Transcript Plate Tectonics Theory

Plate Tectonics Theory
Chapter 9 Notes
Continental Drift
• German Climatologist
and Geophysicist
named Alfred
Wegener.
• Proposed the idea of
continental drift.
• Wrote a book in 1912
called the Origin of
Continents and
Oceans.
Matching a Puzzle
• Others before him
noticed the patterns of
the continental plates.
• Abraham Ortelius (1596)
• Frances Bacon (1625)
• Benjamin Franklin (?)
• Antonio Snider-Pelligrini
(1858) picture above
Pangaea
• Wegener proposed
that at one time in the
distant past, all of the
continental plates
converged into one
supercontinent.
• He called this
supercontinent
Pangaea.
Evidence for Continental Drift
• Matching fossils found on adjacent
continents across the ocean.
• Matching rock types found on adjacent
continents across the ocean.
• Erosion and depositional features found
on adjacent continents across the ocean.
• Magnetic patterns found on the ocean
floor (not discovered until the 1960s).
• Hawaiian Islands produced by Hot Spots.
Matching Fossils
• Mesosaurus – Extinct
crocodile-like reptile.
• Glossopteris – Extinct
seed-fern type plant.
• Lystrosaurus – Extinct
dinosaur.
• Cynognathus – Extinct
dinosaur.
Matching Rock Types
• Matching Rock Types found on adjacent
continents across the ocean.
• Appalachian and Caledonian Mountains
Erosion and Deposition
• Deposits that resemble
glacial till found on
separate continents.
• Large striations found
in solid bedrock.
• This could also be
explained by erosion
and deposition caused
by submarine
landslides.
Magnetic Reversals
• In the 1960’s, scientists discovered that basalt
on the ocean bottom shows that the magnetic
field has reversed polarity in the past.
• It is not understood how this happens and
what affect it might have on living organisms.
• It does appear to be very strong evidence that
the bottom of the ocean has spread.
Hawaiian Hot Spots
Hawaiian Hot Spots
Hawaiian Hot Spots
Geologic Time
• Uniformitarianism vs. Catastrophism
• Current seafloor spreading is moving at about a few
centimeters per year. That is about the speed at which
your fingernails grow.
• So where is all that lithosphere going?
• Subduction and Mountain building.
• The current rates of erosion and uplift vary depending on
the mountain range.
• Some mountains are growing and some are slowly
shrinking due to erosion.
• Appalachians, Alps, Rockies, Andes, Himalayas
Modern Plate Tectonics
• In spite of the numerous evidences, most
scientists rejected Wegener’s idea of
seafloor spreading until the 1960’s.
• With the aid of sonar technology and the
ability to map the ocean floor, substantial
evidence was able to be given.
Plate Boundaries
• The earth’s lithosphere (crust) is made up
of several plates which rest upon a hot
molten magma called the asthenosphere.
• Wherever these plates come in contact
with another plate is called a boundary.
• There are three types of plate boundaries.
1. Divergent Boundaries: Plates move apart
2. Convergent Boundaries: Plates come together
3. Transform Boundaries: Plates slide past each other
Plate Boundaries
• At Divergent boundaries, new lithosphere
is rising up and being produced.
• At Convergent boundaries, old lithosphere
is either subducted or compressed to build
thicker regions of lithosphere (mountains)
• At transform boundaries, two plates slide
past one another. (San Andreas Fault)
Divergent Plate Boundaries
• Divergent plate
boundaries are found
on both oceanic crust
and continental crust.
• Sometimes a rift
valley that begins on
land can develop into
an ocean ridge on the
bottom of the ocean.
• New lithosphere
expands and forms a
ridge. Once it cools
down it shrinks.
Convergent Plate Boundaries
• At Convergent Plate Boundaries, one plate
slides and descends downward toward the
mantle and eventually begins to melt.
• Some of the lithosphere gets compacted into
the crust, causing the plates to thicken.
• This process is called accretion.
• This thickening of the crust creates a
difference in topography at the surface.
• Lithosphere that descends gets recycled.
Convergent Plate Boundaries
• Three types of convergent plate boundaries.
1. Ocean– Continental
2. Ocean–Ocean
3. Continental-Continental
•Differences in density is a major factor
contributing to subduction of lithosphere.
•As dense lithosphere comes in contact with
buoyant lithosphere, it descends below it.
•Where oceanic crust descends below
continental crust you find an ocean trench.
Ocean–Continental Convergence
• Oceanic lithosphere is made of dense basalt.
• Dense basalt descends below buoyant continental
crust rich in silicates (quartz, feldspar, granite etc.)
• Continental Volcanic Arcs form adjacent to
Ocean-Continental Convergent Plate Boundaries.
Oceanic-Oceanic Convergence
• This plate boundary forms volcanic chains
on the bottom of the ocean floor located
adjacent to the plate boundary.
• When the submarine volcanoes reach the
ocean surface, they form island chains
called volcanic island arcs.
Continental-Continental
Convergence
• Sometimes two continental plates collide.
• Continental plates resist subduction due to their
low density, which makes them very buoyant.
• The crust becomes very thick.
• Isostasy and isostatic adjustment and
equilibrium takes place.
• This creates uplift and builds the mountains.
• The largest mountains of the world were formed
in this way. (Himalayas, Everest)
Isostatic rebound (readjustment)
Continental-Continental
Convergence
• Example: The collision of India and Asia to
form the Himalayan mountains.
Transform Plate Boundaries
• Plates slide past one another horizontally.
• Most of these are found along divergent plate
boundaries in the ocean (mid-ocean ridge).
• Some are found on the continents, like the San
Andreas Fault in California.
Mechanism for Plate Tectonics
• One of the main reasons why scientists
remained skeptical of plate tectonics until
the 1960s was because of a lack of a
mechanism that could explain it.
Convection Currents
• The earth’s mantle is
• Two forces that drive
solid, but behaves like
plate movement are the
a ductile fluid.
ridge-push and slab-pull
• Uneven heat
throughout the earth’s
mantle causes
convection currents.
• It is believed that
these convection
currents cause the
earth’s plates to
move.