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THE EARTH THROUGH TIME
TENTH EDITION
H A R O L D L. L E V I N
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1
CHAPTER 7
Plate Tectonics Underlies All Earth History
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EARTHQUAKES
Earthquake = The rapid
release of energy by the
sudden movement of the
Earth. Much of the energy
is released in the form of
seismic waves.
Scientist use seismic waves
to investigate the interior
of the Earth.
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SEISMIC WAVES

Focus (or hypocenter) = the place within the Earth
where the rock breaks, producing an earthquake.

Epicenter = the point on the ground surface directly
above the focus.

Energy moving outward from the focus of an
earthquake travels in the form of seismic waves.
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TYPES OF SEISMIC WAVES
Body waves—Seismic waves that travel through the
interior of the Earth
a. P-waves
b. S-waves
Surface waves—Seismic waves that travel along the
interface between the surface of the crust and
the atmosphere.
a. Love waves
b. Rayleigh waves
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TYPES OF SEISMIC WAVES
Body waves
a.
b.
P-waves
Primary, pressure, push-pull
Fastest seismic wave
(6 km/sec in crust; 8 km/sec
in uppermost mantle)
Travel through solids & liquids
S-waves
Secondary, shear, side-to-side
Slower (3.5 km/sec in crust; 5
km/sec in upper mantle
km/sec)
Travel through solids only
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FIGURE 7-2 P- and S-type seismic waves.
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TYPES OF SEISMIC WAVES
Surface wave types
Love waves—Shear motion
Rayleigh waves—Orbital motion (similar to
ocean waves)
Surface wave characteristics
Slowest
Typically localized near the epicenter
Causes damage to structures during an
earthquake
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SEISMOGRAPHS


Earthquakes are recorded on an instrument called a
seismograph.
The record of the earthquake produced by the
seismograph is called a seismogram.
FIGURE 7-1 Typical seismograph record.
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DETERMINING THE EARTH'S
INTERNAL STRUCTURE
The Earth is a layered body.
The layered structure is determined from studies of
how seismic waves behave as they pass through
the Earth.
P- and S-wave travel times depend on properties of
rock materials through which they pass.
Differences in travel times correspond to differences
in rock properties.
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DETERMINING THE EARTH'S
INTERNAL STRUCTURE
Seismic wave velocity depends on the
density and elasticity of rock.
 Seismic waves travel faster in denser rock.
 Speed of seismic waves increases with
depth (pressure and density increase
downward).

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DETERMINING THE EARTH'S
INTERNAL STRUCTURE
Boundaries between the layers are called
discontinuities (produced by abrupt changes
in seismic wave velocities typically linked to
changes in rock properties).
 Mohorovičić
discontinuity (Moho)
between crust and mantle
 Gutenberg
discontinuity
between mantle and core
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DETERMINING THE EARTH'S
INTERNAL STRUCTURE
Curved wave paths
indicate gradual
increases in density and
seismic wave velocity
with depth.
Refraction (bending of
waves) occurs at
discontinuities between
layers.
FIGURE 7-6 Seismic waves refract (bend)
as they travel through Earth.
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S-WAVE SHADOW ZONE
Place where no S-waves
are received by
seismograph.
Extends across the globe on
side opposite from the
epicenter.
S-waves cannot travel
through the molten (liquid)
outer core.
Larger than the P-wave
shadow zone.
FIGURE 7-6 Seismic waves refract (bend)
as they travel through Earth.
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P-WAVE SHADOW ZONE
Place where no Pwaves are received by
seismographs.
Makes a ring around the
globe. Smaller than the
S-wave shadow zone.
FIGURE 7-6 Seismic waves refract (bend)
as they travel through Earth.
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THE EARTH'S
INTERNAL
STRUCTURE




Crust
Mantle
Outer core
Inner core
FIGURE 7-5 What’s inside Earth.
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CRUST


Continental Crust—A heterogeneous mixtures of rocks
that approximates the composition of granite.
Oceanic Crust—A relatively homogeneous rock of
basaltic composition.
FIGURE 7-10 Generalized crosssection showing Mohorovičić discontinuity.
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CONTINENTAL CRUST



Rock composition mixture that approximate Granite
composition
Averages about 35 km thick; 60 km in mountain ranges
about 2.7 g/cm3.
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OCEANIC CRUST




