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???

• Use your text book and find out at least 4 of the lines of evidence Wegener used to support his theory.

• Explain why his evidence supported his continental drift theory.

• Hints:– Pictures help to explain– Look at Chapter 7

Wegener’s Evidence1. 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 Evidence2. 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 EVIDENCEThe 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_volcanoes_erupt/img/1.jpg

Convection current/Plate motion

Plate Boundaries and Convection Currents

Video Website – Convection Current

• http://www.absorblearning.com/media/item.action?quick=12p

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 Boundariestwo plates move apart

Ocean/OceanFormations- ocean ridge, fissures

Mid-Atlantic Ridgeit is 40,000 miles long

Mid-Ocean Ridge – Magnetic Reversals

Land/landFormations – rift valley

Great African Rift Valley

Video Website - Divergent

• http://www.absorblearning.com/media/item.action?quick=12n

Convergent –two plates come together

Land/oceanFormations- subduction zone, trench, volcanoes,

earthquakes

Video - Subduction

• http://www.absorblearning.com/media/item.action?quick=12s

Mt St. Helens

Ring of Fire

Land/landFormations – Large Mountains (Orogeny)

Video – Convergent Mtns.

• http://www.absorblearning.com/media/item.action?quick=12t

Himalayas

Ocean/oceanFormations – island arc, trench

Eastern Caribbean

Video – Convergent - Island Arc

• http://www.absorblearning.com/media/item.action?quick=12u

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