Chapter 17

Plate Tectonics

Drifting Continents: Early Observations

• Cartographers were the first people to notice matching continents

Antonio Snider-Pelligrini (1858)

Alfred Wegener

• German meteorologist/climatologist• 1912: proposed the idea of continental drift

– Continents started as “Pangea” 200 million years ago

– Slowly moving apart to present positions

Evidence for Continental Drift

• Matching coastlines• Matching rock formations/mineral deposits

separated by oceans• Climate evidence: fossil ferns

Glossopteris) found in Antarctica, South America, and India

• Matching fossils on widely separated continents (Cynognathus, Lystrasaurus, Mesosaurus)

Fossil Evidence

Flaws of Continental Drift

• Did not explain the immense forces required to move continents long distances

• No evidence of continents “plowing through” the ocean crust

• Continental Drift was rejected until the early 1960s, when these flaws were resolved

Seafloor Spreading

• Technology advances in the 1940s and 1950s led to more detailed ocean floor mapping

• Magnetometer: detects changes in magnetic fields

• Sonar: found mid-ocean ridges and deep sea trenches

• Interesting patterns in age of rock samples

Magnetic Patterns and Seafloor Spreading

Age of Ocean Crust and Seafloor Spreading

Sediment Depth and Seafloor Spreading

• Evidence for seafloor spreading corrected a flaw of Wegener’s model of continental drift

• Continents ride along while ocean crust moves away from mid ocean ridges

Types of Plate Boundaries: Divergent

• Plates moving apart• Mid-ocean ridge• On land: rift valley

Types of Plate Boundaries: Convergent

• Continent-continent• Ocean-continent• Ocean-ocean

Types of Plate Boundaries: Transform

• Plates slide horizontally past each other

What Causes Plates to Move?