Download Plate Tectonics and the Distribution of Major Landform Features

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Chapter 9
Plate Tectonics and the Distribution of Major Landform Features
9.1 The Evolution of the Plate Tectonics Theory
I) The Continental Drift Theory (大陸飄移學說)
A) The Drifting Continents
Proponents
 The theory was first postulated by F.B.Taylor in 1858 and further developed
by A. Wegener in 1911.
 He realized that the continents could be fitted together in the manner of a

jigsaw puzzle.
He suggested that these continents had been joined at one time.
drifted apart into their present positions.
Later, they
How did he conceptualize the jigsaw puzzle?
 Wegener had reconstructed a supercontinent named Pangaea, which existed
intact about 300 million years ago.
 Wegener visualized the America as fitted closely against Africa and Europe,
while the continents of Antarctica and Australia, together with the

subcontinents of peninsular India and Madagascar were grouped closely
around the southern tip of Africa.
Starting about 200 millions years ago, continental rifting began as Pangaea
slowly broke up and spilt into two parts: the northern part is called Laurasia
and the southern part, Gondwanaland. The separated fragments floated or
drifted apart on the ocean to form shields. Subsequently both Laurasia and
Gondwanaland broke up into the continents of today. Atlantic, Pacific and
Indian Oceans were also formed between continents. (Please refer to
Figure 9.1)
B) Evidences for Continental Drift
1. Continental outlines on either sides of the Atlantic Ocean are congruent
2. Matching of stratigraphic section and crustal provinces of the eastern border of
South America and western border of Africa
3. Similar fossil plants and animals are found on the either sides of the Atlantic
Ocean
4. Paleoclimatic evidence
5. Paleomagnetic evidence
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Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
Figure 9.1 The Drifting Continents
C) Deficiencies of the Theory
1.
The main problem was that no mechanism was known which could have caused
Pangaea to break up and which could have moved the continents such vast distances, in
different directions.
2.
Wegener had proposed that the continental layer of less dense rock had moved like a
great floating raft through a “sea” of denser oceanic crustal rock. Geologists could
show by use of laws of physics that this mechanism was physically impossible, because
rigid crustal rock could not behave in such fashion. No one, including Wegener, could
explain how continents could be moved.
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II) Theory of Sea Floor Spreading 海底擴張理論 (Hess, 1960)
After studies of the ocean floors in the 1950s and 1960s, the new discoveries about
the deep ocean floor formulated the concept of sea-floor spreading.
Figure 9.2 The Location of the Oceanic Ridges, Fracture Zones Cutting Them
and the Ocean Trenches
A) Evidences for Sea Floor Spreading
1. The Mid-Oceanic Ridge
In the 1950s, seismic investigations have found the mid-oceanic ridge. Its
characteristics:

The mid-oceanic ridge in the Pacific Ocean is the grandest and most extensive
mountain chain on planet earth.

Some of its peaks rise above sea level, forming islands. Other peaks are covered
by many metres of ocean water.

A rift zone, or central crack, marks the middle of the ridge throughout its length.
Earthquakes and volcanic activity are common in the rift zone.

