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Title: Chapter 2: Global Tectonics Our Dynamic Planet


1
Chapter 2 Global Tectonics Our Dynamic Planet
2
Introduction
  • Each rocky body, whether planet or moon, started
    with a hot interior.
  • Each has been kept warm over time by energy
    released by the decay of radioactive isotopes.
  • Despite radioactive heating, rocky bodies have
    cooled considerably since their formation, so
    that their outer layers have stiffened into
    lithospheres (???).

3
Introduction (2)
  • The interior of Earth remains hot and
    geologically active.
  • The mantles of Earth loses internal heat by
    convection (??), the slow flow of solid rock.
  • Hot rock rises upward to near the surface.
  • Earths stiff lithosphere is broken into a
    collection of near-rigid plates.

4
Introduction (3)
  • Most large-scale geologic events, like
    earthquakes or volcanic eruptions, originate
    within Earths interior.
  • Many other processes in the Earth system, such as
    the hydrologic and biogeochemical cycles, are
    profoundly affected by plate tectonics (????).

5
Plate Tectonics (????) From Hypothesis to Theory
  • Plate tectonics is a scientific theory that
    explains two centuries of often puzzling
    observations and hypotheses about our planet
    Earth.
  • The continents are drifting very slowly across
    the face of our planet.
  • Continental drift (????) is a concept with a long
    history.

6
Plate Tectonics From Hypothesis to Theory (2)
  • A century ago geologists puzzled over the fit of
    the shorelines of Africa and South America.
  • They noted that fossils of extinct land-bound
    plants and animals, glacial deposits (????), and
    ancient lava flows (???) could be matched
    together along coastlines that today are
    thousands of kilometers apart.
  • Coal was found in Antarctica.
  • Coal forms in tropical climates, implying that
    Antarctica has moved in the past.

7
Plate Tectonics From Hypothesis to Theory (3)
  • Alfred Wegener proposed the most comprehensive
    early hypothesis for Continental Drift (??????)
    in 1912.
  • His theory was widely rejected because
  • Ocean floor was too strong to be plowed aside.
  • Wegener had not proposed a plausible force that
    could induce the continents to drift.

8
Plate Tectonics From Hypothesis to Theory (4)
  • After many years of scientific observations, the
    theory of Plate Tectonics was born in 1960.
  • Plate tectonics is the process by which Earths
    hot interior loses heat.
  • Nowadays, we can measure the slow drift of plates
    worldwide using satellite navigation systems.
  • The basic premises of plate theory are secure
    because they can be tested against a wide variety
    of observations.

9
Continental Drift versus Plate Tectonics
10
What Earths Surface Features Tell Us
  • The rocks beneath our feet are solid, but they
    are not rigid.
  • Topography the relief and form of the land above
    sea level.
  • Bathymetry topography on the ocean floor.
  • Earth bulges around its equator and is slightly
    flattened at the poles.

11
Isostasy Why Some Rocks Float Higher Than Others
  • The continents average about 4.5 km elevation
    above the ocean floor. They stand notably higher
    than the ocean basins because the thick
    continental crust (????) is relatively light
    (average density 2.7 g/cm3).
  • The thin oceanic crust (????) is relatively heavy
    (average density 3.0g/cm3).
  • The lithosphere (???) floats on the
    asthenosphere (???) .

12
Isostasy (?????) -similar to Principle of
Archimedes' applied to the earth -first noted
when French Bouguer in the 18th century surveyed
the shape of the earth
13
Isostasy (2)
  • The principle of isostasy governs the rise or
    subsidence.
  • All parts of the lithosphere are in a floating
    equilibrium.
  • Low-density wood blocks float high and have deep
    roots, whereas high-density blocks float low
    and have shallow roots.

Fig. 2.2
14
Earths Surface Land Versus Water
  • The ocean covers 71 percent of Earths surface.
  • Sea level fluctuates over time.
  • When climate is colder and water is stored as
    ice
  • Sea level falls. The shoreline moves seaward.
  • When climate gets warmer
  • The ice melts.
  • Sea level rises.
  • The shoreline advances inland.

15
Fig. 2.3
Fig. 2.1
16
Earths Surface Land Versus Water (2)
  • Undersea mid-ocean ridges form a continuous
    feature more than 60,000 km long.
  • Mid-ocean ridges mark where two oceanic plates
    spread apart.
  • New lithosphere forms in the gap.

