Plate Tectonics The Way the Earth Works

1
Plate TectonicsThe Way the Earth Works

• CMU
• Bill Palmer
• Lecture 3

Central Methodist University
2
Continental Drift Hypothesis
Proposed by Alfred Wegener in 1915 Supercontinent
Pangaea started to break up about 200 million
years ago Continents "drifted" to their present
positions Continents "plowed" through the ocean
crust
3
Earth 200 million years ago
http//www.youtube.com/watch?vUtls9WGfcEcfeature
fvw
4
Continental Drift Evidences
1. Geographic fit of South America and Africa 2.
Fossils match across oceans 3. Rock types and
structures match across oceans 4. Ancient
glacial features
5
Continental Drift Evidence 1
Tight fit of the continents, especially
using continental shelves.
6
Continental Drift Evidence 2
Fossil plants and animals
7
Continental Drift Evidence 3
Correlation of mountains with nearly identical
rocks and structures
8
Continental Drift Evidence 4
Glacial features of the same age restore to
a tight polar distribution.
9
Continental Drift Reactions
Received well in Europe and southern
hemisphere. Rejected in U.S., where scientists
staunchly preferred induction (incremental
progress built on observation) over what they
perceived as speculative deduction. Lack of a
suitable mechanism crippled continental drifts
widespread acceptance. Conflict remained
unresolved because seafloors were almost
completely unexplored.
10
The Rise of Plate Tectonics
WW II and the Cold War Military Spending
U.S. Navy mapped seafloor with echo sounding
(sonar) to find and hide submarines. Generalized
maps showed oceanic ridgessubmerged mountain
ranges fracture zonescracks perpendicular to
ridges trenchesnarrow, deep gashes
abyssal plainsvast flat areas
seamountsdrowned undersea islands
Dredged rocks of the seafloor included only
basalt, gabbro, and serpentineno continental
materials.
11
Major Ocean Features
us
12
The Rise of Plate Tectonics
Marine geologists found that seafloor magnetism
has a striped pattern completely unlike patterns
on land.
Black normal polarity White reversed
polarity Both very magnetic
13
The Rise of Plate Tectonics
Hypothesis Stripes indicate periodic reversal
of the direction of Earths magnetic field.
To test this hypothesis, scientists determined
the eruptive ages AND the polarity of young
basalts using the newly developed technique of
Krypton Argon (K-Ar) radiometric dating. K-Ar
looks at the ratio of these two elements and
scientists can then calculate the age of a rock.
The study validated the hypothesis...
14
The Rise of Plate Tectonics
And then (1962-1963) geologists realized that the
patterns are SYMMETRICAL across oceanic ridges.
The K-Ar dates show the youngest rocks at the
ridge.
15
The Rise of Plate Tectonics
Meanwhile, U.S. military developed new, advanced
seismometers to monitor Soviet nuclear tests. By
the late 1950s, seismometers had been deployed in
over 40 allied countries and was recording 24
hrs/day, 365 days/year. Besides the occasional
nuclear test, it recorded every moderate to large
earthquake on the planet. With these
high-precision data, seismologists found that
activity happens in narrow bands.
16
Bands of Seismic Activity Chiefly at trenches and
oceanic ridges
17
The Theory of Plate Tectonics
Group Authorship 1965-1970
Earths outer shell is broken into thin, curved
plates that move laterally atop a weaker
underlying layer. Most earthquakes and volcanic
eruptions happen at plate boundaries. Three
types of relative motions between plates
1. divergent 2. convergent 3.transform
18
Tectonic Plates
19

• Three types of relative motions between plates

20
Divergent Boundaries Chiefly at oceanic
ridges (aka spreading centers)
21
Magnetic Reversals Formed at a spreading center
22
Divergent Boundaries Rip Apart (rift)
Continents
23
Examples of Rifting
East Africa Rift Red Sea Atlantic Ocean
24
Subduction Zone forms at convergent boundaries if
at least one side has oceanic (denser) material.
Major features trench, biggest EQs, explosive
volcanoes
Modern examples Andes, Cascades
25
Another Subduction Zone this one with oceanic
material on both sides.
Modern example Japan
26
Earthquake Depth Indicates subduction zones
27
Collision Zones Form where both sides of a
convergent boundary consist of continental
(buoyant) material
This probably used to be a subduction zone, but
all the oceanic material was subducted.
Modern example Himalayas
28
India meets Tibet Worlds Greatest Subduction
Zone Himalaya Mountains
Mt. Everest
29
Transform Boundary Most are in the
oceans. Some, like the one in California, cut
continents. The PAC-NA plate boundary is MUCH
more complex than this diagram shows.
30
Hotspots such as the one under Hawaii have
validated plate tectonics
31
http//www.youtube.com/watch?v1oLeKTvSIzQfeature
related
32
Why Do the Plates Move?
Two related ideas are widely accepted Slab
pull Denser, colder plate sinks at subduction
zone, pulls rest of plate behind it.
(gravity) Mantle convection Hotter mantle
material rises beneath divergent boundaries,
cooler material sinks at subduction zones. (Heat
Convection) So moving plates, EQs, volcanic
eruptions are due to Earths loss of internal
heat.
33
How Does Convection Work? No one knowsbut they
arent afraid to propose models!
Whole-mantle convection
Two mantle convection cells
Complex convection
34
Wrap-ups

• 1. Who first proposed the idea of Plate
  Tectonics? When?
• 2. What is Pangaea?
• 3. What are FOUR evidences that prove Plate
  Tectonics?
• 4. Summarize the history of scientist acceptance
  of plate tectonics?
• 5. What role did the military play in the Plate
  Tectonics Theory?

35
Wrap-ups

• 6. How does seafloor magnetism support Plate
  Tectonics?
• 7. How do bands of seismic activity support Plate
  Tectonics?
• 8. What are five ocean features related to Plate
  Tectonics?
• 9. What are the three types of motion between
  plates? Make a simple sketch of each.
• 10. What are the three major parts of the theory
  of Plate Tectonics?