Transcript Convergent boundaries

9.1 Continental Drift
Wegener: Early Observations

German Scientist Alfred
Wegener (1912)

Continents had once been
joined to form a single
super continent
Pangaea (‘all land’)

Scientific hypothesis
based on:
1.
Fossil evidence
2.
Rock & mineral
formations
3. Continental puzzle
4. Ancient climates
1. Fossil Evidence: Reptiles

Lystrosaurus:
 Early Triassic Period
(250 mya)
 Land Animal
 Antarctica, India, S
Africa

Cynognathus:
 Late Triassic Period
 “Mammal-like reptile”
 Land Animal
 S Africa, South America,
China, Antarctica
1. Evidence from Fossils
2. Evidence from Rock
Formations

Similar rock
types on opposite
sides of the
Atlantic Ocean.

Appalachian
Mountains in the
US identical to
rock in
Greenland and
Europe.
3. Continental Puzzle

Similarity
between
coastlines on
opposite sides of
South Atlantic
Ocean.

Did not take
erosion into
account
Shows depth of 900 m; brown represents areas of overlap
4. Climate Evidence: Ferns
& Coal



Certain climates
support different
fern types
Land connections
were proved by
same fossils on
different continents
Coal in Northern
US originated in an
equatorial climate
but drifted North
over millions of
years
4. Climate Evidence: Glaciers
• Glacial deposits showing
that between 220-300 mya
ice sheets covered large
areas of S Hemisphere
• Glacial till in S Africa,
S America, India &
Australia
• Below till scratched
bedrock carved by ice
from sea to land
unusual & supports
continental drift
• Glaciation areas lie near
equator
(subtropical/tropical)
Which was NOT a type of evidence
Wegener used to support the theory
of continental drift?
a) rock and mineral formations
b) climate evidence
c) land shape
d) vegetation species
e) fossil evidence
Be prepared to support your choice as to
why these pieces of evidence are used to
support the theory of continental drift.
Why wasn’t it accepted?
 Problems with his
theory:
• Couldn’t explain
how large land
masses were
pushed such great
distances
• Convection
currents!
Accepted Today because:
• Know that asthenosphere (lower mantle)
has convection currents
• Currents push magma which then pushes
plates
• Plates move at different rates
• Move a few centimeters per year (roughly 5)
• Not all pieces of plate move at same rate
or same direction as the plate in general!
How fast do plates move
relative to each other?
a. millimeters per day
b. centimeters per year
c. meters per year
d. centimeters per day
9.2/9.3 Plate Tectonics &
Actions @ Boundaries

Uppermost mantle, along with the overlying crust, behaves
as a strong, rigid layer  lithosphere (divided into plates)

Do plates stay the same size and shape?

Asthenosphere weaker region of mantle

Plates include continents and large areas of the seafloor

Wegener thought that continents moved through the ocean
floor not with it.

Plate movement driven by convection currents within
asthenosphere
Plate Movement
 Earth’s crust: broken
into about 12 major
plates
 Plates move several cm
each year
 Asthenosphere drives
the plate
•
•
Slow moving layer
Convection currents
 Plates can be either:
•
Oceanic: thinner but
MORE dense (sink)
•
Continental: thicker
but LESS dense (rise)
Types of Plate Boundaries
Convergent boundaries :
Compression forces push
plates together
Divergent boundaries:
Tension forces move plates
apart
Transform boundaries:
Shearing forces cause
plates to slide against each
other (opposite directions
or same direction @
different rates)
Divergent Plate boundaries
Processes:
Hot magma rises to fill the 'gap' creating:
new crust and mountain ranges
forms the mid-ocean ridge.
Divergence
Animation
Both earthquakes and volcanoes occur at this type of
boundary.
Rising magma can build up above ocean surface to create a
volcanic island (for example Surtsey, Iceland).
(ex. North American and Eurasian Plate
form the Mid-Atlantic Ridge)
Continental Plates
Diverging
3 different situations for convergent boundaries…
a.
b.
c.
Convergent Boundary Type 1
Continental—Oceanic: denser oceanic crust
subducts under continental crust.
•Once oceanic plate melts as it is driven down, less dense
material punches thru continental plate to create volcanoes
or mountains on edge of continent
•Forms a subduction zone and oceanic trench
•Earthquakes occur as plate is subducted.
Ex: Andes Mtns and Peru-Chile trench
EXAMPLES OF OCEANIC-CONTINENTAL
CONVERGENT BOUNDARIES
1. CASCADE RANGE
(Northwestern US)
2. ANDES MOUNTAINS- W.
coast of S. America.

