Download Chapter 2: Global Tectonics Our Dynamic Planet Introduction

Chapter 2: Global Tectonics
Our Dynamic Planet
Introduction (2)
The interior of Earth and Venus remain hot
and geologically active.
The mantles of Earth and Venus lose internal heat
by convection (對流), the slow flow of solid rock.
Hot rock rises upward to near the surface.
Earth’s stiff lithosphere is broken into a collection
of near-rigid plates.
1
Introduction
3
Introduction (3)
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
(岩石圈).
2
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Most large-scale geologic events, like earthquakes
or volcanic eruptions, originate within Earth’s
interior.
Many other processes in the Earth system, such as
the hydrologic and biogeochemical cycles, are
profoundly affected by plate tectonics (板塊運動).
4
Why Don’t We Live on Venus?
The orbit of Venus is closer to the sun than Earth’s,
but not too close to preclude life.
Venus has water vapor, essential to life.
Venus is a rocky planet like Earth, rich in minerals
to support living organisms.
Venus’ surface, with a temperature of almost 500oC,
is completely shrouded by clouds.
Why Don’t We Live on Venus? (3)
Earth’s convection is characterized by plate
tectonics.
Oceans collect sediment and carbon compounds,
and thereby extract greenhouse gases from the
atmosphere.
Plate tectonic helps maintain carbon cycle.
5
Why Don’t We Live on Venus? (2)
Atmospheric surface pressure on Venus is nearly 100
times that on Earth (100 atm ~ 107Pascal = density ×
(gravity constant) × (water depth)=1000kg/m3 × 10
s/m2 × 1000m, comparable to the pressure one
might feel at a depth of 1 km in the ocean).
In Earth’s atmosphere, carbon dioxide, water, and
several other trace gases cause the greenhouse effect.
In the Venusian atmosphere, the greenhouse effect is
extreme.
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Why Don’t We Live on Venus? (4)
On Venus:
Many meteor craters (隕石坑).
No plate tectonics.
−
Venus has not recycled its surface rocks in perhaps a halfbillion years.
No carbon cycle.
8
Plate Tectonics (板塊學說 :
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 (大陸漂移) is a concept with a long
history.
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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 (冰河沉積), and ancient
lava flows (熔岩流) could be matched together along
coastlines that today are thousands of kilometers
apart.
Coal was found in Antarctica.
−
Plate Tectonics:
From Hypothesis to Theory (3)
Faced with puzzling data, scientist developed
hypotheses to explain them.
Alfred Wegener proposed the most comprehensive
early hypothesis for “Continental Drift (大陸漂移假
說) ” 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.
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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 Earth’s hot
interior loses heat.
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.
Coal forms in tropical climates, implying that
Antarctica has moved in the past.
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12
Figure of the Earth
Continental
Drift
versus
Plate
Tectonics
13
What Earth’s 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.
All evidence points to centrifugal force (離心力)
caused by Earth’s rotational spin.
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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 (大陸地殼) is
relatively light (average density 2.7 g/cm3).
The thin oceanic crust (海洋地殼) is relatively heavy
(average density 3.0g/cm3).
The lithosphere (岩石圈) floats on the
asthenosphere (軟流圈) .
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Isostasy (地殼均衡說)
-similar to Principle of Archimedes' applied to the earth
-first noted when French Bouguer in the 18th century surveyed
the shape of the earth
Earth’s Surface: Land Versus Water
The ocean covers 71 percent of Earth’s surface.
Land occupies only 29 percent.
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.
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19
Isostasy (2)
The principle of isostasy
governs the rise or
subsidence of the crust until
mass is buoyantly balanced.
Because of isostasy, all parts
of the lithosphere are in a
floating equilibrium.
Low-density wood blocks
float high and have deep
“roots,” whereas highdensity blocks float low and
have shallow “roots.”
Fig. 2.2
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Fig. 2.1
Fig. 2.3
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Earth’s 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.
