The World as it Turns

Changes in the Earth’s Crust…Plate Tectonics

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Parts of the Earth…

There are 5 main layers, from surface to center:

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Lithosphere (crust): solid outermost layer, about 5-50km deep;

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Asthenosphere: the lower layer of the earth's crust; less rigid than the lithosphere —more slushy/flexible

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

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

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Solid Inner Core: solid iron/nickel center of the Earth; high temperatures & pressure; like a solid How do we know?

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Earthquakes

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The Theory of Continental Drift

Alfred Wegener, 1912

Continental Drift…

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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…

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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

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Continental Drift: Lack of Support…

Wegener’s theory did not go over well in the scientific community…

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no explanation how the continents “drift” continents plow through sea floor , really???

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What strong force moves huge mass-large distance???

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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

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1940’s, use sonar to map ocean floor Discovery of ridges (underwater mountains), middle Atlantic Ocean

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extended entire length (N S) of ocean

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More ridges found in Indian and Pacific Oceans

Sea Floor Spreading: The Evidence…

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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 S N S N S N S N S N S N S N S

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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 youngest

Mid Ocean Ridge Currents Rising Magma Oceanic Crust (lithosphere)

Sea Floor Spreading: The How…

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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…

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3 types of plate boundary movements

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Convergent

together.

: Boundary between two plates that are pushing –

Divergent:

apart.

Boundary between two plates that are moving –

Transform:

one another.

Boundary between two plates that are sliding past ANIMATIONS

When Boundaries Collide: Convergence…

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Continental vs. oceanic

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More dense oceanic plate dives under less dense; called subduction

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

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

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

When Boundaries Collide: Convergence…

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Oceanic vs. Oceanic

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

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

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this subduction zone forms a curved volcanic mountain chain

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

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When Boundaries Collide: Convergence…

Continental vs. Continental

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

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solid rock is folded and faulted

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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…

Tibet ( \Eurasian Continental Plate) Nepal India (Indian Continental Plate) * Himalayan mountain range is the perfect example of how the highest mountains in the world continues to grow

• Plate Tectonics: Boundaries

They Move! – Convergence

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Turn to your neighbor and explain converging tectonic boundaries

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Use handy supplies to help your explanations

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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

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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 Turns

In Closing…

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

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Use handy supplies to help your explanations Be prepared to share your “demonstrations” with the class

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Review…

As plates move, the structure of the earth changes, creating volcanoes and building mountains