Plate Tectonics

1
Plate Tectonics

• Chapter 8

2
What Is Plate Tectonics?
Section 1

• Theory that describes the formation, movements,
  and interactions of Earths lithospheric plates.
• Lithosphere is broken into large plates that
  move (float) on top of the asthenosphere.
• 3 ways
• Away from each other (divergent plate boundaries)
• Toward each other (convergent plate boundaries)
• Slide past each other (transform plate
  boundaries)

3
Early Ideas About Plate Movements

• Have you ever noticed that the continents look
  like they would fit together like the pieces of a
  puzzle??? If you have, youre not alone.
• 1912 Alfred Wegener
• hypothesis ? continental drift
• Continents have drifted over time
• Observations to support hypothesis
• Shape of continents
• Fossils
• Mesosaurus
• South America Africa
• Distinctive rock formations
• Climate change evidence
• coal deposits

Continental Drift Video
4
Evidence for Joined Continents Interaction
5
The Theory of Plate Tectonics

• Continents (and ocean basins) are part of
  lithospheric plates.
• Plate movement continent movement
• Evidence/Explanations of many geologic processes
• Location of volcanoes earthquakes
• Formation of new crust (ocean floor)

6
Locations of Earthquakes Volcanoes

• Occur in concentrated areas
• Mark locations of plate boundaries
• Where plates move apart, together, or past each
  other
• Strain builds up ? releases causing EQs
• Molten rock rises ? volcanic activity
• Pacific Ocean
• Ring of Fire

Plates Volcanoes Video
7
Locations of Earthquakes Volcanoes
8
Magnetism the Age of the Ocean Floor

• Magnetic properties ages of igneous rocks on
  the ocean floor provide evidence for theory of
  plate tectonics.
• Some igneous rocks contain magnetic minerals.
• Provide record of direction of Earths magnetic
  field when rock formed

9
Magnetism the Age of the Ocean Floor

• Records of Earths magnetic field
• Some rocks recorded reversals in magnetic field
• normal polarity
• what we know today as N S magnetic poles
• reversed polarity
• present N magnetic pole became the S magnetic
  pole
• present S magnetic pole became the N magnetic
  pole
• A of magnetic reversals have taken place at
  different times over millions of years

10
(No Transcript)
11
Magnetism on the Ocean Floor
12
Magnetism the Age of the Ocean Floor

• Mid-ocean ridge long chain of volcanic
  mountains on the ocean floor with a deep central
  valley
• Magnetic reversals are recorded in bands of rock
  on opposite sides of the ridge
• Mirror image
• Center of ridge shows current orientation of
  Earths magnetic field (normal or reversed)
• Rocks at center of ridge are youngest (most newly
  formed)
• As move further from center rocks get older

Magnetic Polarity Recorded in Rocks Animation
13
Magnetism the Age of the Ocean Floor

• Mid-ocean ridges ? boundaries between
  lithospheric plates
• New rock along ridge is formed by hot, molten
  rock which rises up between the plates (because
  it is less dense)
• As new rock forms plates spread
• Older rocks pushed away from ridge (both sides)
• Rock at center of ridge hottest
• Temperature decreases as move out from center

14

• HW
• Read pgs 172-175
• Answer s 1, 2, 3

15
Types of Plate Boundaries
Section 2

• Earths lithosphere is broken in to large plates
• move in 3 ways
• Divergent plate boundaries
• Convergent plate boundaries
• Transform plate boundaries

16
Divergent Boundaries

• Plates move apart
• Sometimes called spreading centers
• Most are along ocean floor
• Creates mid-ocean ridge

17
Divergent Boundaries

• Rift valley
• Center of mid-ocean ridge
• Border between 2 diverging plates
• Sea-floor spreading ? molten rock forces through
  cracks (rifts) in valley
• Oceanic crust forms as rock cools
• older crust moves away from mid-ocean ridge as
  the new crust is formed

18
Divergent Boundaries

• Rift valley (continued)
• Broken into sections
• offset from each other by breaks (fracture zones)
• Perpendicular (right angle) to ridge
• Source of earthquakes at mid-ocean ridges

Divergent Plate Boundary Animation
19
Mid-Ocean Ridges

• Mid-Atlantic Ridge

• East Pacific Rise

20
Convergent Boundaries

• 2 plates move towards each other
• 2 broad classifications
• Subduction boundaries
• Collision boundaries

21
Subduction Boundaries

• Oceanic plate subducts (plunges) below another
  plate
• Boundary between the 2 plates is called a
  subduction boundary
• Features of subduction boundaries
• Deep-sea (or ocean) trenches
• Deepest part of the ocean
• Volcanic activity
• Island arcs
• Along the coast of continents
• Earthquakes

22
Subduction Boundaries

• Ocean-Ocean subduction
• 2 Oceanic plates collide
• Deep-sea trench formed
• accompanied by chain of volcanic islands
  (volcanic island arc) on the overriding plate
• Example
• Pacific Plate subducts under Phillippine Plate
• Pacific Plate pulled down
• Forms Mariana Trench
• Phillippine Plate overrides Pacific Plate
• Forms Mariana Islands (volcanic island chain)

