Plate Tectonics
What is Plate Tectonics?
• Scientists know that Earth’s surface
consists of a number of rigid, yet movable,
pieces called plates.
• The study of the formation and movement
of these plates is called plate tectonics.
• There are about a dozen major plates and
several minor plates.
These
major
plates are
free of
charge on
page 5 of
your ESRT.
We will
learn this
page later
this week.
About the Plates
• Together, the crust and mantle of the Earth
make up a single solid layer called the
lithosphere.
• The lithosphere is rigid, but broken up into
these plates that move.
• It is approximately 100 km thick.
• Recall, there are two types of crust:
oceanic and continental.
About the Plates
• Which type of igneous rock makes up oceanic
crust?
• Yes, that’s right! Basalt
• Which type of igneous rock makes up
continental crust?
• You got it! Granite
• Which one is more dense?
• You’re brilliant! It’s basalt. Of course,
remember, we can go to page 10 of our ESRT
and get that info for free!
About the Plates
• The lithosphere rests on a layer in the
mantle called the asthenosphere.
• This layer is partially melted, thus, it’s what
causes the plates in the lithosphere to
move!
• Guess what it is in the asthenosphere that
forces the movement of the plates?
• Our old friend… Convection Currents!!
About the Plates
• Where convection currents are rising, it
will push apart 2 plates!!
• Where convection currents are sinking, it
will pull 2 plates together!!
• Convection current animation
This is also free of charge on page 10 of your ESRT!!
Evidences For Plate Tectonics
• There are 4 major pieces of evidence that
support the theory of plate tectonics:
• 1. Africa and South America
• 2. Magnetism of Seafloor
• 3. Earthquakes and Volcanoes
• 4. Heat Flow and Seafloor Elevation
Evidences for Plate Tectonics
• Africa and South America
• There is a remarkable similarity in the shape of
the west coast of Africa and the east coast of
South America.
• They look like they fit together like puzzle
pieces.
• This suggest that they were once together.
• This theory is called the continental drift theory.
• Alfred Wegener was the scientist who proposed
this theory.
Evidences For Plate Tectonics
• Wegener also noted that the fossil remains
of the Mesosaurus were only found in 2
places:
• 1. West coast of Africa
• 2. East coast of South America
• This further supported his theory.
• The name for the large land mass that was
all the continents together is known as
Pangaea.
Evidences For Plate Tectonics
Pangea Breakup Animation
More Fossil Evidence
Evidences For Plate Tectonics
• Magnetism of Seafloor
• Some igneous rocks contain magnetic minerals.
• These minerals will align themselves towards
Earth’s poles.
• When we look at the bottom of the ocean floor,
we see a banding pattern of magnetic minerals
in the rocks.
• They are aligned one way for a chunk of time
and then reversed the other way.
Evidences For Plate Tectonics
• This suggests that Earth’s poles have reversed
in the past.
• These bands allow scientist to date the rocks of
the ocean floor.
• The youngest rocks are found at the spreading
center, where new rock is being formed.
• The oldest rocks are found along the coasts of
the continents.
• This proves that the plates have been moving for
millions of years.
Evidences For Plate Tectonics
(Normal)
(Reversed)
•
Fig. 3-12
Age of seafloor by
magnetic stripes
Evidences For Plate Tectonics
• Earthquakes and Volcanoes
• It has been long observed that earthquakes and
volcanoes DO NOT occur randomly on Earth’s
surface.
• Instead, they tend to occur in specific belts.
• These belts just happen to be location of the
boundaries between the plates.
• These boundaries are so active that stress
builds up and causes earthquakes.
• In addition, the boundaries are areas of extreme
heat flow, causing volcanoes.
Location of earthquakes – each black dot is an earthquake
Evidences For Plate Tectonics
• Heat Flow and Seafloor Elevation
• Heat flow at plate boundaries provides evidence
of convection currents in the asthenosphere.
• Heat flow is highest at a spreading center, where
two plates are moving apart.
• At this center, the seafloor is elevated because
the heated materials are expanding.
• These elevated centers create ocean ridges.
The most well-known is the Mid-Atlantic Ocean
Ridge.
Types of Plate Boundaries
• There are 3 main types of plate
boundaries:
• 1. Divergent
• 2. Convergent
• 3. Transform
Types of Plate Boundaries
• 1. Divergent Boundaries
• Also known as spreading centers.
• 2 lithospheric plates are moving APART
(diverging).
• Most are found on the seafloor, forming ocean
ridges.
• Because of this, some of the world’s oceans are
getting bigger!
• Ocean ridges have valleys called rift valleys.
• The rift valleys are broken into fracture zones.
• These are the sources of earthquakes.
Divergent Plate Boundary Animation
Types of Plate Boundaries
• 2. Convergent Boundaries
• 2 tectonic plates are moving TOGETHER.
(converging)
• Three types of convergent boundaries:
• 1. Ocean-Ocean crust convergence
• 2. Ocean-Continental crust convergence
• 3. Continental-Continental crust
convergence
Types of Plate Boundaries
• Ocean-Ocean Convergence and OceanContinental Convergence:
• 2 oceanic plates coming together or 1 ocean
plate and one continental plate coming together.
• One plate will be pushed under the other (the
MORE dense one gets pushed under).
• This is called subduction.
• Subduction creates trenches.
• The subducted plate melts and this will form
volcanoes or island arcs (volcanic islands).
Ocean-Continental Convergence Animation
Ocean-Ocean Convergence Animation
Types of Plate Boundaries
• Continental-Continental Convergence:
• 2 continental plates are coming together.
• When they collide, NO subduction occurs
because continental crust is not very dense and
one will not subduct another.
• Instead, the crust gets crumpled and uplifted and
often creates mountain ranges.
• The most famous is the Himalayas when India
collided with Eurasia.
India colliding with Eurasia animation
Types of Plate Boundaries
• 3. Transform Boundaries:
• 2 plates SLIDE past each other.
• Crust will be deformed and fractured due
to the friction and stress.
• This will create faults and cause
earthquakes at these boundaries.
• Most famous transform fault is the San
Andreas Fault in California.
San Andreas Fault Picture
San Andreas Fault animation
Bridge over the Mid-Atlantic Ridge in Iceland!
Rift Valley
in Iceland
Stop Plate Tectonics
It’s Changing The Earth!