plate tectonics Section 2&3&4

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Alfred Wegener, a

meteorologist, in the early 1900’s proposed the hypothesis that continents were once joined together in a single large land mass he called Pangaea

(meaning “all land” in Greek). He proposed that Pangaea had split apart and the continents had

moved gradually to their present positions - a process that became

known as continental drift.

CONTINENTAL DRIFT

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CONTINENTAL DRIFT

According to the hypothesis of

continental drift

, continents

were once joined together in a

super continent called

Pangaea

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Pangaea

about 245 million years ago, before it began

breaking up.

Wegener named the southern portion of Pangaea

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The position of the continents today. The

continents are still slowly moving, at about the

speed your fingernails grow. Satellite

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Continents fit together like a puzzle….e.g. the

Atlantic coastlines of Africa and South America.

The best fit includes the continental shelves (the continental edges under

water).

Wegener’s Evidence for

Continental Drift

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Wegener’s Evidence for

Continental Drift

Wegener’s Evidence for

Continental Drift

Fossils of plants and animals

of the same species found on

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Wegener’s Evidence for

Continental Drift

Wegener’s Evidence for

Continental Drift

Rock sequences (meaning he looked at the order of rock layers) in South

America, Africa, India, Antarctica, and Australia show remarkable

similarities.

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Wegener’s Evidence for

Continental Drift

Wegener’s Evidence for

Continental Drift

• _{The same three layers are in}

the same order in areas now separated by oceans.

• _{Wegener proposed that the}

rock layers were made when all the continents were part of Pangaea.

• _{He proposed that they}

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Everyone agreed that

Wegener’s evidence was

compelling…

Everyone agreed that

Wegener’s evidence was

compelling…

But wouldn’t we feel the movement?

Also, wouldn’t there be evidence to show that the continents were still moving today?

Wegener was a meteorologist and his

theory was not well accepted (he died on an expedition in Greenland collecting ice

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One reason scientists had a hard

time with Wegener’s theory is

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Seafloor Spreading

In the 1960’s, a scientist named Henry Hess made a discovery that would vindicate Wegener. Using new technology, radar, he discovered that the seafloor has both trenches and

mid-ocean ridges.

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Seafloor Spreading

Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the

mid-ocean ridges.

Then, it flows sideways, carrying the seafloor away from the ridge in both

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Seafloor Spreading

As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created.

The older seafloor moves away from the ridge in opposite directions.

This helped explain how the crust could

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In 1968, scientists aboard the research ship

Glomar Challenger began gathering

information about the rocks on the seafloor. Scientists found that the youngest rocks are located at the mid-ocean ridges.

Evidence for Spreading

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Mechanism for

Plate Tectonics

Mechanism for

Plate Tectonics

Seafloor Spreading provided insight to the mechanism for how the continents moved.

1. The magma which pushes up at the mid-ocean ridge provides the new land

pushing the plates.

2. The subduction zones gobble up the land on the other side of the plates.

3. Convection Currents.

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Plate Tectonic Theory

Both Hess’s discovery and Wegener’s continental drift theory combined into what scientists now call the plate tectonic theory.

Theory of Plate Tectonics: • _{The Earth’s lithosphere is broken}

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3 Different Types of

Plate Boundaries

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1. Divergent

boundary

1. Divergent

boundary

Happens at a seafloor spreading zone.

Plates are separating from each other as a new land mass

forms.

This is seen at mid-ocean ridges_andrifts.

Plate separation is a slow process.

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Divergent Boundary – Iceland

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Divergent Boundary – Arabian and African Plates

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2 .

Convergent

boundaries

3 types

2 . Convergent

boundaries

3 types

Type 1: Continental vs. continental: two

continental plates

collide and crumple at the edges of the plates to form mountains.

Examples: We can see the end result of the collision between the Indian & Eurasian

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Convergent Boundaries

– Continental vs. continental

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Himalayan Mountains

Continental/continental convergent boundary

Himalayan Mountains

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Convergent boundary type 2:

Oceanic vs. Continental

Convergent boundary type 2:

Oceanic vs. Continental

When an oceanic plate goes underneath or is subducted under a continental plate it is called subduction.

This forms a trench, or deep valley, where the plates meet.

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Convergent Boundary type

2– Oceanic & Continental

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Convergent Boundary Type 3:

Oceanic vs. Oceanic: forms island arcs

Convergent Boundary Type 3:

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Island Arc example

The Aleutian Island Chain

Island Arc example

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3. Transform boundary

Two plates slide past each other

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Hot Spots

• _{Areas on the globe where stationary}

mantle plumes rise and melt the

plate above forming volcanoes.

• _{Plates move over these plumes and}

new volcanoes are formed in a line.

• _Examples

_{: the Hawaiian Island chain}

and Yellowstone National Park are

both hotspots.

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34

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Yellowstone Hotspot

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Yellowstone—a

time-bomb

?

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Divergent

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Plate Tectonic Theory

Plate Tectonics explains these geologic

features:

Earthquakes

Mountains

Volcanoes

Trenches

Mid-ocean ridges

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Deforming the

Earth

’

_’

s Crust

_{s Crust}

Deforming the

Earth

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Deformation

_Deformation

Deformation

Stress is the amount of

force per unit area on a given material

The process by which the shape of a rock

changes because of stress is called

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Deformation

Two types of deformation can occur to rocks under stress.

Layers can bend when stress is applied to them.

When too much stress is applied,

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The types of stress that occur at a

plate boundary are compression &

tension...

The types of stress that occur at a

plate boundary are compression &

tension...

• Compression_{is the}

type of stress that occurs when an object is squeezed.

• Tension_{is stress}

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Folding

• Folding_{is the bending of rock}

layers because of stress in the Earth’s crust.

Scientists assume that all rock layers start off horizontal.

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Types of Folds

There are two main types of folds.

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Faulting

Some rock layers

break_{when too}

much stress is applied to them.

The surface along which a rocks break and slide past each other is called a

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Faulting

The two sides of a fault are known as

the hanging wall

and the footwall.

The type of fault

that forms is dependent on

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There are three types of

faults that occur..

.

There are three types of

faults that occur..

.

Normal Faults

Reverse Faults

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Plate Tectonics & Mountain

Building

Plate Tectonics & Mountain

Building

When tectonic plates collide, land features that start as

folds and faults, can

eventually become large

mountain ranges.

When tectonic plates undergo

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Folded Mountains

Form when rock layers are squeezed together and pushed upward.

These mountains form

at convergent plate

boundaries.

Appalachian

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Fault-Block

Mountains

Fault-Block

Mountains

When tectonic forces put enough tension on the Earth’s crust, a large number of normal faults can occur.

Fault-Block mountains_{form when}

tension causes large blocks of the Eart h’s crust to drop down relative to other blocks.

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Volcanic Mountains

Located at convergent plate boundaries where oceanic crust sinks into the

asthenosphere at subduction zones.

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Volcanic Mountains

Sometimes these mountains can rise above the sea and become islands

A majority of the tectonically active volcanic mountains have formed

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