Continental Drift & Plate Tectonics

Plate Tectonics Explains…Plate Tectonics Explains…

The formation of:

1. Oceans

2. Mid-ocean Ridges

3. Trenches

4. Mountains

5. Earthquakes

6. Volcanoes

Continental DriftContinental Drift• (1912) Wegener proposed that continents began to break apart about 200 million

years ago• He called this continental drift, where continents continue to slowly move to their

present positions.

Drifting Continents

• Pangaea, a Greek word that means “all the earth,” refers to the combined landmass. Wegener proposed that the continents were joined as a single landmass called Pangaea.

A Rejected HypothesisA Rejected Hypothesis• In the early 1900s, most scientists rejected Wegener’s

hypothesis of continental drift.

Drifting Continents

• Two unanswered questions—what forces could move continents and how continents could move without shattering—were the main reasons that the hypothesis of continental drift was rejected.

Pangaea

The Tethys Sea existed between the continents of Gondwana and Laurasia before the opening of the Indian Ocean.

Historical Overview

Continental DriftContinental DriftEvidence from Rock Formations

Drifting Continents

a) Wegener reasoned mountain ranges would have fractured as the continents separated.

b) Using this reasoning, Wegener hypothesized that similar rock types are on opposite sides of the Atlantic Ocean.

c) That similar groups of rocks were observed in the South America, Africa, United States, Greenland, and Europe supported Wegener’s idea.

Evidence of Continental DriftRock Record

Evidence of Continental DriftFossil Record

Similar animal and plant fossils were found on opposite sides of the Atlantic Ocean.

Continental DriftContinental DriftEvidence from Fossils

Drifting Continents

Continental DriftContinental DriftAncient Climatic Evidence

Drifting Continents

• Coal deposits found in Antarctica.

• Glacial rock deposits in Africa.

Seafloor SpreadingSeafloor Spreading

• An American scientist named Harry Hess proposed the theory of seafloor spreading.

Seafloor Spreading

– Magma is forced toward the crust along an ocean ridge and fills the gap that is created.

• Seafloor spreading states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.

Seafloor SpreadingSeafloor SpreadingSeafloor Spreading

– Each cycle of spreading and the intrusion of magma results in the formation of another small section of ocean floor, which slowly moves away from the ridge.

– When the magma hardens, a small amount of new ocean floor is added to Earth’s surface.

Theory of Plate TectonicsTheory of Plate Tectonics• The theory of plate tectonics states that

Earth’s crust is broken into slabs called plates.

Theory of Plate Tectonics

• There are a dozen major plates and several smaller ones.

• Tectonic plates move in different directions and at different rates over Earth’s surface.

Theory of Plate TectonicsTheory of Plate TectonicsTheory of Plate Tectonics

Plate BoundariesPlate Boundaries• Tectonic plates interact at places called

plate boundaries.

Theory of Plate Tectonics

• At some plate boundaries:

– Plates come together, or converge – Plates move away from one another, or diverge – Plates move horizontally past one another, or

transform.

Plate Boundaries (3)Plate Boundaries (3)1. Divergent Boundaries

Theory of Plate Tectonics

– Divergent boundaries are places where two tectonic plates are moving apart.

– Most divergent boundaries are found in rifts, or fault-bounded valleys, which form along the axis of an ocean ridge.

– A rift valley, which is a narrow depression, is created when a divergent boundary forms on a continent.

Divergent Boundaries Divergent Boundaries cont…cont…

– Some of the Some of the magma formsmagma forms new oceanic crustnew oceanic crust at at the ridge or is forced back to the surface, forming the ridge or is forced back to the surface, forming an arc of volcanic islands that parallel the trench.an arc of volcanic islands that parallel the trench.

– Oceanic plates are alwaysOceanic plates are always YOUNGERYOUNGER than than continental plates.continental plates.

Plate BoundariesPlate Boundaries2. Convergent Boundaries

Theory of Plate Tectonics

– Convergent boundaries are places where two tectonic plates are moving toward each other.

– There are three types of convergent boundaries:

a. Oceanic crust converging with oceanic crust

b. Continental crust converging with continental crust.

c. Oceanic crust converging with continental crust

Plate BoundariesPlate BoundariesConvergent Boundaries cont…

– Subduction occurs when one of the two converging plates descends beneath the other.

– A subduction zone forms when one oceanic plate, which has become denser as a result of cooling, descends below another plate creating a deep-sea trench.

– The subducted plate descends into the mantle and melts.

Plate BoundariesPlate Boundaries3. Transform Boundaries

Theory of Plate Tectonics

– A transform boundary is a place where two plates slide horizontally past each other, deforming or fracturing the crust.

– Transform boundaries are characterized by long faults and usually offset sections of ocean ridges.

– The San Andreas Fault is an exception to the fact that transform boundaries rarely occur on continents.

Plate Boundaries Stress/Force Description/Facts

1. Divergent

Example: Mid Ocean Ridge (MOR)

TENSION -Plates move apart.-Lava creates new ocean floor.

2. Convergenta.Ocean – ContinentExample: Andes Mtns.

b. Ocean – OceanExample: Aleutian Islands, Alaska. Japan.

c. Continent – ContinentExample: Himalayas.

COMPRESSION -Plates move together.-Ocean floor is destroyed by subduction.-Mountains, volcanoes, and islands can be formed.

3. TransformExample: -San Andreas Fault, CA-MOR.

SHEARING -Plates move past each other.

Plate BoundariesPlate BoundariesConvergent Boundaries

Theory of Plate Tectonics

Activity: When Plates Create 

You need:4 Pages: Instructions, 114, 115, & 116.5 colored pencils: orange, gray (or black), blue, brown, and red. 1. COLOR and LABEL puzzle pieces: a. Orange (Magma)b. Gray or Black(Oceanic Crust)c. Gray or Black (Oceanic Crust)d. Gray or Black(Oceanic Crust)e. Orange (Magma)f. Blue (water) Brown (Islands)g. Gray or Black (Oceanic Crust)h. Orange (Magma)i. Blue (water) Brown (Islands/volcanoes) Orange (Magma) 2. Answer questions and STAPLE on your “Plates,” and turn in.