Basaltic composition
5–12 km thick
About 3.0 g/cm3
Has layered structure consisting of:
 Thin layer of sediment covers basaltic igneous
rock (about 200 m thick)
 Pillow basalts: basalts that erupted under water
(about 2 km thick)
 Gabbro: coarse grained equivalent of basalt;
cooled slowly (about 6 km thick)
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LITHOSPHERE
Lithosphere = outermost 100
km of Earth. Consists of
the crust plus the
outermost part of the
mantle. This layer tends
to behave in a ridged
manner.
Divided into tectonic or
lithospheric plates that
cover surface of Earth
FIGURE 7-5 What’s inside Earth.
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ASTHENOSPHERE





Asthenosphere = low velocity
zone (seismic wave velocity
decreases) below the
lithosphere.
Rocks are at or near melting
point.
Magmas generated here.
Solid that flows (rheid); plastic
behavior.
Slip surface for plate motion
above.
FIGURE 7-5 What’s inside Earth.
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MANTLE





Silica based composed (“rocky”) rich in iron and
magnesium based mineral.
 Peridotite (Mg Fe silicates, olivine)
 Kimberlite (contains diamonds)
 Eclogite
2885 km thick
Average density = 4.5 g/cm3
Solid that flows; plastic behavior.
Not uniform. Several concentric layers with differing
properties.
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ISOSTASY (CRUST-MANTLE INTERACTION)




Buoyancy and floating of the Earth's crust on the mantle.
Denser oceanic crust floats lower, forming ocean basins.
Less dense continental crust floats higher, forming
continents.
As erosion removes part of the crust, it rises isostatically to
a new level.
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CORE

Outer core
 Molten
Fe (85%) with some Ni. May contain
lighter elements such as Si, S, C, or O.
 2250 km thick
 Liquid. S-waves do not pass through outer core.

Inner core
 Solid
Fe (85%) with some Ni
 1220 km radius (slightly larger than the Moon)
 Solid
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CORE AND MAGNETIC FIELD
Convection in liquid outer core plus spin of
solid inner core generates Earth's magnetic
field.
 Magnetic field is also evidence for a
dominantly iron core.

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CRUSTAL STRUCTURES—FAULTS
A fault is a crack in the Earth's crust along
which movement has occurred.
 Types of faults:

 Dip-slip
faults: movement is vertical
Normal faults
Reverse faults and thrust faults
 Strike-slip faults or lateral faults: movement is
horizontal.
 Oblique-slip faults: both vertical and horizontal
movement
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FAULTS
FIGURE 7-57 Types of faults.
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CRUSTAL STRUCTURES—FOLDS

During mountain
building or
compressional
stress, rocks may
deform plastically
to produce folds.

Types of folds
Anticline
Syncline
Monocline
Dome
Basin
A.
B.
C.
D.
E.
FIGURE 7-63 Types of folds.
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ANTICLINE
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R. R. Mudge/USGS
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SYNCLINE
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USGS
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PLATE TECTONICS
Plate Tectonic theory was proposed in mid-twentieth
century. It is a unifying theory showing how a large
number of diverse, seemingly-unrelated geologic facts
are interrelated. The theory was the linkage to two
ideas: Continental Drift and Sea Floor Spreading.
Plate Tectonic theory involves a number of large plates
plus numerous small plates composed of crust and
upper mantle (Lithosphere) that move slowly, change
size, and shape.
The Earth’s surface is a dynamic surface.
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THE DATA BEHIND PLATE TECTONICS
Geophysical data collected after World War II
provided foundation for scientific breakthrough:
 Echo sounding for sea floor mapping discovered
patterns of midocean ridges and deep sea
trenches.
 Magnetometers charted the Earth's magnetic field
over large areas of the sea floor.
 Global network of seismometers (established to
monitor atomic explosions) provided information
on worldwide earthquake patterns.
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EVIDENCE IN
SUPPORT OF THE
THEORY OF PLATE
TECTONICS
1.
Shape of the coastlines:
the jigsaw puzzle fit of
Africa and South America.
FIGURE 7-15 Fit of the continents
about 200 million years ago.
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EVIDENCE IN SUPPORT OF THE
THEORY OF PLATE TECTONICS
2.
•
•
•
•
•
Paleoclimatic evidence: Ancient climatic zones match up
when continents are moved back to their past positions.
Glacial tillites
Glacial striations
Coal deposits
Carbonate deposits
Evaporite deposits
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EVIDENCE IN SUPPORT OF THE
THEORY OF PLATE TECTONICS
3.
Fossil evidence implies once-continuous land connections
between now-separated areas
Mesosaurus
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Glossopteris
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EVIDENCE IN SUPPORT OF THE
THEORY OF PLATE TECTONICS
4.
Distribution of present-day organisms
indicates that they evolved in genetic
isolation on separated continents (such as
Australian marsupials).
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EVIDENCE IN SUPPORT OF THE
THEORY OF PLATE TECTONICS
5.
Geologic similarities between South
America, Africa, and India



