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Plate Tectonics
GLY 2010 – Summer 2012 - Lecture 15
1
Subduction Zones
• Plots of earthquake foci over time
delineate the position of subducting plates
• The plate which is subducted is always
denser than the plate which remains on
the surface
• Earthquakes associated with convergent
plate margins where one plate is
subducting are of shallow, intermediate
and deep focus
• Deep focus earthquakes are not known
except at subduction zones
2
Shallow Subduction Angle
• Plates near the
spreading center will
be much warmer
 They will be only
slightly denser than
surface plate, and the
subduction angle will
be shallow
3
Steep Subduction Angle
• Plates far from the
spreading center
will be relatively
cold, and therefore
dense
 They will subduct at
a steep angle
4
Breakup of Subducting Slabs
• Slabs break off, and are no longer attached
to the subducting plate
• Lack of attachment stops their movement,
and they no longer generate earthquakes
5
Accumulation of Slabs
• Broken slabs are now known to accumulate
within the mantle, stacking up like
pancakes
6
Oceanic Trenches
• Subducting plate drags part of the surface with it
• Creates large oceanic trenches, which also serve to
mark the top of the subduction zones
7
Continental
Volcanic Arcs
• Plates subducted under continents partially melt,
creating long chains of stratovolcanoes on the
continents
 Cascades and Andes are examples
8
Volcanic
Island Arc
• Plates subducted under oceanic plates
create chains of oceanic islands
 Japan, the Philippines, and Indonesia
are examples
9
Plate Motions
• Two plates move relative to each other
 Convergent - Plates move toward each
other, often a head-on collision
 Divergent - Plates move away from each
other
 Sideways (transform) - Plates move past
each other along transform faults
10
Convergent Movement
11
Divergent Movement
12
Transform Motion
13
Plate Types
• At any given point, a plate is either
oceanic or continental
• Interactions between plates are thus:
 Ocean-ocean (O-O)
 Ocean-continent (O-C)
 Continent-continent (C-C)
14
Plate Interactions
Conv.
Divg.
Trans.
O-O
SID Quakes
Oceanic Arc
Volcanism
Japan
SI Quakes
Fissure
Volcanism
MOR
S Quakes
No volcanism
Oceanic
transform faults
O-C
SID Quakes
Stratovolcanic
Chains
Cascades, Andes
Not known
Not known
C-C
SI Quakes
No Volcanism
Himalayas
SI Quakes
Alkaline
volcanism
East Africa
S Quakes
No volcanism
San Andreas
Fault
15
Summary of Plate Interactions
16
Hydrothermal Vents
• Spreading centers are marked by vents
which spew hydrothermal fluids as hot as
650C
• Fluids contain dissolved metals which
precipitate when they hit cold ocean
water, encrusting basalt - vents are called
“black smokers” for this reason
17
Hydrothermal Vent – Black
Smoker
• Black Smoker along
the Juan de Fuca
Ridge
• Temperature: 648ºC
• Courtesy PBS
Station WGBH
Click to
play video
18
Alvin
Alvin photographed
from research vessel
Atlantis
• With room enough
for only one pilot,
a cameraman, and
a massive IMAX
camera, Alvin dove
to depths of
12,000 feet (3,700
meters) during an
ambitious effort to
film hydrothermal
vents on the MidAtlantic Ridge 19
Hydrothermal Vent – White
Smoker
• “White smokers”
release water that is
cooler than their
cousins’ and often
contains compounds of
barium, calcium, and
silicon, which are white
Hydrothermal vents are believed
to play an important role in the
ocean’s temperature, chemistry,
and circulation patterns
20
Vent Biology - Tubeworms
Click to
play video
• A tube worm colony near hydrothermal vents
21
Vent Biology – Vent Crabs
Click to play video
• The vent crab is typically
found among dense clusters
of tubeworms at an average
depth of 1.7 miles and can
tolerate a temperature
gradient that ranges from
77°F in the tubeworm
clumps, to 36°F, which is
the temperature of the
water surrounding the vent
sites
22
Vents of the World
• Some known vent localities on the ocean floor – note
association with mid-ocean ridges
23
Earth’s Magnetic Field
• Earth has a strong
magnetic field
• It is dipolar, with the
poles being called
north and south
24
Earth’s Magnetic Polarity
• Present north magnetic pole is located
near the south geographic pole
• South magnetic pole is located near the
north geographic pole
25
Rock Magnetism
• Rocks often become magnetized because
magnetic mineral grains (usually magnetite) are
aligned