Passive margins have few earthquakes and little
volcanic activity.
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Fig. 2.4
Continental shelves
and slopes (light
blue) take ~25% of
the mass of the
continental crust.
Earth’s Surface: Land Versus Water (3)
The continental shelf (大陸棚) steepens slightly at
100-200 meters below sea level.
The continental slope (大陸斜坡) is the flooded
continental margin.
The continental rise (大陸隆起) descends more
gently from the base of the continental slope.
Earthquakes and volcanoes are common along
active margins.
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Earth’s Surface: Land Versus Water (4)
Ocean trenches (海溝) 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 (深海海床) of the open
ocean lie 3 to 6 km below sea level.
Fig. 2.5
Topography across the
northern Atlantic
Ocean. The Atlantic
coastline is a typical
example for passive
continental margin.
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24
What Earth’s Internal Phenomena Tell
Us
The earth has been losing heat since its formation 4.55
billion years ago.
Heat conduction (熱傳導) and convection (對流) compete
for dominance at all depths beneath the Earth’s surface.
Conduction is dominant when the temperature gradient
in rocks is large like the earth’s surface the and coremantle 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 (地涵對流
或地幔對流).
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Heat Conduction (熱傳導
Conduction is the process by which heat moves
through solid rock via the net effect of molecular
collisions.
It’s a diffusive (擴散) 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 earth’s internal heat through oceanic
crust and lithosphere is largely controlled by
conduction.
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oceanic trench
mid-ocean ridge
Mantle Convection (1)
Earth’s heat can move in a second process called
convection (對流).
Convection can happen in gases, in liquids, or, given
enough time, in ductile solids.
A prerequisite condition for mantle convection is the
thermal expansion (熱膨脹) of hot rock.
Convective heat is transported with the motion of
ductile rock.
Fig. 2.6 mantle convection that shapes the earth’s surface. Heat
source comes from cooling of the earth itself since 4.55 Byr and
decay of radioactive elements.
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28
Mantle Convection (2)
Rock expands as its temperature increases.
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 (黏滯係數) represents the tendency of
rock to ductile flow (延展性流動).
Unit: Newton.second/meter2
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Rock Deformation:
Elastic versus Viscous
For an elastic solid, stress is linearly proportional to strain,
E (elastic constant) = σ ε
In general, 300-1000 atmosphere pressure (1 atm ~ 1 bar = 105 Pascal
=N/m2) is required to compress a rock by 1/1000.
Earthquakes create large elastic deformation in a short time scale, (i.e.,
high strain rate) that travel through mantle and crustal rocks as seismic
waves, transmitting enormous forces over long distances.
Viscosity (µ): measures the resistance of a solid or fluid to ductile flow.
For a Newtonian viscous solid or fluid,
µ = σ ε& , ε& (strain rate) = dε / dt , where σ is stress and ε is strain.
Ductile deformation becomes important tens and hundreds of
kilometers beneath earth’s surface, where rocks are hot and less rigid.
100 atmosphere pressures is estimated to cause the mantle rocks
beneath the plates to deform at a steady rate of 1/106 per year.
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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
Earth’s interior.
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Geothermal Gradient (地溫梯度 of the
Lithosphere
The rock in the lithosphere is too cool for convection to
continue, so heat moves through the lithosphere
primarily by conduction.
The lithosphere-asthenosphere boundary is 13001350oC, depending on depth.
Oceanic lithosphere is about 100 km thick.
The geothermal gradient in oceanic lithosphere is
1300oC/100km, or 13oC/km.
Average continental lithosphere is 200 km.
The average geothermal gradient in continental lithosphere is
about 13500C/200 km, or 6.70C/km.
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Earth’s Convection: Driven From the
Top
Below the lithosphere, rock masses in the deeper
mantle rise and fall according to differences in
temperature and buoyancy.
Earth’s convection is driven mainly by colder
material sinking from the top.
