Download Plate Tectonics

Continental Drift
Evidence: Fossils
Matching Mountain Ranges
Glacier Evidence
“Uh-Oh!”: Problems with
Continental Drift….
…an alternate explanation?
1928: Arthur Holmes suggested magma
rises toward the crust, spreads, and sinks
again. (Convection current)
1930: Wegner dies, unable to adapt
Holmes’ idea to his theory.
Mantle/Crust Structure
A new hope:
Sea-Floor Spreading
MID-ATLANTIC
RIDGE
50,000 km. Long
mountain range on
the ocean floor
Evidence for Plate Tectonics
During WWII, the U.S. in an
effort to find any advantage
that would help out submarines
in the war effort, scientists
found that the iron in the rock at
the mid-ocean ridge sometimes
pointed North and sometimes
pointed South
How could this be?
Basalt is iron rich and contains magnetite.
Magnetite acts like a little magnet and aligns
itself with the orientation of earth’s magnetic
field.
When magma solidifies, the alignment is
“locked in” recording Earth’s magnetic
orientation at the time of cooling.
MAGNETIC STRIPE EVIDENCE
North Oriented Rocks are said to have NORMAL POLARITY
South Oriented Rocks are said to have REVERSED
POLARITY
The earth’s
magnetic field
has reversed
hundreds of
times
POLARITY
FLIPPING
AGE INCREASES
AWAY FROM
OPENING - RIDGE
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?
Sea-Floor Spreading is a
Divergent Plate Boundary
Creates New Crust
SPLITS APART CONTINENTS
TO CREATE NEW OCEANS
Spreading Center
New Ocean
forming –
linear Red
Sea
When it grows
up, it will be
like the
Atlantic
Ocean
EAST AFRICAN RIFT VALLEY – CONTINENTAL RIFT
In Afar, Ethiopia, a 40-mile magmatic rift
that opened up 1 year ago
If new crust is created at divergent
boundaries, is earth getting larger?
CONVERGENT BOUNDARIES – DESTROYS
CRUST
 Oceanic Crust converging with Continental Crust
 Oceanic crust converging with Oceanic Crust
 Continental Crust converging with Continental Crust
OCEAN CRUST - CONTINENTAL CRUST
SUBDUCTION – TRENCH FORMS
Oceanic-Continental
Convergent Boundary - SUBDUCTION
DENSER OCEANIC CRUST
GOES DOWN INTO MANTLE
OCEAN CRUST –OCEAN CRUST
CONVERGENCE - SUBDUCTION
Oceanic Crust – Oceanic Crust
Older, denser oceanic crust is SUBDUCTED (sinks below)
VOLCANIC ISLAND ARC FORMED – ALEUTIAN ISLANDS
TRENCH FORMS
STARTS OUT AS OCEAN-CONTINENT
CONVERGENCE AND THEN AS OCEAN
CRUST IS SUBDUCTED TURNS INTO
CONTINENT TO CONTINENT
Continental Crust – Continental Crust
Convergent Boundary
NO SUBDUCTION - COLLISION ZONE
Collision of India and Asia
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.
Boundaries: Transform
SAN ANDREAS TRANSFORM FAULT
Transform Fault Boundary
AT MID-OCEAN RIDGES
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 Convergent
Continent-Continent collision zones have
shallow earthquakes
Convergent
IndianAustralian
Plate and the
Pacific Plate
(oceanic crust
to oceanic
crust) are
converging
forming the
Tonga Trench.
Deep
EQ’s
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?
SUMMARY
3 PLATE BOUNDARIES
 DIVERGING
 CONVERGING (SUBDUCTION ZONE)
Oceanic Crust – Continental Crust
Oceanic Crust - Oceanic Crust
 CONVERGING (COLLISION ZONE)
Continental Crust – Continental Crust
 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
ARROWS IN THE ASTHENOSPHERE SHOW PLATE
MOVEMENT DUE TO CONVECTION
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
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
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
Hot Spot
This diagram provides evidence that the Pacific Tectonic
Plate is moving toward the
(1) south
(3) southwest
(2) east
(4) northwest