17
Fig. 2.4 Continental shelves and slopes (light
blue) take 25 of the mass of the continental
crust.
Fig. 2.5 Topography across the northern Atlantic
Ocean. The Atlantic coastline is a typical
example for passive continental margin.
18
Earths Surface Land Versus Water (3)
  • The continental shelf (???) steepens slightly at
    100-200 meters below sea level.
  • The continental slope (????) is the flooded
    continental margin.
  • The continental rise (????) descends more gently
    from the base of the continental slope

19
Earths Surface Land Versus Water (4)
  • Ocean trenches (??) occurs where oceanic
    lithosphere and continental (or oceanic)
    lithosphere converge at the boundary between two
    plates (e.g., Ryukyu trench, Mariana trench).
  • Because oceanic lithosphere is the denser of the
    two, it descends under the active continental
    margin and sinks into the deeper mantle.
  • The large, flat abyssal floors (????) of the open
    ocean lie 3 to 6 km below sea level.

20
What Earths Internal Phenomena Tell Us
  • Heat conduction (???) and convection (??).
  • Conduction is dominant when the temperature
    gradient in rocks is large. (earths surface and
    core-mantle boundary)
  • Rocks are poor conductors of heat, so the
    internal heat is transferred by moving the rock
    itself. The circulation of hot rock is
    maintained by mantle convection (?????????).

21
oceanic trench
mid-ocean ridge
Fig. 2.6 mantle convection that shapes the
earths surface. Heat source comes from cooling
of the earth itself since 4.55 Byr and decay of
radioactive elements.
22
Heat Conduction (???)
  • Conduction is the process by which heat moves
    through solid rock via molecular collisions.
  • Its a diffusive (??) process wherein molecules
    transmit their kinetic energy to other molecules
    by colliding with them.
  • Heat is conducted through a medium in which there
    is a spatial variation in the temperature or a
    steep temperature gradient.
  • The loss of the earths internal heat through
    oceanic crust and lithosphere is largely
    controlled by conduction.

23
Mantle Convection (1)
  • Earths heat can move in a second process called
    convection (??).
  • Convection can happen in gases, in liquids, or,
    given enough time, in ductile solids.
  • A prerequisite condition for mantle convection is
    the thermal expansion (???) of hot rock.
  • Convective heat is transported with the motion of
    ductile rock.

24
Mantle Convection (2)
  • Rock expands as its temperature increase, and its
    density thereby decreases slightly.
  • The hot rock is buoyant relative to cooler rock
    in its immediate neighborhood.
  • A 1 percent volume expansion requires an increase
    of 300-400oC and leads to a 1 percent decrease in
    density.
  • Viscosity (????) represents the tendency of rock
    to ductile flow (?????).
  • Unit Newton.second/meter2

25
Rock Deformation Elastic versus Viscous
  • For an elastic solid, stress is linearly
    proportional to strain,
  • In general, 300-1000 atmosphere pressure (1 atm
    1 bar 105 Pascal N/m2) is required to compress
    a rock by 1/1000.
  • Viscosity (m) measures the resistance of a solid
    or fluid to ductile flow.
  • Ductile deformation becomes important at larger
    depth, where rocks are hot and less rigid.
  • 100 atm is estimated to cause the mantle rocks
    beneath the plates to deform at a steady rate of
    1/106 per year.

26
Mantle Convection (3)
  • Rock does not need to melt before it can flow.
  • The presence of H2O encourages flow in solid
    rock.
  • Convection currents bring hot rocks upward from
    Earths interior.

27
Geothermal Gradient (????) of the Lithosphere
  • Heat moves through the lithosphere primarily by
    conduction.
  • The lithosphere-asthenosphere boundary is
    1300-1350oC, depending on depth.
  • Oceanic lithosphere is about 100 km thick.
  • The average geothermal gradient in oceanic
    lithosphere is about 13oC/km.
  • Average continental lithosphere is 200 km.
  • The average geothermal gradient in continental
    lithosphere is about 6.70C/km.

