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 SpreadingDivergent Plate Boundary

Sea-Floor SpreadingDivergent 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 Fault Boundaries

TRANSFORM PLATE 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

MID-ATLANTIC RIDGE

TRENCH

CASCADES

The arrows shown in the asthenosphere represent theinferred 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