Plate Tectonics
I.
II.
III.
IV.
The Discovery of Plate Tectonics
A Mosaic of Plates
Types of Plate Boundaries
“How” Plates Move
I. The Discovery of Plate
Tectonics
A. Continental Drift (Alfred
Wegener)
Proposed largescale movement
of the continents
Evidence:
1. “Puzzle fit” of
continents to form
“Supercontinents”
a. Suess (1900)Gondwana
b. Wegner (1915)Pangea
More Evidence…
2. Similar rock ages
3. Similar geologic
structures such as
mountain ranges
4. Fossil Evidence
from the
Mesosaurus
5. Climate Evidence
such as glacial
deposits
**Wegener’s theory did
not gain much support
because he could not
explain how the
tectonic plates move
B. Seafloor Spreading- Henry
Hess)
1.
2.
3.
4.
Convection currents
move plates around
Mantle source of
heat and convection
proposed new and
recycled seafloor
Evidence from
fathometers on
submarines during
WWII around the
Mid-Atlantic Ridge
showed the seafloor
is not completely flat
**There was no
evidence for this
theory, until…
C. Magnetic “Tape
Recording”
1. Magnetic Reversals
a. Switching strength to the
south
b. Preserved in lava (iron
points towards stronger
pole)
c. Age can be dated and if
the seafloor is spreading
from a center, should be
mirror images on each
side
Magnetic “Tape Recording”
(continued)
2. Magnetic anomaly
a. Normal-positive
anomaly
b. Reverse-negative
anomaly
Mid-ocean ridge
Million years ago (Ma)
4.0
3.0
2.0
Ocean
crust today
Magnetic mapping can measure the rate of seafloor spreading
An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic
Ridge southwest of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates how scientists worked
out the explanation of this pattern.
A sensitive magnetometer
records magnetic anomalies,…
…alternating bands of high
and low magnetism.
Iceland
MidAtlantic
Ridge
Symmetrical bands on both sides.
Why?
Looking at pole reversals
preserved in the ocean floor
provides the evidence needed
to support seafloor spreading.
Seafloor spreading provides
the mechanism for “how” the
continents move! (Wegener)
II. A Mosaic of Plates
Entire Earth’s
surface is made up
of lithospheric plates
B. Both oceanic crust &
continental crust
C. Geologic activities
occur at plate
boundaries:
1. Earthquakes
2. Volcanoes
3. Rifts
4. Folding
5. Faulting
A.
III. Types of Plate Boundaries
A. Divergent Boundaries
1. Continental plate
separation
2. Oceanic plate
separation-Mid
Ocean Ridges
“spreading
centers”
Divergent Boundaries
Continental Plate Separation
East African
Rift Valley
Divergent Boundaries
Continental Plate Separation
Parallel valleys; volcanoes and earthquakes.
East African
Rift Valley
Divergent Boundaries
Oceanic Plate Separation
MidAtlantic
Ridge
Divergent Boundaries
Oceanic Plate Separation
Volcanoes and earthquakes concentrate.
MidAtlantic
Ridge
III. Types of Plate Boundaries
B. Convergent Boundaries
- Conserve Earth’s
surface area
1. Ocean-ocean
convergence
2. Ocean-continent
convergence
3. Continent-continent
convergence
Convergent Boundaries
Ocean-Ocean Convergence
Mariana Islands
Marianas Trench
Convergent Boundaries
Ocean-Ocean Convergence
Deep-sea trench; volcanic island arc.
Mariana Islands
Marianas Trench
Convergent Boundaries
Ocean-Continent Convergence
Andes
Mountains
Peru-Chile Trench
South
American
Plate
Convergent Boundaries
Ocean-Continent Convergence
A volcanic belt of
mountains forms.
Andes
Mountains
Peru-Chile Trench
South
American
Plate
*when oceanic crust is
involved in convergence the
process of subduction
occurs, where the more
dense plate is pushed under
the least dense plate
Convergent Boundaries
Continent-Continent Convergence
Himalaya
Tibetan
Plateau
Main
thrust
fault
Eurasian
Plate
Convergent Boundaries
Continent-Continent Convergence
Crust crumbles, creating high
mountains and a wide plateau.
Himalaya
Tibetan
Plateau
Main
thrust
fault
Eurasian
Plate
III. Types of Plate Boundaries
C. Transform-Fault Boundaries
1. Plates slide past
one another
2. Fracture with
relative
displacement
Transform-Fault Boundaries
Mid-Ocean Ridge Transform Fault
Transform-Fault Boundaries
Mid-Ocean Ridge Transform Fault
Spreading centers offset.
Transform-Fault Boundaries
Continental Transform Fault
Transform-Fault Boundaries
Continental Transform Fault
Offset continental crust.
As plates
move past
each other...
As plates
move past
each other...
…creek beds
are offset
As plates
move past
each other...
…creek beds
are offset
San
Francisco
Los Angeles
http://www.youtube.com/watch?v=ryrXA
GY1dmE
IV. “How” Plates Move
A. Driving Forces
1. Mantle
convection
(heating of a
fluid)- warm
matter rises, cool
matter sinks
2. Gravitational pull
IV. “How” Plates Move
B. Plate Recyling
1. New lithosphere
occurs at ridges
2. Old lithosphere
occurs at
subduction zones
3. ‘Recycling’ of heat
energy within
upper and lower
mantle creates
seismic waves
IV. “How” Plates Move
C. Convection Currents
1. Explain the
movement of
lithospheric plates
2. Create Mantle
plumes (upwelling
and collection of
magma) that fuel
a. Hot spots
b. Volcanoes
http://www.youtube.com/watch?v=NYbT
NFN3NBo
Whole-mantle convection
Upper
mantle
700 km
Lower
mantle
2900 km
Outer core
Whole-mantle convection
Mantle
Outer core
Inner core
Upper
mantle
700 km
Lower
mantle
2900 km
Outer core
Stratified convection
Boundary near
700 km separates
the two convection
systems.