The densest lithosphere is most likely to sink back into
the asthenosphere and the deeper mantle while lighter
continental lithoshere drifts across the earth’s surface.
Ocean floor and the continents are slowly moving (up
to 12 cm/yr).
Fig. 2.7
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35
Adiabatic Expansion (絕熱膨脹 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.
In convective mantle, the mean temperature increases with
depth along an adiabat (絕熱線).
The adiabatic thermal gradient (絕熱溫度梯度) 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.
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Fig. 2.8
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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.
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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
Earth’s surface.
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 Earth’s 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.
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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.
Fig. 2.9 C. surface motion from GPS measurement
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All are moving at speeds ranging from 1 to 12 cm
per year.
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Plates have four kinds of boundaries or
margins (板塊邊界
Divergent margin/spreading center (分離板塊邊界/擴張中
心): magma rises to form new oceanic crust between the
two pieces of the original plate (e.g. East Pacific Rise, MidAtlantic Ridge).
Convergent margin/subduction zone (聚合板塊邊界/隱沒
帶): two plates move toward each other and one sinks
beneath the other (e.g. Japan Trench, Aleutian Trench).
Convergent margin/collision zone (聚合板塊邊界/碰撞
帶): : two colliding continental plates create a mountain
range (e.g. Indo-Himalaya collision zone).
Transform fault margin (轉型斷層邊界): two plates slide
past each other, grinding and abrading their edges.
Fig. 2.10
41
Fig. 2.9 A. Presentday 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.
These earthquakes
delineates the major
plate boundaries.
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Seismology and Plate Margin
Fig. 2.9
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.
Fig. 2.9 B. Surface
motions from
continuous GPS
measurements.
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44
Three Types of Faults and Their
Earthquakes
Strike-slip faults are vertical or near vertical fracture
surfaces (at a plate boundary these are also known as
transform faults).
− Motion is entirely horizontal.
Thrust faults are fracture surfaces that dip at an angle
between the horizontal and the vertical (convergent
motion within a volume of rock).
− Motion is partly horizontal, partly vertical.
Normal faults are fracture surfaces that also dip
(divergent motion with and between bodies of rock).
− Motion is partly horizontal, partly vertical, but
opposite to the motion on a thrust fault.
45
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 (岩漿).
47
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.
Fig. 2.11 Four types of
plate margines
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Fig. 2.12 three
types of faults
48
Animation of Seafloor Spreading
Source: CD of the textbook
Birth of the Atlantic Ocean (2)
Fig. 2.13
Pressure-release partial melting
Seafloor spreading and
magnetic chron
49
Birth of the Atlantic Ocean (1)
When a spreading center splits continental crust:
A great rift (裂谷) forms, such as the African Rift Valley
(東非裂谷).
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).
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The Atlantic Ocean did not
exist 250 million years ago.
The continents that now
border it were joined into a
single vast continent that
Alfred 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.
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Characteristics of Spreading Centers (1)
Earthquakes at midocean ridges occur only in the first 10
km beneath the seafloor and tend to be small.
Normal faults form parallel lines along the rifted margin.
Volcanic activity occurs at midocean ridges and
continental rifts (along narrow parallel fissures).
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.
The thermal cooling model with isostatic equilibrium
predicts the depth of seafloor for nearly all oceanic
lithosphere younger than 70 million years.
52
Seafloor Spreading and Age Map
http://www.windows.ucar.edu/tour/link=/earth/interior/seafl
oor_spreading.html
Topography and Subduction Angles at
Fast and Slow Moving Plates
Topographic axial high is supported from
underneath magma chambers at fast spreading
ridges. The plates are warmer and more buoyant,
so their subduction angles are shallower.
Rift valley forms at slow spreading ridges. The
plates are colder and denser, so their subduction
angles are deeper.
53
Characteristics of Spreading Centers (2)
If the spreading rate is fast:
55
Animation of
Topography and
Subduction Angles at
Fast and Slow Moving
Plates (from CD of textbook)
A larger amount of young warm oceanic lithosphere is
produced.
