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

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?

In your journal

Lithosphere. This layer varies in thickness from 1 to 250 miles. The lithosphere is a cool, strong and rigid layer. Its uppermost part is called the crust and is divided into oceanic and continental-type crusts discussed in the next section.

Asthenosphere. The upper reaches of the mantle are not solid; they are considered plastic and flow very slowly. This is due to the reduction in pressure as we approach the top of the mantle- the rocks are more likely to begin to melt.

Mantle. About 1860 miles thick, the mantle is made of iron and magnesium-rich silicate rocks and has a density of about 4.5 g/cm3. The mantle is hot, but mostly solid because the minerals are under pressure.

Outer Core. About 1240 miles thick, the outer core is also made mostly of iron with a density of about 10 g/cm3. However, a reduction in pressure makes this hot layer a liquid.

Inner Core. About 800 miles in diameter and composed of more than 90% iron with a maximum density of 13 g/cm3. This iron in this layer, although very very hot, is solid due to the immense pressure at the center of the Earth.

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

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

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 and is recorded in the rocks – this provides further evidence of sea floor spreading

• 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

SSSS

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• Sea Floor Spreading from BBC documentary film "Earth The

Power Of The Planet "

Things to Ponder about Sea Floor Spreading…• Use Newton’s Laws of Motion to explain

the movement.• What force is causing the motion?• What happens when the plates hit each

other?• What type of force does there have to be

to cause the movement?

Remember Flip Videos for tomorrow!

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

• There are three types of plate boundaries:– Divergent Boundaries -Boundary between two

plates that are moving apart.

- Convergent Boundaries -Boundary between two plates that are moving apart.

- Transform Boundaries -Boundary between two plates that are sliding past one another.

– animations

Convergent – oceanic to continental • Continental vs. oceanic

– A subduction zone is the area where a more dense oceanic plate dives under less dense continental plate.

– The place where the continental crust is pulled down by the oceanic crust is a deep, v shaped called called a trench.

– Where the oceanic plate subducts and melts under continental plate, the magma and hot gasses are forced upwards creating a volcanic mountain range on the continent.

Mt. St. Helens

Lassen peak

Convergent: oceanic to oceanic• Oceanic vs. Oceanic• If it is 2 oceanic crusts that

meet, then a underwater volcanic range is formed in the ocean.

• As they erupt and the rock cools they will form islands.

Convergent: continental and continental• Continental vs.

Continental

• 2 continental plates converging will form a mountain range like the Himalayan mountain range.

Himalayan Mtns. Taken by Satellite, 2004…

India(Indian Continental Plate)

India(Indian Continental Plate)

Nepal

Tibet (\Eurasian Continental Plate)

Tibet (\Eurasian Continental Plate)

Himalayan Mountain Range

Himalayan Mountain Range

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

India Is still moving into Asia

Today

Divergent boundaries…. • When oceanic moves away from oceanic it forms a mid ocean

ridge. • When continental moves away from continental if forms a rift

valley.

Mid Atlantic ocean ridge

Examples of Divergent Boundaries: Mid-Atlantic Ocean Ridge, East African Rift, and Red Sea

Spreading Centers

Transform…• At transform plate boundaries, plates slide/grind past each other.• A famous example of a transform boundary is found in California. It

is called the San Andreas Fault.

Identify the Plate Boundaries below A. B. C. D. E.