PLATE TECTONICS

Objectives

• Explain how paleomagnetism provided the definitive evidence for continental drift.

• Define seafloor spreading.• Define theory of plate tectonics.• Describe the different types of plate

margins.• Explain the role of mantle convection in

plate tectonics.

A Revolution in Geology• Continental drift• Proposed by Alfred

Wegener• Slow, lateral

movement of continents along Earth’s surface

• The puzzle piece argument• Pangaea• Continental shelf• Continental slope

Matching Geology• Matching rocks• What is the “true” edge of a continent?

Matching Geology• Matching rocks• How well do the

continents fit?

Matching Geology

• Matching rocks• How well do the ages match?

Matching Geology

Matching Geology• Matching rocks• What evidence did the glaciers

leave?

Matching Geology• Matching fossils• What evidence

did the glaciers leave?• Glossopteris:

ancient tree with large seeds found on several continents

• Mesosaurus: small reptile found in Brazil and S. Africa

Matching Geology

Apparent Polar Wandering Paths

• Paleomagnetism– Apparent polar

wandering• Magnetic poles

wandered

– Used to indicate America and Europe were a single continent

Apparent Polar Wandering Paths

Seafloor Spreading

• The missing clue– Seafloor spreading

• The process by which the seafloor splits and moves apart along a midocean ridge

• New oceanic crust forms along the ridge

– Global Positioning Systems (GPS) valuable tool

The Plate Tectonic Model

• Plate tectonic– The movement and interactions of large

fragments of Earth’s lithosphere (i.e. plates)– Below lithosphere- asthenosphere

• Weak due to hot temperature (near melting point)

– Relationship between lithosphere and asthenosphere is isostasy

• Fault– A fracture in Earth’s crust along which

movement has occurred

The Plate Tectonic Model

Types of plate margins• Divergent

margins– A boundary

along which two plates move apart from one another

Types of plate margins• Convergent margin– A boundary along which two plates come

together

Types of plate margins• Subduction zone– A boundary along which one lithosphere plate

plunges into the mantle beneath another plate• “Collision zone” between continents

– Deep oceanic trenches– Arcs of volcanoes

Types of plate margins

• Transform vault– A fracture in the

lithosphere where two plates slide past each other

Earthquake and plate margins

• Earthquake– Tectonic movement produce pressure and friction. – Friction is overcome, the block slips and pent up energy

releases with a huge “snap”– Focus

• Where earthquake begins

– Epicenter• Point on earth’s center directly over the focus

The search for a mechanism

• Earth’s internal heat– Conduction

• Slow release of heat

– Convection• A form of heat

transfer in which hot material circulates from hotter to colder regions, looses its heat, and then repeats the cycle

The tectonic cycle

• Tectonic cycle– Movements/ interactions by which rocks are cycled from

the mantle to the crust and back– Includes earthquakes, volcanism, and plate motion,

driven by convection in the mantle– As a result, seafloor recycles itself every 200 million

years

The tectonic cycle

Hawaiian Islands

Critical Thinking

• What are some of the important questions about plate tectonics that remain unanswered today?

• Why do geologists call plate tectonics a “unifying theory”?

• As Africa slowly separated from Arabia, and the sea began to enter the rift, what kind of sediment would you expect to be deposited?