Transcript Chapter 20: The Earth Through Time

Chapter 20: The Earth Through Time
Introduction: Tracking Past Plate
Motions (1)
The evidence in support of plate motions comes
from measurements made with the Global
Positioning System (GPS).
 However, evidence of past plate motions is not
obtained through GPS.

Introduction: Tracking Past Plate
Motions (2)

Evidence includes:




Great arc-shaped belts of metamorphic rocks formed
by continental collisions.
The eroded remains of island-arc volcanic rocks.
Traces of Earth’s past magnetic field preserved in old
lava flows.
Plates have been moving and changing Earth’s
surface for at least 2 billion years.
Former Ideas About Continents (1)
In the sixteenth century, it became apparent that
the coasts were approximately parallel.
 During the nineteenth century, the favored idea
was that Earth was originally a molten mass that
is cooling and contracting, with the crust being
gradually compressed.

Former Ideas About Continents (2)

Scientists discovered at the beginning of the
twentieth century that the Earth’s interior is
kept hot by radioactive decay.


The Earth might not be cooling but heating up (and
therefore expanding).
Heating would cause the Earth to expand and the
continental crust would then crack into fragments.
The New Idea: Plate Tectonics (1)
By the middle of the twentieth century, a totally
new approach was developed: plate tectonics.
 When great slabs of lithosphere, called plates,
slide sideways across the asthenosphere:


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Some parts of the slabs can be in compression.
Others can be in tension (that is, being pulled apart).
When a plate split in two, the broken edges of
continental crust match perfectly.
The New Idea: Plate Tectonics (2)
The energy needed to move plates comes from
the Earth’s internal heat energy, which causes
great convective flows in the mantle.
 Plate tectonics is the only theory ever developed
that explains all of the Earth’s major features.

Pangaea (1)
Alfred Wegener’s theory of continental drift
originated when he attempted to explain the
match of the shorelines on the two sides of the
Atlantic, especially along Africa and South
America.
 He hypothesized an ancient land mass called
Pangaea.
 The northern half of Pangaea is called Laurasia,
the southern half Gondwanaland.

Pangaea (2)
Laurasia includes Northern America.
 Gondwanaland includes:


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Eurasia.
India.
Africa.
Antarctica.
Australia.
South America.
Figure 20.1 A
Figure 20.1B
Pangaea (3)

During the late Carboniferous Period, about 300
million years ago, a continental ice sheet covered
parts of South America, southern Africa, India,
and southern Australia.

This is explained by continental drift: 300 million
years ago, the regions covered by ice lay in high, cold
latitudes surrounding the south pole, while north
America and Eurasia were close to the equator
Pangaea (4)

Many scientists remained unconvinced because
no one could explain how the solid rock of a
continent could possibly overcome friction and
slide across the oceanic crust.
Apparent Polar Wandering (1)

From the mid-1950s to the mid-1960s,
geophysicists discovered paleomagnetism.

Certain igneous and sedimentary rocks can preserve a
fossil record of the Earth’s magnetic field at the time
and place the rocks formed.
Apparent Polar Wandering (2)

Three essential bits of information are contained
in that fossil magnetic record:

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
The Earth’s polarity (the magnetic field was normal
or reversed at the time of rock’s formation).
The location of the magnetic poles at the time the rock
formed.
The magnetic inclination (indicating how far from the
point of rock formation the magnetic poles lay).
Figure 20.2
Apparent Polar Wandering (3)
The paleomagnetic inclination is a record of the
place between the pole and the equator (that is,
the magnetic latitude) where the rock was
formed.
 In the 1950s, geophysicists determined that the
strange plots of paleopole positions indicated
apparent polar wandering.

Figure 20.3
Seafloor Spreading (1)
In 1962, Harry Hess of Princeton University
hypothesized that the topography of the seafloor
could be explained if the seafloor moves
sideways, away from the oceanic ridges.
 His hypothesis came to be called the theory of
“seafloor spreading,” and was soon proven to be
correct.

