Theory of Plate
Tectonics
Continental Drift
• The geological theory by Alfred Wegner that
states that Earth’s continents were once
ONE large landmass “Pangea” and over time
the continents slowly moved apart.
• Evidence...
▫ Shape of the Earth’s continents
 They look like they fit together like pieces of a
puzzle
Continental Drift
• This theory was NOT CORRECT!!
Plate Tectonics
• The geological theory by Tuzo Wilson that
states that pieces of Earth’s lithosphere are
in constant slow motion.
• Evidence
▫ The SAME FOSSILS on different continents
▫ Magnetic patterns found on the ocean floor at
along a mid-ocean ridge.
Plate Tectonics – Lithosphere
• Lithosphere
▫ Top layer of the Earth which contains the
“plates”
• Asthenosphere
▫ Partially molten layer of Earth right below the
Lithosphere.
▫ There are convection currents in the
Asthenosphere that slowly move Earth’s
plates (the Lithosphere)
Lithosphere and Asthenosphere
Convection
Currents in the
asthenosphere
Convection
Currents in the
asthenosphere
Earth’s Plates – Boundary Map
3 Types of Boundaries
1.Transform
2.Divergent
3.Convergent
Transform Boundaries
• 2 plates slide past each
other, moving in
opposite directions
• Along these boundaries,
crust is neither created
or destroyed
• Earthquakes occur here
• Faults are created
Divergent Boundaries
• 2 plates slide apart or
diverge
• Forms rift valleys and
mid-ocean ridge
• Sea-floor spreading
occurs here
Divergent Boundaries
Sea-Floor Spreading
Rift valley
Convergent Boundaries
• 2 plates slide together or
converge
• Results in a collision
▫ Denser crust will be on the
bottom (oceanic crust)
• Usually forms
mountains, volcanoes,
trenches, and a
subduction zone
Convergent Boundaries
Subduction Zone
The denser plate (oceanic) will go into the Earth
Volcano
Trench
1. Which boundary is this?
2. Which boundary is this?
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9. Which boundary is this?
Plate Tectonics
vs. Continental Drift
Plate Tectonics
Plates
Continental Drift