28
Fig. 2.7
29
Adiabatic Expansion (????) of Rock
  • Adiabatic expansion means expansion without loss
    or gain of energy.
  • Rock is compressed and reduced in volume by
    increasing pressure with depth it is also heated
    by the work done by the pressure force during the
    compression. The associated temperature rise
    causes adiabatic expansion.

30
Adiabatic Expansion (????) of Rock (2)
  • In convective mantle, the mean temperature
    increases with depth along an adiabat (???).
  • The adiabatic thermal gradient (??????) in the
    mantle is the rate of increase of temperature
    with depth as a result of compression of the rock
    by the weight of the overlying rock it is
    approximately 0.5oC/km.

31
Earths Convection Driven From the Top
  • Below the lithosphere, rock masses in the deeper
    mantle rise and fall according to differences in
    temperature and buoyancy.
  • Earths convection is driven mainly by colder
    material sinking from the top.

32
Earths Convection Driven From the Top (2)
  • The densest lithosphere is most likely to sink
    back into the asthenosphere and the deeper mantle
    while lighter continental lithoshere drifts
    across the earths surface.
  • Ocean floor and the continents are slowly moving
    (up to 12 cm/yr).

33
Fig. 2.8
34
Plates and Mantle Convection
  • When continents split apart, a new ocean basin
    forms.
  • The Red Sea was formed this way 30 million years
    ago.
  • Subduction the old lithosphere sinks beneath the
    edge of an adjacent plate.

35
Global Positioning System
  • In the 1960s, the U.S. Department of Defense
    established a network of satellites with orbits
    that could be used for reference in precisely
    determining location.
  • The Global Positioning System (GPS) detects small
    movements of the Earths surface.

Fig. 2.9 C. surface motion from GPS measurement
36
Global Positioning System (2)
  • It is accurate within a few millimeters.
  • Two measurement methods
  • A GPS campaign researchers establish a network
    of fixed reference points on Earths surface,
    often attached to bedrock. The position is
    re-measured every few months or years.
  • Continuous GPS measurement the receivers are
    attached permanently to monuments, and position
    is estimated at fixed intervals of a few seconds
    or minutes.

37
Four Types of Plate Margins and How They Move
  • The lithosphere currently consists of 12 large
    plates.
  • The seven largest plates are
  • North American Plate.
  • South American Plate.
  • African Plate.
  • Pacific Plate.
  • Eurasian Plate.
  • Australian-Indian Plate.
  • Antarctic Plate.

38
Fig. 2.10
39
Fig. 2.9 A. Present-day plat motion based on
many geological data, including lineation of
magnetic anomaly on seafloor, relative motion
along the strike of transform faults, earthquake
slip direction and displacement, etc.. Red dots
mark the location of significant earthquakes
since 1965. Fig. 2.9 B. Surface motions from
continuous GPS measurements.
Fig. 2.9
40
Plates have four kinds of boundaries or margins
(????)
  • Divergent margin/spreading center (??????/????)
    (e.g. East Pacific Rise, Mid-Atlantic Ridge).
  • Convergent margin/subduction zone (??????/???)
    (e.g. Japan Trench, Aleutian Trench).
  • Convergent margin/collision zone (??????/???)
    (e.g. Indo-Himalaya collision zone).
  • Transform fault margin (??????)

41
Seismology and Plate Margin
  • Earthquakes occur in portions of the lithosphere
    that are stiff and brittle.
  • Earthquakes usually occur on pre-existing
    fracture surfaces, or faults.
  • There are distinctive types of earthquakes that
    correlate nicely with motion at plate boundaries.

42
Fig. 2.12 three types of faults
Fig. 2.11 Four types of plate margines
43
Type I Divergent Margin
  • Where two plates spread apart at a divergent
    boundary, hot asthenosphere rises to fill the
    gap.
  • As it ascends, the rock experiences a decrease in
    pressure and partially melts.
  • The molten rock from such pressure-release
    partial melting is called magma (??).

44
Type I Divergent Margin (2)
  • Oceanic crust is formed at mid-ocean ridges
    within 1-2 km of the ridge axes.
  • Found in every ocean.
  • Form a continuous chain that circles the globe.
  • Oceanic crust is about 6-8 km thick worldwide.