Same stratigraphic sequence (same
sequence of layered rocks of same ages in
each place)
Mountain belts and geologic structures
(trends of folded and faulted rocks line up)
Precambrian basement rocks are similar in
Gabon (Africa) and Brazil.
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EVIDENCE IN SUPPORT OF THE
THEORY OF PLATE TECTONICS
6.
7.
8.
Geologic similarities between Appalachian Mountains and
Caledonian Mountains in British Isles and Scandinavia.
Rift Valleys of East Africa indicate a continent breaking up.
Evidence for subsidence in oceans

Guyots: flat-topped sea mounts (erosion when at or
above sea level).

Chains of volcanic islands that are older away from
site of current volcanic activity: Hawaiian Islands and
Emperor Sea Mounts (also subsiding as they go away
from site of current volcanic activity).
6.
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EVIDENCE IN SUPPORT OF THE
THEORY OF PLATE TECTONICS
9.
Mid-ocean ridges are sites of sea floor spreading. They
have the following characteristics:

High heat flow.

Seismic wave velocity decreases at the ridges, due to
high temperatures.

A valley is present along the center of ridge.

Volcanoes are present along the ridge.

Earthquakes occur along the ridge.
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EVIDENCE IN SUPPORT OF THE
THEORY OF PLATE TECTONICS
10.
Paleomagnetism
and Polar
Wandering Curves.
The Earth's
magnetic field
behaves as if there
were a bar magnet
in the center of the
Earth
FIGURE 7-20 Dipole model of Earth’s
magnetic field.
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PALEOMAGNETISM AND POLAR
WANDERING CURVES





As lava cools on the surface of the Earth, tiny crystals of
magnetite form.
When the lava cools to a certain temperature, known as the
Curie point, the crystals become magnetized and aligned
with Earth's magnetic field.
The orientation of the magnetite crystals records the
orientation of the Earth's magnetic field at that time.
As tiny magnetite grains are deposited as sediment, they
become aligned with Earth's magnetic field.
The grains become locked into place when the sediment
becomes cemented.
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PALEOMAGNETISM AND POLAR
WANDERING CURVES
The orientation of Earth's magnetic field is
described by inclination and declination.
Inclination = the angle of the magnetic field with respect to the
horizontal (or the dip of the magnetic field).


Inclination = 90o at poles
Inclination = 0o at the equator
Declination = the angle between where a compass needle
points (magnetic north) and the true geographic north pole
(axis of the Earth).
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APPARENT POLAR WANDERING



Paleomagnetic data confirm that the continents
have moved continuously.
When ancient magnetic pole positions are plotted
on maps, we can see that they were in different
places, relative to a continent, at different times in
the past.
This is called apparent polar wandering. The poles
have not moved. The continents have moved.
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APPARENT POLAR WANDERING

Different polar wandering
paths are seen in rocks of
different continents.

Put continents back
together (like they were in
the past) and the polar
wandering curves match
up.
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FIGURE 7-23 Highly mobile locations of
Earth’s north magnetic pole during the past
43
half-billion years.
THE LITHOSPHERE IS DIVIDED INTO
PLATES (ABOUT 7 LARGE PLATES AND
20 SMALLER ONES)
FIGURE 7-25 Earth’s major tectonic plates
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LITHOSPHERE & ASTHENOSPHERE



Lithosphere = rigid, brittle crust plus uppermost
mantle.
Asthenosphere = partially molten part of upper
mantle, below lithosphere.
Rigid lithospheric plates "float" on flowing
asthenosphere.
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LITHOSPHERE
Two types of crust are present in the upper part of the
lithosphere:
1. Oceanic crust: thin, dense, basaltic
2. Continental crust: thick, low density, granitic
FIGURE 7-26 Earth’s lithosphere.
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TYPES OF PLATE BOUNDARIES



