Chapter 2,Global Tectonics
Our Dynamic Planet
Plate Tectonics:
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 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,and ancient
lava flows 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)
? Faced with puzzling data,scientist
developed hypotheses to explain them.
? Alfred Wegener proposed the most
comprehensive early hypothesis for
“Continental Drift” in 1912.
Plate Tectonics:
From Hypothesis to Theory (4)
? 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.
? Attempts to test Wegener’s hypothesis with
observations had mixed success.
Plate Tectonics:
From Hypothesis to Theory (5)
? 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.
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 caused by
Earth’s rotational spin.
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 is
relatively light (average density 2.7 g/cm3).
? The thin oceanic crust is relatively heavy
(average density 3.0g/cm3).
? The lithosphere floats on the asthenosphere.
Earth’s Topography is Bimodal
Why Earth’s Topography is Bimodal?
Isostasy
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 high-density
blocks float low and have shallow,roots.”
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.
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.
The overflowing Ocean
Earth’s Surface,Land Versus Water (3)
? The continental shelf steepens slightly at 100-
200 meters below sea level.
? The continental slope is the flooded continental
margin.
? The continental rise descends more gently from
the base of the continental slope.
? Earthquakes and volcanoes are common along
active margins.
Topography of the Seafloor
Earth’s Surface,Land Versus Water (4)
? Ocean trenches occurs where oceanic
lithosphere and continental lithosphere
converge at the boundary between two plates.
? 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 of the open ocean
lie 3 to 6 km below sea level.
What Earth’s Internal Phenomena Tell Us
? Rocks are poor conductors of heat,so
Earth moves its internal heat by moving
the rock itself.
? The circulation of hot rock is maintained
by mantle convection.
Convection in the Mantle
Mantle Convection
? Conduction is the process by which heat moves
through solid rock.
? Earth’s heat can move in a second process called
convection.
? Convection can happen in gases,in liquids,or,
given enough time,in ductile solids.
? A prerequisite condition for mantle
convection is the thermal expansion of hot
rock.
? 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 expansion requires an increase
of 300-400oC and leads to a 1 percent
decrease in density.
? Viscosity is the propensity of rock to ductile
flow.
Mantle Convection (2)
? 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.
? The rock in the lithosphere is too cool for
convection to continue.
? Heat moves through the lithosphere primarily
by conduction.
? The lithosphere-asthenosphere boundary is
1300-1350oC,depending on depth.
Mantle Convection (3)
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.
? The densest lithosphere is most likely to
sink back into the asthenosphere and the
deeper mantle,
? Ocean floor and the continents are slowly
moving (up to 12 cm/yr).
Anatomy of a Plate
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.
Geothermal Gradients
? A gradient is a progressive change in
some physical or chemical property.
? The geothermal gradient varies widely
with geography from 5oC/km to 75oC/km.
Deeper and Hotter
Geothermal gradient
? 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.
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.
? 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.
Global Positioning System (2)
Four Types Of Plate Margins
? 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.
Plates have four kinds of boundaries
or margins
? Divergent margin (also called a spreading
center),magma rises to form new oceanic crust
between the two pieces of the original plate.
? Convergent margin/subduction zone,two plates
move toward each other and one sinks beneath
the other.
? Convergent margin/collision zone,two colliding
continental plates create a mountain range.
? Transform fault margin,two plates slide past
each other,grinding and abrading their edges.
Plate Margions
Four kinds Margions,Divergent; convergent (subduction zone
and colliding zone) and Transform
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.
? 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.
Three Types of Faults and Their
Earthquakes
Earthquakes and Deformation near Plate
boundaries
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 is called magma.
Type I,Divergent Margin (2)
? Midocean ridges occur in oceanic crust.
? Found in every ocean.
? Form a continuous chain that circles the
globe.
? Oceanic crust is about 8 km thick.
? When a spreading center splits continental crust:
? A great rift forms (such as the African Rift Valley).
