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

Plate Plate BoundariesBoundaries

•DivergentDivergent•convergent convergent •transformtransform

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

• Plates move towards each other

• Crash or collide

• 3 types based on type of crust

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