Divergent—The plates move apart from one
another. New crust is generated between the
diverging plates.
Convergent—The plates move toward one another
and collide. Crust is destroyed as one plate is
pushed beneath another.
Transform—The plates slide horizontally past
each other. Crust is neither produced nor
destroyed.
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DIVERGENT PLATE BOUNDARIES





Plates move apart from one another
Tensional stress
Rifting occurs
Normal faults
Igneous intrusions, commonly basalt, forming
new ocean crust
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48
SEAFLOOR SPREADING AT
DIVERGENT PLATE BOUNDARY
FIGURE 7-28 Seafloor spreading marks a divergent boundary
between two tectonic plates.
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CONVERGENT PLATE BOUNDARIES
A.
B.
Continental
collision
Subduction
FIGURE 7-32 Convergence: two types of
convergent plate boundaries.
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50
CONTINENTAL COLLISION
Continental collisions
form mountain belts with:
 Folded sedimentary
rocks
 Faulting
 Metamorphism
FIGURE 7-32
 Igneous intrusions
 Slabs of continental crust may override one
another
 Suture zone = zone of convergence between
two continental plates

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SUBDUCTION



An oceanic plate is pushed beneath another plate,
forming a deep-sea trench.
Rocks and sediments of downward-moving plate
are subducted into the mantle and heated.
Partial melting occurs in the mantle. Molten rock
rises to form:
 Volcanic island arcs
 Intrusive igneous rocks
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52
OCEAN-TO-OCEAN SUBDUCTION
An oceanic plate is subducted beneath another oceanic
plate, forming a deep-sea trench, with an associated
basaltic volcanic island arc.
FIGURE 7-32
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53
OCEAN-TO-CONTINENT SUBDUCTION
An oceanic plate is
subducted beneath a
continental plate,
forming a trench
adjacent to a
continent, and
volcanic mountains
along the edge of the
continent.
FIGURE 7-31 The juncture of North
American and Pacific plates.
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54
OCEAN-TO-CONTINENT
SUBDUCTION ZONE INCLUDES
1.
2.
Accretionary prism or accretionary wedge—
Highly contorted and metamorphosed
sediments that are scraped off the descending
plate and accreted onto the continental margin.
Mélange—A complexly folded jumble of
deformed and transported rocks.
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OCEAN-TO-CONTINENT
SUBDUCTION ZONE INCLUDES
3.
4.
Ophiolite suite—Piece of descending oceanic plate
that was scraped off and incorporated into the
accretionary wedge. Contains:
 Deep-sea sediments
 Submarine basalts (pillow lavas)
 Metamorphosed mantle rocks (serpentinized
peridotite)
Blueschists—metamorphic minerals (glaucophane
and lawsonite) indicating high pressures but low
temperatures.
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56
TRANSFORM PLATE BOUNDARIES




Plates slide past one another (Shear stress)
Transform faults link/offset mid-ocean ridges and
convergent boundaries
A natural consequence of horizontal spreading of
seafloor on a curved globe
Example: San Andreas Fault
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TYPES OF
TRANSFORM FAULTS
FIGURE 7-30 Three types of
transform faults.
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PLATE
BOUNDARIES
Red = Midoceanic ridges
Black = Transform faults
Blue = Deep-sea trenches
FIGURE 7-27 Midoceanic ridges (red) and
trenches (blue).
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WILSON CYCLES
Plate tectonic model for opening and closing of an
ocean basin over time.
1.
Opening of new ocean basin at divergent plate
boundary
2.
Seafloor spreading continues and subduction
begins
3.
Final stage of continental collision
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WILSON CYCLES
1.
Opening of a new ocean basin at a divergent
plate boundary.
Sedimentary deposits include:
a. Quartz sandstones
b. Shallow-water platform carbonates
c. Deeper water shales with chert
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61
WILSON CYCLES
2.
Expansion of ocean basin as seafloor
spreading continues and subduction
begins. Sedimentary deposits include:
a.
b.
c.
Graywacke
Turbidites
Volcanic rocks
Also mélange, thrust faults, and ophiolite
sequences near the subduction zone.
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62
WILSON CYCLES
3.
Final stage of continental collision.
Sedimentary deposits include:



Conglomerates
Red sandstones
Shales
Deposited in alluvial fans, rivers, and deltas
as older seafloor sediments are uplifted to
form mountains, and eroded.
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63
WHAT FORCES DRIVE PLATE TECTONICS?
The tectonic plates are moving, but with varying rates
and directions.
What hypotheses have been proposed to explain the
plate motion?
 Convection Cells in the Mantle
 Ridge-Push and Slab-Pull Model
 Thermal Plumes
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64
CONVECTION CELLS IN THE MANTLE



