Continental Drift
Evidence: Fossils
Evidence: Rock Sequences
Matching Mountain Ranges
Glacier Evidence
Evidence: Polar Wander??
“Uh-Oh!”: Problems with
Continental Drift….
…an alternate explanation?
1928: Arthur Holmes suggested fluids rise
toward the crust, spread, and sink again.
1930: Wegner dies, unable to adapt
Holmes’ idea to his theory.
Mantle/Crust Structure
A new hope:
Sea-Floor Spreading
MID-ATLANTIC
RIDGE
Evidence for Plate Tectonics
 The discovery of strips of alternating
polarity, which lie as mirror images across
the ocean ridges, is among the strongest
evidence of seafloor spreading.
Polarity of the Ocean Crust
Sea-Floor Spreading
Divergent Plate Boundary
PILLOW BASALTS FORMING
AT THE EAST PACIFIC RIFT
VALLEY
The earth’s
magnetic field
has reversed
hundreds of
times
EAST AFRICAN RIFT VALLEY – CONTINENTAL RIFT
In Afar, Ethiopia, a 40-mile magmatic rift
that opened up 1 year ago
Spreading Center
Evidence for Plate Tectonics
 Ocean Drilling
• The data on the ages of seafloor sediment
confirmed what the seafloor spreading
hypothesis predicted.
• The youngest oceanic crust is at the ridge crest,
and the oldest oceanic crust is at the continental
margins.
Which graph best represents the geologic age of the surface bedrock on the ocean bottom?
Mid-ocean ridges (rifts) normally form where tectonic plates are
(1) converging
(3) stationary
(2) diverging
(4) sliding past each other
The motion of the convection currents in the mantle beneath the Atlantic Ocean
appears to be mainly making this ocean basin
(1) deeper
(3) wider
(2) shallower
(4) narrower
If new crust is created at divergent
boundaries, is earth getting larger?
CONVERGENT BOUNDARIES
 Oceanic crust - Oceanic Crust
older, denser crust subducts forming ocean trench
 Oceanic Crust – Continental Crust
denser oceanic crust subducts forming ocean trench
 Continental Crust – Continental Crust
resists subduction - folding, faulting, and mountain
building
Oceanic Crust – Oceanic Crust
Older, denser oceanic crust is SUBDUCTED
VOLCANIC ISLAND ARC FORMED – ALEUTIAN ISLANDS
Oceanic-Continental
Convergent Boundary - SUBDUCTION
DENSER OCEANIC CRUST
GOES DOWN INTO MANTLE
Continental Crust – Continental Crust
Convergent Boundary
NO SUBDUCTION - COLLISION ZONE
Collision of India and Asia
More Evidence for Plate Tectonics
 Earthquake Patterns
• Scientists found a close link between
deep-focus earthquakes and ocean
trenches.
• The absence of deep-focus
earthquakes along the oceanic ridge
system was shown to be consistent
with the new theory.
Convergent plate (subduction) boundaries
have deeper earthquakes
Divergent plate boundaries and collision
zones have shallow earthquakes
f07_58_pg196
The greatest number of earthquakes in the cross section occurred at:
1.
Sea level
2.
Between sea level and a depth of l00 kms.
3. at a depth between 100 – 300 kms.
4. At a depth between 300 – 600 kms.
Which cross section has arrows that best represent the relative
motion of the crustal plates along the Wadati-Benioff zone
beneath the Tonga Trench?
TRANSFORM PLATE BOUNDARIES
Transform Fault Boundaries
 At a transform fault boundary, plates grind
past each other without creating or
destroying the lithosphere.
 SHALLOW EARTHQUAKES
 Transform faults
• Most join two segments of a mid-ocean ridge.
• At the time of formation, they roughly parallel
the direction of plate movement.
Transform Fault Boundary
AT MID-OCEAN RIDGES
SUMMARY
3 PLATE BOUNDARIES
 DIVERGING
 CONVERGING (SUBDUCTION ZONE)
Oceanic Crust – Continental Crust
Oceanic Crust - Oceanic Crust
 CONVERGING (COLLISION ZONE)
Continental Crust – Continental Crust
 TRANSFORM
Boundaries: Diverging
Boundaries: Converging
SUBDUCTION
CONVERGING –- COLLISION ZONE
No subduction – continental crust less dense - buoyant
Boundaries: Transform
So, what causes plates to move?
• Convection currents: drag and move the
lithospheric plates above the asthenosphere
(three sources of heat produce the convection currents):
(1)Leftover heat from earth’s formation
(2)Decay of Radioactive elements
(3)Plate friction
Convection Cells
Convection Currents in the mantle
drag and pull the lithospheric plates
above them
3 types of plate boundaries caused by convection currents
CASCADES
MIDATLANTIC
RIDGE
TRENCH
The arrows shown in the asthenosphere represent the
inferred slow circulation of the plastic mantle by a process
called
(1) insolation
(3) conduction
(2) convection
(4) radiation
More Evidence for Plate Tectonics
 Hot
Spots
• A hot spot is a concentration of heat in the
mantle capable of producing magma, which
rises to Earth’s surface; The Pacific plate
moves over a hot spot, producing the
Hawaiian Islands.
• Hot spot evidence supports that the plates
move over the Earth’s surface.
HOT SPOTS
• Major regions of volcanic activity in the interior of
plates away from plate boundaries.
• The cause of these hot spots is thought to be
plumes of magma rising up from mantle
producing sites of active volcanism.
• As the plate moves over a hot spot, a chain of
volcanic mountains forms, like Hawaiian Islands
Which cross section best represents the plate boundaries
and mantle movement beneath line AB?
Hot Spot
This diagram provides evidence that the Pacific Crustal
Plate was moving toward the
(1) south
(3) southwest
(2) east
(4) northwest
SUMMARY OF P.T.:
• The plate tectonic model suggests:
(1) continents can move across the
surface of the globe
(2) patterns of volcanism can
change and shift across the globe
as plates and their boundaries
evolve and move
(3) new oceans may grow
(4) oceans basins close and are
deformed to produce mountains