Theory of
Plate Tectonics
How do we know the plates exist?
 Earthquake and Volcano Zones
 Ocean floor features (Trenches and Mid-Oceanic
ridges)
Lithospheric Plates
7 major Plates
(continental
crust, oceanic
crust)
The Earth is made up of about 7 major plates and numerous minor plates.
Tectonic plates are constantly on the move. The fastest plate races along at 15
centimeters (6 inches) per year while the slowest plates crawl at less than 2.5
centimeters (1 inch) per year.
Lithospheric Plate Cross Section
Tectonic Plate
How do we know the plates move?
 Evidence Supporting Continental Drift
 Movement of the Pacific Plate (Hawaiian island chain formed
over a hotspot)
Three Types of Plate Boundaries
 Divergent
 plates move apart; sea floor
spreading
 Convergent
 Plates collide together;
subduction
 Transform (strike-slip)
 Plates slide past each other;
San Andreas fault
Plate Boundaries
Transform Boundaries
Transform Boundary: San Andreas Fault
Transform Boundary
 Places where plates slide past each other
 Lithosphere is neither created or
destroyed
 Most found at sea floor
 Connect segments of
diverging mid-ocean ridges
Divergent Boundaries
Divergent Boundaries
 Where plates pull apart
 New lithosphere is created – old
lithosphere spreads away from each other
 Create mid-ocean ridges
 Hot mantle material wells up to form new
lithosphere
Iceland
This process is called sea floor
spreading
Sea Floor Spreading
 As plates move apart heat from the
mantle softens the crust
 Less dense material rises until crust
cracks
 Crack is filled with magma which cools to
create new ocean floor (igneous rock)
 New oceanic crust forms on each side of
the valley creating Mid-Ocean Ridges.
 Example: Mid-Atlantic Ridge
Sea Floor Spreading
rift valley
mid-ocean ridges
newer less dense crust
older more dense crust
lithospheric plate
magma
asthenosphere
The Discovery of Sea Floor Spreading
Evidence of Sea Floor Spreading
 Rock samples are younger closer to a mid-ocean ridge
 Magnetic Reversals
What Is Sea-Floor Spreading?
 In sea-floor spreading, the sea floor spreads
apart along both sides of a mid-ocean ridge as
new crust is added. As a result, the ocean floors
move like conveyor belts, carrying the continents
along with them.
Evidence for Sea-Floor Spreading
 Several types of evidence supported Hess’s
theory of sea-floor spreading: eruptions of
molten material, magnetic stripes in the rock of
the ocean floor, and the ages of the rocks
themselves.
Convergent Boundaries
Convergent Boundaries
 Where plates push together or collide
 Old lithosphere is destroyed by subduction
 Mountains created by collision
Plate Collisions
Oceanic Continental
Subduction
Oceanic Oceanic
Subduction
Continental–
Continental
No Subduction
Collision Zone
What Destroys Crust?
Subduction Zones
 Occurs when oceanic crust collides with oceanic or
continental crust at a Convergent Boundary
 The older, more dense, oceanic crust plunges back into the
mantle at a deep ocean trench.
 Forms deep ocean trenches & volcanic mountains or island
arcs.
Subduction
Trenches of the World
How do the plates move?
The surface of the Earth is always in
motion
Driven by motion
within the Earth’s
interior
What Causes the Plates to Move?
Convection Currents
Movement of molten material deep within
the Earth
Slow churning
Carry heat from
lower mantle and
core to lithosphere
Recycle lithosphere
material back to
the mantle
Putting it all together
 When Alfred Wegener first proposed the idea
of Continental Drift what was missing?
 How and why
 He couldn’t explain how large continents
could move across the oceans
 Our modern plate tectonic view gives us the
answer. How?
 The oceans are in motion, pushing the
continents along with them.
 It is not just the continents that move, but
continent and ocean together as a single
tectonic plate, driven by convective motion
from below.
Explain this diagram: