The Theory of Plate The Theory of Plate TectonicsTectonics

66thth Grade Grade

Remember what we learned yesterday Remember what we learned yesterday about continental drift? about continental drift?

Alfred Wegener found a variety of evidence that suggested that the seven continents used to be joined together as one large supercontinent.

What was the ONE PART of his theory that he was not able to explain?

Alfred Wegener was not able to Alfred Wegener was not able to explain the explain the forceforce that moved the that moved the

continentscontinents

What What forceforce is great enough to is great enough to move the heavy continents?move the heavy continents?

Geologists think that the movement of Geologists think that the movement of convection currents convection currents in the mantle is the major in the mantle is the major

force that causes plate motion! force that causes plate motion!

The Theory of Plate The Theory of Plate TectonicsTectonics::

• States that pieces of the Earth’s lithosphere called plates are in slow constant motion because of convection currents in the mantle

What are What are plates?plates?

Plates:Plates:

• The lithosphere is broken into separate sections called plates

• Plates move around on top of the mantle like rafts

Plates fit together along cracks Plates fit together along cracks in the in the lithospherelithosphere

Review: What is the Review: What is the LithosphereLithosphere??

• The solid upper part of the mantle = lithosphere

–Less dense than the material below it so it “floats” on top of the asthenosphere

Review: What is the Review: What is the AsthenosphereAsthenosphere??

AsthenosphereAsthenosphere: semi-solid layer : semi-solid layer below the lithospherebelow the lithosphere

***The plates of the ***The plates of the lithospherelithosphere float float on the on the asthenosphereasthenosphere******

2 Types of Plates2 Types of Plates

• Oceanic plates - plates below the oceans

• Continental plates - plates below the continents

Plate BoundariesPlate Boundaries

Remember that the Remember that the convection convection currents currents (heat transfer) that are (heat transfer) that are

occurring in the mantle are occurring in the mantle are causing these causing these plates to moveplates to move

Scientists use instruments on Scientists use instruments on satellitessatellites to measure plate to measure plate

movement. movement.

Plates move at slow ratesPlates move at slow rates: ex. The : ex. The North American and Eurasian North American and Eurasian

plates are moving apart at 2.5 cm plates are moving apart at 2.5 cm per year –about as fast as your per year –about as fast as your

fingernails growfingernails grow

3 types of tectonic plate movement3 types of tectonic plate movement::

1. Divergent1. Divergent

2. Convergent2. Convergent

3. Transform3. Transform

Divergent BoundariesDivergent Boundaries

• Boundary between two plates that are moving apart or rifting

• RIFTING causes SEAFLOOR

SPREADING

What is What is seafloor spreading?seafloor spreading?

• When oceanic plates diverge, (move apart) leaving room for molten material to erupt and cool

• This leads to the constant creation of new ocean floor

• This always occurs at the mid-ocean ridge

Seafloor Spreading at the Mid-Seafloor Spreading at the Mid-Ocean RidgeOcean Ridge

Mid-Ocean RidgeMid-Ocean Ridge

Mid-Ocean RidgeMid-Ocean Ridge

Life at the Mid-Ocean RidgeLife at the Mid-Ocean Ridge

Features of Divergent Features of Divergent BoundariesBoundaries

• Mid-ocean ridges

• Rift Valleys

Rift Valley: when divergent Rift Valley: when divergent boundaries occur on land boundaries occur on land

Great Rift Valley in AfricaGreat Rift Valley in Africa• Volcanic mountains found along the rift

include Mount Kilimanjaro, Mount Kenya and Mount Elgon

• extending from Jordan in southwestern Asia through eastern Africa to Mozambique

• 4,000 miles long, 30 miles wide

Great Rift ValleyGreat Rift Valley

Convergent BoundariesConvergent Boundaries

• Boundaries between two plates that are colliding

• Oceanic plate colliding with a less dense Continental plate

• the dense oceanic crust sinks under the continental crust

• Causes TRENCHES

• Oceanic plate colliding with another Oceanic plate

• The more dense plate slides under the less dense plate, creating a TRENCH

When 2 continental plates collide, both are about the same density

Collision squeezes the crust into mountain ranges

Transform Fault BoundariesTransform Fault Boundaries

• Boundary between two plates that are sliding past each other

• Creates “faults” or breaks in the Earth’s crust

• What do you think that this movement causes?

EarthquakesEarthquakes!!

San Andreas Fault, CASan Andreas Fault, CA

Questions...Questions...

• What are the three types of boundaries?

• What direction do plates go for each?

Understanding Plate TectonicsUnderstanding Plate Tectonics

• http://www.youtube.com/watch?v=tcPghqnnTVk