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Types of Plate Boundaries

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Title: Types of Plate Boundaries


1
Plate Tectonics
Types of Plate Boundaries
2
Alfred Wegner - Continental Drift Hypothesis
Alfred Wegener, a German climatologist, developed
the Continental Drift hypothesis in 1915
3
Continental 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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Some of Wegeners Evidence at the Time
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Evidence for continental drift
8
Rocks in Newfoudland are same age/type as Sweden,
Norway, Scotland.
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Problems 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

11
Mid-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.

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  • 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

13
The 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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  • Lithosphere (crust upper mantle) broken into
    lithospheric plates (tectonic plates)

17
How plates move - Convection Currents
18
Mantle 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

20
TYPES 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

22
A 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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DIVERGENT 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

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Diverging Plates
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How ocean basins formed
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CONVERGENT 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

32
Converging Plates - Subduction
33
CONVERGENT 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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Earthquake focus increases in depth along
subducting plate
The further from the edge of plate, the deeper
the earthquake
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CONVERGENT 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

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Converging Continental Plates
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TRANSFORM 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

43
Transform Boundaries
44
Streams offset by San Andreas Fault
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GATHERING 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
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