Transcript plate tectonics

PLATE TECTONICS
SCIENTIFIC METHOD
Observations (Facts)
Hypothesis (Interpretation – incompletely tested)
Testing (More critical observations)
SCIENTIFIC THEORY (Tested and accepted
interpretation). The foundations of our
understanding
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
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
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
THE BIG PICTURE
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
Appalachians, Urals
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)
THE BIG PICTURE
Convergent Boundaries
Continental Collision
DRIVING MECHANISM?
Convection Currents in Mantle
Simple Convection Currents In
Mantle
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. 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
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