Download The Theory of Continental Drift

The Theory of
Continental Drift
Continental Drift Theory
Alfred Wegener proposed in 1912
that the continents had once been
joined together in a single
landmass and have drifted apart
since.
Wegener named this
supercontinent Pangaea.
Wegener’s theory was rejected by
scientists because he could not
explain what force pushes or pulls
continents.
Evidence To Support
• Continents look like
they could be part of
a giant jigsaw
puzzle
Evidence To Support
Distribution of Fossils
• Plant and animal
fossils found on
the coastlines of
different continents
Evidence To Support
• Same rock patterns
found in South
America, India, Africa,
Antarctica and
Australia
Sequence of Rocks
Evidence To Support
• Tropical plant remains
(coal deposits) found in
Antarctica
• Glaciation in Africa,
South America, India,
and Australia during the
same time
Ancient Climates
Plate Tectonics
What is Plate Tectonics
• The Earth’s crust and upper mantle
are broken into sections called plates
• Plates move around on top of the
mantle like rafts
What is the Lithosphere
Lithosphere??
• The crust and part of the upper
mantle = lithosphere
–100 km thick
–Less dense than the material below it
so it “floats”
What is the Asthenosphere
Asthenosphere?
?
• The plastic layer below the
lithosphere = asthenosphere
• The plates of the lithosphere float
on the asthenosphere
• Lithosphere :
broken in to 7 large
rigid pieces
1. African
2. North American
3. South American
4, Australian
5. Antarctic,
6. Pacific plates.
7. Eurasian
$
$Several minor plates also exist
( the Arabian, Nazca, and
Philippines plates )
2 Types of Plates
• Ocean plates - plates below the
oceans
• Continental plates - plates below
the continents
Questions...
•
•
•
•
What is the theory of plate tectonics?
What is the lithosphere?
What is the asthenosphere?
What is the connection between the
two?
• What are the two types of plates?
Plate Boundaries
Plate tectonics
The plates are all moving
in different directions
and at different speeds
Plates only move a few
centimeters each year,
so collisions are
very slow and last
millions of years
~2 to10 cm per year
(about the speed at which your fingernails grow)
grow
Mountains, earthquakes, and volcanoes form where plates collide.
Divergent Boundaries
• Boundary between two plates that
are moving apart or rifting
• RIFTING causes SEAFLOOR
SPREADING
Divergent Boundaries
Places where plates are coming apart
The Lithosphere is pulled
apart
Plates separate along that
boundary
A block between the faults
cracks and drops down into the soft, plastic interior
Divergent Boundaries
Places where plates are coming apart
A rift is formed
Magma seeps upward to fill
the cracks
New crust is now formed
along the boundary
Earthquakes occur along the faults
volcanoes form where the magma reaches the surface.
Features of Divergent Boundaries
•
•
•
Mid$
Mid$ocean ridges
Rift valleys
Fissure volcanoes
Seafloor spreading
Movement of two oceanic plates away from each other
Ocean floor spreading was first suggested
by Harry Hess and Robert Dietz in the 1960's.
Zone of divergence: where the oceanic
plates are moving away from each other
Seafloor spreading
Seafloor spreading
Mid$
Mid$ocean Ridge
Results in the formation of new oceanic
crust from magma that comes from within
the Earth's mantle along a a mid$
mid$ocean
ridge.
Mid Atlantic ridge
Rift valley:
valley: Deep crack that runs
along the top of this chain of
mountains This is where new ocean
floor is continuously created.
Divergence along the Mid
Atlantic ridge causes the Atlantic
Ocean to widen at only about 2
centimeters per year
Divergent Boundaries: Sea floor spreading
Mid Atlantic ridge
The Earth's longest mountain chain is
an underwater chain of mountains
47,000 miles long
Runs down the middle of the Atlantic
Ocean (surfacing at Iceland), around
Africa, through the Indian Ocean,
between Australia and Antarctica, and
north through the Pacific Ocean.
SEAFLOOR
SPREADING
• The image shows the
surface of the Earth
minus its water
• Notice the distinct
mountain chains
which circles the
world
• It wasn't until the
50's and 60's that
this detail became
known
subduction
New crust is continually being pushed away from divergent boundaries
(where sea$floor spreading occurs), increasing Earth's surface. But the
Earth isn't getting any bigger.