• Rock’s magnetic field is fixed at the time magma
cools below the Curie point for igneous rocks, or
at the time of lithification for sedimentary rocks
• Magnetism of older rocks is called
“paleomagnetism”
26
Magnetic Inclination and Declination
27
Magnetic Stripes
• In the early 1960’s
oceanographic
research uncovered
a curious
phenomenon, called
magnetic stripes
• Measurements of the
earth’s magnetic
field show small
variations from
place to place
28
Magnetic Anomalies
• Magnetic Anomaly = Average regional
magnetic field of the earth - magnetic field
at a point
• Plotting magnetic anomalies lead to a
curious pattern of “stripes”, first seen in the
Atlantic, later in the Pacific
29
Explanation of Stripes
• The first to propose an
explanation that was scientifically
accepted was L. Wilson Morley, a
Canadian geoscientist
• Morley sent his paper to the
British journal Nature in January,
1963
• Nature rejected the article, and it
was not published until more than
a year later in another journal
30
Morley’s Idea
• Sea-floor spreading - new magma
emerging at a MOR and hardening into
rock, which then spread away from the
ridge with time (Hess-Dietz hypothesis)
• Polarity reversals - the North and South
magnetic poles changing position suddenly
31
Polarity Reversal
32
Vine-Matthews
Frederick J. Vine
• Working completely
independently of Morley, and
unaware of his idea, D.H.
Matthews and his graduate
student, Frederick Vine,
formulated an explanation often
called the Vine-Mathews theory
33
Vine-Matthews Publication
• Published “Magnetic Anomalies Over Ocean
Ridges (in Nature!) in September, 1963
• Matthews was a Research Fellow of King's
College, Cambridge
34
Vine-Matthews-Morley Theory
• If we assume sea-floor spreading is
occurring, the magnetic field of the rock
is fixed, in alignment with the earth’s
field, at the time the rock cools
• The measured field above such rocks
equals the earth’s field plus the rock’s
field (because they are aligned)
35
After Polarity Reversal
• If the earth’s field reverses, the field of
the previously created rock will be
aligned against the earth’s field, slightly
decreasing it
• A second reversal will again align the
field of the rock and earth
36
Magnetic
Stripes
• As magma rises,
it hardens and its
magnetic field
matches the
present field of
the earth - after a
polarity reversal,
it will be aligned
against the earth’s
field
37
Sea Floor Magnetism Animation
38
Anomalies
• When the rock’s field and the earth’s
field are aligned, the field at a point will
be greater than the regional average - a
positive anomaly
• When the rock’s field and earth’s field
are in opposite directions, the field at a
point will be less than the regional
average - a negative anomaly
39
Anomaly
Diagram
• Anomalies are
really just
regions of high
and low
magnetic
intensity
40
Magnetic Stripe Theory
• By postulating a series of irregular (in time)
polarity reversals, with continuous eruption
of magma at the spreading center, Morley
and later Vine-Mathews offered an
explanation for magnetic stripes
• After several years of discussion, this
explanation was accepted by most earth
scientists in a series of conferences in 196768
41
Magnetic
Reversal
Record
• Recent
magnetic
field data
from lava
samples of
known age
42
Rate of Plate Movement
• Once Plate Tectonics was accepted, it
became necessary to determine how
fast plates move
• Three methods have been used
 Hot spots
 Satellite Tracking
 Magnetic reversal
43
Hot Spots
• Hot spots, which generate magma in the
asthenosphere, below the moving
lithospheric plates, may be used as a
reference since they are effectively
stationary relative to lithospheric plate
44
Hot Spots
“Movement”
• Hot spots produce
volcanoes, like the
Hawaiian Islands or
many seamounts or
guyots (mountains that
made it above sea-level,
then were flattened by
wave erosion)
45
Hot Spot
Diagram
• Diagram showing
creation of several
Hawaiian Islands
• Age of islands
should be
progressively
older as they
move away, and
this is observed
46
Hot Spot Animation
47
Pacific Ocean Near Hawaii
48
Satellite Tracking
• Lasers bounce beams off satellite in
geosynchronous orbit
• Time necessary to make the round-trip
journey can be used to measure the distance
of the laser station from the satellite
• The amount this changes from year to year
can be used to determine plate motions
49
GPS
Receiver
• Receivers such as this receive signals