? 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
Birth of the Atlantic Ocean (2)
? 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.
? 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
? 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.
Characteristics of Spreading Centers (2)
? The principle of isostasy applies,lower-
density rock rises to form a higher
elevation.
? The seafloor around midocean ridges for
nearly all oceanic lithosphere is younger
than 70 million years.
Characteristics of Spreading Centers (3)
? If the spreading rate is fast,
? 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 is fast by
comparison,6-20 cm/yr.
Role of Seawater at Spreading Centers
? Seawater circulates through cracks
beneath the ocean floor.
? Cold water percolates过滤 through these
cracks,warms in contact with subsurface
rock,and rises convectively to form
undersea hot springs.
Role of Seawater at Spreading Centers (2)
? 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.
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,
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.
? The principle of isostasy demands that the plate
subsides as it grows denser.
? The process by which lithosphere sinks into the
asthenosphere is called subduction.
Type II,Convergent
Margin/Subduction Zone (2)
? 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.
Type II,Convergent
Margin/Subduction Zone (3)
? Under elevated temperature and pressure,the
crust expels a number of chemical compounds.
? Water (H2O).
? Carbon dioxide (CO2).
? Sulfur compounds.
? A small addition of these volatile substances can
lower the melting point of rock by several
hundred degrees Celsius.
Type II,Convergent
Margin/Subduction Zone (4)
? 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,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.
The CO2 Connection,Again (2)
? 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.
Volcanoes At Subduction Zones
? At a plate boundary,the plunging plate
draws the seafloor down into an ocean
trench,often 10 km deep or more.
? If the overriding plate is oceanic
lithosphere,volcanoes form a series of
islands called a volcanic island arc.
? Mariana Islands.
? Aleutian Islands.
Volcanoes At Subduction Zones (2)
? If the overriding plate is continental
lithosphere,a continental volcanic arc
forms,while sediment washed from the
continent tends to fill the offshore trench.
? Cascade Range of the Pacific Northwest.
? The Andes of South America.
Volcanoes At Subduction Zones (3)
? Chemical analyses of subduction-arc rocks
disclose unusual concentrations of rare
elements,such as boron硼,best explained
by the expulsion of water and other
volatile substances from the descending
slab.
? Most subduction-arc volcanoes occur
roughly where earthquakes indicate that
the top of the slab is 100 km deep.
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.
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 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.
? The Alps.
? The Himalayas.
? The Appalachians.
Type IV,Transform Fault Margin
? Along a transform fault margin,two plates
grind 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.
Transform Fault in Continents
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.
? Iceland.
? The oceanic trenches.
Comparing Venusian Topography
? 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 tectonicc is not plate tectonicc.
Comparing Venusian Topography (2)
? 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.
Fig,2.19
Comparing Venusian Topography (3)
? 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.
An Icy Analogue to Earth Tectonics
? The closest approximation to Earth tectonic in
our solar system is found on Europa木卫二 (one
of Jupiter’s 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.
? 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.
Hot Spots And Absolute Motion
During the nineteenth 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.
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,
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.
The World’s Hot Spots (2)
? 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.
Volcanic Domes and Coronae on Venus
? There is abundant evidence for hot
spots on Venus.
? Numerous elevated domes and ring-
shaped features called coronae have
been detected by Magellan’s radar.
Volcanic Domes and Coronae 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.
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 small size has limited their tectonic
histories.
? Too small to retain internal heat for billions of
years,the Moon,Mercury,and Mars now have
thick immobile lithospheres.
Plume Volcanism on Mars (2)
? 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,
Olympus Mons
Plume Volcanism on Mars (3)
?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.
What Causes Plate Tectonics?
? 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).
What Causes Plate Tectonics? (2)
? 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).
What Causes Plate Tectonics? (3)
? 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.
What Causes Plate Tectonics? (4)
? 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.
