Transcript Document

CONTINENTAL DRIFT
http://www.tectonics.caltech.edu/movi
es/outreach/sumatra/pangaea.mov
The Story of Alfred L.
Wegener
 There once was a German
Geologist who noticed similar
rocks & fossil remains were
found on continents which
seemed to fit together
 He called this “super” continent
called Pangaea
 In 1912 Wegener
published the
first version &
died defending
his theory
PANGAEA
the large landmass that included all of Earth’s
present day continents
WEGENERS CONTINENTAL
DRIFT
 Alfred Wegener in
the early 1900’s
proposed the
hypothesis that
continents were
once joined together
in a single large land
mass he called
Pangaea (meaning
“all land” in Greek).
WEGENERS CONTINENTAL
DRIFT
He proposed that
Pangaea had split
apart and the
continents had
moved gradually to
their present
positions - a
process that
became known as
continental drift.
Pangaea about 200 million years ago, before it began
breaking up. Wegener named the southern portion of
Pangaea Gondwana, and the northern portion Laurasia.
The continents about 70 million years ago. Notice that
the breakup of Pangea formed the Atlantic Ocean.
India’s eventual collision with Eurasia would form the
Himalayan Mountains.
The position of the continents today. The continents are
still slowly moving, at about the speed your fingernails
grow. Satellite measurements have confirmed that every
year the Atlantic Ocean gets a few inches wider!
Support for Continental Drift
1. SHAPE OF THE CONTINENTS
Continents fit together like puzzle pieces
Support for Continental Drift
1. SHAPE OF THE CONTINENTS
 Wegener was not the only
one to notice the fit of the
continents.
 1858: Antonio Pelligrini
depicted Africa and South
America connected.
 He was one of the first
scientists to publish maps
depicting the apparent fit
of the continents.
Support for Continental Drift
2. FOSSIL EVIDENCE
SAME FOSSILS: DIFFERENT CONTINENTS
Mesosaurus was incapable of swimming across a large
ocean.
Evidence for Continental Drift
 Lystrosaurus: mammal
like reptile, herbivore,
about the size of a
small dog.
 Mesosaurus:
freshwater reptile,
carnivore.
 Glossopteris: extinct
seed fern was about
12 ft (3.6 m) tall.
 Cynognathus: metrelong predator of the
Lower Triassic. It was
one of the more
mammal-like of the
"mammal-like reptiles",
Support for Continental Drift
3. ROCK EVIDENCE – Mountain ranges
 SAME ROCK:
DIFFERENT RANGE
Existing mountain
ranges separated by
vast oceans contain
rocks of identical
mineral content.
 A prime example are
the Appalachian
Mountains in the
eastern U.S and the
Caledonian Mountains
in the British Isles.
Support for Continental Drift
3. ROCK EVIDENCE – Mountain ranges
Rock sequences
(meaning he looked at
the order of rock
layers) in South
America, Africa, India,
Antarctica, and
Australia show
remarkable
similarities.
Wegener showed that
the same three layers
occur at each of these
places.
Picture from
http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/p
art4.html
Support for Continental Drift
4. GLACIAL SCARS/
CLIMATE CHANGE
SAME SCARS:
DIFFERENT CONTINENTS
• The land area
that shows
evidence of this
glaciation now
lies near the
equator in a
subtropical or
tropical climate.
Support for Continental Drift
5. LOCATION OF COAL
tropical
DEPOSITS/ CLIMATE • Large
swamps existed
CHANGE
during the same
time in the
Northern
Hemisphere with
lush vegetation
which eventually
became the coal
fields of the
eastern U.S.,
Europe, and
Siberia.
Support for Continental Drift
5. LOCATION OF COAL DEPOSITS/
CLIMATE CHANGE
Coal deposits have been found in temperate and
polar regions; however, coal is formed in tropical
regions.
CONTINENTAL DRIFT IN DOUBT
Why didn’t people believe in
continental drift?
People couldn’t image how the earth
could be millions of years old
People couldn’t image a force great
enough to move the continents
Wind and currents could possibly
move fossils
Theory was not accepted by
scientists
CONTINENTAL DRIFT IN DOUBT
 Everyone agreed that Wegener’s evidence
was compelling. But wouldn’t we feel the
movement?
