Theory of Theory of Plate TectonicsPlate Tectonics

How do we know the plates exist?How do we know the plates exist?

    Earthquake and Volcano ZonesEarthquake and Volcano Zones

Ocean floor features (Trenches and Ocean floor features (Trenches and Mid-Oceanic ridges)Mid-Oceanic ridges)

Lithospheric PlatesLithospheric Plates

7 major Plates 7 major Plates

(continental (continental crust, oceanic crust, oceanic crust)crust)

The Earth is made up of a dozen or so major plates and several minor plates. Tectonic plates are constantly on the move. The fastest plate races along at 15 centimeters (6 inches) per year while the slowest plates crawl at less than 2.5 centimeters (1 inch) per year.

Lithospheric Plate Cross SectionLithospheric Plate Cross Section

How do we know the plates move?How do we know the plates move?

Evidence Supporting Continental Evidence Supporting Continental DriftDrift

Movement of the Pacific Plate Movement of the Pacific Plate (Hawaiian island chain formed over a (Hawaiian island chain formed over a hotspot)hotspot)

Continental Drift & PangeaContinental Drift & Pangea

Fossil EvidenceFossil Evidence

How do the Plates Move?How do the Plates Move?

Sea Floor SpreadingSea Floor Spreading

mid-ocean ridgesrift valley

lithospheric plate

asthenosphere

magma

newer less dense crust older more dense crust

Sea Floor SpreadingSea Floor Spreading A A rift valleyrift valley is volcanically active forming is volcanically active forming

new oceanic crust on each side of the new oceanic crust on each side of the valley creating valley creating Mid-Ocean Ridges.Mid-Ocean Ridges.

The new crust pushes the older crust away The new crust pushes the older crust away from the mid-ocean ridge, moving the from the mid-ocean ridge, moving the ocean floor & the continentsocean floor & the continents

Ex: Mid-Atlantic RidgeEx: Mid-Atlantic Ridge

Divergent Boundary Formation

Evidence of Sea Floor SpreadingEvidence of Sea Floor Spreading Rock samples are younger closer to a mid-Rock samples are younger closer to a mid-

ocean ridgeocean ridge Magnetic stripes in oceanic crust match on Magnetic stripes in oceanic crust match on

both sides of a mid-ocean ridge both sides of a mid-ocean ridge

Transform faultsTransform faults Gives mid-ocean ridges their shapeGives mid-ocean ridges their shape

San Andreas FaultSan Andreas Fault Transform faults can become plate Transform faults can become plate

boundaries (sliding past each other)boundaries (sliding past each other)

What Destroys Crust? What Destroys Crust? Subduction ZonesSubduction Zones

Occurs when oceanic crust collides with Occurs when oceanic crust collides with oceanic or continental crustoceanic or continental crust

The older, more dense, oceanic crust The older, more dense, oceanic crust plunges back into the mantle at a plunges back into the mantle at a deep deep ocean trenchocean trench..

Forms Forms deep ocean trenchesdeep ocean trenches & volcanic & volcanic mountainsmountains or or island arcsisland arcs..

Oceanic - Oceanic - Continental Continental SubductionSubduction

Oceanic - Oceanic - Oceanic Oceanic

SubductionSubduction

Trenches of the WorldTrenches of the World

When Continents CollideWhen Continents Collide

Continental–ContinentalContinental–Continental

(No Subduction)(No Subduction)

Formation of the Himalaya MountainsFormation of the Himalaya Mountains

Three Plate BoundariesThree Plate Boundaries DivergentDivergent

– plates move apart; sea floor plates move apart; sea floor spreadingspreading

ConvergentConvergent– Plates collide together; Plates collide together;

subductionsubduction

TransformTransform (strike-slip) (strike-slip)– Plates slide past each other; Plates slide past each other;

San Andreas faultSan Andreas fault

Motion at Plate Boundaries

Tectonic Settings and Volcanic Activity

Plate BoundariesPlate Boundaries

What Causes the Plates to Move?What Causes the Plates to Move?

Convection Convection Currents in mantleCurrents in mantle

Ridge PushRidge Push

Slab PullSlab Pull

Convection and Tectonics