Learning Objectives
After reading, studying,
and discussing the chapter, students should be able to:
·
Briefly discuss the evidence
used by Alfred Wegener to support his theory of continental drift.
·
Briefly explain the theory of
plate tectonics.
·
Compare and contrast the
distribution and geologic characteristics of tectonic plate boundaries,
including divergent, convergent, and transform boundaries.
·
Discuss the evidence used to
test the plate tectonics model, including ocean drilling and hot spots.
·
Describe how the continents were
arranged in the past.
·
Discuss mantle convection and
the various mechanisms proposed to explain plate motion.
Chapter Outline___________________________________________________________________
I. Continental drift
A. Alfred Wegener
1. First
proposed hypothesis, 1915
2. The
Origin of Continents and Oceans
B. Supercontinent called Pangaea began
breaking apart about 200 mya
C. Continents "drifted" to present
position and broke through oceanic crust
D. Evidence used by
Wegener and others
1. Fit of
South America and Africa
2.
Fossils
3.
Rock structures
4.
Ancient climates
E.
Objections
1. Inability
to provide a mechanism
2. No evidence that continents
broke through ocean crust
II. Plate tectonics
A. More
encompassing than continental drift
B. Associated with
Earth's rigid outer shell
1. Called
lithosphere
2. Consists
of about 20 slabs called plates
a.
Plates are moving slowly
b.
Largest plate is the Pacific plate
c.
Plates are mostly beneath the ocean
C. Asthenosphere
1. Beneath
lithosphere
2. Hotter and
weaker than lithosphere
3. Allows for motion of
lithosphere
D. Plate boundaries
1. Associated
with plate boundaries
a.
Seismic activity
b.
Volcanism
c. Mountain building
2. Types of plate boundaries
a.
Divergent (spreading)
1)
Most along oceanic ridge crests
2) Seafloor spreading
forms ridge crest fractures which fill with molten material
3) On the continents
rifts or rift valleys form
b.
Convergent
1) Lithosphere subducted
into mantle
2) Types of convergent boundaries
a)
Oceanic-continental forms a subduction zone
(1) Deep-ocean trench
(2) Continental volcanic arcs
b) Oceanic-oceanic
forms volcanoes on ocean floor that merge as a volcanic island arc
c)
Continental-continental often produces mountains
(1) e.g., Himalayas
(2) Other possibilities
(a) Alps
(b) Appalachians
(c) Urals
c.
Transform
1)
Plates slide past one another
a) No new crust created
b) No crust destroyed
2)
Transform faults
a) Most in oceanic crust
b) Join two segments
of mid-ocean ridge
c) Parallel motions of plates
E. Evidence
1. Ocean
drilling
a. Deep Sea Drilling Project
(Glomar Challenger)
b. Age
of oldest sediments
1)
Youngest near ridges
2)
Older with distance from ridge
c. Ocean
basins are geologically young
2. Hot spots
a.
Rising plume of mantle material
b.
Volcanoes can form over
1)
i.e., Hawaiian Island chain
2)
Chains mark plate movement
3.
Paleomagnetism
a.
Probably most persuasive evidence
b. Ancient magnetism found in
rocks
c.
Paleomagnetic records show
1)
Polar wandering
2)
Evidence for movement
3)
Earth's magnetic field reversals
a. Recorded in sea floor rocks
b. Confirms seafloor spreading
F. Driving
mechanism
1. No one model explains all plate
motions
2. Earth's heat is the driving
force
3. Several
models have been proposed
a.
Convection currents in mantle
b. Slab-pull, ridge push, and
slab suction
1)
Descending oceanic crust pulls
2)
Elevated ridge system pushes
c.
Mantle plumes
1) Extend from
mantle-core boundary
2) Spread laterally
under lithosphere
