Download Chapter 22.4: Plate Tectonics

Plate Tectonics
http://www.scotese.com/earth.htm
Continental Drift Theory
Who:
When:
What:
Theory suggests that …
Evidence:
Weakness:
Alfred Wegener & Continental Drift
Who: Alfred Wegener
When: Early 1900s
What:
Theory suggests that
continents were once joined
together in a supercontinent, called Pangaea
and have since drifted apart
Weakness: Couldn’t explain
the MECHANISM … HOW
the continents moved.
Continental Drift: Supporting Evidence
1. Continents seem to
fit together like
puzzle pieces
2. Fossil records show
similar plant and
animal fossils on
continents separated
by oceans …
suggesting that
continents were once
one land mass
(mesosaur; lystrosaur)
on different continents
Fossil Evidence Supports Continental Drift
Continental Drift:
Evidence
3. Similar
Geological
Formations (Mt.
Chains and rock
formations ) on the
margins Different
Continents match.
•
Same minerals
•
Same age
Continental Drift:
Evidence
4. Evidence of glacial
ice sheets in
tropical/subtropical
locations like India
•
Glaciers did not reach
equator in last ice age.
•
Therefore … India
must have been
located closer to the
poles!
 Pangaea…200 mya
•Harry Hess’s Builds on Wegener’s C. Drift:
1940s-50s
Uses echo location (sonar) to map the sea floor
Discovers that the sea floor is not flat.
It has….
A. Oceanic Ridges (Volcanic Mt. Chains)
B. Fissures – cracks where lava exits.
C. Trenches – deep canyons
D. Seamounts – underwater mountains.
http://www.youtube.com/watch?v=6CsTTmvX6mc
Harry Hess’s: SEAFLOOR SPREADING Theory
•The sea floor is continually remade at Spreading Zones (M.O.R) and
destroyed at subduction zones.
•Ocean crust is NEW at the M.O.R. and then gets pushed away. OLD crust is
subducted.
•SFS provides the HOW for C.Drift. The continents can move as the Sea floor
spreads apart.
Movies:
http://ocean-ridge.ldeo.columbia.edu/Other_stuff/PAR_Movie/parmovie.html
Theory of Plate Tectonics
= Earth is made up of a system of rigid
plates that move slowly due to convection
currents of rock in the mantle.
• 1960s
• Built upon Wegener’s C.Drift evidence
(fossils & mt. chains); Hess’s S.F.S.
evidence (age of rocks); and seismic data
Evidence for Plate Tectonics:
1. Isotope dating of rock reveals that rock near M.O.R.
is younger than ocean floor far away from plate
boundary. Old crust gets pushed away as new crust
is made. The seafloor moves!
Red = < 20 ma
Blue = 200 ma
Oldest seafloor
~ 200 ma
Evidence for Plate
Tectonics:
2.Magnetic “stripes” of sea floor
Magnetic banding on sea floor show that the sea floor moves.
Iron in basalt lines up w. E’s mag. Poles
Pattern of iron FLIPS as E’s mag. pole flips = Ocean floor
made CONTINOUSLY
http://www.youtube.com/watch?v=YIAXiE8RedA
Evidence for Plate
Tectonics: Magnetic
banding on sea floor show
that the sea floor moves.
If sea floor did not move …
the entire ocean floor
would have the same
polarity.
Geomagnetic Data from NOAA: http://www.ngdc.noaa.gov/geomag/
subchrons:
Evidence for Plate Tectonics:
3. Earthquake & Volcano Data
•
Locations of Eqs & Volanoes outline plate boundaries
•
•
http://earthquake.usgs.gov/earthquakes/recenteqsww/
http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/quakes_all.php
Why do the plates move?
• Convection currents of soft rock in the mantle cause the
plates floating on top to move.
• Convection currents are powered by differences in the
temperatures within Earth’s interior
– Magma near core heats up, expands, and rises because it is
less dense
– Magma closer to crust, cools, becomes denser, and sinks
• Heat needed for convection comes from:
1. Heat trapped in Earth’s interior from its formation
2. Radioactive decay of heavy elements in the mantle
Why do the plates move?
Convection Currents
http://www.youtube.com/watch?v=IpnHAj4R-Z8&feature=related
Why do the plates move?
Ridge Push
Slab Pull
@ spreading
centers
where ..
Hot, less dense
rock rises
pushing
plates apart.
Ridge forms
from lava
that builds
volcanic mts.
@ subduction
zones
where...
Cooler, denser
rock sink
due to
gravity
Sinking rock in
convection
current pulls
the overriding plate
with it.
Ocean Depth and Age
Younger crust is
warmer (less time
cooling) … and
more bouyant
(less dense).
Older crust is cooler
(more cooling
time) and has
accumulated
more sediments
= DENSER
Ridge: Higher elevation (younger, hotter rock, less dense rock)
Basins (away fr. Ridge): Lower elevation (older, cooler rock, denser, sinks)
The ocean floor gets recycled by sea-floor spreading
and subduction
Old ocean crust gets destroyed
/ partially melted
Takes 200 million yrs to recycle
ocean floor
New ocean crust forms at fissures at
the Mid-Ocean Ridge
Plate Movements
Diverging Plate Boundaries: 2 plates move apart.
New crust form btw. plates
Sea floor Spreading (Ocean Plates
separate)
•Results: New ocean crust forms @
fissure & old crust pushed away
•.Shallow Eqs, Underwater
volcanoes
Locations: M.O.R.s & Iceland
Rifting (Land Plates Separate)
•Results: Crust thins  Land
plates pull apart –> Rift valley
forms  new ocean crust 
narrow sea develops
• Shallow Eqs
•Volcanoes
•Locations: Great Rift Valley (E.
Africa)
Rifting and
Extensional
Tectonics
“unsuccessful”
rifts
“successful” rifts
Converging Plate Boundaries: 2 plates collide
SUBDUCTION (ocean-land)
•Results: Dense (iron rich basalt) ocean plate sinks back into mantle
• Ocean crust destroyed
• Volcanoes on over-riding slab = Coastal volcanoes & Mts.
• Deep focus EQs
•Locations: Andes (S.Ame); Washington, US (Mt. St. Helens); Alaska
•SuBDUCTION (ocean-ocean)= Older (cooler & denser) ocean plate subducts
Results: same as above + volcanic island arcs
Location = Aleutian islands (Alaska); Japan; Indonesian Islands; Caribbean
Converging Plate Boundaries: 2 plates collide
Mt. Building (Land &
Land collide)
•Results: Two land
plates collide &
crumple – uplift
•.EQs
•Large Mt. Chains
Locations: Himalayas
(Nepal, India)
Transform Plate Boundaries
Transformation
Plates slide past one
another.
• Results: EQs, faults,
fault-block Mts.
• Locations: San Andreas
Fault, CA; along MORs
Movie: http://oceanridge.ldeo.columbia.edu/Other_stuff/PAR_Movie/parmovie.html
Basics:
http://geoinformatics.sdsc.edu/doe/student/tectonics/sea_floor_spreading_i.ht
ml
1.
2.
3
Eurasian
Plate
Indian
Plate
4.
5.
C
B
A
D
Use this diagram to help you EXPLAIN HOW the seafloor gets
Which
recycled. (5pts)
~ Use a topic sentence; vocabulary terms; & supporting details!
~ You can include a diagram to help you.
~ Be sure to include the following terms in your explanation:
mantle; plates or lithosphere; convection currents; fissure; Midocean ridge; sea-floor spreading; trench; subduction; volcanic
activity, ridge push, slab pull, less or more dense, new/old
crust, ocean crust, continental crust