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Slide 01
Tectonic Plates
- The Restless Foundations of Our Dynamic Earth
- Beneath our feet, enormous slabs of rock drift, collide, and pull apart at rates measured in centimeters per year - yet over geological time, they have reshaped continents, raised mountains, opened oceans, and triggered the most powerful forces on Earth.
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Slide 02
Earth's Layered Structure
- Inner Core
- Solid iron-nickel sphere, 1,220 km radius. Temperature: 5,400C (as hot as the Sun's surface).
- Outer Core
- Liquid iron-nickel, 2,180 km thick. Convection here generates Earth's magnetic field.
- Mantle
- 2,900 km thick. Solid but flows over geological time. Convection drives plate motion.
- Crust
- Thin outer shell: oceanic (5-10 km) and continental (30-70 km). Broken into tectonic plates.
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Slide 03
What Are Tectonic Plates?
- Tectonic plates are massive, rigid segments of Earth's lithosphere (crust + uppermost mantle) that float on the semi-fluid asthenosphere beneath. There are 7 major plates and numerous smaller ones. They range from 15 to over 200 km thick and move at 1-16 cm per year. Their interactions at boundaries produce earthquakes, volcanoes, mountain ranges, and ocean trenches.
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Slide 04
The Seven Major Plates
- Pacific Plate
- Largest oceanic plate (103M km2). Entirely oceanic. Surrounded by the "Ring of Fire."
- North American
- Includes most of North America, Greenland, and part of the Atlantic Ocean floor.
- Eurasian
- Most of Europe and Asia. Collides with Indian, African, and Pacific plates.
- African
- Currently splitting along the East African Rift. Will eventually divide into two plates.
- Also: South American, Antarctic, and Indo-Australian (sometimes split into Indian + Australian)
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Slide 05
Continental Drift: The Idea
- 1596 - Abraham Ortelius notes that Americas and Africa/Europe fit together like puzzle pieces
- 1912 - Alfred Wegener proposes continental drift and the supercontinent "Pangaea"
- 1920s-50s - Scientific establishment rejects Wegener; no known mechanism for movement
- 1960s - Seafloor spreading, paleomagnetism, and GPS data vindicate the theory
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Slide 06
Alfred Wegener's Evidence
- Coastline fit - South America's east coast perfectly matches West Africa's coast
- Fossil match - Identical fossils (Mesosaurus, Glossopteris) on now-separated continents
- Rock belts - Mountain chains and geological formations align across ocean gaps
- Glacial evidence - Tropical regions show ancient glacial scratches; makes sense if they were once at poles
- Climate fossils - Coal deposits (tropical origin) found in Antarctica
- Rejected in his lifetime - He died in 1930 on a Greenland expedition, vindicated decades later
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Slide 07
Seafloor Spreading
- In 1962, Harry Hess proposed that new oceanic crust forms at mid-ocean ridges, where magma rises from the mantle. As new rock forms, it pushes older seafloor outward in both directions. This was confirmed by the discovery of magnetic striping: alternating bands of normal and reversed magnetic polarity in ocean floor basalt, symmetric around ridges, recording Earth's magnetic field reversals.
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Slide 08
What Drives Plate Motion?
- Mantle Convection
- Heat from the core drives slow circulation in the mantle, dragging plates along like conveyor belts.
- Ridge Push
- Elevated mid-ocean ridges push plates outward under gravity, like sliding down a slope.
- Slab Pull
- Cold, dense oceanic crust sinks at subduction zones, pulling the rest of the plate behind it. Strongest force.
- Basal Drag
- Friction between the convecting mantle and the base of plates drags them along.
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Slide 09
Three Types of Plate Boundaries
- Divergent
- Plates move apart. New crust forms. Mid-ocean ridges, rift valleys. Example: Mid-Atlantic Ridge.
- Convergent
- Plates collide. Subduction, mountain building, deep trenches. Example: Himalayas, Andes.
- Transform
- Plates slide past each other. Earthquakes but no volcanism. Example: San Andreas Fault.
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Slide 10
Divergent Boundaries
- At divergent boundaries, plates pull apart and magma rises to fill the gap, creating new oceanic crust. The Mid-Atlantic Ridge runs 16,000 km from Arctic to Antarctic - the longest mountain range on Earth, mostly underwater. Iceland sits directly on this ridge, allowing us to observe divergence on land. The ridge spreads at approximately 2.5 cm per year.
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Slide 11
Convergent Boundaries: Subduction
- When oceanic crust meets continental crust, the denser oceanic plate dives beneath in a process called subduction. This creates deep ocean trenches (Mariana Trench: 10,994m deep), explosive volcanic arcs (Andes, Cascades, Japan), and powerful megathrust earthquakes. The subducting slab carries water into the mantle, lowering the melting point of rock above and generating magma.
