Transcript Chapter 2

Chapter 2: Global Tectonics
Our Dynamic Planet
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
(岩石圈).
Introduction (2)
The interior of Earth remains hot and geologically
active.
The mantles of Earth loses 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.
Introduction (3)
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).
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.
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.

Coal forms in tropical climates, implying that
Antarctica has moved in the past.
Plate Tectonics:
From Hypothesis to Theory (3)
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.
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.
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.
Continental
Drift
versus
Plate
Tectonics
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.
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) .
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
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 highdensity blocks float low and
have shallow “roots.”
Fig. 2.2
Earth’s Surface: Land Versus Water
The ocean covers 71 percent of Earth’s 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.
Fig. 2.1
Fig. 2.3
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.
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.
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
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.
What Earth’s Internal Phenomena Tell
Us

Heat conduction (熱傳導) and convection (對流).
Conduction is dominant when the temperature
gradient in rocks is large. (earth’s 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 (地涵對流或地幔對流).
oceanic trench
mid-ocean ridge
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.
Heat Conduction (熱傳導)
Conduction is the process by which heat moves through solid
rock via 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.
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.
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 (黏滯係數) represents the tendency of
rock to ductile flow (延展性流動).
Unit: Newton.second/meter2
Rock Deformation:
Elastic versus Viscous
For an elastic solid, stress is linearly proportional to strain,
E (elastic constant)   
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.
m    ,  (strain rate)  d / dt, where  is stress and  is strain.
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.
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.
Geothermal Gradient (地溫梯度) of the
Lithosphere
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 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.
Fig. 2.7
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.
Adiabatic Expansion (絕熱膨脹) of
Rock (2)
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.
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.
Earth’s 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 earth’s surface.
Ocean floor and the continents are slowly moving (up
to 12 cm/yr).
Fig. 2.8
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.
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.
Fig. 2.9 C. surface motion from GPS measurement
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.
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. 2.10
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
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) (e.g. East Pacific Rise, Mid-Atlantic Ridge).
Convergent margin/subduction zone (聚合板塊邊界/隱沒
帶) (e.g. Japan Trench, Aleutian Trench).
Convergent margin/collision zone (聚合板塊邊界/碰撞帶)
(e.g. Indo-Himalaya collision zone).
Transform fault margin (轉型斷層邊界)
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.
Fig. 2.11 Four types of
plate margines
Fig. 2.12 three
types of faults
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).
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.
Animation of Seafloor Spreading
Source: CD of the textbook
Pressure-release partial melting
Seafloor spreading and
magnetic chron
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).
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.
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
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 …
Seafloor Spreading and Age Map
http://www.windows.ucar.edu/tour/link=/earth/interior/seafloor_spreading.html
Animation of
Topography and
Subduction Angles at
Fast and Slow Moving
Plates (from CD of textbook)
Role of Seawater at Spreading Centers
Seawater circulates through cracks beneath the ocean
floor.
Cold water percolates (滲透) 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.
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 Earth’s climate.
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.
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.
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 Earth’s climate.
Volcanism tends to replace the CO2 that is lost
from the atmosphere into the ocean and stored in the
seafloor..
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.
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.
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.
Distribution of earthquake epicenters from 1975 to 1995. Depth of the
earthquake focus is indicated by color.
Animation of Subduction Process
(from CD of textbook)
Fig. 2.16 Wadati-Benioff Zone
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.
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
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.
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. (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)
Fig. 2.10
Fig. 2.18
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)
Iceland
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).
Fig. 2.19
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.
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 Earth’s
plates.
Topography at Europa’s plate margins suggests
convergence, divergence, and transform-fault
motion, just as with Earth’s plate margins.
HOT SPOT
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.
Hot Spots And Absolute Motion (2)
In the 1960’s, 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
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.
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.
Animation of Formation of Hotspot Track
(from CD of textbook)
Fig. 2.22 Yellow dots mark hotspot volcanism associated with rising mantle
plumes.
Volcanism Associated with Plate Tectonics
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.
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.
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.
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.
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.
Evolution of Mantle Plumes
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.
http://www.geos.ed.ac.uk/undergraduate/prospectus/earthsci/PlumesCartoon_Helge_Gonnermann.jpg
Cartoon of Earth’s interior
Seismic tomography
低速
（熱）
高速
（冷）
Images of Subducting slabs（隱沒板塊）
Grand S.P., van der Hilst R.D., and Widiyantoro S., 1997. Global seismic tomography a
snapshot of convection in the Earth, GSA Today.
Images of Subducting slabs（隱沒板塊）
The thickness of
continental lithosphere ?
SAW16AN
Seismically defined lithosphere is at most 200250 km thick under old continents.
- consistent to other geophysical studies.