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

valleyfaults

North Atlantic Ridge

Mid

-Atla

ntic

R

idge

Eas

t Pac

ific

R

idge

Indi

an

Rid

ge

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 crustolder 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 TrenchPacific Plate

(older)Philippine Plate

(younger)

Japa

n Tr

ench

Mar

iana

Tre

nch

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 OceanSpreading

Ocean

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. Americanplate

Pacificplate

What are Hotspots?

Volcanism normally at divergent or convergent plate boundariesMelting 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 Hawa

ii

OregonNevada

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

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

a. Subduction removes intervening oceanic crust

b. Accretion and volcanism adds buoyant material to overriding plate

c. 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% SiO2Degree 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 volcanoesCascades

Emplaced within crustintrusionsSierra 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 ValleyRed 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

1. Rift breaks continent apart2. New divergent boundary/ridge forms3. Grows into mature ocean basin4. Subduction begins as oceanic crust

becomes older, colder, and denser 5. Terranes accreted to continent6. 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

1. What are the three types of plate boundaries? Describe the geologic processes occurring at each boundary.

2. Provide a modern-day example of each type of plate boundary.

3. How does plate tectonic activity build continents and make continental crust?

4. How does plate tectonic activity break continents apart and make oceanic crust?

5. Describe the tectonic setting of the Pacific Northwest.

Questions for Review