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Plate Tectonic Model
Did you read chapter 30 before coming to class?
A. YesB. No
So how do you move a continent?
What drives plate motion?
Forces driving plate motion• Ridge Push• Slab Pull• Basal Drive?
Forces resisting plate motion• Basal Drag?• Transform Fault Friction• Collision• Mantle Resistance
Plate Boundaries 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 crust2. Ocean crust meets continental crust3. 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 SmokersPillow 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
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) Divergent plate
boundary
Continental Rift Zone
ConvergingContinental
The interior structure of Earth has been determined mostly from
a) Drillingb) Explorationc) X-Raysd) Earthquake
wavese) Chemical
analysis
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