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Slide 12
Convergent Boundaries: Collision
- When two continental plates collide, neither subducts (both are too buoyant). Instead, the crust crumples, folds, and thrusts upward, building immense mountain ranges. The Himalayas formed when the Indian plate crashed into Eurasia beginning 50 million years ago. India continues moving north at 5 cm/year, and Everest grows about 4mm annually as a result.
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Slide 13
Transform Boundaries
- At transform boundaries, plates grind past each other horizontally. No crust is created or destroyed, but enormous friction builds stress that releases as earthquakes. The San Andreas Fault in California is the most famous example: the Pacific Plate moves northwest past the North American Plate at 46mm/year. Los Angeles will be adjacent to San Francisco in about 15 million years.
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Slide 14
The Ring of Fire
- 75%Of world's active and dormant volcanoes are on the Ring of Fire
- 90%Of world's earthquakes occur along this 40,000 km horseshoe
- 452Volcanoes line the Ring of Fire around the Pacific
- 40,000 kmLength of the Ring from New Zealand to Chile via Japan and Alaska
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Slide 15
Earthquakes: Release of Stress
- 1960 Chile - Magnitude 9.5, strongest recorded. Generated tsunami reaching Japan and Hawaii
- 2004 Indian Ocean - Magnitude 9.1. Tsunami killed 230,000+ people across 14 countries
- 2011 Tohoku, Japan - Magnitude 9.1. Tsunami triggered Fukushima nuclear disaster
- Mechanism - Stored elastic energy in rock releases suddenly when friction is overcome at faults
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Slide 16
Volcanoes and Plate Tectonics
- Subduction zones produce explosive stratovolcanoes (Mt. St. Helens, Pinatubo, Fuji)
- Divergent boundaries produce effusive basaltic eruptions (Iceland, mid-ocean ridges)
- Hotspots produce volcanoes away from boundaries (Hawaii, Yellowstone)
- 80% of eruptions occur underwater at mid-ocean ridges, unseen
- Volcanic gases (CO2, SO2, water vapor) have shaped Earth's atmosphere for 4.5 billion years
- Super-eruptions (Toba, 74,000 years ago) can alter global climate for years
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Slide 17
Hotspots: Plumes from the Deep
- Some volcanoes exist far from plate boundaries, above mantle plumes - columns of anomalously hot rock rising from deep in the mantle (possibly the core-mantle boundary). As a plate moves over a stationary hotspot, it creates a chain of progressively older volcanoes. The Hawaiian island chain stretches 6,000 km, with active volcanism only at the southeast end where the plate currently sits over the plume.
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Slide 18
Mountain Building (Orogeny)
- Himalayas
- India-Eurasia collision. Still rising. Contains all 14 peaks above 8,000m. 50M years of convergence.
- Andes
- Nazca plate subducting under South America. 7,000 km long. World's longest continental range.
- Alps
- African plate pushing into Eurasia. Still active. Mediterranean is closing.
- Appalachians
- Ancient (480-300 Mya). Once Himalaya-height. Erosion has reduced them. Formed when Pangaea assembled.
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Slide 19
Pangaea: The Last Supercontinent
- Approximately 335-175 million years ago, all continents were assembled into a single landmass called Pangaea ("all lands"), surrounded by a single ocean, Panthalassa. Pangaea began rifting apart in the Jurassic period. The Atlantic Ocean opened as the Americas separated from Africa and Europe. Dinosaurs walked freely across continents that are now separated by thousands of kilometers of ocean.
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Slide 20
The Supercontinent Cycle
- Vaalbara (~3.6 Ga) - Possibly Earth's first supercontinent
- Rodinia (~1.1-0.75 Ga) - Assembled and broke up, triggering Snowball Earth glaciations
- Gondwana (~550-180 Ma) - Southern supercontinent: Africa, South America, India, Australia, Antarctica
- Pangaea (~335-175 Ma) - Most recent supercontinent. Surrounded by Panthalassa ocean
- Pangaea Proxima (~250 Mya future) - Predicted next supercontinent as Atlantic closes
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Slide 21
The Mid-Ocean Ridge System
- The global mid-ocean ridge system is Earth's longest mountain range at 65,000 km - spanning every ocean. It is essentially a continuous wound in Earth's surface where new oceanic crust is born. Hydrothermal vents along ridges support unique ecosystems independent of sunlight, fueled by chemical energy from superheated mineral-rich water (up to 400C). These may be where life on Earth originated.
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Slide 22
Ocean Trenches
- Mariana Trench
- 10,994m deep. Deepest point on Earth (Challenger Deep). Pacific plate subducting under Philippine plate.