The rift is about 35 to 45 km wide. It is bordered by vertical walls that plunge
downward for 1.5 km.
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Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
Figure 9.3 Diagrammatic Cross-section of a Mid-Oceanic Ridge
2.
Magnetic Anomaly (磁場異常)
Oriented samples from many basaltic lava flows were dated and their direction of
magnetisation measured. It was found that all flows of a particular age, irrespective
of their geographic location, indicate the same polarity for the earth’s magnetic field
and that both polarities are indicated for different times. By studying a great number
of flows from many parts of the world, it was possible by 1966 to suggest a
geomagnetic reversal time-scale for the past three and a half million years.
During the 1950’s and 1970’s, geologists discovered that the magnetic anomalies
present within
symmetrical
belts of the mid-oceanic ridges surface on either
side of the ridge crests.
Figure 9.4 Patterns of Palaeomagnetism from the Rocks of the Ocean Floor
(The width of each band was found to be proportional to the duration of the
palaeomagnetic phases established from the studies of rocks of known age on land.)
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3. Young Ocean Floor
In 1969 the US Deep-Sea Drilling “JOIDES” (Joint Oceanographic Institution
Deep-Sea Earth Sampling) commenced its phase at some fifty locations in the Atlantic
and Pacific Oceans. The cores of sediment overlying the basaltic lava flows were
examined.
It was found that the age of the oldest sediments which overlay and are intermingled
with top of the cooled lava is directly proportional to the distance of the sample from
the ridge. The amount of overlying ocean sediment also increases with distance
from the ridge.
4. Heat Flow
Geothermal heat constantly seeps out through the earth’s crust from its interior.
Results obtained in thermistor probe measurement in 1950 revealed that the heat flow
through the ocean floor is in general comparable to that determined previously for the
continents. Over the oceanic ridge, however, it is several times higher. These
anomalously high values reflect the emplacement of hot mantle-derived material in
the vicinity of ridge crests.
B) Idea of Spreading Sea Floor
What is sea floor spreading?
Sea floor spreading is the name given to the theory that continents have been moved
apart because new crust has been added to the ocean floor. Many scientists now
believe that continents have moved apart as result of sea floor spreading.
How have sea floor spreading been taking place?
The mid-oceanic ridge is the place where new crust is added from the mantle.
Mantle material rises on radioactive convection current. Then it breaks through
the crust and cools within the rift zone. As more new material is added to the ocean
floor, the ocean bottom becomes wider and wider. Thus, continents are moved
farther and farther apart.
C) Unsolved Problem
If one accepts theories on sea-floor spreading then one is also obliged to accept one or
other of two alternatives. Either:
a) the earth has expanded to accommodate the increase area of crust; or
b) as fast as crust is being produced at one site it is being consumed at another
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Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
9.3 Plate Tectonics
A) Structure of the Upper Mantle and the Crust
1. Upper Mantle
The upper mantle consists mainly of liquid magma which may intrude into the
crust.
It is the source region of most of the earth’s internal energy and of the forces
responsible for ocean floor spreading, continental drift and major earthquakes.
A part of the upper mantle is now in fact believed to be partially melted. This is
the asthenosphere (軟流圈) about 100 km beneath the earth’s surface.
-
Geophysicists believe it to be melted because it only transmits earthquake waves
slowly. It is sometimes called the low-velocity layer.
2.
-
The Crust
It is the outermost and thinnest of the earth zones, extending down to 30 to 40
km below the continents and to about 10 km beneath the oceans.
The base of the crust, where it contacts the mantle, is sharply defined.
The crust is solid in nature. The dominant rocks occurring in the crust fall into
contrasted groups:
Sial
Sima
Types of plate
Continental plate
Oceanic plate
Colour
Light rock (e.g. granite)
Dark rock (e.g. sandstones
and shales)
Density
Lower (2.7 g/cm³)
Higher
(about
2.8-3.0
g/cm³)
What is Lithosphere?
The solid crust and uppermost part of the mantle (which is solid) act as a single unit.
It is called lithosphere (岩石圈). The lithosphere is about 100 km thick. It is
broken into many large units, called lithospheric plates.
Figure 9.5
Asthenosphere and Lithosphere
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B) Main Idea of Plate Tectonics Theory
Plate tectonics replaced the older ideas of continental drift and sea-floor spreading
when it was realized that:
- The earth’s crust can be divided into several plates and the world’s main tectonic
features are related to activity at the edges of the plates.
- typical plates include both continental and oceanic crust.
- the sial-sima boundary was not a suitable surface for continents to move on, and
that movement took place in the low-velocity layer, the asthenosphere,.
- the lithosphere with its appreciable strength and rigidity includes both Sial and
Sima, continent and sea floor, so “plates” which may likewise contain both
components convey a better image of the process than continental drift or
sea-floor spreading which emphasize one or the other.
- It is supposed that new crust is created at spreading sites, and that the crust is
destroyed at subduction zone.
Main Summary
The general theory of lithospheric plates with their relative motions and boundaries’
interactions is plate tectonics.
B) Plates
Major Plates:
Pacific Plate, American Plate, African Plate, Australian-Indian
Plate, Eurasian Plate, Antarctic Plate
Minor Plates:
Arabian Plate, Philippine Plate, Cocos Plate, Nasca Plate,
Caribbean Plate, Scotia Plate
Figure 9.6 Major Plates of the World
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Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
C) Cause of Movement
In broad terms it may be connected with slow convection currents fueled by
radioactive processes. These could be deep-seated in the mantle or they may be
shallow in the asthenosphere (i.e. 100-400 km depth).
Volcanic activities
mid-oceanic ridge
lithosphere
asthenosphere
Upswelling
magma
Figure 9.7
A generalized model of plate tectonics
D) Plate Boundaries and Related Landform Features
1. Constructive Plate Margin / Zone of Spreading
The edges of the plates where plates move apart.
Results:

new molten rock material (basaltic magma) is injected into the gap,

solidifying to basalt, creating new ocean floor and

thus enlarging the plates.
a) Continental Rupture
Continental rupture is the rifting apart of a single continental lithosphere plate.
According to Figure 9.8, describe the stages of continental rupture:
At first the crust is both lifted and stretched apart as the lithosphere plate is arched
upward. In this stage, block mountains are formed. They are result of tensional
tectonics.
Next a long narrow valley, called a rift valley, appears. The widening crack in its
center is continually filled in with magma rising form the mantle below. The
magma solidifies to form new crust in the floor of the rift valley. Crustal blocks slip
down along a succession of steep faults, maintaining a mountainous landscape.
As separation continues, a narrow ocean appears; down its center runs a spreading
p;ate boundary. Plate accretion takes place to produce new oceanic crust and
lithosphere. We find in the Red Sea today an example of a narrow ocean formed by
continental rupture.
The widening of the ocean basin can continue until a large ocean has formed and
the continents are widely separated.
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Advanced Level-Geography-Landform System
New ocean
Figure 9.8
Schematic block diagrams showing stages in continental rupture
and opening up of a new ocean basin and formation of mid-oceanic ridge
Reading (Example – The East African rift valley)
About East African Rift Valley
The term rift valley was first used in 1920 to describe the structural elements of the
faulted area in eastern Africa.

Its Future (Figure 9.10)
The speculative suggestion has been made that the rift system will eventually become
the boundary of a detached lithospheric plate, the Somalian plate. According to the
interpretation of future relationships, a spreading plate boundary will open up along
the Lake Nyasa rift valley, making a new ocean basin, while a transform boundary
will follow the northern part of the rift system to the Gulf of Aden. The entire plate
will travel northeastward, moving past the Arabian plate.

Figure 9.9
Development of a typical rift valley in East Africa
(A) Normal faulting has produced a tilted fault block on the left
(B) Renewed normal faulting has broken the valley floor into narrow blocks
(C) After another episode of minor faulting, extrusive activity has built volcanoes in
the rift valley and on the flank of the uplift.
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Figure 9.9
Figure 9.10
Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
Figure 9.10
Present and future tectonics of eastern Africa
(A) Sketch map of the East African rift-valley system
(B) Continental rifting is just beginning in East Africa, where the rift-valley system
is shown by a spreading boundary symbol
(C) A prediction of the tectonic map 50 million years from now
b)
i)





Other Features associated with Constructive Margins
Vulcanicity
volcanic eruptions
vertical dykes are intruded in great numbers (parallel to the rift)
submarine lava flows and hills
the basaltic lava (basic) is normally very free-flowing thus it forms bubbling lava
lakes or fast-moving rivers of molten rock
the volcanoes formed are more in a shield shape.
ii) Earthquakes
Small, shallow depth earthquakes (less than 70 m) occur in the top few kms of the
crust and show essentially tensional and vertical movement.
iii) Mid-oceanic Ridges