 Also, wouldn’t there be evidence to show
that the continents were still moving
today?
 Wegener was a meteorologist and his
theory was not well accepted. (He died on
an expedition in Greenland collecting ice
samples)
 BUT by the 1960’s evidence would prove
continental drift is TRUE and…. The story
continues (as does all good science!)
Seafloor Spreading
 One reason scientists
had a hard time with
Wegener’s theory is
that there was no
mechanism for the
continents motion.
 Then comes along
seafloor spreading!
Seafloor Spreading
Picture from USGS
http://pubs.usgs.gov/gip/dynamic/HHH.html
 In the 1960’s, a scientist
named Henry Hess made
a discovery that would
vindicate Wegner.
 Using new technology,
radar, he discovered that
the seafloor has both
trenches and mid-ocean
ridges.
 Henry Hess proposed the
sea-floor spreading
theory.
Seafloor
Spreading
 Hess proposed that
hot, less dense
material below Earth’s
crust rises toward the
surface at the midocean ridges.
 Then, it flows
sideways, carrying the
seafloor away from the
ridge in both
directions.
Picture from http://library.thinkquest.org/17457/platetectonics/4.php
Seafloor Spreading
 As the seafloor
spreads apart at a
mid-ocean ridge,
new seafloor is
created.
 The older seafloor
moves away from
the ridge in
opposite
directions.
 This helped explain how the crust could
move—something that the continental drift
hypothesis could not do.
Evidence for Spreading
 In 1968, scientists aboard the
research ship Glomar
Challenger began gathering
information about the rocks on
the seafloor.
 Scientists found that the
youngest rocks are located at
the mid-ocean ridges.
Mechanism for
Plate Tectonics
 Seafloor Spreading
provided insight to the
mechanism for how the
continents moved.
 The magma which
pushes up at the midocean ridge provides
the new land pushing
the plates, and the
subduction zones
The mechanism that
gobble up the land on made continental drift
the other side of the
was convection
plates.
currents!
Picture from
http://library.thinkquest.org/17457/platetectonics/2.php
Plate Tectonic Theory
 Both Hess’s discovery and
Wegner’s continental drift
theory combined into what
scientists now call the
Plate Tectonic Theory.
 Theory of plate tectonics:
• The Earth’s crust and part of
the upper mantle are broken
into sections, called plates
which move on a plastic-like
layer of the mantle
Tectonic plates move. The
boundaries created where
they meet are called
FAULTS
Three types of plate
boundary
 Divergent
 Convergent
 Transform
There are three types of plate
movement boundaries
1. Divergent
 Also known as seafloor
spreading
 Plates are separating
from each other as a
new land mass forms
 This is seen at midocean ridges and rifts
 Plate separation is a
slow process. For
example, divergence
along the Mid Atlantic
ridge causes the
Atlantic Ocean to widen
at only about 2
centimeters per year.
Picture from www.geology.com
Author Hobart M. King
Rift Valleys are areas
where the crust is
pulling away slowly.
There are three types of plate
movement boundaries
2. Convergent
 Two continental
plates collide. &
crumple the edges of
the plates & form
mountains.
 We can see the end
result of the collision
between the Indian &
Eurasian plates
which are the
Himalayan
Mountains.
Picture from USGS
Picture from www.geology.com Author Hobart M. King
Continent-Oceanic Crust
Collision
 SUBDUCTION is another type of a
convergent plate movement.
Subduction
 Oceanic crust and
lithosphere subducts
underneath the
continental crust and
lithosphere
 Oceanic lithosphere
heats and dehydrates
as it subsides
 The melt rises forming
volcanism
 E.g. The Andes
Ocean-Ocean Plate
Collision
 When two oceanic plates collide, one runs
over the other which causes it to sink into
the mantle forming a subduction zone.
 The subducting plate is bent downward to
form a very deep depression in the ocean
floor called a trench.
 The worlds deepest parts of the ocean are
found along trenches.
 E.g. The Mariana Trench is 11 km deep!
There are three types of plate
movement boundaries
3. Transform
 Two plates slide
past each other
 Example: San
Andreas Fault in
California
Picture from www.geology.com
Author Hobart M. King
Divergent
Convergent
Transform
Plate Tectonic Theory
 Plate Tectonics explains
 Earthquakes
 Mountains
 Volcanoes