Seafloor Spreading (2)
Hess postulated that magma rose from Earth’s
interior and formed new oceanic crust along the
midocean ridges.
 Three geophysicists (Frederick Vine, Drummond
Matthews, and Lawrence Morley) proposed that
lava extruded at any midocean ridge becomes
magnetized and acquires the magnetic polarity
that exists at the time the lava cools.

Seafloor Spreading (3)
The oceanic crust contains a continuous record
of the Earth’s changing magnetic polarity.
 Successive strips of oceanic crust are magnetized
with normal and reversed polarity.
 The magnetic striping allows the age of any place
on the seafloor to be determined.
 Magnetic striping also provides a means of
estimating the speed with which the seafloor had
moved.

Figure 20.4
Magnetic Record And Plate Velocities
(1)
The most recent magnetic reversal occurred
730,000 years ago.
 The oldest reversals so far found date back to the
middle Jurassic, about 175 million years ago.
 From the symmetrical spacing of magnetic time
lines it appears that both plates move away from
a spreading center at equal rates.

Figure 20.5
Magnetic Record And Plate Velocities
(2)
All that can be deduced from magnetic time lines
is the relative velocity of two plates.
 Absolute velocities requires information from
GPS measurements (for present-day plate
motions), or hot spot tracks, or past plate
motions;

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Plates with only oceanic lithosphere tend to have high
relative velocities (Pacific and Nazca plates).
Magnetic Record And Plate Velocities
(3)
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Plates with a great deal of thick continental
lithosphere, such as the African, North American, and
Eurasian plates, have low relative velocities.
Plate velocities vary with the geometry of motion of a
sphere.

Plates of lithosphere are pieces of a shell on a spherical Earth.
Magnetic Record And Plate Velocities
(4)
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One consequence of different plates velocities is that
the width of new oceanic crust bordering a spreading
center increases with the distance from the rotation
pole.
Each transform fault lies on a line analogous to a line
of latitude around the rotation pole.
Figure20.6
Figure 20.7
Figure 20.8
Relict Plate Boundaries In The
Geologic Record (1)
Seafloor magnetic strips help us reconstruct
plate motion only as far back in time as the
Jurassic, some 175 million years ago.
 All large expanses of older oceanic crust have
been subducted back into the mantle at
convergent plate boundaries.

Relict Plate Boundaries In The
Geologic Record (2)
The paleomagnetism of continental rock can be
used to follow plate motion further back in time,
but without the breadth and continuity of
seafloor data.
 Today’s continents were assembled from many
distinct plates or plate fragments.
 Small fragments of continental crust that have
drifted as a single unit in the Earth history are
called terranes.

Ophiolites (1)
The igneous rock formed at a spreading center,
known as midocean ridge basalt (MORB), has a
distinctive chemistry;
 When MORB is found on land, it usually lies
within a body of rock that appears to be a
fragment of oceanic crust caught up in a
continental collision.

Ophiolites (2)
The minerals that characterize basalt, if buried
deep within the collision zone, transform into a
assemblage dominated by a distinctive green
fibrous mineral called serpentine.
 These serpentine-dominated fragments of
oceanic crust found on continents are called
ophiolites, from the Greek word for serpent,
ophis.

Ophiolites (3)

Their structure matches well the crustal
structure expected at a midocean ridge:
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At the top is a thin veneer of sediment that was
deposited on the ocean floor.
Beneath the sediment is a layer of pillowed basalt.
Still deeper are sills of gabbro, the plutonic equivalent
of basalt.
Ophiolites (4)
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Many ophiolites also contain the apparent source
rocks of the basalts and gabbros.
Beneath the gabbro sills there is often a layer of
peridotite.
The contact between gabbro and peridotite is
interpreted to be the former Moho at the base of what
was formerly oceanic crust.
Subduction Mélange And Blueschists
(1)
Along convergent margins, a distinctive feature
is the development of a mélange: a chaotic
mixture of broken, jumbled, and thrust-faulted
rock.
 A sinking plate drags the sedimentary rock
formed from accumulated sediment downward
beneath the overriding plate.

