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** In 1912 Alfred Wegener proposed that the Earth‘s

continents are moving around the surface.

* Two hundred million years ago all of Earth's landmasses were connected to form a super continent called PANGAEA.

*Continental Drift

Earth Science Reference Tables

Pg. 8

Pangaea formed during Triassic Period

then moved to current position today.

**

1. Certain continents seem to fit together like a jigsaw puzzle

2. Some fossils are found on separated continents

EXAMPLE: Mesosaurus is found in South America and Africa

3. Warm climate fossils and coal beds are found in cold climates

EXAMPLE: Glossopteris fossils found in Antarctica

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4. Similar mountain ranges and mineral deposits are found in separated continents.

EXAMPLE: Appalachian Mts. in New York match-up with Caledonian Mts. in Europe (Scandinavia)

** * Fossils

* * “Uh-Oh!”: Problems with Continental Drift….

*Problems With Wegener's Hypothesis

*Wegener did not consider the ocean crust just the continents

*Wegener did not explain what force was moving the continents

*Continental Drift is rejected, but helps create theory of Plate Tectonics

1928: Arthur Holmes suggested magma rises toward the crust, spread, and sinks again. (Convection current)

1930: Wegner dies, unable to adapt Holmes’ idea to his theory.

*An Alternate Explanation

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*Mantle/Crust Structure *Took all ideas from continental drift and….

1. The temperature of the oceanic crust decreases as the distance increases from the Mid-Ocean Ridge (Convection currents in Mantle).

2. The age of the oceanic crust increases as the distance increases from the Mid-Ocean Ridge (SEA-FLOOR SPREADING).

*Sea-Floor Spreading

MID-ATLANTIC RIDGE

50,000 km. Long mountain range on the ocean floor

*

3. Pattern of corresponding stripes, on either side of the Mid-Ocean Ridge, of normal and reverse polarity (SEA-FLOOR SPREADING).

Magnetic Pole changes – changing polarity (North/South Pole)

Crust created at ridge, so gets older away from ridge

4. Volcanoes, earthquakes, and mountains occur in specific bands or belts on the Earth. These belts correspond with the plate boundaries.

*

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5. Hawaiian Islands were formed and are forming in the direction of Pacific plate movement (HOTSPOT).

Magma

* *

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

* *

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.

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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.

*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

*

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.

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Which graph best represents the geologic age of the surface bedrock on the ocean bottom?

*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.

Caused by convection currents:

MID-ATLANTIC RIDGE

TRENCH

CASCADES

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ARROWS IN THE ASTHENOSPHERE SHOW PLATE MOVEMENT DUE TO CONVECTION

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

*

*Earth’s plate borders are known as boundaries. There are 3 types of boundaries:

*Divergent

*Convergent

*Transform (Faults)

*Sea-Floor Spreading is aDivergent Plate Boundary

*Sea-Floor Spreading is aDivergent Plate Boundary

Divergent Boundaries: plates move apart

Creates new crust

**Splits apart continents to create new oceans

*

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New Ocean forming – linear

Red Sea;

Eventually, it will be like the Atlantic Ocean.

EAST AFRICAN RIFT VALLEY – CONTINENTAL RIFT

*In Afar, Ethiopia, a 40-mile magmatic rift that opened up several years ago

*If new crust is created at divergent boundaries, is Earth getting larger?

NO - CONVERGENT BOUNDARIES DESTROY CRUST

*

*Plates come together (collide) at convergent boundaries.

*This destroys crust & ‘makes-up’ for the new crust created by divergent boundaries on the other side.

*Oceanic Crust converging with Continental Crust

*Oceanic crust converging with Oceanic Crust

*Continental Crust converging with Continental Crust

*

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*

DENSER OCEANIC CRUST GOES DOWN INTO MANTLE

*

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

*

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*

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

* *

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.

f07_58_pg196

Convergent:

Indian-Australian Plate and the Pacific Plate (oceanic crust to oceanic crust) are converging forming the Tonga Trench.

Deep EQ’s

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* *

* *

*

DIVERGING

CONVERGING (SUBDUCTION ZONE)

Oceanic Crust – Continental Crust

Oceanic Crust - Oceanic Crust

CONVERGING (COLLISION ZONE)

Continental Crust – Continental Crust

TRANSFORM

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.

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*Which cross section has arrows that best represent the relative motion of the crustal plates along the Wadati-Benioff zone beneath the Tonga Trench?

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