Plate Boundaries; subduction; magnetic polarity

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Plate Tectonics
Ann C. Cloutier
Pangaea
Continental Drift
Sea Floor Spreading
Isochron Maps – Geomagnetic Core
GPS
Sonar

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Continental Drift
from a Hypothesis by Alfred Wegener and was
developed into a Theory with supporting evidence
 Super continent called Pangaea >Greek for “all land”
 Began to brake apart over 200 million years ago
 Puzzle fit of South America and Africa
 Collected evidence in rocks, fossils, and climatic data to
support his hypothesis

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Evidence to collect to support hypothesis but, no means to
explain how the continents were moved…yet !

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Glossopteris and Mesosaurus

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Ancient climates
evidence of coal deposits found in Antarctica
 coal is formed from swamp plant or other vegetation that
has decomposed
 this meant a temperate rainy climate was once on the
surface of Antarctica
 glacial deposits of till and striation scratches on bedrock
found in Afrrica, 290 million year old land formations
 today Africa is covered by huge deserts where the
evidence of continental glaciers was documented

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How is coal formed ?

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Today 290MYBP

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Seafloor Spreading
 Technology sonar echo-
location > sound beams /
time returned
 WWII submarine warfare
developed technology to
detect more than an
enemy sub
 Detailed maps and a safe
way to navigate the
bottom

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Seafloor spreading Harry Hess

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Mid Ocean Ridges & Trenches
 Ocean ridges are long
mountain ranges
 Continuous wrap around
planet
 Deepest and longest
mountain range on the
planet
 ~46,000 miles long (~ 72km)
 Mariana Trench
 11 km deep deepest
trench in all oceans

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Continental Crust - Oceanic Crust
 Oldest age 3.8 BYBP
 Billions of years before
present
 Less dense than oceanic
crust
 Continental mountains
keep getting folded and
faults break up crust
 Young age ~ today’s lava
>180 MYBP
 Millions of years before
present
 More dense than
continental crust
 The further away from the
ridge the thicker and
older the oceanic crust
becomes

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Earth quakes occur at plate boundaries

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Seismic information

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Richter Scale
Earthquake magnitude of seismic wave data

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Safety for Earthquake

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Causes of Plate Movement
 Mantle convection are thought to be the driving mechanism of plate tectonic
movement
(continental drift caused by seafloor spreading and subduction)
 The transfer of heat, called thermal energy is the cause of the movement of
hot matter (magma) moving to cold matter; this is called convection
 Warm matter move toward cold matter
 Cooler matter sinks because it is more dense
 “G force” gravity pulls it down into the mantle
 Ridge push and slab pull are the results of this process of heat moving away
from the radiation of the core out towards the crust

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Convection current of Earth’s mantle
the bulldozer pushes plates around the planet surface

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Isochron Map
maps with magnetic fields showing Earth’s polarity for
magnetic north
 A line graph defined by data
for rocks of the same age;
 With the same isotopic
composition
 Polarity event is recorded in
rocks a period of no more
than 100,000 years when
Earth’s magnetic polarity was
opposite to the existing
polarity

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Check out and watch this movie on your iPad
 http://science.discovery.com/tv-shows/greatest-
discoveries/videos/100-greatest-discoveries-magnetic-field-
reversal.htm

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http://www.youtube.com/watch?
v=1sNL-XaBmBk

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http://mysciencebox.org/files/images/magnetic_reversal_animation.gif

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Questions?
Going further time for the iPad technology
 Do scientist know if convection currents stay in
the same places ?
 Are hot spots located in the same places ?
For how long ?
 Is Cape Cod located on a hot spot?
 What state is located over a hot spot ?
 When was the last earthquake Cape Cod felt ?