The ridge will be wider.
A slow-spreading ridge will be narrower.
The Atlantic Ocean spreads slowly, growing wider at
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 it is bordered by
convergent plate boundaries.
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56
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 (black smokers).
Seawater reacts chemically with lithospheric rock,
leaching many metallic elements from it.
A small fraction of the seawater remains in the rock,
chemically bound within hydrous (water-bearing)
minerals like serpentine and clays.
Type II: Convergent
Margin/Subduction Zone
Over 70 million years, oceanic lithosphere can drift
1500 to 3000 km from the spreading center. 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. These are active continental margins.
The sinking slab warms, softens, and exchanges material
with the surrounding mantle.
57
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 countless 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 Earth’s climate.
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59
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).
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.
60
The CO2 Connection, Again
Water, carbon dioxide, and sulfuric gases like sulfur
dioxide (SO2) and hydrogen sulfide (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 Earth’s climate.
Volcanism tends to replace the CO2 that is lost
from the atmosphere into the ocean and stored in the
seafloor.
Warmer or cooler episodes in past climates can be
deduced from the fossils of ancient plants and animals.
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Volcanoes At Subduction Zones
At a plate boundary, the plunging plate draws the
seafloor down into an ocean trench, often 10 km or
more.
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.
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Volcanoes At Subduction Zones (2)
If the overriding plate is oceanic lithosphere, volcanoes
form a series of islands called a volcanic island arc (火
山島弧) , 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.
Chemical analyses of subduction-arc rocks disclose
unusual concentrations of rare elements, such as
boron, best explained by the expulsion of water and
other volatile substances from the descending slab.
63
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.
64
Animation of
Subduction Process
(from CD of textbook)
Type III: Convergent Margin/Collision
Zone (2)
Fig. 2.16 Wadati-Benioff Zone
Collision zones that
mark the closure of
a former ocean
form spectacular
mountain ranges.
For example, the
Alps, Himalayas,
and Appalachians.
Fig. 2.15 Earthquakes in theTonga
subduction zone, where the Pacific
plate subducts beneath the
Australian-Indian Plate
Fig. 2.17
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 (擠壓摺皺)
together to form a collision zone.
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67
Type IV: Transform Fault Margin
Along a transform fault margin, two plates
grind(摩擦) 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 Earth’s 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. They are both strike-slip, with a right-lateral
sense of motion, and have total fault lengths of ~1000 km.
The North Anatolian Fault (NAF) is in east-west trend while
the San Andreas Fault (SAF) is approximately north-south
trend. The fault slip rate is about 24 mm/yr for NAF and 2034 mm/yr for SAF.
Animation of Movement Along Transform Fault
(from CD of textbook)
69
Topography of the Ocean Floor
Two main features:
Midocean ridges
−
Some 64,000 km in length.
The spreading centers separate plates.
− 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. The
exact location is called CHALLENGER DEEP and
the water there is 11,033 m deep.
−
71
Topography, Volcanoes, and Earthquakes
in Iceland
Fig. 2.18
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72
Topography, Volcanoes, and
Earthqakes in Southeast Asia
Fig. 2.19
73
Comparing Venusian Topography (1)
Venus resembles Earth in size and chemical
composition.
The Magellan project mapped its surface over several
years.
Observation of volcanoes, extensional fissures, and other
indicators of surface motion.
Venusian tectonics is not plate tectonics.
Venusian topography does not exhibit long midocean ridges
and subduction zones.
Venus has no water ocean because of the extreme
temperature of its surface (around 450-500oC).
Venus has no ocean floor.
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75
Comparing Venusian Topography (2)
By comparing craters abundances, geologists
estimate that the Venusian surface is roughly 500
million years old.
Approximately 900 meteor craters on Venus.
On the Earth, the continents (continental crust)
can be billions of years old.