from satellites…using a computer, the
distance to each satellite is computed
• The position on the earth’s surface is then
calculated
50
Magnetic Reversal Times
• The age of the rock at each reversal can
be dated
• The distance from the spreading center,
together with the age, can be used to
calculate a velocity (units of which are
distance/time)
51
Building Continents
• Craton - The original nucleus of the
continent - broken into two categories
 Continental shield - Broad areas of exposed
crystalline rocks that have not changed in
more than a billion years. Often exposed by
glaciation
 Continental platform - Areas surrounding the
shield, where layers of younger rock cover the
shield
52
Formation of Early Landmasses
• Early islands probably collided to form
larger landmasses, and heat and pressure
of collision may have metamorphosed
some of the rock
• The mafic/ultramafic rocks might have
heated to the point where low-melting
minerals, usually felsic, formed magmas
• Intermediate to felsic magmas cooled to
form the first continental crust
53
Early Atmosphere and Weathering
• Volcanic gases modified the early
atmosphere
• Weathering and erosion would have
created sediment to fill in sedimentary
basins
54
Accretion
• As more continental land masses formed,
collisions between them became inevitable
• Subduction followed, leading to further
differentiation of felsic/intermediate
magma by partial melting at 100-150 km
55
Addition of Terranes
56
Identification of
Exotic Terranes
• The displaced, or exotic,
terranes can be
distinguished from
surrounding rock based
on:




Their age
Fossil assemblages
Stratigraphy
Paleomagnetic data
57
Ophiolites
• Accretion also adds ophiolite materials to
continents
• Ophiolites are rocks that comprise the
oceanic crust
• They are mafic, but often include
serpentinite, formed by metasomatitism
of the mafic rock
• Serpentinite is soft, green, and often
contorted into a snake-like appearance
58
Formation of Ophiolite
• Ophiolite sequences are formed near
spreading centers, and include evidence of
metasomatism
59
Ophiolite Sequence
60
Suture Zone
The Great Lakes
Tectonic Zone which
bounds gneisses to the
south and Greenstones
to the north
•
• A boundary
between colliding
continents, in
which the
continental crust if
thickened as one
continent slides
under the other
61
Exotic Terrane
• USGS video about Hells Canyon Idaho region
62
Continental Collisions
• As the continents accreted, some joined to
become supercontinents
• Southern supercontinent = Gondwana
• Northern supercontinent = Laurasia
63
Formation of Pangaea
• By 225 mybp, at the
end of the Paleozoic
era, Pangaea was a
vast supercontinent,
which included
essentially all land
on earth, and
stretched from pole
to pole
64
Animation of Pangaea Breakup
65
Rifting, Stage 1
• Plumes of hot magma
originating beneath the
lithosphere penetrate the
lithosphere and cause
triangular cracks
66
Rifting, Stage 2
• Mantle currents
usually separate and
widen two of the
cracks
• These cracks may
flood with seawater
67
Rifting, Stage 3
• The other crack often
becomes inactive
• The inactive branch is
often called a failed arm
68
Rift Rivers
• The Benue Trough
is occupied by the
Benue River, a
tributary of the
Niger River
69
Modern Rifting
• The Red Sea, Gulf
of Aden, and the
East African Rift
Zone are a modern
day example of a
rifting event
70
Current Rifting in North America
• The New Madrid earthquake may
represent release of pent-up stress along
the failed arm
• Currently, rifting is occurring
 In the Rio Grande area between Colorado
and N. Mexico
 In the Basin and Range of Arizona, Nevada,
and Utah
71
Active Branches
• The active branches are marked by high
heat flow, normal faulting, frequent
shallow-focus earthquakes, and
widespread basaltic volcanism
• The great basalt plateaus of the continents
(Columbian, Deccan) mark current or
ancient rift zones
72
Formation of Graben Valleys
• Rifts widen, crating grabens
• The grabens are invaded by
the sea
• The Gulf of Aden and the
Red Sea form an example
• The Great Rift Valley of East
Africa, where many believe
man originated, is believed
to be the inactive arm
73
Incomplete Rifting
• Rifting may start, with a great outpouring
of mafic lavas, and then cease
• This happened in N. America about 1100
MYBP around Lake Superior
74
The Last 600 MY
75
Plate Boundary Process Correlation
76