45
Animation of Seafloor Spreading Source CD of the
textbook
Pressure-release partial melting
Seafloor spreading and magnetic chron
46
Birth of the Atlantic Ocean (1)
  • When a spreading center splits continental crust
  • A great rift (??) forms, such as the African Rift
    Valley (????).
  • As the two pieces of continental crust spread
    apart
  • The lithosphere thins.
  • The underlying asthenosphere rises.
  • Volcanism commences.
  • The rift widens and deepens, eventually dropping
    below sea level. Then the sea enters to form a
    long, narrow water body (like the Red Sea).

47
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48
Birth of the Atlantic Ocean (2)
Fig. 2.13
  • The continents that now border it were joined
    into a single vast continent that Wegener named
    Pangaea which means all lands.
  • About 200 million years ago, new spreading
    centers split the huge continent.
  • The Atlantic continues to widen today at 2-4
    cm/yr.

49
Characteristics of Spreading Centers (1)
  • Earthquakes at midocean ridges occur only in the
    first 10 km beneath the seafloor and tend to be
    small.
  • Volcanic activity occurs at midocean ridges and
    continental rift.
  • The midocean ridges rise 2 km or more above
    surrounding seafloor.The principle of isostasy
    applies lower-density rock rises to form a
    higher elevation at ridges and the cooling
    results the subsidence of seafloor

50
Characteristics of Spreading Centers (2)
  • If the spreading rate is fast
  • A larger amount of young warm oceanic lithosphere
    is produced, and the ridge will be wider.
  • A slow-spreading ridge will be narrower.
  • The Atlantic Ocean spreads slowly (2-4 cm/yr).
  • The Pacific spreading center (East Pacific Rise)
    is fast by comparison 6-20 cm/yr. The Pacific
    Ocean basin is shrinking because

51
Seafloor Spreading and Age Map http//www.windows.
ucar.edu/tour/link/earth/interior/seafloor_spread
ing.html
52
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53
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54
Animation of Topography and Subduction Angles at
Fast and Slow Moving Plates (from CD of textbook)
55
Role of Seawater at Spreading Centers
  • Seawater circulates through cracks beneath the
    ocean floor.
  • Cold water percolates (??) through these cracks,
    warms in contact with subsurface rock, and rises
    convectively to form undersea hot springs.
  • A small fraction of the seawater remains in the
    rock, chemically bound within hydrous
    (water-bearing) minerals like serpentine and
    clays.

56
The CO2 Connection
  • As oceanic lithosphere ages, it accumulates a
    thick layer of sediments such as clay and calcium
    carbonate (CaCO3) from the shells and internal
    skeletons of marine organisms.
  • The formation of calcium carbonate consumes
    carbon dioxide (CO2) that is dissolved in
    seawater.
  • Seafloor sediments remove CO2 from the
    atmosphere, and thus have a long-term influence
    on the greenhouse effect and Earths climate.

57
Type II Convergent Margin/Subduction Zone
  • As the plate cools, it grows denser and the
    principle of isostasy demands that the plate
    subsides. The process by which lithosphere sinks
    into the asthenosphere is called subduction.
  • The margins along which plates are subducted are
    called subduction zones.
  • The sinking slab warms, softens, and exchanges
    material with the surrounding mantle.

58
Type II Convergent Margin/Subduction Zone (2)
  • Under elevated temperature and pressure, the
    crust expels a number of chemical compounds.
  • Water (H2O), Carbon dioxide (CO2), and Sulfur
    compounds (S).
  • A small addition of these volatile substances can
    lower the melting point of rock by several
    hundred degrees Celsius ?The hot mantle rock
    immediately above the sinking slab starts to
    melt.
  • Magma rises to the surface to form volcanoes.
  • Subduction zones are marked by an arc of
    volcanoes parallel to the edge of the plate.

59
The CO2 Connection, Again
  • Water, CO2, and sulfuric gases like SO2 and H2S
    return to the atmosphere.
  • Subduction zone volcanic activity raises the
    carbon dioxide level in the atmosphere, exerting
    a strong influence on the greenhouse effect and
    Earths climate.
  • Volcanism tends to replace the CO2 that is lost
  • from the atmosphere into the ocean and stored in
    the seafloor..

