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Lecture 30
Tectonics
Chapter 15.1  15.7
• Tectonic Movements
• Plate Tectonics
Crust Deformations
The Earth’s 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.
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).
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 Earth’s 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:
1. There are more sedimentary materials in the mountains.
2. They are crumpled by intense folding along faults.
Therefore, mountain formation should involve intense
horizontal compressional forces.
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.
Plate Tectonics
Plate Tectonics
The Earth’s 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).
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.
Plate Tectonics
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.
Summary
The Earth’s 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 Earth’s surface.