What happens, then, to keep the Earth the same size?
subduction
PLATES ARE DESTROYED
Subduction causes partial melting of both the ocean crust and mantle as
they slide past one another
Most of the subducting plate continues into the mantle, perhaps to
reappear much later at a distant divergent boundary.
Convergent Boundaries
* Places where plates crash or crunch together
Think about it like this :
2 plates colliding is like 2 cars crashing into each other
The front ends of cars fold and bend in a collision, so do
the "front ends" of colliding plates.
Convergent Boundaries
There are 3 types/
1) Ocean plate colliding with a less dense continental
plate
2) Ocean plate colliding with another ocean
plate
3) Continental plate colliding with another continental
plate
1) oceanic plate
continental plate
• Ocean plate is less dense continental plate
• Subduction Zone:
Zone: where the less dense plate
slides under the more dense plate
VOLCANOES occur at
subduction zones
subduction
Convergent: Places where plates crash or crunch together
The edge of this
continental plate has
folded into a huge
mountain range
The edge of the oceanic
plate has bent downward
and dug deep into the
Earth.= Trench
oceanic plate
continental plate
subduction
• All that folding and bending makes rock in
both plates break and slip, causing
earthquakes.
• As the edge of the oceanic plate digs into Earth's hot interior, some of
the rock in it melts.
•The
The melted rock rises up through the
continental plate, causing more
earthquakes on its way up, and forming
volcanic eruptions where it finally
reaches the surface.
Convergent Boundaries
Subduction
If the activity continues, the volcano may grow tall enough to breech the surface of the ocean
creating an island.
West coast of South
America :
Oceanic Nazca Plate
is crashing into the
continent of South
America.
The crash formed the
Andes Mountains,
the long string of
volcanoes along the
mountain crest,
and the deep trench
off the coast in the
Pacific Ocean.
Andes Mountains
2) oceanic plate
oceanic plate
• The less dense (older) plate slides under the
more dense plate creating a subduction zone
called a TRENCH
The plate with the older (cooler, more dense) crust descends
into the subduction zone. A chain of volcanic islands (island
arc) forms on the overriding plate.
Trenches often lie adjacent to chains of islands (island arcs)
formed by magma from the subducted plate.
Aleutian Islands
3) Continental plate
Continental plate
Have Collision Zones:
a place where folded and thrust faulted
mountains form.
Collisional bounderies
When two land masses meet, neither will slide under the other
They crumple and fold.
Some pieces of land are thrust
over or under other pieces
An ocean floor pushed toward a land
mass will always slide under the land
mass. This is because the land mass is more buoyant, or lighter,
than the ocean floor.
The result is a mountain range.
range.
Collisional bounderies
The highest mountains in the world
The Himalayas
All these mountains were
formed when two land masses came
together.
European Alps
Appalachian Mountains
Transform Fault
Boundaries
• Boundary between two plates that are sliding past
each other
EARTHQUAKES along faults
Transform Boundaries
plates slide past each other
Cause:
Linear valleys along the
boundary where rock
has been ground up by the sliding.
Stream beds that have been split in half and the two
halves have moved in opposite directions.
Transform Boundaries
San Andreas fault
The slice of California
to the west of the fault is slowly
moving north relative to the rest of
California
Motion along the fault is sideways
Los Angeles is creeping towards San Francisco at about 6 cm per
year.
In about ten million years, the two cities will be side by side!
San Andreas fault
The strongest and most famous earthquake along the San Andreas fault hit San Francisco in
1906. Many buildings were shaken to pieces by the quake, and much of the rest of the city
was destroyed by the fires that followed. More than 600 people died as a result of the quake
and fires.
Questions
1.
2.
3.
4.
5.
6.
What are the three types of boundaries?
What direction do plates go for each?
Which boundary has a subduction zone…what
occurs at a subduction zone?
What type of boundary is involved with seafloor
spreading?
What type of boundary is at the San Andreas
fault?
The Mid Atlantic Ridge is an example of what
boundary?
Causes of Plate
Tectonics
Convection Currents
• Hot magma in the Earth moves
toward the surface, cools, then sinks
again.
• Creates convection currents beneath
the plates that cause the plates to
move.
Questions...
1. What causes plates to move?
2. How is a convection current formed?