60
Volcanoes At Subduction Zones
  • At a plate boundary, the plunging plate draws the
    seafloor down into an ocean trench.
  • When the slab gets down to about 100 km, water
    squeezed out of the subducted materials begins to
    react with the ambient mantle rock and causes
    some of the mantle to melt. Molten rock that
    makes it all the way to the surface erupts to
    form a line of volcanoes spaced about 70 km apart
    from the trench.

61
Volcanoes At Subduction Zones (2)
  • If the overriding plate is oceanic lithosphere,
    volcanoes form a series of islands called a
    volcanic island arc (????) , e.g., Mariana
    Islands, Aleutian Islands.
  • If the overriding plate is continental
    lithosphere, a continental volcanic arc forms.
    Sediment washed from the continent tends to fill
    the offshore trench, e.g., Cascade Range of the
    Pacific Northwest, the Andes of South America.

62
Earthquakes in Subduction Zones
  • The largest and the deepest earthquakes occur in
    subduction zones.
  • The location of most earthquakes define the top
    surface of a slab as it slides into the mantle
    (the surface to as deep as 670 km).
  • Quakes deeper than 100 km are more likely
    associated with faults caused by stresses within
    the slab.

63
Distribution of earthquake epicenters from 1975
to 1995. Depth of the earthquake focus is
indicated by color.
64
Animation of Subduction Process (from CD of
textbook)
Fig. 2.16 Wadati-Benioff Zone
65
Type III Convergent Margin/Collision Zone
  • Continental crust is not recycled into the
    mantle.
  • Continental crust is lighter (less dense) and
    thicker than oceanic crust.
  • When two fragments of continental lithosphere
    converge, the surface rocks crumple (????)
    together to form a collision zone.

66
Type III Convergent Margin/Collision Zone (2)
  • Collision zones that mark the closure of a former
    ocean form spectacular mountain ranges. For
    example, the Alps, Himalayas, and Appalachians.

Fig. 2.17
67
Type IV Transform Fault Margin
  • Along a transform fault margin, two plates
    grind(??) past each other in horizontal motion.
  • These margins involve strike-slip faults in the
    shallow lithosphere and often a broader shear
    zone deeper in the lithosphere.
  • Most transform fault occur underwater between
    oceanic plates.

68
Type IV Transform Fault Margin (2)
  • Two of Earths most notorious and dangerous
    transform faults are on land.
  • The North Anatolian Fault in Turkey.
  • The San Andreas Fault in California.
  • Both transform faults are similar in slip rate,
    length and straightness. (Right-lateral
    strike-slip, fault lengths of 1000 km.) The
    fault slip rate is about 24 mm/yr for NAF and
    20-34 mm/yr for SAF.

Animation of Movement Along Transform Fault (from
CD of textbook)
69
Fig. 2.10
70
Fig. 2.18
71
Topography of the Ocean Floor
  • Two main features
  • Midocean ridges
  • Some 64,000 km in length.
  • The oceanic ridge with its central rift reaches
    sea level and forms volcanic islands, e.g.,
    Iceland.
  • Oceanic trenches
  • the deepest parts of the ocean.
  • The deepest spot on Earth is located in the
    western Pacific, near Guam in the Mariana Trench.
    (depth 11,033 m at the CHALLENGER DEEP)

72
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73
Iceland
74
Comparing Venusian Topography
  • Venus resembles Earth in size and chemical
    composition.
  • The Magellan project mapped its surface over
    several years.
  • Venusian tectonics is not plate tectonics.
  • Venusian topography does not exhibit long
    midocean ridges and subduction zones.
  • Venus has no water ocean or ocean floor because
    of the extreme temperature of its surface (around
    450-500oC).

75
Fig. 2.19
76
An Icy Analogue to Earth Tectonics (1)
  • The closest approximation to Earth tectonic in
    our solar system is found on Europa (one of
    Jupiters (??) four largest moons).
  • Europa is 3138 km in diameter, large enough to be
    discovered in 1610 by Galileo with his early
    telescope.
  • Europas interior has rocky composition with
    density similar to Earth.

77
An Icy Analogue to Earth Tectonics (2)
  • Its surface layer consists mainly of water ice,
    perhaps more than 100 km deep.
  • Plates on Europa are much smaller than Earths
    plates.
  • Topography at Europas plate margins suggests
    convergence, divergence, and transform-fault
    motion, just as with Earths plate margins.