Subduction Mélange And Blueschists
(2)
Caught between the overriding plate and the
sinking plate, the sediment becomes shattered,
crushed, sheared, and thrust-faulted, forming a
mélange.
 As the mélange thickens, it undergoes
metamorphism, common in many mélange
zones, to form low-temperature metamorphism
blueschists and eclogites.

Figure 20.10
Back-arc Basins And Plate Extension
(1)
When the sinking of a subducting plate is faster
than the forward motion of the overriding plate,
the margin of the overriding plate can be
subjected to tensional (pulling) stress.
 If the overriding plate is oceanic or if the
extension of a continental margin has progressed
to an extreme state, an arc-shaped basin forms
behind and parallel to the magmatic arc of the
subduction zone.

Figure 20.11
Figure 20.12
Back-arc Basins And Plate Extension
(2)

Basaltic magma may rise into such a back-arc
basin at a newly formed spreading center, and
new oceanic crust may form.
Supercontinents And Vanished Oceans
(1)
The average composition of continental
lithosphere is quartz-rich, compared to olivine
and pyroxene-rich oceanic lithosphere.
 Continental lithosphere is always less dense than
oceanic lithosphere and is not subducted.
 Seismic evidence suggests that a root of mantle
rock is attached to the base of old, cool
continental crust.

Supercontinents And Vanished Oceans
(2)
Continental buoyancy can be enhanced by the
detachment and loss of this dense mantle root
during the final stages of a continental collision.
 When plate movement bunches continental
fragments together, the heat of the mantle
beneath still must escape Earth’s interior.

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A supercontinent impedes heat flow from the deep
mantle to the surface, effectively forming a layer of
insulation.
Supercontinents And Vanished Oceans
(3)

Convective motion within Earth’s mantle:
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Will accumulate heat and cause thermal buoyancy at
the base of the continental lithosphere.
Warms and softens the lithosphere, which begins to
rift.
The opening of the Atlantic ocean was heralded by the
eruption of large basalt flows;
 In some cases, these eruptions persisted and became hot spots
(example: the Parana flood basalt in Brazil).

Supercontinents And Vanished Oceans
(4)

Two supercontinents existed in the past:
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Pangaea, formed roughly 350 million years ago,
Rodinia, an earlier Proterozoic continent formed
roughly 1100 million years ago and split apart roughly
750 million years ago.
The sutures between continental fragments can
be found in the many mountain ranges and belts
of ophiolites that formed in the late Paleozoic
period, roughly between 450 and 350 million
years ago.
Supercontinents And Vanished Oceans
(5)
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Examples include:
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The Appalachian mountains in eastern North
America.
The Atlas mountains in Morocco.
The Ural mountains in Russia.
The Hercynian mountains in Europe.
Each of these mountain ranges is heavily eroded
now.
Supercontinents And Vanished Oceans
(6)

As Pangaea formed, sediment sequences on
continental shelves around the world record
evidence that global sea level fell, draining
shallow continental seas and exposing large areas
of continental shelves.
Supercontinents And Vanished Oceans
(7)

Geologists interpret the sea level drop as
evidence for:
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A general slowdown in plate movement.
A slowdown in the rate of formation of young,
buoyant oceanic lithosphere at midocean ridges.
The oceanic lithosphere at the time of Pangaea
was, on average, older, colder, and denser,
leading to a deeper world ocean.
Supercontinents And Vanished Oceans
(8)
Pangaea lasted as a supercontinent for at least
150 million years.
 New oceans formed when Pangaea separated:
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The Atlantic.
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Southern Oceans.
Figure 20.13
Ice Ages in Earth History (1)

Large continental ice sheets have existed several
times during Earth’s history:
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In the Carboniferous Period, 286 to 360 million years
ago.
In the Ordovician and Silurian periods, roughly 450
to 425 million years ago.
Both of these glacial periods occurred at times
when large continental landmasses lays over
Earth’s south pole, providing a platform for the
accumulation of vast quantities of snow and ice
at high latitudes.
Figure 20.14 A
Figure 20.14 B
Figure 20.14 C
Ice Ages in Earth History (2)