- Tonga Trench
- 10,882m. Fastest plate convergence on Earth (24 cm/yr). Most seismically active subduction zone.
- Japan Trench
- Source of devastating 2011 Tohoku earthquake. Pacific plate subducts at 8-8.5 cm/yr.
- Peru-Chile Trench
- 8,065m deep, 5,900 km long. Source of 1960 M9.5 earthquake - strongest ever recorded.
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Slide 23
The East African Rift
- Africa is splitting apart. The East African Rift System stretches 3,000 km from the Red Sea to Mozambique. In 5-10 million years, East Africa (the Somali Plate) will separate completely, creating a new ocean. The rift has already produced the Red Sea and the Gulf of Aden. Volcanic activity along the rift includes Kilimanjaro, Mt. Kenya, and the active Erta Ale lava lake in Ethiopia.
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Slide 24
Plate Tectonics and Life
- Continental positions control ocean currents, which determine global climate patterns
- Plate separation isolates populations, driving speciation (Australia's unique marsupials)
- Mountain building creates new habitats and rain shadows, diversifying ecosystems
- Volcanic outgassing regulated early atmosphere; CO2 cycling controls long-term climate
- Mass extinctions correlate with massive volcanic events (Siberian Traps, Deccan Traps)
- Without tectonics, Earth's carbon cycle would fail and planet would become uninhabitable
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Slide 25
Plate Tectonics and Resources
- Minerals
- Ore deposits concentrate at plate boundaries: copper in subduction zones, diamonds in ancient cratons.
- Fossil Fuels
- Oil and gas form in sedimentary basins created by rifting and passive margins.
- Geothermal
- Iceland, New Zealand, and other rift/volcanic regions tap heat from the mantle for energy.
- Fertile Soils
- Volcanic soils (andisols) are among Earth's most productive agricultural lands.
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Slide 26
Measuring Plate Motion
- Today, GPS satellites measure plate motion with millimeter precision. The fastest-moving plate is the Tonga microplate at 24 cm/year. Australia moves north at 7 cm/year (the fastest major continent). India moves northeast at 5 cm/year. Seafloor magnetic striping, satellite laser ranging, and very long baseline interferometry (VLBI) all confirm these measurements independently.
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Slide 27
Earthquake Prediction and Preparedness
- Cannot Predict
- Despite decades of research, reliable short-term earthquake prediction remains impossible.
- Can Forecast
- Probabilistic hazard maps identify high-risk zones. San Andreas has 72% chance of M6.7+ by 2043.
- Early Warning
- Seconds to minutes of warning from P-waves before destructive S-waves arrive. Japan leads this technology.
- Engineering
- Base isolation, flexible steel frames, and strict building codes save lives in Japan, Chile, and California.
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Slide 28
Tsunamis: Waves from the Deep
- When a megathrust earthquake vertically displaces the ocean floor, it generates tsunamis - waves that travel at 800 km/h in deep water (jet aircraft speed) but are barely perceptible at open sea. Upon reaching shallow coastal waters, they slow down and build to devastating heights. The 2004 Indian Ocean tsunami reached 30m in Banda Aceh and killed over 230,000 people across 14 countries.
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Slide 29
Plate Tectonics on Other Worlds
- Mars
- No current tectonics. May have had early plate activity. Olympus Mons formed over a stationary hotspot.
- Venus
- "Stagnant lid" regime. Periodic catastrophic resurfacing events rather than continuous recycling.
- Europa (Jupiter)
- Ice shell may have plate-like tectonics, with subduction of ice plates into subsurface ocean.
- Earth
- Only known planet with active plate tectonics. May be essential for long-term habitability.
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Slide 30
The Future of Earth's Plates
- +50 Million Years - Africa collides with Europe, closing the Mediterranean Sea
- +100 Million Years - Australia merges with Southeast Asia
- +200 Million Years - Atlantic Ocean begins closing as new subduction zones form
- +250 Million Years - Next supercontinent "Pangaea Proxima" or "Amasia" assembles
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Slide 31
Key Numbers
- 15Major and minor tectonic plates currently recognized
- 500KDetectable earthquakes per year (~100K felt, ~100 cause damage)
- 4.5 GaYears plate tectonics has likely been active on Earth
- 2.5 cm/yrAverage rate of plate motion (speed fingernails grow)
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Slide 32
A Living Planet
- Plate tectonics makes Earth unique in our solar system. It recycles carbon, generates magnetic shielding, creates diverse habitats, and maintains the conditions for life over billions of years. Every mountain you see, every earthquake you feel, every volcanic island that rises from the sea tells the same story: our planet is alive, dynamic, and constantly remaking itself from the inside out.
- The End
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