonics

Chapter 15.1 ? 15.7

• Tectonic Movements
• Plate Tectonics

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Crust Deformations
The Earths crust looks firm, but it is in a
constant motion. Strong forces act in the crust
to move the rocks around. Such forces and the
changes they cause are called tectonic (Greek for
carpenter)
Cracks in rock formations are due to cooling of
molten rocks and mechanical stresses. Motions
occur along fracture lines called faults. Faults
form in rocks when the stresses overcome the
internal strength of the rock resulting in a
fracture.
These motions usually take place as a series of
small displacements with intervals of years to
centuries. Immediate motions create cliffs (fault
scrapes), which can be leveled off by erosion
before the next motion.
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Crust Deformations
Slow, continuous, motion produces rocks
folding. Folding may produce hills and
depressions directly, but erosion usually erases
them quickly. Indirectly folds produce parallel
ridges and valleys, which are resistant to the
action of water streams.
Large-scale motions in the crust may involve the
entire continents or their large parts (coastal
features).
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Mountain Building
The ways of building mountains include
accumulations of lava and pre-existing surface
material ejected by a volcano as well as motions
along faults. Great Earths mountains (the
Himalayas, the Alps, the Appalachians) have a
long and complex history that involves a
combination of motions.

• Analysis of layers of sedimentary rocks in
  mountains and valleys shows that
• There are more sedimentary materials in the
  mountains.
• They are crumpled by intense folding along faults.

Therefore, mountain formation should involve
intense horizontal compressional forces.
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Continental Drift
The continental coastal lines suggest that the
continents may have been joined together in the
past. The first detailed theory of continental
drift, based on biological and geologic evidence,
was proposed by Alfred Wegener in 1912.
Wegener tried to explain the parallel evolution
of animals and plants in many now-distant
regions. Only during the last 200 million years
living organisms in different continents
developed in different ways. Wegener suggested
that the continents were once part of one large
landmass called Pangaea.
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Plate Tectonics
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Plate Tectonics
The Earths lithosphere (the crust the
uppermost part of the mantle) is broken into a
dozen plates. The lithosphere gradually turns
into the softer asthenosphere (100 km thick).
Most major earthquakes and volcanic eruption
occur along plate boundaries.
Plate tectonics carries rock from the mantle,
transports it across the seafloor, and returns it
back. New crust is spread through mid-ocean
ridges. The old crust is returned back through
trenches (subduction).
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Tectonic Plate Collisions
Ocean floors, mapped only recently, consist of
much younger (200 million years) rocks than
continents (up to 3.8 billion years).
It was found that ocean floor is continuously
spreading by rising molten rock along mid-ocean
ridges. However, the Earth does not
expand. Therefore, the ocean floor spread should
be balanced by other large-scale processes in the
lithosphere, which is split along the ridges,
trenches, and fracture zones.
Tectonic plates move apart, creating new
lithosphere, and collide, destroying the old one.
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Plate Tectonics
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Moving Plates
The plates move with speeds of a few centimeters
per year.
The continents merged together 200 million years
ago and formed a single continent Pangaea
(Historical perspective).
Volcanoes usually exist near plate
boundaries. Many islands emerged at places of
subduction.
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Summary
The Earths crust looks firm, but is being
deformed by strong forces over long periods of
time. The crust is broken in a dozen tectonic
plates that are constantly, but very slowly,
moving causing the continental drift. Plate
tectonics, a unique phenomenon among the planets
of the Solar system, is responsible for the
changing appearance of the Earths surface.