I. Continental drift: an idea before its
time
A. Alfred Wegener
1. First proposed hypothesis, 1915
B. Wegener's
continental drift hypothesis
1. Supercontinent
called Pangaea began breaking apart about 200 million years ago
2. Continents "drifted"
to present positions
3. Continents "broke"
through the ocean crust
4. Evidence used by Wegener
a. Fit of
b.
Fossils match across the seas
c. Rock types and structures
match
d. Ancient climates
5. Main objection to Wegener's proposal was its inability to provide a mechanism
II. Plate tectonics: the new paradigm
A. More encompassing than continental
drift
B. Associated with Earth's rigid outer
shell
1. Called the lithosphere
2. Consists of several plates
a. Plates are moving slowly
C. Asthenosphere
1. Exists beneath the lithosphere
2. Hotter and weaker than
lithosphere
3. Allows for motion of lithosphere
D. Plate boundaries
1. All major
interactions among plates occur along their boundaries
2. Types of plate
boundaries
a. Divergent
plate boundaries (constructive margins)
1. Two
plates move apart
2.
Mantle material upwells to create new seafloor
3. Ocean
ridges and seafloor spreading
4.
Continental rifts form at spreading centers within a continent
b.
Convergent plate boundaries (destructive margins)
1.
Plates collide, an ocean trench forms and lithosphere is subducted
into the mantle
2. Types of convergence
a.
Oceanic-continental convergence
1.
Denser oceanic slab sinks into the asthenosphere
b.
Oceanic-oceanic convergence
1.
Two oceanic slabs converge and one descends beneath the other
2.
Often forms volcanoes on the ocean floor
c.
Continental-continental convergence
1.
When subducting plates contain continental material,
two continents collide
2.
Can produce new mountain ranges such as the Himalayas
c. Transform
fault boundaries
1.
Plates slide past one another
a.
No new crust is created
b.
No crust is destroyed
E. Evidence for the
plate tectonics model
1. Paleomagnetism
a. Paleomagnetic records show
1. Polar
wandering (evidence that continents moved)
2.
Earth's magnetic field reversals
a. Recorded
in rocks as they form at oceanic ridges
b.
Record of reversals across ocean ridges confirms seafloor spreading
2. Earthquake
patterns
3. Ocean drilling
a. Age of
deepest sediments
1.
Youngest are near the ridges
2. Older
are at a distance from the ridge
4. Hot spots
a. Rising
plumes of mantle material
b. Volcanoes
can form over them
1. e.g.,
Hawaiian Island chain
2.
Chains of volcanoes mark plate movement
F. Driving mechanism
of plate tectonics
1. No one model
explains all facets of plate tectonics
2. Earth's heat
is the driving force
3. Several models
have been proposed
a. Slab-pull
and slab-push model
b. Plate-mantle
convection
1. Mantle plumes
extend from mantle-core boundary and cause convection
within the mantle
2.
Models
a.
Layering at 660 kilometers
b.
Whole-mantle convection
c. Deep-layer model