Subduction and Collision Boundary Animations
23
Ocean-Ocean Subduction Boundary
24
Ocean-Continent Subduction Boundary

• Ocean-Continent Subduction
• The denser oceanic plate subducts below the
  less-dense continental plate
• Ocean trenches form.
• Volcanoes mountains are formed on the
  continental plate.
• Example
• Nazca Plate (off west coast of S. America)
  subducts under the South American Plate
• Nazca Plate pulled down
• Forms Peru-Chile Trench
• South American Plate overrides Nazca Plate
• Forms Andes Mountains and volcanoes (along
  western edge of S. America)

Subduction and Collision Boundary Animations
25
Ocean-Continent Subduction Boundary
26
Collision Boundaries

• Continent-continent collision
• Neither plate subducts, so the crust piles up
  (because the continents join to form a single
  larger continent)
• Mountains are formed.
• Example
• Indian subcontinent colliding into Eurasian Plate
• Formed Himalaya Mountains

Subduction and Collision Boundary Animations
27
Continent-Continent Collision
28
Transform Plate Boundary

• 2 plates slide past each other at a fault.
• Stress is released as an earthquake.
• Example
• CaliforniaSan Andreas Fault
• Pacific Plate (Southwestern Cali.) is moving NW
• N. American Plate (rest of N. Amer.) moving SE

29
Transform Plate Boundary
Transform Boundary--San Andreas Fault Animation
N
30

• HW
• Read pgs 176-179
• Answer s 1, 2, 3, 4, 6

31
Causes Of Plate Movement Ch. 8.3
Convection cells (caused by density differences)
in the asthenosphere (mantle) cause hot material
to rise, move outward (pushing the plates/ridge
push), sink as the material cools (pulling
plates down/slab pull).
Convection in the Mantle Animation
32
The Big Picture
33

• HW
• Read pgs 180-181
• Answer 1

34
Plate Movements Continental Growth Sec 4

• Reconstructing the Past
• Many different kinds of evidence give clues as to
  what Earth looked like in the past
• Rocks in Ural Appalachian Mountain
• evidence of past subduction ? formed at
  convergent boundary, but neither mountain range
  is located near a plate boundary today
• Ages of rocks that form in ocean basins
• Magnetic record of igneous rocks
• Can reveal latitude of formation
• Fossils
• Organisms that lived in shallow seas ? fossils
  found on high mountaintops
• Rocks that show evidence of having been covered
  by glaciers, but now are in tropical areas

35
Plate Movements Continental Growth

• Plate Tectonics Pangaea
• Evidence suggests 250 million years ago (mya)
  that all the continents were welded together into
  one land mass ? Pangaea

36
Plate Movements Continental Growth

• Formation of Pangaea
• Use data from continents to make models of what
  Earth may have looked like before Pangaea
• A large continental mass stretched between the
  south pole the equator ? Gondwana
• made of smaller landmasses (S. America, southern
  Europe, Africa, the Near East, India, Australia,
  New Zealand, Antarctica)
• Other small landmasses ranged across the globe
• Gondwana moved northward and converged w/ the
  other landmasses to form Pangaea

37
Plate Movements Continental Growth

• Break up of Pangaea
• 2 separate landmasses
• Gondwana
• Laurasia
• Over time both broke into smaller landmasses
  which began to resemble the continents today
• Pangaea is still breaking up
• The formation break up may have happened many
  times in the past

Break up of Pangaea animation
38
(No Transcript)
39
Plate Movements Continental Growth

• Plate Tectonics Continental Growth
• P.T. affects shape of continents as well as
  position
• Added rock materials to the margins of ancient
  continent cores (cratons)
• 2.5 billion years ago (bya) core continental
  material stabilized
• Ex. North American craton
• at surface in most of eastern Canada ? Canadian
  Shield
• Some of the oldest rock material (4 billion
  years old)
• Rest of the craton is buried under a platform of
  sediments
• Remainder of the continent is material added to
  the craton

40
(No Transcript)
41
Plate Movements Continental Growth

• Sources of Growth Material
• Deep-sea sediments
• Added when an oceanic plate plunges under a
  continental plate at a subduction boundary
• Sediments from ocean floor scraped off left
  behind on edge of continent
• Igneous Rock
• Magma rises beneath surface cools ? pluton
• Volcanoes at subduction boundaries add to edges

42
Plate Movements Continental Growth

• River Sediments
• Weathered eroded materials deposited at edges
• Terrane
• large block of lithospheric plate that has been
  moved attached to continent
• 3 characteristics to ID a terrane
• 1. bounded on all sides by faults
• 2. rocks fossils found in terrane do not match
  those of neighboring terranes
• 3. magnetic record of terrane does not match
  surround terranes

Growth of a Continent Animation
43

• HW
• Read pgs 182-187
• Answer 3