Large-scale thermal convection cells in the mantle may
move tectonic plates.
Convection cells transfer heat in a circular pattern. Hot
material rises; cool material sinks.
Mantle heat probably results from radioactive decay.
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65
RIDGE-PUSH MODEL
FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement.
Crust forms at mid-ocean ridge spreading center where it is hot
and thermally expanded. Crust tends to slide off the thermal
bulge, pushing the rest of the oceanic plate ahead of it. This is
called ridge-push..
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66
SLAB-PULL MODEL
FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement.
Near subduction zones, oceanic crust is cold and dense
(typically denser than the asthenosphere below it), and tends
to sink into the mantle, pulling the rest of the oceanic plate
behind it. This is referred to as slab-pull.
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67
THERMAL PLUMES


Thermal plumes are concentrated areas of heat rising from
near the core-mantle boundary. Hot spots are present on
the Earth's surface above a thermal.
The lithosphere expands and domes upward, above a
thermal plume. The uplifted area splits into three radiating
fractures and the three plates move outward away from the
hot spot.
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68
THERMAL PLUMES
A triple junction
over a thermal
plume. Afar
Triangle.
FIGURE 7-39 Rising plumes of hot mantle may
severely rift the crust, often at 120 angles.
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69
THERMAL PLUMES



Thermal plumes do not all produce triple junctions.
Hot spots are present across the globe. If the lava from the thermal plume makes
its way to the surface, volcanic activity may result.
As a tectonic plate moves over a hot spot, a chain of volcanoes is formed.
FIGURE 7-52 Major
worldwide hotspots.
Red dots are hot spot
locations.
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70
PALEOMAGNETIC EVIDENCE


Magnetic reversals (magnetic north switches with
magnetic south) have occurred relatively frequently
through geologic time.
Magnetization in older rocks has different
orientations (as determined by magnetometer
towed by a ship).
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71
PALEOMAGNETIC EVIDENCE
Normal (+) and reversed (-)
magnetization of the seafloor
about the mid-ocean ridge. Note
the symmetry on either side of
the ridge.
FIGURE 7-41 Magnetic field of
seafloor near Iceland.
FIGURE 7-42 Normal (+) and reversed (-)
magnetizations of the seafloor.
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Magnetic stripes on the
sea floor are symmetrical
about the mid-ocean
ridges.
72
MAGNETIC REVERSAL
TIME SCALE
Reversals in sea floor basalts match the
reversal time scale determined from rocks
exposed on land.
Continental basalts were dated
radiometrically and correlated with the
oceanic basalts. Using this method,
magnetic reversals on the sea floor were
dated.
FIGURE 7-43 Reversals of Earth’s magnetic
field during the past 70 million years.
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73
RATES OF SEAFLOOR SPREADING
The velocity of plate movement varies around the
world.
 Plates with large continents tend to move more
slowly (up to 2 cm per year).
 Oceanic plates move more rapidly (averaging 6–9
cm per year).
 Ocean basins & sea floor are young compared to
continental crust


Only a thin layer of sediment covers the sea floor basalt.
Sea floor rocks date to less than 200 million years (most
less than 150 million years).
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74
MEASUREMENT OF PLATE TECTONICS
FROM SPACE



Lasers
Man-made satellites in orbit around Earth—Global
Positioning System
By measuring distances between specific points on
adjacent tectonic plates over time, rates of plate
movement can be determined.
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75
SEISMIC EVIDENCE FOR
PLATE TECTONICS



Inclined zones of earthquake foci dip at about a
45o angle into the mantle, near a deep-sea trench.
Benioff Zones, (or Wadati-Benioff Zones).
The zone of earthquake foci marks the movement
of the subducting plate as it slides into the mantle.
The Benioff Zone provides evidence for subduction
where one plate is sliding beneath another, causing
earthquakes.
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76
GRAVITY EVIDENCE



A gravity anomaly is the difference between the
calculated theoretical value of gravity and the
actual measured gravity at a location.
Strong negative gravity anomalies occur where
there is a large amount of low-density rock beneath
the surface.
Strong negative gravity anomalies associated with
deep sea trenches indicate the location of less
dense oceanic crust rocks being subducted into
the denser mantle.
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77
GRAVITY EVIDENCE
FIGURE 7-50 Gravity variation over a deep-sea trench.
Negative gravity anomaly associated with a deep sea trench.
Sediments and lower density rocks are subducted into an area
that would otherwise be filled with denser rocks. As a result, the
force of gravity over the subduction zone is weaker than normal.
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78
THERMAL PLUMES, HOT SPOTS, AND
HAWAII





