Dr. James Wittke
James.Wittke@nau.edu
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Alfred Wegener
◦ First proposed hypothesis in
1915
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Supercontinent called
Pangaea broke apart about
200 million years ago
◦ Continents "drifted" to present
positions
◦ Substantial supporting evidence
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Fit of edges of continents
Fossils match across oceans
Rock types and structures (mountain
chains, shield areas) match across oceans
Ancient climates (glaciers, coal)
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American geologists particularly critical of
Wegener
European and South African geologists less
critical
◦ Observed evidence for Pangaea first hand
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Main criticisms
◦ No credible mechanism for continental drift
◦ Continental rocks less strong than oceanic rocks
 continents can’t “plow” through oceans
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Arthur Holmes (1931)
speculated that
continental drift resulted
from mantle convection
currents
Crust carried along atop
the moving mantle
New seafloor forms in
center of oceans
Mountains form where
material is dragged back
into mantle
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Earth’s has dipole
magnetic field (like
a bar magnet)
Magnetic poles
align closely with
geographic poles
Lines of force
extend from pole to
pole
Intersection of lines
with Earth surface
varies with latitude
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Compasses and magnetic
minerals align with lines of
magnetic force
Magnetite
T > 585 C
◦ Common in basaltic lava flows
◦ Not magnetic at high temperatures
when lava is still liquid)
T < 585 C
◦ Aligns with Earth’s field when it
becomes magnetic below Curie
point (585 C)
◦ Lava solidifies and magnetite
locks orientation of magnetic field
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Paleomagnetic study rocks of
different ages showed the pole
changed location with time 
polar wandering
Wandering consistent with
continental drift
Polar wandering curves different
for Europe and North America
Curves coincide when continents
are moved to reconstruct
Pangaea
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Ships equipped with sonar
discovered oceanic ridge
system (1940s and 1950s)
Harry Hess (1960s)
proposed ridges represent
zones of mantle upwelling
Stretching produces cracks
into which molten rock
intrude making new
seafloor
Material sinks into mantle
at deep sea trenches
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Discovered that poles of
magnetic field switch
periodically in magnetic
reversals
◦ Rocks with poles oriented
like that of today  normal
polarity
◦ Rocks with poles oriented
opposite that of today 
reversed polarity
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Radiometric dating of
the rocks yielded a
magnetic time scale
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Intervals of one type
of polarity called
chrons
When reversals
occur appears
random
Chrons last a million
years on average,
but may be much
shorter
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Research vessels towed
sensitive magnetometers
behind them  discovered
variations in intensity of
magnetic field
Variations were aligned
parallel to mid-ocean ridges
and symmetrical across
ridges
Vine and Mathews (1963)
proposed these formed
when new lava was added in
narrow zones at at centers
of mid-ocean ridges
Reversal pattern
south of Iceland
Note: Width of
a reversal
depends on
spreading rate
and duration of
chron.
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Slow movements of large
parts of Earth’s
lithosphere
Plates are rigid (no
internal deformation)
Lithosphere detached
from underlying mantle
by hot weak
asthenosphere
◦ Temperatures in
asthenosphere near melting
point
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About 20 plates; 7 major
ones…
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Divergent (constructive) – plates moving
apart
◦ Upwelling mantle
◦ Basalt forms new seafloor
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Convergent (destructive) – plates moving
together
◦ Subduction causes volcanism
◦ Mountain building
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Transform fault (conservative) – plates
move past one another
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Most broad, linear swells called mid-oceanic ridges
(MOR)
◦ Occupy elevated positions
◦ Extensive normal faulting and shallow earthquakes
◦ High heat flow and numerous volcanic structures
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Some divergent margins on continents
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Ridge structure depends
on spreading rate
Slow rifting  rift valley
Fast rifting  no rift
valley, more magmatism
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Mantle material rises
 decompression
melting
Magma rises to
create new ocean
lithosphere
Distinct rock
assemblage called
“ophiolite sequence”
New lithosphere is
hot; cools with time
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Layer 1 – unconsolidated
sediments
Layer 2 – pillow lavas
Layer 3 – numerous
interconnected dikes
called sheeted dikes
Layer 4 – gabbro and
layered gabbro
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Tensional forces
stretch and thin
continental crust
Upwelling of
astheosphere
(decompression
melting) 
volcanism
Rift may develop
into ocean basin
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Produced by continental rifting
No longer associated with plate boundaries
Little volcanism and few earthquakes
Found along most coastal areas that
surround Atlantic ocean
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Continental shelf
◦ Flooded extension of continent
◦ Gently sloping
◦ Contains important mineral deposits (oil, gas)
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Two plates move together
Subduction of denser oceanic lithosphere
(subduction zone)  deep-ocean trench
Slab descends into mantle at 45-90º angle
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Depends on age of
ocean lithosphere
◦ Older (cooler, denser)
lithosphere  steep
◦ Younger (hotter, less
dense) lithosphere 
shallow
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Shallow subduction
