Transcript 4.5 The Theory of Plate Tectonics

4.5 The Theory
of Plate
Tectonics
Objectives
• Explain the theory of plate
tectonics
• Describe the three types of plate
boundaries.
Engage/Explore
• What is a plate?
• Think of other contexts in which the
word is used.
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Metal plates that cover machinery
Home plate in baseball,
a reptile’s plates or scales
plate of photographs in a textbook
Introduction
• J. Tuzo Wilson Canadian
scientist.
• He discovered
that there are
cracks in the
continents similar
to those on the
ocean floor.
• Wilson proposed
that the
lithosphere is
broken into
separate sections
called plates.
• Plates can carry
both continents or
parts of the ocean
floor.
Great Rift Valley in East
Africa
A Theory of Plate Motion
• Wilson combined the thoughts of
sea-floor spreading, Earth’s plates,
and continental drift into a theory.
• A scientific theory is a well-tested
concept that explains a wide range
of observations.
• Plate tectonics is the geological
theory that state that pieces of
Earth’s lithosphere are in constant,
slow motion, driven by convection
currents in the mantle. It explains
the formation, movement, and
subduction of Earth’s plates.
How can Earth’s plates
move?
• The plates of the
lithosphere float on
top of the
asthenosphere.
Convection currents
rise in the
asthenosphere and
spread out beneath
the lithosphere.
Earth’s Lithospheric Plates
Affects of the plates
• Collide
• Pull apart
• Grind past each
other
• Volcanoes
• Mountain ranges
• Deep-sea
trenches
Earth’s Lithospheric Plates
• P. 133 Figure 23
• Which plates include only ocean
floor?
• Which plates include both
continents and ocean floor?
Plate Boundaries
• Demonstration
• Materials - 2 woooden blocks
• Three types of blocks
1. Transform boundary - slide past
2. Divergent boundary - pull away
3. Convergent boundary - push two
blocks together.
• Draw the blocks with labels and arrows
showing the direction of each block’s
movement.
Flat rock demonstration
• Demonstrate the boundaries using
rocks.
• Why do you think earthquakes occur
frequently at transform boundaries?
• Answer: The plates cannot move
smoothly past one another because of
the irregular nature of faults.
• How fast do you think Earth’s plates are
moving?
• Answer: Only a few centimenters per
year.
Plate Boundaries
• Faults - breaks the Earth’s crust
where rocks have slipped past
each other which form along these
boundaries.
• Three kinds of boundaries
Transform Boundary
• A place where two plates slip past each
other, moving in opposite directions.
• Earthquakes occur along these
boundaries.
• San Andreas Fault is an example. The
Pacific plate is sliding past the North
American plate.
• Crust is neither created nor destroyed.
San Andreas Fault
San Andreas Fault
Divergent Boundary
• The place where two
plates move apart.
• Most occur at the
mid-ocean ridge and
on land.
• The boundary forms
rift valleys.
• Example - Great Rift
Valley has a 3000
km crack.
Rio Grande Rift
• Extends
from central
Colorado to El
Paso, Texas.
Exploring Exercise - Plate
Tectonics
Plate Tectonics - p. 134-135
What is magma?
What is the magma coming from
that is shown erupting through the
mid-ocean ridge and the rift
valley?
Answer: asthenosphere
What new process are shown?
Convergent Boundaries
• Two plates come together. This is called
a collision
• When two continental plates collide,
why isn’t one subducted beneath the
other?
Appalachian Mountains formed when
two continental plates collided.
-The density of plates determines which
plate comes out on top.
Appalachian Mountains
Appalachian Mountains
Convergent Boundaries
Oceanic/oceanic
Subduction occurs
Oceanic/continental
Oceanic plate sinks
Continental/continental
Mountain ranges form
Candy Bar Inquiry
Challenge
• Material - candy bar, paper towel
• Procedure
– Wash hands
– Give out Snickers
– How can you use this candy bar to
model what happens at divergent and
convergent boundaries?
– Make a sketch of what you see.
The Continents’ Slow
Dance
• The continents move at slow rates:
one to ten centimeters per year.
• Pages 136-137 Read Figure 26
• Pangea began to drift apart about
225 million years ago.