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
1960s 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, cant 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 Earths 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)
Earths 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 plates 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
Earths 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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