Title: Types of Plate Boundaries
1Plate Tectonics
Types of Plate Boundaries
2Alfred Wegner - Continental Drift Hypothesis
Alfred Wegener, a German climatologist, developed
the Continental Drift hypothesis in 1915
3Continental Drift
- Continents have drifted to present locations
- Continents once joined as supercontinent
Pangaea - Pangaea formed 250 mya
- 200 mya, tectonic forces began pulling Pangaea
apart
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6 Some of Wegeners Evidence at the Time
7Evidence for continental drift
8Rocks in Newfoudland are same age/type as Sweden,
Norway, Scotland.
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10Problems with Continental Drift Hypothesis
- Continents drift -- but what about the ocean
floor? - What force could move continents?
- Studies of the ocean floor in the 2 decades
following WWII led to the development of the
plate tectonic theory
11Mid-Atlantic Ridge
- Mountain range running N-S on floor of Atlantic
Ocean - Magnetic polarity of ocean floor striped with
alternating N/S poles - This is called Magnetic striping
- How does this occur?
- 1. New ocean forms when basaltic magma from
mantle rises and hardens at the ocean ridge. New
magma coming up moves older rock away from ridge
like conveyer belt.
12- 2. Basalt, rich in iron, becomes magnetic
- 3. Minerals line up with magnetic north of earth
- 4. Earths magnetic field flips every 500,000
years - New portions of the ridge will have reverse
polarity - Result alternating bands of normal and reverse
polarity in rock around ridge
13The Theory of Plate Tectonics
- The Earth is constantly changing
- The Earths crust is divided into 8 large plates
(and several small plates) - Almost all major earthquake or volcano activity
occurs along the plate boundaries - Because each plate moves as a unit, the interiors
of the plates are generally stable. - Really not a theory due to overwhelming
evidence!!!!
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16- Lithosphere (crust upper mantle) broken into
lithospheric plates (tectonic plates)
17How plates move - Convection Currents
18Mantle convection
- Convection in the mantle brings hot material
upward in some places. Elsewhere, cooler rock
sinks. - Upwelling hot material can cause lithosphere to
rift (split) and plates drift apart (usually at
oceanic ridges)
19- The plates pushed apart contact another plate
- This often forms subduction zones
- Denser plate (usually oceanic) forced underneath
less dense plate (continental) - A valley, called a trench, is formed
- Subducting plate pulls rest of plate slab pull
20TYPES OF PLATE BOUNDARIES
- Divergent boundaries -- plates move away from
each other - Convergent boundaries -- plates move toward each
other - Transform boundaries -- plates try to slide past
each other
21 A Divergent B Convergent C Transform
Types of plate Boundaries
- plates are moving apart
- new crust is created
- magma is coming to the surface
- plates are coming together
- crust is returning to the mantle
- plates are slipping past each other
- crust is not created or destroyed
22A Divergent B Convergent C Transform
- almost always found under the ocean
- Forms mid-ocean ridges
- Iceland is a rare example of one on land
- usually ocean plate colliding with land plate
- ocean plate goes under land plate
- pushes up mountains and forms deep ocean
trenches (subduction zones)
- rare on the planet
- famous one is the San Andreas Fault in California
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24DIVERGENT BOUNDARIES
- Plates move apart along a system of fractures
- As magma rises and cools, it pushes older rock
away - Newer rock found closer to spreading oceanic
ridge and older rock farther away - Spreading on land rift
- Blocks of rock are down-dropped along fractures
(faults) - rift valleys - Seafloor spreading mid-ocean ridges volcanic
activity produces new seafloor as plates drift
apart - Examples E. African Rift, mid-Atlantic ridge
25Diverging Plates
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28How ocean basins formed
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31CONVERGENT BOUNDARIES Oceanic-Continental
- Oceanic plate (dense basalt) subducts under
continental plate (less dense granite) - Subducting plate pushed into mantle and
meltsthis forms cone volcanos - Force also causes formation of mountains as
continental crust crumples - Causes small?large earthquakes
- BC is over a subduction zone oceanic Juan de
Fuca plate subducting under continental North
American plate ? formed Coast Mountains
32Converging Plates - Subduction
33CONVERGENT BOUNDARIES Oceanic-Oceanic
- Rock densities similar, so one plate forced under
other - Volcanoes produced
- Form long chain of island island arc
- Eg Japan, Aleutian Islands
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36Earthquake focus increases in depth along
subducting plate
The further from the edge of plate, the deeper
the earthquake
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38CONVERGENT BOUNDARIES Continental-Continental
- Plates are same density
- No subduction!
- Instead, we get mountain building
- Plates crumple and fold as they collide
- No volcanos formed
- Do get earthquakes
- Eg Himilayas (Mt. Everest) formed by India
colliding with Eurasian plate
39Converging Continental Plates
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42TRANSFORM BOUNDARIES
- Plates slide past each other in opposite
directions - No volcanoes or mountains
- Do get many large, shallow earthquakes
- Examples San Andreas fault zone, southern CA
- oceanic Pacific Plate sliding past North American
plate - Also found at divergent plate boundaries
43Transform Boundaries
44Streams offset by San Andreas Fault
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46GATHERING EVIDENCE
- Field work - geologists sampling rocks, drilling,
mapping formations - Remote Sensing - observing from a distance
(satellite photos, sonar mapping of ocean floors) - Seismology - study of earthquakes and seismic
waves - Volcanology - study of volcanoes