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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
Mt St. Helens
Ring of Fire
Land/land
Formations – Large Mountains (Orogeny)
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