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Continental Drift, Continental Drift, Seafloor Spreading & Seafloor Spreading &
Plate TectonicsPlate Tectonics
Standard 3 Standard 3
(pink test review packet)(pink test review packet)
Continental DriftContinental Drift• Earth’s continents were once joined as
single landmass; broke apart and continents drifted to present position
• Pangaea: supercontinent (break up ~ 200 mya)
• Wegner
• Not accepted - lack of mechanism for the movement of continents (why and how)
Evidence for Continental DriftEvidence for Continental Drift
1. Jigsaw puzzle fit of continents (S.A. & Africa)
2. Rock formations on different continents – same age, similar structure
3. Fossils of land dwelling animals on different continents
4. Climate – coal beds (form in humid swamps) found in Antarctica & tropical plants
Seafloor SpreadingSeafloor Spreading
• New oceanic crust forms at mid-ocean ridges
• Destroyed at subduction zones (deep-sea trenches)
• Magma rises, forced upwards, lava fills in ridge, hardens and new seafloor moves away from the center of the ridge.
Evidence for Seafloor SpreadingEvidence for Seafloor Spreading
• Discovery of mid-ocean ridges
• Seafloor youngest at mid-ocean ridges
• Magnetic pattern is the same on both sides of the ridge (mirror image)
• Hess
• Technology: Sonar (uses sound waves)
Theory of Plate TectonicsTheory of Plate Tectonics
• Earth’s crust and rigid upper mantle are broken into tectonic plates
• Movement of plates creates most volcanoes and major mountain ranges
• Movements cause earthquakes
• Plates move because of Convection in the mantle
Types of crust/platesTypes of crust/plates• Continental Crust
– Older– Lighter– Granite– 30 miles thick
• Oceanic Crust– Younger– Denser– Basalt– 5 miles thick
DivergentDivergent– Plates move away from each other– Most found on seafloor (mid-ocean ridges)– Found on continental crust – stretches crust
to form a rift valley (African Rift Valley)– Shallow earthquakes
Convergent Boundaries: plates Convergent Boundaries: plates converge/collideconverge/collide
• Continental – continental– high mountain ranges– Himalayas
• Continental – oceanic– volcanic mountain ranges on land, deep-sea trenches– Cold more dense plate sinks– Andes, Cascades
• Oceanic – oceanic– volcanic islands, deep-sea trenches– Colder, denser plate sinks– Mariana Island, Japan
Transform BoundariesTransform Boundaries
• Two plates slide past each other
• Example – San Andreas fault
Mantle ConvectionMantle Convection• ConvectionConvection: hot less dense material
rises & cold, denser material sinks
• Magma rises because it is less dense than surrounding rock & it forces itself upwards
• Driving force of plate tectonicsDriving force of plate tectonics
HOT SPOTSHOT SPOTS• Some volcanoes form over hot spots
• As tectonic plate moves chain volcanoes form
• Hawaiian Islands – Kilauea located over hot spot
• Yellowstone located over hot spot