Download plate tectonics

PLATE
TECTONICS
INTERNAL STRUCTURE
CRUST
Thin, cool, brittle
5-40 miles thick
CORE
Outer part liquid
Inner part solid
Down to 4000 miles
6700oC
MANTLE
Hot, plastic
Down to1800 miles
Evidence from analysis of earthquake waves
DENSITIES
ATMOSPHERE
< 1 g/cm3
CRUST
2.7-3.0 g/cm3
HYDROSPHERE
1g/cm3
6700oC
CORE
11-14 g/cm3
MANTLE
3.3-5.6 g/cm3
Average density 5.5 g/cm3
THEORY OF
PLATE TECTONICS 1968
Continental Drift –Hypothesis first
proposed by Alfred Wegener in 1915
Sea Floor Spreading –Hypothesis first
proposed by Harry Hess in 1962
Plate Tectonics 1968 – combination of
these two hypotheses
BASIC PROPOSITIONS
Earth’s crust is divided into rigid plates that
float buoyantly on the denser plastic
mantle
Elevation of the crust depends on density
(rock type and temperature) & thickness
Plates move laterally over surface
generating unstable earthquake belts
where they interact
Isostacy
Recall that the crust (density 2.7-3.0 g/cm3) floats on the
upper mantle (density 3.3 g/cm3) with elevation
dependent on:
Density (Rock type & Temperature)
Thickness
Earthquake Foci
STABLE & UNSTABLE CRUST
Stable Plates
Granitic Crust – Continental Craton
Basaltic Crust – Abyssal Plains
Unstable Plate Boundaries
Divergent – Rift Zones (Midocean Ridges)
Convergent – Subduction Zones
Shear Zones – Transform Faults
Eight Major Plates
North America
South America
Africa
Eurasia
Antarctica
Consist of both granitic
& basaltic crust
Australia – India
Two granitic continents
plus basaltic crust
Pacific- all basaltic
crust
Nazca – all basaltic
crust
A World Cruise
A Note
Lithosphere
Rigid outer layer of
Earth down to depth
of about 60 mls.
Includes crust and
uppermost mantle.
Forms plates, generates
earthquakes due to
faulting
Asthenosphere
Hot plastic zone in
upper mantle (60-400
mls) which forms
base of lithospheric
plates
No earthquakes
generated
PLATE TECTONICS
& THE EVOLUTION
OF OCEANS & CONTINENTS
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Red Sea
The Red Sea Rift
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Red Sea
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Red Sea
Atlantic
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Mature
Red Sea
Atlantic
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Mature
Red Sea
Atlantic
Pacific
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Mature
Senile
Red Sea
Atlantic
Pacific
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Mature
Senile
Red Sea
Atlantic
Pacific
Mediterranean
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Mature
Senile
Dead
Red Sea
Atlantic
Pacific
Mediterranean
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Mature
Senile
Dead
Red Sea
Atlantic
Pacific
Mediterranean
Himalayas
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Mature
Senile
Dead
Dead, rotted
and dismembered
(Yuck!)
Red Sea
Atlantic
Pacific
Mediterranean
Himalayas
Evolution of Ocean Basins
according to Dr. Chowns
Baby Ocean
Adolescent
Mature
Senile
Dead
Dead, rotted
and dismembered
(Yuck!)
Red Sea
Atlantic
Pacific
Mediterranean
Himalayas
Urals, Appalachians
Age, Temperature & Depth of
Oceans
Hot rock is less dense than cold rock
Density affects ocean depth
Age and depth increase away from Ridges
Young, hot basalt forms high Mid-Ocean
Ridges
Old, cold basaltic crust is subducted in
trenches
Heat Flow from Pacific Sea-floor
Three types of convergent
boundary
Basalt – Basalt (Ocean-Ocean)
Island Arc-Trench (Japan, Marianas)
Basalt-Granite (Ocean-Continent)
Mountain Chain-Trench (Andes)
Granite-Granite (Continent-Continent)
Mountain Chain (Himalayas)
Convergent Boundaries
Continental Collision
OPHIOLITES
Usually oceanic crust is subducted butSometimes a slab of upper mantle and
ocean crust gets caught up in continental
collisions (obducted) and preserved in the
mountains
Examples Cyprus, Alps, Appalachians
DRIVING MECHANISM?
CONVECTION CURRENTS IN
MANTLE
Driven by internal heat resulting from
radioactivity
Analogy of convection in a pot of soup
Estimates of temperature made from
analysis of Earthquake waves
Exact details unknown because it is
difficult to collect data from the mantle
Simple Convection Currents In
Mantle
More Complex Models
PLATE TECTONICS THE
SUPPORTING EVIDENCE
Wegener’s original evidence for
Continental Drift 1913
Evidence compiled since 1950, especially
paleomagnetism
WEGENER’S EVIDENCE
1. Distribution of Mountains &
Earthquakes
Himalayan
AlpineCircumpacific
Ring of Fire
2. Remarkable fit between
continental margins (especially
across Atlantic)
3. Continuation of old mountains
from one continent to another (eg.
Appalachians)
4. Similarities in rock layers and
history between continents that
were formerly contiguous (eg.
North America & Europe, Africa &
South America)
5. Similarity in fossils between
continents ( eg. Mesosaurus)
6. Peculiarities in distribution of
modern organisms
Distribution of Marsupials
200 Ma
7. Paleoclimatic Evidence (eg.
Ancient Glacial Deposits)
EVIDENCE SINCE 1950
1. Direct measurements of plate movement
by laser, satellite and GPS
2. Tracking of hot spots like Hawaii
The Hawaiian Hot Spot
Hot Spots and Island Chains
Movement of plates over hot spots in
mantle produces chains of volcanoes.
As old volcanoes move away from hot
spots they cool and subside to form atolls
or guyots
Eg. Hawaiian chain & Emperor seamounts
3.Tracking of continents by position
relative to the magnetic poles
PALEOMAGNETISM
Modern Magnetic Pole
Two modern continents
with magnetic rocks aligned
to magnetic poles
Modern Magnetic Pole
Pole positions inferred
from ancient magnetic
rocks
Ancient Magnetic Pole
Continents repositioned
so that ancient pole
matches modern
pole position
Apparent Polar Wandering
4. Proof of spreading rates from
record of magnetic reversals in
seafloor basalts
Magnetic stripes produced by sea
floor spreading
Age of the Ocean Floor
THE BIG PICTURE
Evolution of the Oceans
200 Ma
Breakup of Pangaea 200-100 Ma
Modern oceans 50-0 Ma