Transcript Evidence of Plate Tectonics

Unit 5 – The Dynamic Earth
Chapter 17 – Plate Tectonics
Major focus for the Regents Exam!
ESRT pages we will study in this unit:
• Page 5 – Tectonic Plates
• Page 10 – Inferred Properties of the Earth
• Page 11 – Earthquake wave travel time & crust
composition
Plate Tectonics
Plate tectonics is a proven “theory” that
describes the formation, movement, and
interaction of lithospheric plates over a
plastic asthenosphere.
• Lithosphere – is broken into plates that ride
on the asthenosphere. (rigid mantle and
crust)
• Asthenosphere – is the “plastic” mantle.
(movement mechanism for lithosphere)
Drifting Continents
What caused people to consider that the
earth’s surface might be moving?
• Shape of South America and Africa
• Fossil evidence
• And much later…bands of reverse
magnetism in igneous rock on the ocean
floor.
Principles of Plate Tectonics
• The lithospheric plates are made of both
continental and oceanic crust, and cover the
entire earth surface.
• As plates move…so do the continents.
• Plate tectonics explains how new crust
forms and why earthquakes and volcanoes
are located in concentrated belts.
Evidence of Plate Tectonics
Evidence that we use today to observe plate
tectonics for minor crustal movement:
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Folds
Faults
Displaced fossils
Displaced strata
Evidence
• Folds – occur when rocks are deformed slowly
due to pressure.
Evidence
• Faults – happen when rocks are deformed
quickly due to pressure of tension.
Evidence
Displaced fossils –
• Marine fossils found high above sea level.
• Uplift of land (Grand Canyon, Alps, Rocky
Mountains
• Fossils of land organisms (plants and
animals) found below sea level.
• Subsidence of land (Gulf of Mexico)
Evidence
Displaced Strata – Sedimentary rocks found
above sea level due to uplift.
Evidence
Evidence that we use today to observe plate
tectonics for major crustal movement:
• Movement of plates (G.P.S.)
• Mountain Ranges
• Rift Zones
How does it work?
How can we assume that the processes that
occur today…formed the “evidence” that
we see from events of the past?
Uniformitarianism – the laws of nature have
not changed (ex. Gravity, weathering,
erosion and deposition) …”the present is
the key to the past”
Start of Plate Tectonics
Where did the theory of plate tectonics start?
• Observations
1. Similarities in the shape of continents.
2. Mesosaurus (lived about 270 m.y.a) fossils are found
in both South America and Africa.
• Theory of Continental Drift – proposed by Alfred
Wegner in 1912
He used observations above to propose plate
movement
Downfalls of his theory…was he could never
explain the mechanism for movement.
5 Pieces of Evidence to explain plate tectonics:
1. Fit of Continents (modified from old theory of
continental drift ~ Wegner)
2. Glaciation – evidence of glaciation in lower latitudes.
(ex. Scratches, glacial erratics, till)
3. Correlation of rocks and mountain ranges on both sides
of the Atlantic Ocean.
4. Age of ocean floor (Mid-Ocean Ridge)
5. Magnetic Reversals (Mid-Ocean Ridge)
Theory of Plate Tectonics
Plate Boundaries – tectonics plates touch at
locations called plate boundaries. There
are 3 main types of plate boundaries,
based on how the plates are moving in
relation to each other.
1. Convergent boundaries
2. Divergent boundaries
3. Transform boundaries
Convergent Boundaries
•
Locations where two plates are moving towards
one another.
• There are 3 specific types of convergent
boundaries
1. Continent – Continent boundary
2. Ocean – Ocean boundary
3. Continent – Ocean boundary
Continent – Continent
• A location where two plates of continental
crust are coming together.
Ocean - Ocean
• A location where two plates of oceanic crust
are coming together.
Continent - Ocean
• A location where continental crust is
colliding with oceanic crust
Subduction
• Why does one plate usually sink below
another plate?
Subduction
• The more dense plate sinks below the less
dense plate.
Continental crust = 2.8 g/cm3
Oceanic crust = 3.0 g/cm3
Divergent Boundaries
• A location where two plates are moving apart, due to
magma rising up and creating new crust. Sea-floor
spreading.
Transform boundary
• A location where two plates are sliding past
one another.
Animations of Plate Boundaries
http://www2.wwnorton.com/college/geo/egeo/
flash/2_6.swf
http://www2.wwnorton.com/college/geo/egeo/
flash/2_6.swf
Causes of Plate Motion
• Still much to be discovered in this area, but
we have a better explanation of a
mechanism for plate movement.
Mantle Convection
Convection explained…
Convection: the transfer of thermal energy by
the movement of heated matter.
• Heated matter will expand.
• Matter that expands becomes less dense.
• Less dense matter rises in more dense
matter.
Convection Current
Matter is heated = density is decreased = matter rises
Matter is cooled = density is increased = matter sinks
Mantle Convection
• The mantle is heated unevenly by deeper
layers of the asthenosphere.
• As the lower parts of the mantle are heated
they start to rise.
• As they rise, they enter colder regions of the
mantle and begin to cool.
• This cooler mantle material will begin to
sink…thus creating a convection cell.
Mantle Convection
• These convection cells in the mantle can
move the lithosphere (plates) in several
ways.
Types of Movement
Push – the weight of the uplifted rock at the
ridge will push the crust away from the
ridge.
Pull – the weight of a subducted plate pulls
the rest of the plate down into the Earth.