78
HOT SPOT
79
Hot Spots And Absolute Motion (1)
  • American geologist James Dwight Dana (1813-1895)
    observed that the age of extinct volcanoes in the
    Hawaiian Island chain increases as one gets
    farther away from the active volcanoes on the
    big island.
  • The only active volcanoes are at the southeast
    end of the island chain, and the seamounts to the
    northwest are long extinct.

80
Hot Spots And Absolute Motion (2)
  • In the 1960s, J. Tuzo Wilson proposed that a
    long-lived hot spot lies anchored deep in the
    mantle beneath Hawaii.
  • A hot buoyant plume of mantle rock continually
    rises from the hot spot, partially melting to
    form magma at the bottom of the lithospheremagma
    that feeds Hawaiis active volcanoes.
  • If the seafloor moves over the mantle plume, an
    active volcano could remain over the magma source
    only for about a million years.

81
Hot Spots And Absolute Motion (3)
  • As the plate moves, the old volcano would pass
    beyond the plume and become dormant, and a new
    volcano would sprout periodically through the
    plate above the hot spot, fed by plume magma.

82
Animation of Formation of Hotspot Track (from CD
of textbook)
83
Fig. 2.22 Yellow dots mark hotspot volcanism
associated with rising mantle plumes.
84
Volcanism Associated with Plate Tectonics
85
Hot Spots, mantle plumes
  • Several dozen hot spots have been identified.
  • Because hot spot volcanoes do not form tracks on
    the African Plate, geologists conclude that this
    plate must be very nearly stationary.
  • Hot spots transport roughly 10 percent of the
    total heat that escapes Earth.
  • Mantle plumes were probably more numerous 90-110
    million years ago than today, because extinct
    seamount volcanoes of that age crowd together in
    the central Pacific.

86
What Causes Plate Tectonics? (1)
  • There is more agreement on how plate tectonics
    works than on why it works.
  • Hot, buoyant, low-viscosity material rises in
    narrow columns that resemble hot spot plumes.
  • Cooler, stiffer material from the surface sinks
    into the mantle.

87
What Causes Plate Tectonics? (2)
  • Three forces seem likely to have a part in moving
    the lithosphere
  • Ridge push the young lithosphere sits atop a
    topographic high, where gravity causes it to
    slide down the gentle slopes of the ridge.
  • Slab pull at a subduction zone, as the cold,
    dense slab is free to sink into the mantle, it
    pulls the rest of the lithosphere into the
    oceanic trench behind it.
  • Friction
  • Slab friction drags the top, the bottom, and the
    leading edge of descending lithosphere in the
    subduction zone.
  • Plate friction drags elsewhere at the base of the
    plate.

88
Why Does Plate Tectonics Work?
  • The theory of plate tectonics does not explain
    why the plates exist.
  • At the present time, a number of scientific clues
    point to water as the missing ingredient in the
    plate tectonics.
  • Water molecules can diffuse slowly through solid
    rock.
  • Water can weaken rock in several ways.

89
Fig. 2.25 Computer simulation of mantle
convection. Viscosity decreases as temperature
increases by 800C from the coolest (blue) to
warmest (red) regions. Hot buoyant rock (red)
flows more readily and rises upward in narrow
plumes. Cooler rock (blue) is stiffer and sinks
in interconnected sheets. The arrows show the
lateral velocity of ductile flow, diverging from
the hot plumes and converging on the cooler
sheets.
90
Evolution of Mantle Plumes
91
Fig. 2.26 Computer simulation of mantle
convection with stiff plates. Rock viscosity is
formulated to maintain narrow weak zones at the
plate boundaries, so that plates remain distinct
and can move relative to each other. The
viscosity varies by a factor of 30 between red
(stiff) and blue (weak) regions. Arrows indicate
surface velocity.
92
http//www.geos.ed.ac.uk/undergraduate/prospectus/
earthsci/PlumesCartoon_Helge_Gonnermann.jpg
93
Cartoon of Earths interior
94
Seismic tomography
95
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98
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99
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100
Images of Subducting slabs(????)
Grand S.P., van der Hilst R.D., and Widiyantoro
S., 1997. Global seismic tomography a snapshot
of convection in the Earth, GSA Today.
101
Images of Subducting slabs(????)
102
The thickness of continental lithosphere ?
SAW16AN
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