Volcanoes develop over hot spots or thermal plumes.
As the plate moves across the hot spot (appears to be
stationary), a chain of volcanoes forms.
The youngest volcano is over the hot spot.
The volcanoes become older away from the site of volcanic
activity.
Chains of volcanic islands and underwater sea mounts extend
for thousands of km in the Pacific Ocean as well as other
oceans.
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79
THERMAL PLUMES, HOT SPOTS, AND
HAWAII
FIGURE 7-51 The
Hawaiian Island chain.
A new volcano, Lo'ihi, is forming above the hot spot, SE of the
island of Hawaii.
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80
EXOTIC TERRAINS

Small pieces of continental crust surrounded
by oceanic crust are called microcontinents.
 Examples:
Greenland, Madagascar, Crete, New
Zealand, New Guinea.
Microcontinents are moved by seafloor spreading,
and may eventually arrive at a subduction zone.
They are too low in density and too buoyant to be
subducted into the mantle, so they collide with (and
become incorporated into the margin of) a larger
continent as an exotic terrain.
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81
EXOTIC TERRAINS
Exotic terrains are present along the margins of
every continent.
They are fault-bounded areas with different
structure, age, fossils, and rock type, compared
with the surrounding rocks.
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82
EXOTIC TERRAINS



Green terrains probably
originated as parts of other
continents.
Pink terrains may be displaced
parts of North America.
The terrains are composed of
Paleozoic or older rocks
accreted during Mesozoic and
Cenozoic.
FIGURE 7-55 The western margin of North
America is a jumble of exotic terranes.
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83
IMAGE CREDITS
• FIGURE 7-2 P- and S-type seismic waves. Source: Harold Levin.
• FIGURE 7-1 Typical seismograph record. Source: Harold Levin.
• FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. Source: Harold Levin.
• FIGURE 7-5 What’s inside Earth. Source: Harold Levin.
• FIGURE 7-10 Generalized cross-section showing Mohorovičić discontinuity. Source: Harold Levin.
• FIGURE 7-57 Types of faults. Source: Harold Levin.
• FIGURE 7-63 Types of folds. Source: Harold Levin.
• FIGURE 7-15 Fit of the continents about 200 million years ago. Source: Thomas Brucker for John Wiley & Sons, Inc.
• FIGURE 7-20 Dipole model of Earth’s magnetic field. Source: Harold Levin.
• FIGURE 7-23 Highly mobile locations of Earth’s north magnetic pole during the past half-billion years. Source: Harold Levin.
• FIGURE 7-25 Earth’s major tectonic plates. Source: Harold Levin.
• FIGURE 7-26 Earth’s lithosphere. Source: Harold Levin.
• FIGURE 7-32 Convergence: two types of convergent plate boundaries. Source: Harold Levin.
• FIGURE 7-31 The juncture of North American and Pacific plates. Source: Harold Levin.
• FIGURE 7-30 Three types of transform faults. Source: Harold Levin.
•FIGURE 7-27 Midoceanic ridges (red) and trenches (blue). Source: Harold Levin.
• FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement. Source: Harold Levin.
• FIGURE 7-39 Rising plumes of hot mantle may severely rift the crust, often at 120 angles. Source: Harold Levin.
• FIGURE 7-52 Major worldwide hotspots. Source: Harold Levin.
•FIGURE 7-41 Magnetic field of seafloor near Iceland. Source: Harold Levin.
• FIGURE 7-42 Normal (+) and reversed ( -- ) magnetizations of the seafloor. Source: Harold Levin.
• FIGURE 7-43 Reversals of Earth’s magnetic field during the past 70 million years. Source: After Heirtzler, J., 1968, Jour.
Geophysical Res., 73:2119–2136. Modified by permission of American Geophysical Union.
•FIGURE 7-50 Gravity variation over a deep-sea trench. Source: Harold Levin.
• FIGURE 7-51 The Hawaiian Island chain. Source: Harold Levin.
• FIGURE 7-55 The western margin of North America is a jumble of exotic terranes. Source: After Ben-Avraham, Z., 1981,
American Scientist, 69:228. Modified by permission of American Geophysical Union.
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84