Factors leading to an ice sheet:
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The amount of carbon dioxide and other greenhouse
gases in the atmosphere.
The tilt of the Earth’s axis.
Changes in the Earth’s orbit around the sun.
The height of continents and their positions in relation
to the north and south poles.
Phanerozoic Glaciations (1)

Three major periods of high-latitude glaciation
have been recognized in the Phanerozoic:
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
Pleistocene (0 to 5 million years ago).
Carboniferous (roughly 300 million years ago).
Ordovician (roughly 455 million years ago).
Rapid carbon burial and reduced volcanic
activity lower concentrations of CO2 in the
atmosphere, reducing the greenhouse effect and
encouraging glaciation.
Phanerozoic Glaciations (2)
Carboniferous coal deposits occur within
repetitive layered sedimentary sequences of
marine and nonmarine sediments called
cyclothems.
 Cyclothems indicate repeated transgressions by
the sea.
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Timing connected to the longer Milankovitch orbital
cycles of 100,000 years.
glacial and interglacial climate may have alternated in
a regular pattern.
Phanerozoic Glaciations (3)

New continental glaciers:
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Would have stolen water from the oceans, causing sea
level to drop.
Drained Coastal coal swamps as the sea level fell.
Decreased carbon consumption.
Atmospheric CO2 levels could return to normal
levels.

An increase in greenhouse warming would melt the
glaciers and raise the sea level, rejuvenating the coal
swamps and repeating the glacial cycle.
Phanerozoic Glaciations (4)

There is evidence for continental glaciation near
the end of the Proterozoic Eon, between 600 and
800 million years ago.

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The extensive low-latitude glaciation occurred during
a climatic extreme in which the whole Earth must
have experienced glaciation.
Extensive low-latitude glaciation did not occur at
any time during the Phanerozoic.
Regional Structures Of Continents (1)

Because the most ancient oceanic crust known to
exist in the ocean dates only from the midJurassic Period, the only direct evidence
concerning geologic events more ancient than the
mid-Jurassic comes from the continental crust.
Regional Structures Of Continents (2)

Two kinds of structural units can be
distinguished within the continental crust:
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The craton is a core of very ancient rock that has
attained tectonic and isostatic stability.
Orogens are elongate regions of crust that have been
intensely folded and faulted during continental
collision.
Only the youngest orogens are mountainous today.
 Ancient orogens reveal their history through the kinds of
rock they contain and the kind of deformation present.

Figure 20.15
Regional Structures Of Continents (3)
An assemblage of cratons and ancient orogens
that has reached isostatic equilibrium is called a
continental shield.
 Because it is a stable platform, a continental
shield is covered by a thin layer of littledeformed sediments.

Regional Structures Of Continents (4)

North America has a huge continental shield at
its core, and around the shield are four younger
orogens:
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The Caledonide.
Appalachian.
Cordilleran.
Innuitian.
Each is younger than 600 million years.
Regional Structures Of Continents (5)

Because the North American shield crops out in
Canada, and is mostly covered by flat-lying
sedimentary rocks in the United States,
geologists often refer to it as the Canadian
Shield.
Regional Structures Of Continents (6)

Geologists have identified several ancient cratons
and orogens in the Canadian Shield;
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Within the cratons, all rocks are older than 2.0 billion
years.
The small cratons within the Canadian Shield were
probably minicontinents during the Archean Eon and
early part of the Proterozoic Eon.
By about 1.6 billion years ago, these minicontinents
had become welded together.
Regional Structures Of Continents (7)

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Each time two cratonic fragments collided, an orogen
was formed between them.
The existence of ancient collision belts is the best
evidence that plate tectonics operated at least as far
back as 2 billion years ago.
Regional Structures Of Continents (8)

The fragmentation, drift, and welding together
of pieces of continental are responsible for the
five types of continental margins we know of
today:
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Passive.
Convergent.
Collision.
Transform fault.
Accreted terrane.
Passive Continental Margins (1)

A passive continental margin occurs in the stable
interior of a plate.