Computer simulation of convection in an idealized Earth Mantle
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,
Our Dynamic Planet
Plate Tectonics:
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 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,and ancient
lava flows 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)
? Faced with puzzling data,scientist
developed hypotheses to explain them.
? Alfred Wegener proposed the most
comprehensive early hypothesis for
“Continental Drift” in 1912.
Plate Tectonics:
From Hypothesis to Theory (4)
? 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.
? Attempts to test Wegener’s hypothesis with
observations had mixed success.
Plate Tectonics:
From Hypothesis to Theory (5)
? 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.
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 caused by
Earth’s rotational spin.
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 is
relatively light (average density 2.7 g/cm3).
? The thin oceanic crust is relatively heavy
(average density 3.0g/cm3).
? The lithosphere floats on the asthenosphere.
Earth’s Topography is Bimodal
Why Earth’s Topography is Bimodal?
Isostasy
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 high-density
blocks float low and have shallow,roots.”
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.
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.
The overflowing Ocean
Earth’s Surface,Land Versus Water (3)
? The continental shelf steepens slightly at 100-
200 meters below sea level.
? The continental slope is the flooded continental
margin.
? The continental rise descends more gently from
the base of the continental slope.
? Earthquakes and volcanoes are common along
active margins.
Topography of the Seafloor
Earth’s Surface,Land Versus Water (4)
? Ocean trenches occurs where oceanic
lithosphere and continental lithosphere
converge at the boundary between two plates.
? 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 of the open ocean
lie 3 to 6 km below sea level.
What Earth’s Internal Phenomena Tell Us
? Rocks are poor conductors of heat,so
Earth moves its internal heat by moving
the rock itself.
? The circulation of hot rock is maintained
by mantle convection.
Convection in the Mantle
Mantle Convection
? Conduction is the process by which heat moves
through solid rock.
? Earth’s heat can move in a second process called
convection.
? Convection can happen in gases,in liquids,or,
given enough time,in ductile solids.
? A prerequisite condition for mantle
convection is the thermal expansion of hot
rock.
? 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 expansion requires an increase
of 300-400oC and leads to a 1 percent
decrease in density.
? Viscosity is the propensity of rock to ductile
flow.
Mantle Convection (2)
? 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.
? The rock in the lithosphere is too cool for
convection to continue.
? Heat moves through the lithosphere primarily
by conduction.
? The lithosphere-asthenosphere boundary is
1300-1350oC,depending on depth.
Mantle Convection (3)
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.
? The densest lithosphere is most likely to
sink back into the asthenosphere and the
deeper mantle,
? Ocean floor and the continents are slowly
moving (up to 12 cm/yr).
Anatomy of a Plate
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.
Geothermal Gradients
? A gradient is a progressive change in
some physical or chemical property.
? The geothermal gradient varies widely
with geography from 5oC/km to 75oC/km.
Deeper and Hotter
Geothermal gradient
? 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.
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.
? 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.
Global Positioning System (2)
Four Types Of Plate Margins
? 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.
Plates have four kinds of boundaries
or margins
? Divergent margin (also called a spreading
center),magma rises to form new oceanic crust
between the two pieces of the original plate.
? Convergent margin/subduction zone,two plates
move toward each other and one sinks beneath
the other.
? Convergent margin/collision zone,two colliding
continental plates create a mountain range.
? Transform fault margin,two plates slide past
each other,grinding and abrading their edges.
Plate Margions
Four kinds Margions,Divergent; convergent (subduction zone
and colliding zone) and Transform
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.
? 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.
Three Types of Faults and Their
Earthquakes
Earthquakes and Deformation near Plate
boundaries
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 is called magma.
Type I,Divergent Margin (2)
? Midocean ridges occur in oceanic crust.
? Found in every ocean.
? Form a continuous chain that circles the
globe.
? Oceanic crust is about 8 km thick.
? When a spreading center splits continental crust:
? A great rift forms (such as the African Rift Valley).
