17.1 Drifting Continents Plate Tectonics

Early Observations In the late 1500s, mapmakers noticed the apparent “fit” of the continents on either side of the Atlantic Ocean. But what could be the cause?

Continental Drift- Pangea Proposed in 1912 by German scientist Alfred Wegener States that the continents were once joined as a single landmass and broke apart 200 mya

Continental Drift- Evidence Similarities on either side of the Atlantic Ocean Coastal Shapes Rock Formations Fossils Ancient Climates Glaciation

Continental Shapes The coastal shapes of the continents fit together like pieces in a puzzle.

Rocks Rock formations in Africa and South America are of the same age and structure.

Ancient Plants and Animals Fossil evidence suggests landmasses were once joined.

Past Environments Ancient Climates including swamps, deserts, ice caps, and bodies of water line up. As evidenced by coal, sand, and evaporite (gypsum and salt) deposits

Glaciation Rocks in South America, Africa, India, Australia and Antarctica possess similar deep scratches explainable by glacial movement.

Hypothesis Rejected? Wegener could not prove his hypothesis of continental movement during his lifetime. Many scientists maintained their view of a static, flat Earth surface. However… New evidence found on the seafloor was revealed in the 1960s.

17.2 Seafloor Spreading Plate Tectonics

Studying the Ocean Floor Technology Sonar- uses sound waves Magnetometer- detects small changes in magnetic fields

Ocean Floor Topography Not flat as scientists once thought! Ocean Ridges- longest mountain range on Earth Earthquakes and volcanism are common Deep-Sea Trenches- deepest places on Earth Marianas Trench in the Pacific Ocean is > 11 km deep!

Ocean Rocks and Sediment Rocks near ridges are younger than those near trenches. Age of rocks and depth of sediment increases with distance from ridges.

Magnetism Paleomagnetism- study of the Earth’s magnetic record using iron-bearing minerals that record changes in the magnetic field through time.

The Geomagnetic Time Scale Magnetic reversal- change in Earth’s magnetic field North and South Pole swap Magnetic symmetry- parallel lines on either side of ridges Isochrons- connects points of the same age

Seafloor Spreading New ocean crust is created at ocean ridges and destroyed at deep-sea trenches Hotter and less dense, magma rises at the ridges and hardens to form new oceanic crust Old crust plunges and is recycled in the deep-sea trenches

Hypothesis to Theory! Seafloor spreading finally answered the “How” question looming over Wegener’s Continental Shift Hypothesis.

17.3 Theory of Plate Tectonics

Theory of Plate Tectonics States that the Earth’s crust and rigid upper mantle are broken into enormous slabs called plates.

Plate Boundaries- Types Converge- come together Diverge- move apart Transform- slide past

Divergent Boundaries NOT FINISHED