PLATE TECTONICS17.1 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 OceanCoastal ShapesRock FormationsFossilsAncient ClimatesGlaciation

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.

PLATE TECTONICS17.2 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 fieldNorth and South Pole

swap Magnetic symmetry-

parallel lines on either side of ridgesIsochrons- connects

points of the same age

Seafloor Spreading New ocean crust is

created at ocean ridges and destroyed at deep-sea trenchesHotter 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.

PLATE TECTONICS17.3 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

PLATE TECTONICS17.4 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?