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

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 Japa n Hawa ii Oregon Neva da

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% SiO 2 Oceanic crust (basalt) 50-55% SiO 2 Continental crust (granite) 60-75% SiO 2 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 1.Rift breaks continent apart 2.New divergent boundary/ridge forms 3.Grows into mature ocean basin 4.Subduction begins as oceanic crust becomes older, colder, and denser 5.Terranes accreted to continent 6.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