MYSTERIES OF PLANET EARTH

1

• Chapter TwelvePlate Tectonics Creating Oceans
  and Continents

2
Key elements of Plate Tectonics

• Earths lithosphere consists of rigid plates
• Plates move relative to one another by
  divergence, convergence, or transform motion
• Oceanic lithosphere forms at divergent plate
  boundaries and are consumed at subduction zones
• Most earthquakes, volcanism, faulting and
  mountain building take place at plate boundaries.
• Plate centers tend to be geologically stableKey

3
Basic Assumptions of Plate Tectonic Theory

• Lithosphere consists of rigid plates (100 km
  average 70 km for ocean 150 km for continents)
• Plates move relative to one another by
  Divergence, Convergence, or Transform motion
• Formation of Oceanic lithosphere at divergent
  plate boundaries and is consumed at subduction
  zone
• Most earthquake activity, volcanism, faulting,
  and mountain building take place at plate
  boundaries
• Centers of plates are stable

4
Plate velocity - determination

• Satellite based studies- plate motion and
  geomagnetic research
• Velocity of plates measures directly by
  ground-based laser which beams off reflectors on
  selected satellites
• Hot Spots- can serve as fixed reference point to
  measure absolute plate velocities
• Structures formed as plates move over hot spots
  also reveal speed and direction of plates
• Marine Magnetic Anomalies are used to estimate
  rates of divergence, and therefore plate motion
• Eco-sounder sonar- topography of sea floor and
  seismic profiling- study underlying layers

5
Relative Plate velocities
6
Relative Plate velocities-contd.
7
Satellite Global system
8
Mid-Pacific Hot spot
9
Tracking Magnetic Field Reversals

• DISCOVERY OF MARINE MAGNETIC ANOMALIES (WW-II
  MAGNETOMETER OBSERVATIONS IN THE SHIP)
• VINE MATHEWS HYPOTHESIS OF MAGNETIC REVERSALS
  (EVIDENCE OF SEA-FLOOR SPREADING AT DIVERGENT
  PLATE BOUNDARIES)
• MEASURING PLATE MOVEMENT BY MAGNETIC ANOMALIES
  (ANOMALYS DISTANCE FROM SPREADING RIDGE)
• PLATE SPEED 1 TO 10 CM PER YEAR

10
Marine magnetic anomalies
11
Marine magnetic anomaly-contd.
12
Marine magnetic anomaly-contd.
13
Directions and Rates of Plate movement
14
Nature and Origin of the Ocean Floor

• Rifting
• rising mantle over hot spot- 3 radiating valleys.
  One fails to open and later becomes filled with
  sediments- aulacogen. When rifting stops- the
  rift edge becomes inactive tectonically and
  therefore have passive continental margin
• Divergent Plate Boundaries
• as divergence continues, full seaway forms and
  new oceanic lithosphere forms at the mid-oceanic
  ridge as up-welling ultramafic melt produce
  basaltic magma
• Transform Boundaries/offset mid-oceanic ridges
• transform fault becomes divided into short offset
  segments by ocean
• Information about the Ocean Floor Echo-Sounding
  sonar, Seismic Profiling, Deep-Sea Drilling
  Project Submersible vessels

15
Active Rifting
16
Rifting and Origin of Ocean Basins

• Active Arms marked by
• High heat flow
• Normal faulting
• Frequent shallow earthquakes
• Widespread basaltic volcanism
• Rift valley widening leads to the development of
  new seaways and evaporite belts
• In Ocean, it leads to the formation of sediment
  deposition and development of Passive continental
  margins

17
The East Africa Rift Zone
18
The growth of oceanic basin
19
Growth of ocean basin contd.
20
Growth of ocean basin contd.
21
Nature and Origin of the Ocean floor contd.

• Oceanic trench
• forms where dense oceanic plate plunges
  (subducts) under less dense plates forming a
  depression in the earths surface
• Melange
• mixture of oceanic sediments and ophiolite rocks-
  form massive accretionary wedge that may be
  attached to the edge of the overriding plate
• Convergent
• collision of two plates resulting in suture zones
• Volcanic arc
• chain of volcanoes formed from subduction

22
Structure of Oceanic Lithosphere

• Upper surface 200 M sediment of siliceous or
  carbonate ooze and/or reddish clay
• 200 m- 2 km Oceanic basalt with top layer of
  pillow structures
• 3-6 km Gabbro
• Below Gabbro is Peridodite
• Serpentinite formed by alteration of rocks by
  water Entire sequence of ocean-floor rock may
  be altered its faults fissures

23
Structure of Oceanic Lithosphere-contd.

• H2O Pyroxene (in basalt, gabbro) chlorite
• H2O Mg olivine (Umafic peridotite)- Serpentine
  (Mg-Si mineral)

24
Layer of Ophiolite suite
25
Transform Boundaries Offset Mid-Ocean Ridges

• Occur where plates slide past one another in
  opposite direction
• 15 total length of plate margins
• Faulted blocks move in opposite directions ---
  produce stress --- leads to earthquakes

26
Direction of motion of a plate
27
Subduction-zone feature
28
Breadth of arc-trench gap
29
Breadth of arc-trench gap-contd
30
Anatomy of a continent
31
Continental shield
32
Origin of a Supercontinent
33
Origin of the supercontinent Pangaea
34
Earths plates before Pangaea

• Gondwana 500 million yrs BP Near South Pole 4
  Northern landmasses
• Laurasia 3 Landmasses in the Northern
  Hemisphere North America, Northern Europe,
  Southern Europe Parts of Africa Siberia
• Pangaea Laurasia and Gondwana colloided to form
  Pangaea

35
Future Events

• 50-100 Million yrs from now Australia will
  collide with Southeast Asia
• Western part of California will separate from
  North America along the San Andreas fault and
  become a separate microcontinent
• Mediterranean will close as the African and
  Eurasian Plates collide
• Subduction may occur along the East coast of
  North America
• All of the Earths landmasses may reunite into
  another Pangaea-like supercontinent

36
Driving Forces of Plate Motion

• Uncertainty on what drives Plate Motion
• Slow Asthenosphere convection currents
• Evidence against plate pushing by rising magma
  wedges at divergent boundaries
• Evidence in favor of plate pulling by descending
  slabs at subduction zones
• Gravity forces plates away from uplifted
  mid-ocean ridge

37
Convection Cells Hypothesis

• Deep convection cell hypothesis Lower mantle to
  the surface of the earth
• Shallow convection cell hypothesis Cells are
  driven within the Asthenosphere

38
Three factors that may drive plate tectonics
39
Convection cell hypothesis
40
Convection cell hypothesis-contd.
41
Configuration cell hypothesis
42
Seismic Topography
43
Seismic topography-contd.
44
Thermal plumes
45
Two patterns of Marine anomalies