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Plate Tectonics

And

Plate Boundaries

HORRAH!!!

History of Plate Tectonics

and Alfred Wegener

History

• In the 1500’s, a Dutch mapmaker, Abraham Ortelius noticed the continents across the Atlantic Ocean fit like puzzle pieces.

• He proposed that North and South America had been separated from Europe and Africa by earthquakes and floods.

Abraham Ortelius’ Map

Edward Suess

• An Austrian geologist,

in the late 1800’s,

proposed the southern

continents had once

been joined together in

a single landmass

• He named this

landmass

Gondwanaland.

Gondwanaland

Alfred Wegener

• In 1912 a German meteorologist, Alfred Wegener, had an idea about continental movement.

• His hypothesis was called Continental Drift.

• His hypothesis

proposed that the

continent’s had

once been joined

as a single

landmass, called

PANGAEA.

• ‘Pangaea’ is a

greek word

meaning ‘All the

Earth’

PANGAEA

• Wegener believed

that Pangaea began

to break apart 250

million years ago.

• The continents have

been continuously

moving since then to

where they are now.

Wegener’s Evidence 1. Rocks

– Believed there should be similar rock types on opposite sides of the Atlantic

– Appalachian Mtns. are similar to the Scottish Highlands

– Rocks in Brazil matched those in Africa (similar age of 2.2 billion years old)

More Evidence 2. Fossils

– Animal Fossils

• Lystrosaurus – land reptile

• Mesosaurus – freshwater reptile

• Cynognathus – land reptile

• ALL couldn’t have swam across

Atlantic Ocean (salt water)

Fossil Evidence • Plants

– Glossopteris

• Seeds – too heavy to blow

across ocean

– Too fragile to float across

ocean

• Animals

– Land animals couldn’t swim

– Freshwater animals can’t

swim in ocean (salt water)

– Couldn’t have evolved the

same way in different

locations!

Even More Evidence…

• 3. Climate

– Coal deposits

found in

Antarctica

– Glacier deposits

found in Africa,

India, Australia,

and South

America

Science Haters

• Other scientists rejected his hypothesis because he couldn’t explain what was causing the continents to move (a.k.a. HOW?)

• He thought rotation of the Earth but

physicists said no

• Also thought continents plowed through ocean floor but there was no evidence for that.

Arthur Holmes

• 1929, a geoscientist,

Holmes proposed that

convection occurred

in the mantle.

• This convection

could push and pull

plates (continents)

apart or together.

• But NO evidence!!!

FINDING EVIDENCE The first way to map the ocean floor used Sonar.

This way stunk because it didn’t accurately map

the floor.

The then invented the Magnetometer. This

could detect small changes in the magnetic field

in rocks and map them.

Seafloor Spreading

• Mid-Atlantic Ridge

Seafloor Spreading – new ocean crust is formed

at ocean ridges and destroyed at deep sea

trenches.

Harry Hess

• Used paleomagnetism and seafloor mapping to prove

convection is a source for ‘continental drift’

Plate motion is thought to be

caused by convection

currents in the

asthenosphere. Since the

hotter material deep in the

asthenosphere is less dense it

will slowly rise, as it reaches

the base of the lithosphere it

begins to cool, become more

dense, and will sink. Rising

convection currents will

move plates apart

(divergent), while sinking

convection currents will

move plates together

(convergent). http://news.bbc.co.uk/nol/shared/spl/hi/pop_ups/04/world_how_volca

noes_erupt/img/1.jpg

Convection current/Plate motion

Plate Boundaries and Convection Currents

These new maps showed the different magnetism

in different layers of rock.

As new crust is created the iron bearing minerals

point to where the magnetic pole is at the time of

creation.

This is called Paleomagnetism

Mid-Ocean Ridge • Isochron – line of same aged rock

Magnetic Pole Reversals

• Random reversal of Earth’s Polarity

– Average irregular 300,000 year cycles

– Last one was over 780,000 years ago = overdue

• Unknown as to why

– Maybe external, maybe internal…

This proved that new sea floor was being created

at mid-ocean ridges.

Identical magnetism on opposite sides of ridges.

Thus, the plates where moving.

Plate Boundaries and Convection Currents

Types of Boundaries

Divergent Boundaries

two plates move apart

Ocean/Ocean

Formations- ocean ridge, fissures

Mid-Atlantic Ridge it is 40,000 miles long

Mid-Ocean Ridge – Magnetic Reversals

Land/land

Formations – rift valley

Great African Rift Valley

Video Website - Divergent

• http://www.absorblearning.com/media/item.

action?quick=12n

Convergent –two plates come

together

Land/ocean

Formations- subduction zone, trench, volcanoes,

earthquakes

Video - Subduction

• subduction

Mt St. Helens

Ring of Fire

Land/land

Formations – Large Mountains (Orogeny)

Video – Convergent Mtns.

• converging mts

Himalayas

Ocean/ocean

Formations – island arc, trench

Eastern Caribbean

Video – Convergent - Island Arc

• island arc

Transform – the plates slide past

each other, they form faults

Friction builds up between the plates until they

snap and cause EARTHQUAKES

San Andreas Fault

Hotspot

Hotspots The hotspot is also known as a

magma chamber, it is located in the

mantle

The Hotspot then explodes,

creating a volcano

The plate then moves. Notice the volcano moves

with the plate, but the hotspot stays were it was

The Hotspot then explodes again, forming a new

volcano

The plate moves again, starting the process over

Another volcano is then created forming a

Volcanic Chain

An example of a hotspot is

Hawaii