The World as it TurnsChanges in the Earth’s Crust…Plate Tectonics

Parts of the Earth…• There are 5 main layers, from surface

to center:– Lithosphere (crust): solid outermost

layer, about 5-50km deep; – Asthenosphere: the lower layer of the

earth's crust; less rigid than the lithosphere—more slushy/flexible

– Mantle: region below crust extending down to Earth's core; some molten rock; less flexible

– Liquid Outer Core: made of molten iron/nickel; flows & creates Earth’s magnetic field

– Solid Inner Core: solid iron/nickel center of the Earth; high temperatures & pressure; like a solid

How do we know?

Earthquakes

The Theory of Continental Drift

Alfred Wegener, 1912

Continental Drift…• proposed – continents slowly drift in

relation to one another; continents were once joined, single super-continent (Pangaea)

Interactive Map of Pangaea

Alfred Wegener

Continental Drift: The Evidence…• shape of the continents - puzzle-like fit• same animal fossils – fresh water fossils-opposite

shores of Atlantic Ocean • same sequence of rock layers - found on opposite

shores of Atlantic Ocean along coastlines• fossils of tropical plants - found in polar areas• glacial deposits - found in tropical areas

More on the evidence

Continental Drift: Lack of Support…• Wegener’s theory did not go over well in the scientific

community…– no explanation how the continents “drift” – continents plow through sea floor , really???– What strong force moves huge mass-large distance???

• To read an excellent historical account of Continental Drift with informative graphics, visit the U.S. Geological Survey.

Crustal History Confirmation

A.Describe the evidence Wegener used to support his theory of Continental Drift.

B.Why did the scientific community not believe in Wegener’s theory?

Sea Floor Spreading

More Evidence is Found

Sea Floor Spreading• 1940’s, use sonar to map

ocean floor• Discovery of ridges

(underwater mountains), middle Atlantic Ocean– extended entire length (N -

S) of ocean– More ridges found in Indian

and Pacific Oceans

Sea Floor Spreading: The Evidence…• 1968, scientists drill cores of rock from ocean floor

– youngest rock at the mid-ocean rift; oldest at continental coasts

– Earth’s magnetic field constantly shifting, • On average, Atlantic Ocean spreads ~ 1.25 cm/yr, similar to

fingernail growth

Polarity reversals same

distance from rift on either

side

N N N NN N N S SS SS SS S

Sea Floor Spreading: the How…• Occurs at MID-OCEAN RIDGES • CONVECTION – Hot, less dense magma rises, flows sideways ,

cools & sinks• Magma/crust friction pulls crust; rift fills with magma/lava, cools -

creating NEW crust• Ocean lithosphere (crust) is youngest NEAR mid-ocean ridge

Mid Ocean Ridge

Convection Currents

Oceanic Crust (lithosphere)Rising

Magma

youngest

oldest

Sea Floor Spreading: The How…• Sea Floor Spreading animation• Remember: convection currents CAUSE sea floor spreading

—gravity causes convection currents• Convection Currents and Plate Movement animation

Crustal History Confirmation

A.Explain sea-floor spreading.B.When examining rock samples from

the ocean floor, identify 2 things that can be found that support the hypothesis of sea floor spreading.

Where are we today?

The Plate Tectonic Theory

The Plate Tectonics Theory• Plate Tectonics Theory combines continental drift & seafloor

spreading• Lithosphere floats on denser, liquid rock in mantle called

asthenosphere.• Lithosphere broken into tectonic plates• Tectonic plates move about 1-8 cm/yr

• This movement is the “continental drift” referred to by Wegener

The Plate Tectonics Theory• New plate added by rising magma at divergent boundaries (think

mid-ocean ridges)• Plates are destroyed in subduction zones at convergent

boundaries

• Transform boundaries slide past each other

Plate Boundary Movements…• 3 types of plate boundary movements

– Convergent: Boundary between two plates that are pushing together.

– Divergent: Boundary between two plates that are moving apart.

– Transform: Boundary between two plates that are sliding past one another.

• ANIMATIONS

When Boundaries Collide: Convergence…• Continental vs. oceanic

– More dense oceanic plate dives under less dense; called subduction

– dense, leading edge of the oceanic plate actually pulls the continental plate down forming a trench

– Crust heats and melts as it is forced downward below the continental crust

– Hot gases and magma forced upward creating a volcanic mountain range on the continent

When Boundaries Collide: Convergence…• Oceanic vs. Oceanic

– the farther a plate gets from the mid-ocean ridge that created it, the colder and denser it gets

– when two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink (subduct)

– this subduction zone forms a curved volcanic mountain chain

– Aleutian Peninsula of Alaska is an example of a very volcanically-active island arc

When Boundaries Collide: Convergence…• Continental vs.

Continental– two continental plates

meet head-on, neither can sink; both plates are too buoyant

– solid rock is folded and faulted

– huge chunks of rock many kilometers wide are thrust on top of one another, forming a towering mountain range

Himalayan Mtns. Taken by Satellite, 2004…

India(Indian Continental Plate)

Nepal

Tibet (\Eurasian Continental Plate)

Himalayan Mountain Range

* Himalayan mountain range is the perfect example of how the highest mountains in the world continues to grow

Plate Tectonics: Boundaries• They Move! – Convergence

• Turn to your neighbor and explain converging tectonic boundaries

• Use handy supplies to help your explanations

When Boundaries Collide: Divergence…• hot magma moves slowly upward, cooler magma near surface

moves slowly downward forming convection currents within the asthenosphere

• Rising convection currents diverge where they approach the surface pulling on the plate above it creating a divergent plate boundary

• two sides move away in opposite directions, cracks between the diverging plates fill with molten rock which cools and quickly solidifies, forming new oceanic crust

When Boundaries Collide: Transform…• At transform plate boundaries, plates slide/grind past each other.• separates the North American plate from the Pacific plate along the

San Andreas fault, a transform plate boundary responsible for many California’s earthquakes

The World as it TurnsIn Closing…

• Turn to your neighbor and explain converging, diverging and transform boundaries tectonic boundaries

• Use handy supplies to help your explanations• Be prepared to share your “demonstrations”

with the class

Review…• As plates move, the structure of the earth changes, creating

volcanoes and building mountains