Plate Tectonics
Earth Science
Charlotte Carter
Unit Review
Drifting Continents
• Evidence of drifting
– Fossils found on both of the
continents of Africa and South
America that are of the same
organism
• Fossils: The preserved remains of
ancient organisms
– German scientist Alfred Wegner
came up with the theory of
continental drift, he was actually a
meteorologist and not a geologist
• The theory called the giant landmass that
once was on Earth Pangaea
– Pangaea: Means all Earth
•Early Earth & Plate Tectonics
Fossil Evidence
• Fossils play a key role in
supporting the theory of
Continental drift
– Glossopteris is an extinct
plant
• They are found in rocks that
are around 250,000,000 years
old ( 250 million)
• The seeds were to large to be
carried by wind and too fragile
to have survived ocean waves
• The fact that they are found in
Antarctica implies that they had
to be in a better climate than
what is present Antarctica
Fossils
• Glossopteris: a fern found on the
southern continents
• Cynognathus: a land reptile found in
South America and Africa
• Lystrosaurus: a land reptile found in
Africa, Antarctica, and India
• Mesosaurus: a freshwater swimming
reptile found in Africa and South America
Geologic Evidence
• From Rocks
– Not only fossils provide
evidence for theory support
• Rocks in Africa and South
America
– When you examine the
continents of Africa and
South America,they look as if
they could piece together like
a puzzle
– The layers of coal depth line
up with each other
– Rock deposits from glaciers
also match
Naval oceanography made two significant
contributions in the advancement of the theory of
plate tectonics.
• .
1. The charting of the ocean floor using echo sounding refined the
understanding of seafloor structure
2. Studies of the sea floor using instruments called magnetometers
revealed strong evidence in support of seafloor spreading.
•
•
Just as maps of the oceans and coastlines initiated the idea of
continental drift, maps of the ocean floor propelled science into the
investigation of seafloor spreading. Charts and studies of the mid
ocean ridge system, trenches, seamounts, and continental shelves—
have been examined and catalogued.
Using these and other data, Harry Hess and Charles Dietz proposed
the idea of seafloor spreading. They suggested that the worldwide
system of undersea mountains called the mid-ocean ridges is where
new ocean floor is created as plates move apart. As they diverge, they
move large crustal plates along with the continents that sit atop of
them.
Spreading Sea Floor
Divergent Boundary
•
Divergent
Boundaries
– Constructive-add
new land
– Mid-ocean Ridge
•
•
•
Ocean floor moves
away on either side of
the ridge
Called sea-floor
spreading
Often offset by transform Sea-Floor Spreading/Mid-Ocean Ridge
faults which causes a
Basic Plate Boundaries
curve in the ridge
Transform Faults
Magnetic Rock Strips
•Some minerals (such as
magnetite) have
magnetic properties
•These minerals line up
with the Earth’s magnetic
poles
•When the molten rock
hardens, a permanent
record of the Earth’s
magnetism remains
•The Earth’s magnetic
poles have reversed
themselves from time to
time
•Animation
Convergent Boundaries
• Plates collide-destructive
• Two continents colliding
build mountains or
plateaus
• The Indian Plate and the
Eurasian Plate collide to
form the Himalayas
which have Mt. Everest ,
the highest mountain on
Earth.
Convergent Boundary
• Destructive-crust is
destroyed (melted by the
mantle)
• Called Subduction
• When oceanic and
continental crust collide
• Oceanic crust is pushed
down into the mantle and
melted
• Some of this melted
material surges upward
Convergent Boundaries
• The continental
crust is also forced
upward producing
volcanoes
• The Cascade Range
in Washington and
Oregon are an
example of this
Convergent Boundaries
Convergent Boundaries
• When two oceanic
plates collide subduction
occurs
• The older denser plate
is pushed into the
mantle and melted
• Some of the material
rises upward and erupts
on the ocean floor
forming an island arc
Theory of Plate Tectonics
• Tectonics: refers to the branch of geology that is
concerned with plate movements
• Theory of Plate Tectonics: Links together the ideas of
continental drift and ocean floor spreading to explain
how the Earth has evolved over time.
– It explains the formation, movements, collisions, and
destruction of the Earth’s crust
– According to the theory the Earth’s uppermost layer, called
the lithosphere, is made up of plates
Lithospheric Plates
• There are seven major plates
– The Pacific Plate• which covers 1/5 of the Earth
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–
–
–
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North American
South American
Eurasian
African
Indo-Australian
Antarctic Plates
• There are also Many smaller
plates
– Caribbean
– Arabian
• Plate Tectonics Review
SUMMATION
• The theory of plate tectonics is relatively new
• Early 1900s, Alfred Wegener first developed a theory of
continental drift based on a collection of evidence from
rocks and fossils found on continents separated by vast
distances
• 1928, Arthur Holmes proposed a mechanism to explain
how the continents could have moved across Earth's
surface
• Circular convection currents of molten material deep
within Earth could provide ample force to divide and
move continents
• By the late 1960s, evidence of seafloor spreading helped
to support the now well-accepted theory of plate
tectonics.