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Plate Tectonic Model
Did you read chapter 30
before coming to
class?
A. Yes
B. No
So how do you move a continent?
What drives plate motion?
 Forces driving plate motion
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•
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Ridge Push
Slab Pull
Basal Drive?
 Forces resisting plate motion
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•
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Basal Drag?
Transform Fault Friction
Collision
Mantle Resistance
Plate Boundaries
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The action in plate tectonics is almost always at the plate boundaries.
Two plates moving towards each other form a convergent boundary
1. Ocean crust meets ocean crust
2. Ocean crust meets continental crust
3. Continental Crust meets continental crust
Two plates moving away from each other form a divergent boundary
Divergent Plate Boundaries in the oceans
 Extensional Stresses stretching ocean crust
 Creates New Seafloor
 Volcanoes
•
Eruptions of basalt
•
Low in Si, high in Fe
 Earthquakes
•
•
Shallow foci
Low to modest magnitudes
Partial melting of peridotite in the mantle produces basalt.
Why doesn’t this process just produce more peridotite?
 Different mineral
components in the
peridotite melt at
different T’s
 What will happen if only
a small fraction of the
rock melts?
 Will the melted part
have the same
composition as the
original rock?
The mid ocean ridges
Iceland
 Mid Atlantic Ridge
above the water
The mid ocean ridges have a rich variety of
life and geologic structure
Tube Worms
Spider Crabs
Formations on the ridges
Black Smokers
Pillow Basalt
Divergent Plate Boundaries under
continents
 Extensional Stresses
stretching
 Eventually creates New
Seafloor
 Volcanoes
• Basalt and granite (Bimodal
composition)
 Earthquakes
• Shallow foci
• Modest magnitudes -larger than oceanic rifts.
The rift valley in Africa is one of the most
clear examples of continental rifting
Oldoinyo Lengai
Erta Ale
Killimanjaro
The rift occurring under the Red Sea is
more advanced
Why do we get both high & low Si rocks at continental
rifts?
 Basalt is dense and ponds at
base of crust
 Heat from basalt melts
some of the Si-rich crust
producing granite/rhyolite
magma
 Both types of magma may
eventually erupt at the
surface
Convergent Plate Boundaries
 Compressional Stresses
 Old Seafloor Destroyed (Subduction)
 Volcanoes
• Explosive and Very Dangerous (lots of gas)
 Earthquakes
• Shallow to Deep foci
• Large magnitudes
 Mountain Belts/High Deformation
 Accretion – Building of the Continents
Ocean-Ocean Collision
Island arcs: Indonesia, Japan, Taiwan, Philippines,
Kamchatka Peninsula, Aleutian Islands.
Ocean-Continent Collision
The Andes mountains are the result of a
ocean-continent collision
Deep, severe earthquakes and explosive
volcanoes occur as oceanic crust subducts
Continent-Continent Collision
 Why are there no volcanoes when continents
collide?
 No plate is sinking into the mantle and melting!
The Indian subcontinent slammed into Asia
several million years ago
India is now a part of Asia. The Asian
continent has grown.
Appalachian mountains were similar to the Himalaya mountains
200-300 MYA (collision between North America and Africa)
Build-up of North America through time
Pieces added to western
North America in the last
200 million years
Transform Plate Boundaries
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Shear Stresses
No destruction or creation of crust
Volcanoes are rare
Earthquakes
• Shallow foci
• Large magnitudes
 Occur both in ocean basins and on
continents
Transform faults are evident in the ocean
ridges
The San Andreas fault is a transform
boundary
Island Chains
 Created by plates drifting over hot
spots.
Other “Hot Spots” around the world
Major plate boundaries today
Lets play “Name that boundary”!
Island Arc
Transform
Boundary
Subduction
(Trench)
Converging
Continental
Continental
Rift Zone
Divergent plate
boundary
The interior structure of Earth has been
determined mostly from
a)
b)
c)
d)
e)
Drilling
Exploration
X-Rays
Earthquake waves
Chemical analysis