Continental Drift and Plate Tectonics
Reminders:
Did you read chapter 27
before coming to class?
A. Yes
B. No
 Some of you still have not
registered your clicker.
 Technical glitches with scoring
the first two vocabulary
quizzes.
As the continents drift apart, Bob tries
one last time to save his faithful dog Rex.
How are Mountains and Continents
Formed?
 Prior to the early 1900’s it was thought that
mountains were pushed up from below somehow,
but there weren’t good explanations.
 So how would you go about trying to come up with
an explanation (a theory)?
• Collect evidence and make a hypothesis
• Review the evidence to see if it fits the hypothesis
• Revise the hypothesis and formulate a theory
Glaciers leave distinctive patterns as they
flow
Jigsaw fit of the Continents
 Many people had
noticed the
similarities of the
coasts.
 Becomes even more
compelling when you
consider the edges of
the continental shelf.
 Places where it didn’t
fit were often
located near river
deltas.
Patterns of Ancient Glaciation
1
Paleoclimate Indicators and structural
trends
Patterns of Fossil Occurrence
Rocks (magnetite) can record the
orientation of Earth’s magnetic field
You can tell whether the continents or the
poles wander by comparing two continents
Fixed Continents,
Wandering Pole
So how do you move a continent?
Fixed Pole,
Wandering Continent
Topography of Seafloor
2
The ocean crust is youngest near the
ridges
Thickness of Sediment on Seafloor
Magnetic Reversals
There are stripes of oppositely magnetized
rock running parallel to the ridges.
The ocean floor structure indicates that it is
created at a ridge and then spreads
What drives plate motion?
 Forces driving plate motion
•
•
•
Ridge Push
Slab Pull
Basal Drive?
 Forces resisting plate motion
•
•
•
•
Basal Drag?
Transform Fault Friction
Collision
Mantle Resistance
3
Since the surface area isn’t getting any bigger, we
know that ocean crust is destroyed at trenches
Plate Boundaries



Divergent Plate Boundaries in the oceans
 Extensional Stresses stretching ocean crust
 Creates New Seafloor
 Volcanoes
•
Eruptions of basalt
•
Low in Si, high in Fe
 Earthquakes
•
•
Shallow foci
Low to modest magnitudes
Divergent Plate Boundaries under
continents
 Extensional Stresses
stretching
 Eventually creates New
Seafloor
 Volcanoes
• Basalt and granite (Bimodal
composition)
 Earthquakes
• Shallow foci
• Modest magnitudes
larger than oceanic
rifts.
The action in plate tectonics is almost always at the plate boundaries.
Two plates moving towards each other form a convergent boundary
1. Ocean crust meets ocean crust
2. Ocean crust meets continental crust
3. Continental Crust meets continental crust
Two plates moving away from each other form a divergent boundary
Partial melting of peridotite in the mantle produces basalt.
Why doesn’t this process just produce more peridotite?
 Different mineral
components in the
peridotite melt at
different T’s
 What will happen if only
a small fraction of the
rock melts?
 Will the melted part
have the same
composition as the
original rock?
The rift valley in Africa is one of the most
clear examples of continental rifting
Oldoinyo Lengai
Erta Ale
Killimanjaro
4
The rift occurring under the Red Sea is
more advanced
Ocean-Ocean Convergance
Island arcs: Indonesia, Japan, Taiwan, Philippines,
Kamchatka Peninsula, Aleutian Islands.
Ocean-Continent Convergance
The Andes mountains are the result of a
ocean-continent collision
5
Continent-Continent Collision
Subduction (Benioff) Zones


Named for U.S. geophysicist Hugo Benioff
He observed a pattern of earthquake focal
depths near trenches, which he concluded
confirmed the idea of recycling of the
oceanic crust and its convection.
 Why are there no volcanoes when continents
collide?
 No plate is sinking into the mantle and melting!
Hugo Benioff
The Indian subcontinent slammed into Asia
several million years ago
India is now a part of Asia. The Asian
continent has grown.
Appalachian mountains were similar to the Himalaya mountains
200-300 MYA (collision between North America and Africa)
Build-up of North America through time
Pieces added to western
North America in the last
200 million years
6
Transform Plate Boundaries
Transform faults are evident in the ocean
ridges
 Shear Stresses
 No destruction or
creation of crust
 Volcanoes are rare
 Earthquakes
• Shallow foci
• Large magnitudes
 Occur both in ocean
basins and on continents
The San Andreas fault is a transform
boundary
Island Chains
Other “Hot Spots” around the world
 Created by plates drifting over hot
spots.
7
Major plate boundaries today
Lets play “Name that boundary”!
Island Arc
Transform
Boundary
Subduction
(Trench)
The interior structure of Earth has been
determined mostly from
a)
b)
c)
d)
e)
Drilling
Exploration
X-Rays
Earthquake waves
Chemical analysis
Converging
Continental
Continental
Rift Zone
Divergent plate
boundary
What causes the S-wave shadow zone?
A. The distance is too far to
sustain any amplitude
B. The waves refract and are
directed away
C. The waves reflect back to
the epicenter
D. Shear waves cannot pass
through the liquid outer
core
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