76
An Icy Analogue to Earth Tectonics (1)
The closest approximation to Earth tectonic in our
solar system is found on Europa (one of Jupiter’s
(木星) four largest moons).
Europa is 3138 km in diameter, large enough to be
discovered in 1610 by Galileo with his early telescope.
Europa’s 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.
Large fragments of the icy surface appear to be rigid.
Plates on Europa are much smaller than Earth’s
plates.
Topography at Europa’s plate margins suggests
convergence, divergence, and transform-fault
motion, just as with Earth’s plate margins.
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Hot Spots And Absolute Motion (1)
During the 19th century, 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.
Earthquakes occur only near the active volcanoes.
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Hot Spots And Absolute Motion (2)
In the 1960’s, J. Tuzo Wilson proposed that a longlived 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 lithosphere—magma that feeds
Hawaii’s 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.
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Hot Spots And Absolute Motion (3)
As the plate moves, the old volcano would pass
beyond the plume and become dormant.
A new volcano would sprout periodically through the
plate above the hot spot, fed by plume magma.
The Hawaiian Islands connect with a chain of
seamounts to the northwest.
These are dormant seafloor volcanoes that have sunk below
the sea surface by erosion and isostasy.
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Fig. 2.21
Animation of Formation of Hotspot Track
(from CD of textbook)
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Fig. 2.22 Yellow dots mark hotspot volcanism associated with rising mantle
plumes.
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Volcanism Associated with Plate Tectonics
Volcanic Domes and Coronae on Venus
(1)
There is abundant evidence for hotspots on Venus.
Numerous elevated domes and ring-shaped features
called coronae have been detected by Magellan’s
radar.
ƒ Several hypotheses have been advanced for the
origin of domes and coronae on Venus but their
circularity can be explained well by a variation of
the hot spot model.
Blobs of hot mantle (know as diapirs) that rise to the base
of the lithosphere should spread evenly in all directions.
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The World’s Hot Spots
Several dozen hot spots have been identified
worldwide.
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.
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87
Volcanic Domes (火山丘 and Coronae
(日冕 on Venus (2)
As diapir rises and spreads, it first forms a steep-
sided dome, then a broad plateau, and finally,
when the center collapses, a ring-like ridge.
The large number of domes and coronae indicates that the
Venusian mantle convects strongly beneath its unbroken
lithosphere.
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Fig. 2.23A. Coronae: ring-like topographic
structures ranging in diameter 60-1000 km.
Fig. 2.23B. Physical model for the evolution
of Venusian topography in response to an
isolated hot rising diapir of mantle rock.
Figure 2.24
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Plume Volcanism on Mars
Other rocky bodies in the solar system, such as Mars,
Mercury, and our Moon have also had volcanism in the
past.
Their sizes are too small to retain internal heat for billions
of years, and has limited their tectonic histories. The
Moon, Mercury, and Mars now have thick immobile
lithospheres.
At least 20 huge volcanoes and many smaller cones have
been identified on Mars. The largest is Olympus Mons (27
km high). It is a complex caldera that is 80 km across.
Mauna Loa in Hawaii, the largest volcano on Earth, has a
similar shape, but it is only 9 km high.
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91
Plume Volcanism on Mars (2)
The presence of a huge volcanic edifice such as
Olympus Mons implies:
A long-lived mantle plume.
The plume must have remained connected to the
volcanic vent for a very long time.
The Martian lithosphere has been stationary (no plate
tectonics).
The Martian lithosphere must be thick and strong.
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What Causes Plate Tectonics? (1)
There is more agreement on how plate tectonics
works than on why it works.
Mantle convection occurs in a variety of patterns,
large-scale and small-scale.
Hotter rock is less viscous than cooler rock
(critical to convective circulation).
Hot, buoyant, low-viscosity material rises in
narrow columns that resemble hot spot plumes.
Cooler, stiffer material from the surface sinks into
the mantle in sheets (similar to subducting slabs).
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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:
−
−
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.
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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.
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.
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96
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.
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