angles results in
interaction with
overlying plate
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Ocean-continent
◦ Destruction of oceanic
lithosphere
◦ Trench along continent
margin
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Ocean-ocean
◦ Destruction of oceanic
lithosphere
◦ Trench in ocean
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Continent-continent
◦ Mountain building
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Formation of continental volcanic arcs
◦ Examples include Andes, Cascades, and Sierra
Nevadan system
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Subduction beneath continent
◦ Dense oceanic slab sinks into asthenosphere
◦ Hydration melting in mantle above slab (fluids from
slab)  andesite
◦ Blueschist metamorphism (low T-high P)
◦ Melting and assimilation in crust  granite
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Formation of volcanic island arcs (as
volcanoes emerge from sea)
◦ Examples include Aleutian, Mariana, and Tonga
islands
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Two oceanic slabs converge
◦ Denser (older) oceanic slab sinks
◦ Hydration melting in mantle above slab (fluids
from slab)  basalt and andesite
◦ Blueschist metamorphism (low T-high P)
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Always preceded by ocean-continent
convergence (volcanic arc on continent)
Converging plates both contain continental
material
Subducting plate carries continent  two
continents collide
◦ Regional metamorphism
◦ Deformation
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Can produce new mountain ranges such as
the Himalayas
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Plates slide past one
another
No new crust is
created or destroyed
Transform faults
◦ Most join two
segments of midocean ridge
◦ Aid movement of
oceanic crustal
material
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Ocean drilling
◦ Ages and thickness of ocean bottom
sediments
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Hot spots (mantle plumes)
◦ Fixed locations of volcanism
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Paleomagnetism
◦ Ancient magnetic field measurements
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Drill directly into ocean-floor
sediment
◦ Ocean floors are young (no
rocks older than 180 million
years)
◦ Age of deepest sediments
increases away from midocean ridges
◦ Thickness of ocean-floor
sediments increases away
from mid-ocean ridges
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Caused by rising plumes of
mantle material
Volcanoes form over hot
spots (Hawaiian Island
chain)
Long-lived structures
Some originate at great
depth, perhaps at mantlecore boundary (CMB)
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Magnetometer
towed by ships
used to detect
submarines
Detected variations
in magnetism of
ocean floor lavas
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Widths of magnetic stripes
◦ Averaged over millions of years
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Hotspot tracks
◦ Hawaiian Island - Emperor Seamount chain
◦ Depends on hotspots being truly “fixed”
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Direct measurement
◦ Very Long Baseline Interferometry (VLBI)
◦ Global Positioning System (GPS)
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Unequal distribution of Earth's heat drives
movements of plates and mantle
Convective flow of mantle
Mantle convection and plate movements
part of same system
◦ Sinking cool subducting plates  downward
portion of convective flow
◦ Upwelling of hot rock at mid-ocean ridges and in
plumes  upward portion of convective flow
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Precise nature of convective flow not known
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Subduction of cold
lithosphere is main
driving force of plate
motion
Sinking slabs pull
trailing plate after
them
Relatively rapid
movement if >20% of
plate boundary is
subduction zone
◦ Pacific, Cocos, and
Nazca plates
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Forces produced
by drag of
subducting slab
Induced mantle
flow pulls both
subducting and
overriding plates
towards trench
Even detached
plates continue to
create flow in
mantle
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Mid-ocean ridges elevated because they are hot
Plates slide down flanks of ridges
Much less important than slab pull
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Horizontal motion of lithosphere away from
mid-ocean ridges causes mantle upwelling
Plate movement integral part of convective
flow
Models of flow in Earth’s interior
◦ Layering at 660 kilometers (two convective
systems)
◦ Whole-mantle convection (one convective
system)
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“Layer cake” model
Two zones of
convection  two
compositions
Explains why MOR
and plume magmas
are different
Problem: evidence
that slabs do not
stop at 660 km
boundary
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Slabs descend to
core-mantle
boundary (CMB)
and stir entire
mantle  one
composition
Slabs accumulate
at CMB
Reheated slab
material at CMB
rises in plumes
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Ductile slab
folds and
stacks up at
CMB
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Plate movements in past documented by
evidence including…
◦ Paleomagnetism
◦ Age dating
◦ Matching rock sequences, fossils, etc.
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Confident reconstruction of plate movement
since Cambrian (540 Million years ago)
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Rifting and dispersal of one supercontinent
Followed by long period as fragments are
reassembled
Before Pangaea
◦ Rodinia split apart between 750 and 550 million
years ago
◦ Some fragments eventually formed Gondwana;
others became continental landmasses in
Northern Hemisphere
◦ Most of these landmasses were reassembled into
Pangaea
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Extrapolate plate movements into future 50
million years
Some predictions:
◦ Areas west of San Andreas Fault slide northward
past North American plate
◦ Africa collides with Eurasia, closing Mediterranean
and initiating mountain building
◦ Australia and new Guinea on collision course with
Asia