17.1 Drifting Continents

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

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  Where 2 plates are moving apart  Crust is created  aka- rift or rift valley

Convergent Boundaries  Where 2 plates move together  Subduction  Crust is destroyed  Any combo of oceanic and continental plates

Transform Boundaries  Where 2 plates slide past each other  Crust is only deformed or fractured

17.4 Causes of Plate Motion

Mantle Convection  Currents transfer thermal energy in unequally heated matter  Produces a pattern of motion

Push and Pull  Ridge Push- new crust pushes plates towards the trenches  Slab Pull- pulls trailing lithosphere into the trench

Push and Pull  Ridge Push- new crust pushes plates towards the trenches  Slab Pull- pulls trailing lithosphere into the trench

Mantle Convection  Extremely slow  Deep within the Earth Unanswered Questions…  How do they start?  Are they permanent features?  How deep do they have an affect?