? 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
Birth of the Atlantic Ocean (2)
? 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.
? 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
? 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.
Characteristics of Spreading Centers (2)
? The principle of isostasy applies,lower-
density rock rises to form a higher
elevation.
? The seafloor around midocean ridges for
nearly all oceanic lithosphere is younger
than 70 million years.
Characteristics of Spreading Centers (3)
? If the spreading rate is fast,
? 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 is fast by
comparison,6-20 cm/yr.
Role of Seawater at Spreading Centers
? Seawater circulates through cracks
beneath the ocean floor.
? Cold water percolates过滤 through these
cracks,warms in contact with subsurface
rock,and rises convectively to form
undersea hot springs.
Role of Seawater at Spreading Centers (2)
? 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.
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,
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.
? The principle of isostasy demands that the plate
subsides as it grows denser.
? The process by which lithosphere sinks into the
asthenosphere is called subduction.
Type II,Convergent
Margin/Subduction Zone (2)
? 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.
Type II,Convergent
Margin/Subduction Zone (3)
? Under elevated temperature and pressure,the
crust expels a number of chemical compounds.
? Water (H2O).
? Carbon dioxide (CO2).
? Sulfur compounds.
? A small addition of these volatile substances can
lower the melting point of rock by several
hundred degrees Celsius.
Type II,Convergent
Margin/Subduction Zone (4)
? 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,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.
The CO2 Connection,Again (2)
? 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.
Volcanoes At Subduction Zones
? At a plate boundary,the plunging plate
draws the seafloor down into an ocean
trench,often 10 km deep or more.
? If the overriding plate is oceanic
lithosphere,volcanoes form a series of
islands called a volcanic island arc.
? Mariana Islands.
? Aleutian Islands.
Volcanoes At Subduction Zones (2)
? If the overriding plate is continental
lithosphere,a continental volcanic arc
forms,while sediment washed from the
continent tends to fill the offshore trench.
? Cascade Range of the Pacific Northwest.
? The Andes of South America.
Volcanoes At Subduction Zones (3)
? Chemical analyses of subduction-arc rocks
disclose unusual concentrations of rare
elements,such as boron硼,best explained
by the expulsion of water and other
volatile substances from the descending
slab.
? Most subduction-arc volcanoes occur
roughly where earthquakes indicate that
the top of the slab is 100 km deep.
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.
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 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.
? The Alps.
? The Himalayas.
? The Appalachians.
Type IV,Transform Fault Margin
? Along a transform fault margin,two plates
grind 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.
Transform Fault in Continents
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.
? Iceland.
? The oceanic trenches.
Comparing Venusian Topography
? 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 tectonicc is not plate tectonicc.
Comparing Venusian Topography (2)
? 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.
Fig,2.19
Comparing Venusian Topography (3)
? 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.
An Icy Analogue to Earth Tectonics
? The closest approximation to Earth tectonic in
our solar system is found on Europa木卫二 (one
of Jupiter’s 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.
? 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.
Hot Spots And Absolute Motion
During the nineteenth 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.
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,
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.
The World’s Hot Spots (2)
? 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.
Volcanic Domes and Coronae on Venus
? There is abundant evidence for hot
spots on Venus.
? Numerous elevated domes and ring-
shaped features called coronae have
been detected by Magellan’s radar.
Volcanic Domes and Coronae 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.
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 small size has limited their tectonic
histories.
? Too small to retain internal heat for billions of
years,the Moon,Mercury,and Mars now have
thick immobile lithospheres.
Plume Volcanism on Mars (2)
? 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,
Olympus Mons
Plume Volcanism on Mars (3)
?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.
What Causes Plate Tectonics?
? 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).
What Causes Plate Tectonics? (2)
? 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).
What Causes Plate Tectonics? (3)
? 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.
What Causes Plate Tectonics? (4)
? 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.
Computer simulation of convection in an idealized Earth Mantle
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,