The eastern coast of North America, for example, is in
the stable interior of the North American Plate.
Passive continental margins develop when a new
ocean basin forms by the rifting of continental
crust (for example, the Red Sea).
Figure 20.16
Figure 20.17
Passive Continental Margins (2)
Passive continental margins are places where a
great thickness of sediment accumulates.
 This accumulation apparently occurs in the
following manner:

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Basaltic magma, associated with formation of the new
spreading edge, splits the continent.
A plateau forms as the lithosphere expands, with an
elevation of as much as 2.5 km above sea level.
Passive Continental Margins (3)



Tensional forces cause normal faults and form a rift
(pronounced topographic relief between the plateau
and the floor of the rift).
Before the rift floor sinks low enough for sea water to
enter, clastic nonmarine sediments shed from the
steep valley walls accumulate in the rift.
Basaltic lavas, dikes, and sills form by magma rising
up the normal faults.
Passive Continental Margins (4)



As the rift widens, a point is reached where seawater
enters.
The high rate of evaporation results in the deposition
of a strata of evaporate salts laid down on top of the
clastic nonmarine sediments
The formation of three-armed rifts with one of
the arms not developing into an ocean is a
characteristic feature of passive continental
margins.
Figure 20.18
Passive Continental Margins (5)

The Gulf of Aden, the Red Sea, and the northern
end of the African Rift Valley meet at angles of
1200.

Such a meeting point formed by three spreading edges
is called a plate triple junction.
Continental Convergent Margins (1)
At a continental convergent margin, the edge of
a continent coincides with a convergent plate
margin in which oceanic lithosphere is being
subducted beneath continental lithosphere.
 Subduction produces intense deformation of a
continental margin.

Continental Convergent Margins (2)

The Andean coast of South America is an
example.


The Nazca Plate (capped by oceanic crust) is being
subducted beneath the South American Plate.
Wet partial melting in the mantle activated by water
released by the subducted Nazca Plate produced the
andesitic magma that formed the Andes ( a
continental volcanic arc).
Continental Convergent Margins (3)

Sediment subjected to deformation in such a
setting forms a mélange.


Sediments subjected to high-pressure and lowtemperature metamorphism.
Adjacent and parallel to the belt of mélange, the crust
beneath the continental volcanic arc is
metamorphosed, but here the metamorphism is
regional. One distinctive feature of a continental
convergent margin, therefore, is a pair of parallel
metamorphic belts.
Figure 20.19
Continental Collision Margins (1)

The edges of two continents, each on a different
plate, come into collision at a continental
collision margin.

This results in intense folding and thrust faulting.


The Himalayan Mountain chain.
Within a fold-and-thrust mountain system,
strata are compressed, faulted, folded, and
crumpled, commonly in an exceedingly complex
manner.
Figure 20.20
Continental Collision Margins (2)

Metamorphism and igneous activity are always
present.


The Alps, the Himalayas, and the Carpathians are all
young fold-and thrust mountain systems formed
during the Mesozoic and Cenozoic eras.
The Appalachians and the Urals are older,
Paleozoic-aged, examples.
Fold-And Thrust Mountain System (1)

These systems develop from piles of sedimentary
strata commonly 15,000 m or more in thickness.


Sediments are predominantly marine, accumulated
along passive continental margins.
The American geologist J.D. Dana coined the
term geosyncline to describe a great trough that
received thick deposits of sediment during slow
subsidence through long geologic periods.
Fold-And Thrust Mountain System (2)

One distinctive feature of mountain systems
formed by collision is that an ocean disappears
and a new mountain system lies in the interior of
a major landmass.
Transform Fault Margins

A transform fault continental margin occurs
when the margin of a continent coincides with a
transform fault boundary of a plate.


The San Andreas Fault.
The San Andreas Fault apparently arose when
the westward-moving North American continent
overrode part of the East Pacific Rise.

