Transcript Chapter 10: Continental Drift and Plate Tectonics

CONTINENTAL DRIFT
AND PLATE TECTONICS
HISTORY OF CONTINENTAL DRIFT
• In German "die Verschiebung der Kontinente"
• As long as 400 years ago, map makers
noted that the continents’ shapes
looked to fit together like a puzzle.
• LOOK!
• Cartographer Abraham Ortelius in his work
Thesaurus Geographicus ... suggested that the
Americas were "torn away from Europe and
Africa ... by earthquakes and floods"
THE COMPLETED PUZZLE
THEORY OF CONTINENTAL DRIFT
• 1912: Alfred Wegener proposed that
continents move!
• Reaction by the scientific community• HAHAHAHAHAHA. ROFL - ARE YOU KIDDING?
WHY DID WEGENER THEORIZE
CONTINENTAL DRIFT?
#1 Shape of the continents
#2 Fossil clues:
• Wegener found similar fossils on different
continents
• The fossils that he found could probably not
have spread between continents
• Freshwater reptile fossil
• Plant fossil
• Similarly evolved species
• Why are these two significant? Would finding
a bird on two continents support continental
drift as well?
• #3 Climate clues
• Wegener found warm climate fossils
in the Arctic and cold climate fossils
near the equator.
• Warm weather fossils
• Evidence of glaciers
• #4 Rock clues
Wegener found similar strata (layers) of
rocks on different continents.
• Could this be a coincidence?
MATCHING ROCK LAYERS
• Pangaea – (“all land”) existed
about 250 million years ago and
began separating around 200
million years ago.
• Problem - Wegener’s theory proposed only
that the continents were once one
supercontinent and they had “moved” apart
but could not explain HOW.
• Things Wegener didn’t know:
• What the inside of the Earth looked like
• That the Earth was broken into plates
• That the location of earthquakes and
volcanoes shows plate boundaries
• Convection Currents
• Was his theory accepted by other
scientists?
• His theory did not become accepted
until the 1950’s/1960’s. Why?
• Wegener was appreciated
posthumously
• 1960’s: Harry Hess given credit for
providing some of the proof
necessary to support Wegener’s
theory
• Hess’s theory of seafloor spreading
• Believed that material moved up at midocean ridges and pushed material
outward
Mid-Ocean Ridges
Discovered in 1950’s
• Why Did Hess believe that the
seafloor was spreading?
•
The discovery of the midocean ridges
•
Core samples of ocean floor
showed that the age of
rocks INCREASED the
farther they were from midocean ridges
•
Core samples also showed
corresponding magnetic
reversals on each side of
the mid-ocean ridge
ROCK AGES
MAGNETIC REVERSALS
SO WHERE ARE WE AT THIS POINT?
• We believe the continents have moved.
• These findings provided proof for Hess’s theory and
further supported Wegener’s theory
• Theory of Plate Tectonics has still not been
developed.
• But no one still understands why or how the plates
move. We are still lacking the MECHANISM.
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• 1960’s Wilson and other scientists gave
mechanism to explain HOW the plates are
moved.
• Scientists proposed that the Earth’s
lithosphere is broken into plates, and that
CONVECTION CURRENTS in the plastic
asthenosphere moved the lithospheric plates
above.
• All of this information, plus LOTS more,
developed into our current theory of Plate
Tectonics.
MAPPING VOLCANOES
MAPPING EARTHQUAKES
PLATE BOUNDARIES:
• This is where two or more
lithospheric plates meet
• Called faults or boundaries
• Lots of tectonic activity (i.e.
volcanoes and earthquakes) occurs at
these places
• Three types:
CONVERGENT BOUNDARIES
• Two plates colliding (moving together)
• If the plates are ocean-continental:
Subduction zones, mountains, volcanoes
and ocean trenches will form
• Oceanic crust is destroyed here
• More dense (basalt vs. granite)
• Examples are Ring of Fire and Andes Mts
• If the plates are both continental
• No subduction, only mountain building
• Himalayas are example
• Crust is neither created nor destroyed
“Ring of Fire”
ANDES MOUNTAINS
HIMALAYAN MOUNTAINS
DIVERGENT BOUNDARIES
TWO PLATES MOVING APART
• If the plate is an ocean plate:
• Forms mid-ocean ridge
• Mid-Atlantic Ridge is example
• If the plate is continental:
• Forms rift valleys
• Great Rift Valley is example
• New rock is created here
MID-ATLANTIC RIDGE
GREAT RIFT VALLEY
TRANSFORM BOUNDARIES
A.K.A. STRIKE-SLIP BOUNDARIES
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Two plates sliding/scraping past each other
Earthquakes are common
Crust is neither created nor destroyed
San Andreas Fault and Haiti our examples
SAN ANDREAS FAULT
DOES NOT INVOLVE THE JUAN DE FUCA PLATE
HAITI 2010
• Where would each form?
• Volcano
• Seafloor Spreading
• Mountains
• Subduction Zone
• Rift Valley
• Name two places where each type of boundary
would form using your text.
Plate Tectonics – late 1960’s
Post Harry Hess’ Seafloor Spreading
• Plate tectonics (from the Latin tectonicus, from
"pertaining to building”) is a scientific theory that
describes the large-scale motion of Earth's
lithosphere on a molten asthenosphere, due to
convection currents caused by the transfer of
heat from the Earth’s core.
• Plate motions range up to a typical 10–
40 mm/year (Mid-Atlantic Ridge; about as fast as
fingernails grow), to about 160 mm/year (Nazca
Plate; about as fast as hair grows).
WHY DO THE PLATES MOVE?
• Solid lithospheric plates move because the
plastic asthenosphere is moving.
• Radioactive reactions in the core heat
magma deep in the Earth which becomes
less dense and rises.
• As magma cools it becomes more dense,
sinks, then reheats, forming CONVECTION
CURRENTS (50 million years, though deeper
convection can be closer to 200 million years).
MEASURING PLATE MOVEMENT
• Scientists NOW use lasers, GPS and satellites to
measure plate movement over time.
How far would you have to drill through
the lithosphere to get to the
asthenosphere?
7 – 70 km (4 – 45 miles)