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Plate Tectonics
- Part A
- Theory of Plate Tectonics
Old 7th Grade Text
Chap 14:3
Continental Drift
Hypothesis
• In 1912 Alfred Wegener suggested that all the
continents were once connected as one large
landmass now called Pangaea
• Pangaea means “all land”
• The land mass broke apart about 200 million
years ago and
• The continents slowly drifted to their present
positions
Seafloor Spreading
-explains the motion of the continents
• Hot dense material below the Earth’s crust rises upward to
the surface at the mid-ocean ridges
• Then it flows sideways, carrying the seafloor away from
the ridge.
• As the new seafloor spreads apart, magma moves up and
flows from the cracks, cools, and forms new seafloor.
• Youngest rocks are located at the mid-ocean ridges and
become increasingly older the further out you go.
Plate Tectonics
• In the 1960’s scientists developed a new theory
that combined continental drift and seafloor
spreading.
• It is called The Theory of Plate Tectonics
Plate Tectonics
• The Earth’s crust and upper mantle are broken
into sections.
• The sections, called plates, move on a plasticlike layer of the mantle
– Geologists use the term Plastic to describe the
consistency of rock that flows but is not a liquid
like Silly Putty, modeling clay, or toothpaste.
Plate Tectonics
• The plates and upper mantle
form the lithosphere
– The lithosphere is about 100km thick and generally
is less dense than the material underneath.
• The plastic-like layer is called the asthenosphere
– The rigid plates of the lithosphere float and move
around on the asthenosphere
Plate boundaries
• When plates move, they can interact in
several ways.
– They can collide into each other, or pull apart, or
slide alongside each other
• When plates interact, the result of their
movement can be seen at the plate
boundaries
Plates moving apartDivergent Boundaries
• Plates moving apart at the mid-ocean
ridges are examples of divergent plate
boundaries
– In the Atlantic ocean, the North American
Plate is moving away from the Eurasian
and the African plates.
– The Great Rift Valley in eastern Africa
may become a divergent boundary. The
continental plate is being pulled apart.
Plates moving togetherconvergent boundaries
• As new crust is added in one place, it
disappears below the surface at another.
Plates moving
togetherconvergent boundaries
• When Ocean and Land Plates
meet:
– The denser oceanic plates sink under less dense
continental plates
– The area where the ocean plate moves down is called a
subduction zone
– This type of convergent boundary forms a deep sea
trench
Plates moving togetherconvergent boundaries
• When Ocean and Land Plates meet
(continued):
– Newly formed hot magma is forced
upward along the subduction zones and
forms volcanic mountains
– The Andes Mountain Range in South
America contains many volcanoes that
were formed at the convergent
boundary of the Nazca and South
American plates
Plates moving togetherconvergent boundaries
• When Two Oceanic Plates Collide
– The older, denser plate will subduct and sink
down into the mantle.
– The Mariana Islands in the Western Pacific are a
chain of volcanic islands formed from two oceanic
plates colliding.
Plates moving togetherconvergent boundaries
• When Two Continental (Land) Plates Collide
– There is usually no subduction because both of
these plates are less dense than the material of the
asthenosphere.
– Plates will crumple up to form mountain ranges.
– Earthquakes are common here, but not volcanoes.
– The Himalayas in Asia are forming where the
Indo-Australian plate collides with the Eurasian
Plate
Plates Sliding Past each otherTransform Boundary
• When one plate slips past another suddenly
Earthquakes occur
– The Pacific Plate is sliding past the North
American Plate forming the famous San Andreas
Fault in California.
Causes of
Plate Tectonics
• Convection inside the Earth
– Cycle of heating, rising, cooling and sinking is
called a convection current.
– This process occurring inside the mantle of the
earth is the driving force behind plate tectonics.
Convection currents
occur inside the
mantle. They are the
driving force behind
plate tectonics.
Features caused by Plate
Tectonics
• The interaction of plates produces forces that build
mountains, create ocean basins, and cause
volcanoes.
• When rocks in the Earth’s crust break and move the
energy is released as Seismic waves
• We feel this release as Earthquakes
Features caused by
Plate Tectonics
• Mid-Ocean Ridges, Faults and Rift Valleys – are
formed at divergent boundaries where plates are
pulled apart.
• Mountains and Volcanoes – are formed at
convergent boundaries where plates collide.
• Strike-Slip faults- cause of Earthquakes are
found at transform boundaries where plates
grind past each other.
Testing for Plate
Tectonics
• Using lasers and satellites, scientists can
measure movements as little as 1 cm per
year.
– Hawaii is moving toward Japan at a rate of
about 8.3 cm per year.
– Maryland(?) is moving away from England at a
rate of 1.7 cm per year
• Scientists have observed that the plates
move at rates from about 1cm to 12cm per
year.
Section A Assessment
Using a separate sheet, answer the following
questions.
• What happens to plates at a transform boundary?
• What occurs at plate boundaries that are
associated with seafloor spreading?
• Describe the three types of plate boundaries
where volcanic eruptions can occur.
• How are convection currents related to plate
tectonics?
• Using a flow map- describe the three theories
that were discussed at the beginning of the
section.