The San Andreas is the transform fault that connects
the two remaining segments of the old spreading
center.
Figure 20.21
Accreted Terrane Margins (1)
An accreted terrane continental margin is a
former convergent or transform fault margin
that has been further modified by the addition of
rafted-in, exotic fragments of crust such as island
arcs.
 The northwestern margin of North America,
from central California to Alaska, is an example
of an accreted terrane margin.

Accreted Terrane Margins (2)
Eventually, any fragment not consumed by
subduction is added (accreted) to a larger
continental mass.
 In the western Pacific Ocean, there are several
examples:




Taiwan.
The Philippine Islands.
The many islands of Indonesia.
Accreted Terrane Margins (3)
Each fragment, called a terrane, is a geologic
entity characterized by a distinctive stratigraphic
sequence and structural history.
 An accreted terrane is always fault-bounded and
differs in its geologic features from adjacent
terranes (also called suspect terranes).
 Some terranes have moved 5000 km or more.

Figure 20.22
Mountain Building
Today’s fold-and-thrust ranges are the orogens
that formed during the last few hundred million
years.
 Examples include:




The Appalachians.
The Alps.
The Canadian Rockies.
The Appalachians (1)
The Appalachians are a Paleozoic fold-andthrust mountain system 2500 km long, that
borders the eastern and southeastern coasts of
North America.
 They contain mud cracks, ripple marks, fossils of
shallow-water organisms, and, in places, fresh
water materials such as coal.
 The sedimentary strata thicken from west to
east.

Figure 20.23
Figure 20.24
The Appalachians (2)
Many of Pennsylvania’s oil pools were found in
these gently folded strata.
 In the region known as the Valley Ridge
Province, the strata have been bent into broad
anticlines and synclines.
 In the west, the strata are nearly flat lying, but as
one moves further east the strata dip more
steeply to the east.

Figure 20.25
The Appalachians (3)

The surface along which movement occurred is
known as a detachment surface, and the slice that
moved is commonly referred to by its French
name, decollement.
Figure 20.26
The Alps (1)
The Alps and associated mountain ranges in
southern Europe were formed during the
Mesozoic and Cenozoic eras, as a consequence of
a collision between the European and African
plates.
 The Jura Mountains, which mark the
northwestern edge of the Alps, have the same
folded form and origin as the Valley and Ridge
Province in North America.

Figure 20.27
The Alps (2)
The Jura Mountains were formed from shallowwater sediments deposited on an ancient
continental shelf.
 In the high Alps, thrusting appears to have
developed on a much grander scale than in the
Appalachians.
 The high Alps are composed of deeper-water
marine strata.

Figure 20.28
The Canadian Rockies
In the Canadian Rockies, a central zone has been
intensely metamorphosed.
 The thrust sheets in the Canadian Rockies
moved eastward away from the core zone.
 Each sedimentary unit becomes thinner from
west to east.

Figure 20.29
Revisiting Plate Tectonics And The
Earth System (1)
For more than 2000 years, Arab traders sailed to
India during the hot summer months, because
summer winds blow from the west. They sailed
back while winter winds blow from the east.
 The Arabic name for this seasonal reversal of
wind and weather is mausim, from which we
derive our word monsoon.

Revisiting Plate Tectonics And The
Earth System (2)
The monsoon winds of India and Southeast Asia
are a consequence of the collision between the
Eurasian Plate and the Australian-Indian Plate.
 The high mountains and plateau divert the
normal flow of westerly winds.

Figure 20.31
Revisiting Plate Tectonics And The
Earth System (3)

Chinese geologists have discovered that these
high lands are rather recent topographic
features.

They base their conclusion in part on evidence of
plant fossils of the Pliocene Epoch (5.3 to 1.6 million
years ago), collected at altitudes of 4000 to 6000 m,
that include many subtropical forms that today exist
only at altitudes below 2000 m;
Revisiting Plate Tectonics And The
Earth System (4)

Sediment cores from the northern Indian Ocean
show rates of sedimentation until about 10
million years ago.

Two peaks in sediment supply (9 to 6 and 4 to 2
million years ago) are evidence of major intervals of
Himalayan uplift.