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Plate Tectonics
Evolution of
Continents,
Ocean Basins,
Mountains
Continental Margins
Plate Margins - Where Stuff
Happens…..
Divergent
Convergent
Transform
Relative Movements of Plates
a) Divergent, (b) Convergent-continent/ocean basin, © transform, (d)
convergent-continent/continent
Plate Boundaries
• Seismic activity
• Volcanic activity
• Mountain building
Ocean-Ocean Divergent Margin
• Mid Ocean Ridges - long,
sinuous ridge system,
central rift valley
• New ocean crust
• Active volcanism
• Shallow earthquakes < 100
km
• Rounded “Pillow” basalts
• Ridge offset by transform
faults
Ocean-Ocean
Divergent Margin
Lava fountains (10 m high) spouting from
eruptive fissures during the October 1980
eruption of Krafla Volcano.
• MOR cools by:
• Conduction
• Convection - cold sea water circulates
• Hydro-thermal springs - >350°C
• Circulating waters rich in H2S, SiO2,
Mn, CO2, H2, CH4, K, Ca, etc.
• Precipitate - form black / white
smokers
• Adaptations to Vents
• Hydrogen-sulfide- (H2S) oxidizing
bacteria - base of food chain
Pillow Basalts
• Hot magma cools quickly in
cold ocean water
ContinentContinent
Divergent
Margin
• ACTIVE - Red Sea began ~10 Ma
• INACTIVE - East African Rift Zone; parts
up to 20 Ma
• Example of stable triple junction - 3
breaks. One arm fails
http://earthview.sdsu.edu/trees/tecqest.htm
Continent-Continent Divergent
Margin
• Rio Grande Trough (USA) and African Rift
Valley
• - < 100 km deep
• Volcanism - basaltic and rhyolitic
Oldoinyo
Lengai,
active
volcano in
the East
African Rift
Zone
Seafloor Spreading at Mid-Ocean Ridges
• Sediment accumulation increases away from
ridge - “pelagic rain”
• Depth of crust increases away from ridge
• Age of crust increases away from ridge
Sea Floor Age (less than 200 Ma)
How Do Divergent Margins Develop?
DOMING
•
High heat flow, thermal expansion of crust - few
km high / 1000’s km across.
•
High, dry climate
RIFTING
•
Normal faults accommodate stretch
•
NO ocean crust
•
Possible formation of triple junction
•
High, dry climate: non-marine sandstones,
conglomerates in basin
•
East African Rift Zone
DRIFTING
•
Crust thins and initial intrusion of basalts as
new seafloor
•
Shallow ocean; high evaporation
•
Later “normal” marine sediment
•
Drainage reverses in to the sea.
•
Gulf of California (5 Ma)
•
Red Sea (10 Ma)
Convergent Boundaries
• Plate Boundaries where plates
are moving together ….. or
converging
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High seismicity to depths of 700 km
Volcanism and/or metamorphism
Areas of continental accretion / growth
Subduction of crust / mountain building
• Types:
– Ocean to ocean and ocean to continent
– Continent to continent
Types of Convergence
Ocean-Ocean, Ocean-Continent,
and
Continent-Continent
Seismic Activity Along Descending Plate
Ocean - Continent Collision
•
Oceanic crust more dense than continental crust
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Continental margin / Volcanic Arc
Deep trench on ocean side; Fore arc attached to continental crust
Volcanoes - mountain range parallel to / 300-400 km from trench
High volcanic activity- Andesitic
High seismic activity; earthquake foci 0 to 700 km deep
•
W margin of North / South America
– Andes and Cascades
Components:
• Oceanic plate
• Trench
• Fore-arc ridge (melange)
• Fore-arc basin
• Continental plate with volcanic mountain range (e.g. Andes)
The Ring of Fire- Volcanics associated with
Pacific plate boundaries
Mountain Building-e.g. the Andes
Ocean to Ocean Collision
• Two plates of same thickness (? Age/temperature) Which one wins?
• Deep trench on “ocean” side
• Arcuate line of islands ~100 Km land ward from trench - Island Arc
• Zone is long and narrow (1000’s Km x 200-300 Km)
• High volcanic activity- Andesitic
• High seismic activity; earthquake foci 0 to 700 km deep
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Aleutian Trench
Marianas Trench (10,860 m deep)
Tonga Trench (10,800 m deep)
Lesser Antilles
Components:
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Oceanic plate
Trench
Fore-arc ridge (melange)
Fore-arc basin, Island arc
Back-arc basin (down-going plate melts / accom. of
spreading)
Skinner and Porter, 2000
Continent to Continent Collision
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Two pieces of continental crust of equal - light density
Neither subducts; mountain building, continental
accretion
Australia/India and Eurasia - Himalayas
Africa and Europe - Alps (early Mesozoic)
Africa and North America – Appalachians (Paleozoic)
Building Continental Material
Precambrian Shields