Oceanic—Oceanic: Older and more dense
plate sinks below the other.
Ex. Kuril Islands and Marianas Trench: plunges deeper into the
Earth's interior (~11,000 m) than Mt Everest, the world's tallest
mountain, rises above sea level (~8,854 m).
EXAMPLES OF OCEANIC-OCEANIC CONVERGENT
BOUNDARIES
1. Aleutian Islands, Alaska
EXAMPLES OF OCEANIC-OCEANIC CONVERGENT
BOUNDARIES
2. Japan
c. Continental – Continental –
________
OLDER and more ________
DENSE plate sinks below
the other.
Cont-Cont Animation
EXAMPLES OF CONTINENTAL-CONTINENTAL
CONVERGENT BOUNDARIES
HIMILAYAN MOUNTAINS
(like MT EVEREST and K2)
The 10 highest
mountains in the
world are found here
* Convergent Boundaries create ____________
VOLCANOES
and ________________,
EARTHQUAKES especially around the
“RING OF FIRE”
_________
PACIFIC Ocean in the _______________.
TRANSFORM boundary:
3. ___________________
• Occurs where two plates slide past each
other horizontally- along a fault
Fault = a break in a body of rock. One
block slides relative to the other (lots of
friction)
FAULT
TRANSFORM FAULT BOUNDARY
What do you see? How did this happen?
EXAMPLE OF A TRANSFORM FAULT BOUNDARY
San Andreas FaultCalifornia/Mexico
A Review of Earth’s Interior and Plate Tectonics Video
Divergent
Continental crust
 rift valley
Convergent
Cont/Cont. plates 
mountain range
Transform
Plates move against
each other.
Stress builds up…
Stress is released
earthquake!
Oceanic crust  midocean ridge
Oceanic/Oceanic or
Oceanic/Continental
subduction
What type of boundary is
shown?
a. an oceanic ridge
b. a continental-continental boundary
c. a transform boundary
d. an oceanic-continental boundary
9.4 Testing Plate Tectonics

Several pieces of evidence discovered after
hundreds of years of testing the theory of plate
tectonics
1. Paleomagnetism: symmetrical bands of
magnetized rock on ocean floor/volcanic
regions
2. Earthquake Patterns: deep-focus
earthquakes & ocean trenches
3. Ocean Drilling: age of oceanic crust
4. Hot Spots
1. Magnetism & Seafloor

Paleomagnetism: natural
remnant magnetism in rock bodies
 Permanent magnetization
acquired by rock that can be used
to determine the location of the
magnetic poles ay the time it
became magnetized.

Certain rocks contain iron-rich
minerals (ex: magnetite):
 Heated to certain temp.
minerals lose their magnetism
 Cooled to certain temp.
magnetized in direction parallel to
existing magnetic field
 Similar to a compass needle!
Polarity
Magnetism & Seafloor (cont)
Polarity
•
But the Earth's magnetic
field reverses every
several thousand years
Creates
new material on
the sea floor which is
oriented magnetically
opposite the older
material.
•
Flow changes→
magnetic reversal→
compass points S
Magnetism & Seafloor (cont)
•
Oceanic crust contains basaltic rock
-contains large amts of Fe-bearing minerals

So it’s MAGNETIC!
•This and the discovery of
magnetic reversal led to
the understanding that we
can study rock samples far
from ridge and understand
what the core was like in
magnetic polarity millions
of years ago.
Magnetism & Seafloor (cont)
•
Ocean floor shows record of past reversals
•
•
→ imaginary “strips” along seafloor
Magnetometers are used to detect the “strips”
Found the “strips” are mirror images on either
side of ridge→ symmetrical!! WOW!
Magnetic Seafloor Symmetry
2. Earthquake Patterns

Close link between deep-focus earthquakes and ocean
trenches



When the depths of earthquake foci and their locations within
the trench systems are plotted a pattern emerges
Most shallow focus
EQ occur within
or adj. to trench
Int. or deep focus
occur toward
mainland
3. Age of Rock (Ocean Drilling)
• Rock NEAR ocean ridge= YOUNGER
• Rock FAR from ocean ridge= OLDER
• Oceanic rocks are relatively young (180 million years)
compared to continental rocks (3.8 billion years) old
• because oceanic is recycled (or “destroyed”) @ subduction zones
• Thickness increases with distance from ridge
Isochron
• Isochron→ imaginary line on map showing
points that have same age (formed @ same time)
Approximately how long
did the Gauss epoch last?
a. 5 million years
b. 3 million years
c. 1 million years
d. 100,000 years
Which epoch saw the most
fluctuation between normal and
reverse polarity?
a. Gauss
b. Matuyama
c. Gilbert
d. Brunhes
Which letter on the map
shows the oldest seafloor?
a) A
b) B
c) C
d) D
Be prepared to support why an isochron map provides evidence
for seafloor spreading?
4. Hot Spots
What is special about Hawaii?
Hot Spot
Animation
9.5 Mechanisms of Plate
Motion

Causes of plate
motion:
1. Slab-Pull and
Ridge-Push
2. Mantle Convection
1. Slab-Pull/Ridge-Push

•
Divergent – new crust forming at the ocean ridge
pushes older crust to the sides toward trenches
(called ridge push)
Convergent – older, thicker crust is pushed to the
trenches where it subducts
•
The overlying weight causes the plate to pull the
rest of it down (called slab pull)
2. Convection Currents
•
Believed to be cause of plate motion
•
Transfer thermal energy from warmer  cooler
regions of matter
•
Thermal energy comes from radioactive decay
of the mantle itself and the core
In which layer of Earth do
convection currents occur?
a) lithosphere
b) asthenosphere
c) crust
d) core
Be prepared to support your choice as to why convection currents
drive plate movement.
Which is modeled by the
water movement?
Cold Water
a. subduction
b. continental drift
c. magnetic reversal
d. mantle convection
Which is not a force causing
plates to move?
a. ridge push
b. slab pull
c. volcanism
d. convection