Plate Tectonics

Earth Science Mr. Ahearn March 06

Continental Drift Alfred Wegner proposed in 1912 that all continents were once connected as one large landmass called Pangaea that broke apart about 200 million yrs ago, this was his theory of continental drift.

Breakup of Pangaea

Evidence for Continental Drift • Fossil Clues – fossils from animals such as the

Mesosaurus

found in S. America and Africa

Evidence for Continental Drift Climate Clues glacial deposits and grooved bedrock indicate that ice once covered southern areas of S. America, Africa, India, and Australia. Northern continents have fossils of tropical plants.

Glacial Evidence

Evidence for Continental Drift Rock Clues there are similar rock structures found on different continents. ex. The Appalachian Mtns are similar to the mtns. of Greenland and Western Europe.

Matching Mountain Ranges

Review Questions 1.) What is the theory of continental drift? Who came up with it?

2.) What is the evidence that supports continental drift?

3.) Why would you expect to see similar rocks and rock structures on two landmasses that were connected at one time?

Seafloor Spreading Not until the invention of echo-sounding did scientist discover the ocean is full of mountains and valleys.

Mid-Ocean Ridges form an underwater mtn. range that extends through the center of much of Earth’s oceans.

Seafloor Spreading • Early 1960’s Princeton University’s Harry Hess came up with explanation for mid ocean ridges, known as: Seafloor Spreading- hot, less dense material in the mantle is forced upward to the surface at a mid-ocean ridge where the material flows sideways, carrying the seafloor away from the ridge.

Seafloor Spreading Evidence Age-

Glomar Challenger

drilled into seafloor rocks, discovering the rocks became older as they moved away on each side of the mid-ocean ridge.

Seafloor Spreading

Magnetic Clues -

Earth has reversed magnetic field in the past.

-Fe aligns in ocean basalt according to Earths Field.

- rocks show many field reversals moving away from ridges.

Seafloor Spreading • Deep sea sediments and rocks were collected • Ocean rock  young 180 myo (compared to continental 3.8 byo) • Ocean sediment thin compared to continental • Young rock and thinner sediments @ ridges • Old rock and thicker sediment @ trenches

Isochron • Lines connecting points of same age

The Missing Link • Seafloor spreading explained the mechanism that made continents drift, completing Wegner’s model.

Review • How does ocean ridges and trenches support the theory of seafloor spreading?

• How is ocean rock & sediments evidence for seafloor spreading?

• How does an isochron map of the ocean floor support seafloor spreading?

Plate Tectonics According to the

plate tectonics

theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere.

• A

plate

is one of numerous rigid sections of the lithosphere that move as a unit over the material of the asthenosphere.

Plate Boundaries

Divergent boundaries

(also called spreading centers) are the place where two plates move apart.

Convergent boundaries

plates move together.

form where two

Transform fault boundaries

are margins where two plates grind past each other without the production or destruction of the lithosphere.

Types of Plate Boundaries

Divergent Boundaries

Oceanic ridges

are continuous elevated zones on the floor of all major ocean basins. The rifts at the crest of ridges represent divergent plate boundaries.

Rift valleys

are deep faulted structures found along the axes of divergent plate boundaries. They can develop on the seafloor or on land.

Seafloor spreading

produces new oceanic lithosphere.

Spreading Zones

Continental Rifts When spreading centers develop within a continent, the landmass may split into two or more smaller segments, forming a rift.

East African Rift Valley

Convergent Boundaries • A

subduction zone

occurs when one oceanic plate is forced down into the mantle beneath a second plate.

Oceanic-Continental • Denser oceanic slab sinks into the asthenosphere • • Pockets of magma develop and rise

Continental volcanic arcs

form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent • Examples include the Andes, Cascades, and the Sierra Nevadas

Ocean-Continent

Ocean-Ocean

Continent-Continent

Collision of India

Transform Boundary • At a transform fault boundary, plates grind past each other without destroying the lithosphere

Evidence for Plate Tectonics • •

Paleomagnetism

is the natural remnant magnetism in rock bodies; this permanent magnetization acquired by rock can be used to determine the location of the magnetic poles at the time the rock became magnetized.

Normal polarity

—when rocks show the same magnetism as the present magnetism field

Reverse polarity

—when rocks show the opposite magnetism as the present magnetism field

Paleomagnetism in Lava Flows

Ocean Crust Polarity

Evidence for Plate Tectonics • Earthquakes • Ocean drilling • Hot Spots

Hotspots

Complete #38- 50 Review Book p.69

Causes of Plate Tectonics • Convection in the Mantle

Causes of Plate Tectonics • Slab pull – ridge push

Complete Questions 51-57 Review Book p. 71 The End