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Chapter 7 Plate Tectonics

Describe the models that have been proposed to explain the driving mechanism for plate motion

Continental Drift: An Idea Before Its Time

Fit together like pieces of a puzzle

1915, Alfred Wegener plublished

" The origin of Continents and oceans"

continental drift

Pangaea means "all land" fig. 7.2, p. 181

200 million years ago

fossils, rocks, ancient climates

 

Evidence

Similarity of coastlines on opposites sides of South Atlantic

1960’s challenged by other scientists

figure 7.3, p. 182

streams and sediments, enlarging continental shelf

 

Evidence: Fossils Match Across the Seas

Identical fossils on widely separated land masses

Mesosaurus, fig. 7.4, p. 182

Glossopteris = fossil fern

large seeds, can’t be blown far

diverse climates

Distribution of present day organisms

Australian marsupial

 

Evidence: Rock Types and Structure Match

The picture and the continental drift puzzle

the pieces must fit and be continuous

Rock match

Mountain belts, coastline to coastline

Appalachians to Newfoundland fig. 7.6 A, p. 184

Reformation = continuous belt Figure 7.6 B, p. 184

 

Evidence: Ancient Climates

Paleoclimatic = changes

glacial deposits 220-300 million years ago

ice sheets cover Southern Hemisphere

glaciers found in southern Africa and South America and in India and Australia striated and grooved bedrock

Fossils and coal fields

Figure 7.7, p. 185

 

The Great Debate

1924 criticism

After his death in 1930

Hostile critque T.C. Chamberlin

Moon and tides - Harold Jeffreys

Solution - continental drift

 

Plate Tectonics

More encompassing than continental drift

Associated with Earth’s rigid outer shell

Called the lithosphere

Consists of about 20 slabs (plates)

Plates are moving slowly

Largest plate is the Pacific plate

Plates are mostly beneath the ocean

 

Asthensophere

Exists beneath the lithosphere

Hotter and weaker than lithosphere

Allows for motion of lithosphere

 

Plate Boundaries

Associated with plate boundaries

Seismic activity

Volcanism

Mountain building

 

Types of plate boundaries

Divergent (spreading) boundary

Most exist along oceanic ridge crests

Seafloor spreading occurs along the boundary

Forms fractures (openings) on the ridge crest

Fractures fill will molten material

When the boundary occurs on a continent, rifts or rift valleys form

 

Type of plate boundaries

Convergent boundary

Lithosphere is subducted into the mantle

Types of convergent boundaries

Oceanic-continental boundary

Oceanic-oceanic boundary

Continental-continental boundary

 

Oceanic-continental boundary

Forms a subduction zone with a deep-ocean trench

Volcanic arcs form

Andes

Cascades

Sierra Nevada system

 

Oceanic-oceanic boundary

Often forms volcanoes on the ocean floor

Island arc forms as volcanoes emerge

Aleutian Islands

Alaskan Peninsula

Philippines

Japan

 

Continental-continental boundary

Neither plate will subduct

Can produce mountains

Himalayas

Other possibilities

Alps

Appalachians

Urals

 

Transform boundary

Plates slide past one another

No new crust is created

No crust is destroyed

Transform faults

Most are in oceanic crust

Parallel the direction of plate movement

Aids movement of crustal material

 

Evidence that supports
plate - tectonics

Paleonmagnetism

Probably the most persuasive evidence

Ancient magnetism preserved in rocks

Paleomagnetic records show

Polar wandering (evidence that continents moved)

Earth’s magnetic field reversals

Recorded in the sea floor rocks

Confirms seafloor spreading

 

Earthquake patterns

Associated with plate boundaries

Deep-focus earthquakes along trenches provide a method for tracking the plate’s descent

 

Ocean drilling

Deep Sea Drilling Project (ship: Glomar Challenger)

Age of deepest sediments

Youngest are near the ridges

Older are at a distance from the ridge

Ocean basins are geologically young

 

Hot spots

Rising plumes of mantle material

Volcanoes can form over them

Hawaiian Island chain

Chains of volcanoes mark plate movement

 

Breakup of Pangaea

Migrations of continents over the past 500 million years has been determined

Breakup begins about 200 million years ago

North American and Africa began separating between 200 and 165 million years ago

Africa and South America begin splitting apart about 135 million years ago

Landmasses also had fragmented prior to the formation of Pangaea

Fragments that formed Pangaea began collecting 500 and 225 million years ago

Driving mechanism of plate tectonics

No one model explains all plate motions

Earth’s heat is the driving force

Several models have been proposed

Convection currents in mantle

Slab-pull and slab-push model

Descending oceanic crust pulls the plate

Elevated ridge system pushes the plate

Hot plumes

Extend from mantle-core boundary

Spread laterally under lithosphere

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