Transcript The Theory of Continental Drift

Plate Tectonics
Mrs. Griffin
Drifting Continents
• Average human lifetimedrift is slow
• On a geologic time scaleWhoa Nelly!
• South Africa is moving
away from Africa- 2 to 3
cm/y
• Hawaii volcanic islands are
migrating NW ~8 to 9 cm/y
• What’s the cause?
Early Observations
• Eduard Suess- single
land mass know as
Gondwanaland.
• 1st hypothesis taken
seriously was proposed
by Alfred Wegener
– Called his hypothesis
continental drift
– 200 mya supercontinent
Pangaea began to break
apart
Evidence from Rock Formation
• If mountains were once formed together
then fractured……
• Wegener hypothesized similar rocks types
must exist on opposite sides of the Atlantic
Ocean.
– Appalachian Mountains
– Greenland and Europe
Evidence from Fossils
• Wegener also
gathered evidence
from fossils.
• Land dwellers could
not have swam the
great distances that
exist today.
• Glossopteris fossils
have been found on
many continents
Ancient Climatic Evidence
• What types of rocks
• Glacial deposits in Africa,
provide clues to the past?
India, Australia, and
South America.
– Sedimentary
– Continents once near the
• Wegener also found
south pole?
evidence of vast climatic
changes on some
continents.
– Coal deposits in Antarctica,
forms from dead swamp
plants…..???
– Was Antarctica close to the
equator?
A Rejected Hypothesis
• Wegener's theory was not accepted by scientists
due to two major flaws, what were they?
– What is the driving force behind the movement of the
continents?
– How were the continents moving without fracturing?
• Wegener died in 1930, and it wasn’t until the
early 1960’s that new evidence was revealed
that could explain how and why. Where do you
think this evidence was found?
– Seafloor
Alfred Wegener
Review
• The theory of continental drift was proposed by which
individual?
• This supercontinent means “all the Earth.”
• What type of evidence did the above individual base his
hypothesis on?
• What were the two “flaws” regarding the theory of
continental drift?
• Where was the evidence found to explain the two
missing flaws?
Seafloor Spreading
• Help from technology:
– Development of echo-sounding methods.
• Sonar, uses sound waves to measure water depth.
• Magnetometer- a device that can detect
small changes in magnetic fields, towed by
ships, records the magnetic field strength
in rocks on the ocean floor.
– Used to make maps of the seafloor.
Sea-floor Topography
Ocean Floor Topography
• What did these maps reveal?
– Underwater mountain chains called ocean
ridges.
• Form the longest continuous mountain ranges on
Earth; earthquakes and volcanoes are common
among these areas.
– Maps also revealed deep-sea trenches,
counterparts to ocean ridges.
• Mariana Trench is over 11 km deep.
Ocean Rocks and Sediments
• Rock samples taken from areas near
ocean ridges are younger than samples
taken from areas near deep-sea trenches.
• Ocean floor rocks are much younger than
continental rocks. Why is this so?
Seafloor Spreading
• Topographic, sedimentary, age, and
magnetic data combined laid the
foundation for Harry Hess to propose his
theory:
– Seafloor spreading: new ocean crust is
formed at ocean ridges and destroyed at
deep-sea trenches.
Seafloor Spreading
• Seafloor spreading
provides the answer
to the how question.
• Continents are merely
passengers that ride
with ocean crust as it
slowly moves away
from ocean ridges.
The Theory of Plate Tectonics
• Why are some regions of Earth dotted with
many active volcanoes while other regions
have none? Why do earthquakes occur
frequently in certain areas?
– The theory of plate tectonics: Earth’s crust
and rigid upper mantle are broken into
enormous slabs called plates.
• They move in different directions and at
different rates.
Earth’s Tectonic Plates
Plate Boundaries
• Tectonic plates interact at places called
plate boundaries which include divergent,
convergent, and transform boundaries.
Plate Boundaries
• Places where two tectonic
plates are moving apart are
called divergent boundaries.
– Found mostly on the
seafloor, form ocean
ridges.
– Some form on continents;
when crust separates, the
stretched crust forms a
long narrow depression
called a rift valley.
Convergent Boundaries
• Places where two tectonic
plates are moving toward each
other are convergent
boundaries, 3 types.
– Oceanic-oceanic
– Oceanic-continental
– Continental-continental
• In subduction zones- one of
the two plates descends
beneath the other ->this
creates a deep-sea trench.
– Recycling of the plates begins
Transform Boundaries
• Where two plates
slide horizontally past
one another.
– Here crust is only
deformed or fractured.
– Characterized by long
faults.
– Rarely occurs on
continents…..except
for where?
• San Andreas Fault
Causes of Plate Motions
• Convection currents are thought to be the driving
mechanism of plate movements.
• Convection: the transfer of thermal energy by the
movement of heated matter.
– Convection currents in the asthenosphere are set in
motion by the transfer of energy b/w Earth’s hot
interior and its cooler exterior.
Mantle Convection
• How is mantle convection
related to movement of plates?
• Hypothesized: ridge
push/slab pull.
– Ridge push: formation of
ocean ridge causes
asthenosphere to rise; the
weight of uplifted trench
pushes plate.
– Slab Pull: sinking region of
a mantle convection could
be pulled down at
subduction zones.