Lecture 4 Outline: Plate Tectonics – Mechanisms and Margins
Learning Objectives:
What are the types of plate boundaries?
What processes occur at different types of plate boundaries?
What are hotspots?
How does tectonics build continents and ocean basins?
What Happens at Plate Boundaries?
Plate interiors stable – geologic activity limited to surface processes
But interactions between plates at plate boundaries results in
Magma and volcanism
Faulting and earthquakes
Mountain building
Production of new crust
Recycling of old crust
What are the Types of Plate Boundaries?
Divergent
plates pulled apart
Convergent
plates collide
Transform
plates sheared
Each plate surrounded by different types of boundaries
What are the Types of Plate Boundaries?
What are Divergent Plate Boundaries?
Ridges
Crust pulled apart
Magma by decompression melting in asthenosphere
Cools to make new oceanic crust
Oceanic crust
lithosphere
asthenosphere
magma
central rift valley
faults
North Atlantic Ridge
Mid-Atlantic Ridge
East Pacific Ridge
Indian Ridge
Antarctic Ridge
Where are Divergent Plate Boundaries Found?
Ocean ridge above sea level in Iceland
Where are Divergent Plate Boundaries Found?
What are the Major Geologic Features of the Ocean Ridge?
Shield Volcano
Edge of North American Plate
Fault
Down-dropped fault block
Central rift valley
Filled by lava flows
What are Convergent Plate Boundaries?
Two plates collide with each other – two types
Subduction zone
Between two plates of different density – denser plate subducted
melting in mantle by addition of water from subducted plate
Trench and volcanic arc – chain of volcanoes on overriding plate
Earthquakes
What are Convergent Plate Boundaries?
Collision zone
between plates too buoyant to subduct
Crust thickened and mountains raised instead
Earthquakes but no volcanoes
Indian Plate
Eurasian Plate
Younger and weaker
Older and stronger
deformed
Which Plate gets Subducted?
If both plates composed of oceanic crust
older and denser crust subducted by younger and lighter crust
Overriding plate
Plate boundary
Where Can We Find an Example of an Oceanic Plate Subducted by Another Oceanic Plate?
Pacific Plate subducted by Philippine Plate at Mariana Trench
Pacific Plate
(older)
Philippine Plate
(younger)
Japan Trench
Mariana Trench
Challenger Deep
Eurasian Plate
Which Plate gets Subducted?
If one plate of continental crust and one of oceanic crust
denser oceanic crust subducted by lighter continental crust
Material too light to subduct added to continent as accreted terranes
sediments, volcanic islands, fragments of continental crust
Where Can We Find an Example of a Collision Zone?
Indian and Eurasian Plates
Collision began 45 mya when subduction completely closed ocean basin
Himalaya and Tibetan Plateau
Recent or continuing collisions produce Earth’s tallest mountains
50 mya
today
Closing Ocean
Spreading Ocean
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Oblique motion between plates – without convergence or divergence
Faulting and earthquakes – no volcanism
What are Transform Plate Boundaries?
Where Can We Find an Example of a Transform Plate Boundary?
San Andreas Fault
Transform boundary between Pacific and North American plates
N. American
plate
Pacific
plate
What are Hotspots?
Volcanism normally at divergent or convergent plate boundaries
Melting of mantle by decompression or addition of water
But some volcanoes located in middle of plates!
What’s the explanation?
Hotspot: plume of hot rock rising from deep mantle
What are Hotspots?
Source of magma well below lithosphere
Doesn’t move with plate – rather plate passes over magma source
Results in age progression to volcanism – hotspot track
Example: Hawaiian Islands
What is a Tectonic Setting?
Geologic environment of area relative to any nearby plate boundaries or hotspots
Each setting associated with specific geologic processes types of volcanoes, earthquakes, etc
Japan
Hawaii
Oregon
Nevada
Divergent
Plates pulled apart
lithosphere created
Convergent
Plates move together
lithosphere recycled
What are the Different Tectonic Settings?
Transform
Plates slide past one another
Lithosphere neither created nor recycled
Hotspot
Plate passes over deep mantle plume
What are the Different Tectonic Settings?
What is the Tectonic Setting of Oregon?
Convergent – between North America and Juan de Fuca plates
Subduction, terrane accretion, earthquakes, volcanic arc
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Accretion
buoyant material from subducting plate
Coast Range
Volcanism partial melting in mantle due to addition of water
Cascades
Coast Ranges and Cascades both result of subduction
Specific processes are different
What is the Tectonic Setting of Oregon?
| Continents assembled from pieces of crust too light to subduct |
How Does Plate Tectonics Build Continents?
By accretion and collision
Subduction removes intervening oceanic crust
Accretion and volcanism adds buoyant material to overriding plate
Intervening oceanic crust removed – continents collide
How is Continental Crust Made?
strongly enriched in silica relative to oceanic crust/mantle
Subduction zones: recycling centers that sort out continental crust
Mantle (peridotite)
45% SiO2
Oceanic crust (basalt)
50-55% SiO2
Continental crust (granite)
60-75% SiO2
Degree of Silica Enrichment
How do Subduction Zones Make Continental Crust?
1. Accretion of buoyant materials as ocean crust subducts
Builds continent outward over time
Buoyant materials often added as terranes – block of crust with different geologic origin and history from adjacent areas
Volcanic island arcs, marine sediments, thick oceanic crust, fragments of continental crust
Moved great distances on subducting plate
How do Subduction Zones Make Continental Crust?
2. melting of subducted plate and mantle and fractional crystallization
Makes magmas richer in silica than oceanic crust
Erupted at surface volcanoes
Cascades
Emplaced within crust
intrusions
Sierra Nevada
| Rifting of continent to create new divergent boundary Caused by mantle upwelling beneath continent Example: East African Rift Valley Active rifts grow over time to become new oceans |
How does Plate Tectonics Make Ocean Basins?
African Rift Valley
Red Sea advanced rift
Rift Valley lakes
rifts
How do Rifts Start and Grow into Ocean Basins?
Crust heated by upwelling mantle causing uplift
Uplift collapses with continued stretching to form rift valley
Rift floods to form narrow sea
Widens by seafloor spreading into new ocean basin
| Processes of continent assembly, breakup, and re-assembly Continental rifting, seafloor spreading, subduction and accretion, collision Rift breaks continent apart New divergent boundary/ridge forms Grows into mature ocean basin Subduction begins as oceanic crust becomes older, colder, and denser Terranes accreted to continent Continents collide when intervening oceanic crust completely subducted Stages in Tectonic Cycle with modern-day examples of each stage |
What is the Tectonic Cycle?
1. Continental rifting
2. Spreading center develops
3. Ocean basin
4. Subduction
5. Terrane Accretion
6. Continental collision
| What are the three types of plate boundaries? Describe the geologic processes occurring at each boundary. Provide a modern-day example of each type of plate boundary. How does plate tectonic activity build continents and make continental crust? How does plate tectonic activity break continents apart and make oceanic crust? Describe the tectonic setting of the Pacific Northwest. |