Characteristics of Oceanic Ridges:
1. Faulted topography
2. Precisely in the centre of the ridge, at its highest point, is an axial rift which is a
trench-like feature. The form of this rift suggests that the crust is being pulled
apart along the line of the rift axis.
3. High heat flow at the centre of the ridge, decreasing at the flanks.
4. Widespread volcanic activities – because of the rising conventional current in the
mantle, isolated magma pockets are found at oceanic ridges. Where there is a
release of pressure along tensional faults, the magma can get to the surface.
5. The centre of the ridge is seismically active. There are shallow earthquakes
6.
(<70 km depth) due to the friction between the upwelling magma and the crustal
materials.
The median position, e.g. the Mid-Atlantic Ridge, constructive margin on land in
the East African Valley
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
Example – Mid-Atlantic Ridge
Location
It is found between the American plate and African plate or Eurasian plate.
Future
The upwelling of magma at divergent plate margins makes them major zones of
volcanic activity. Iceland, Surtsey, and the Azores, all of them areas of recent
volcanic activity, are all on the mid-Atlantic ridge.
asthenosphere
Figure 9.11 Constructive Margins
Figure 9.12 The Mid-Atlantic Ridge, its
Transform Faults and the Age of Ocean
2. Destructive Margins
Floor
a) Features of Destructive Margins
i) Zone of Subduction
When the two plates meet, the thinner and denser oceanic crust apparently bends
plunges down beneath the continental plate to be absorbed by the mantle.
The process of downplunging of one plate beneath another is called subduction.
ii) Earthquakes
The theory of subduction is supported by the location of the earthquake foci in the
broad, inclined seismic zone beneath the continent. So, along this broad seismic
zone (Benioff zone), the ocean floor is disappearing, being consumed as it plunges
downwards into the earth’s mantle.
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Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
Strong pressure builds up at the downslanting contact of the two plates, and these are
relieved by sudden fault slippages that generate earthquakes of large magnitude,
shallow ones near the coast, deepening inland to as much as 700 km (deep focus
earthquakes).
Line of active
vulcanicity
oceanic crust
continental crust
Depth in km
Location of most
earthquakes
Figure 9.13
The Relationship of Earthquakes, Trenches and Benioff Zone
iii) Trenches (海溝)
Commonly, where the oceanic plate plunges down into the mantle, there is an oceanic
trench found. The ocean trenches of the world are almost all found along the edges
of the Pacific Ocean.
Example – Tonga Island Arc and Trench
Tonga trench itself is a long narrow feature, stretching for about 1200 km.
maximum depth of the Tonga trench is 10882 m.
The
Apart from this distinctive topography, trench-island arc systems have several other
features worthy of note.
1.
2.
The distinctive lava type in this instance is andesite, which form acid lava
volcanic cones.
The heat flow pattern indicates a distinct low over the trench as well as high over
the island arc. Seismic data are also of interest. Hugo Benioff discovered that
earthquake foci are found at increasing depth moving from the trench towards
the island arcs. The foci lying in such a tight linear grouping has been called
the Benioff zone after its discoverer.
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A – Map of deep-focus epicentres,
active volcanoes and coutours of
the Benioff Zone
B – Cross Section of All
Earthquakes Foci along XY
Figure 9.14 Distribution of Earthquakes between the Tonga Trench and the
Fuji Islands
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Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
Figure 9.15
Trenches
and Benioff Zones of the
Western Pacific.
to
the
indicated
Depth
Benioff
by
100
Zone
km
contours.
vi) Vulcanicity
STAGE 1 As the plate sinks, the plate and its overlying sedimentary cover are
melted by the surrounding asthenosphere and thus it softens at greater
depth. Here, the plate experiences massive increases in pressure and
temperature until, as a depth of 40-600 km, it melts and is absorbed into
the mantle.
STAGE 2 The descending ocean floor is therefore destroyed. The underportion,
which is mantle rock in composition, simply reverts to asthenosphere as
it softens. The thin upper crust, formed of less dense mineral matter,
actually melts and become pockets of low density andesitic magma.
This magma tends to rise because it is less dense than the surrounding
material.
STAGE 3 Also, the top surface of the plate has water within its structure. The
presence of water lowers the melting point of the rocks. Magma reaches
the surface in bubbles known as plutons. Reaching the earth’s surface,
quantities of this magma build volcanoes, which tend to form volcanic
island arc along the destructive boundary, behind the trenches.
STAGE 4 The lavas extruded from these volcanoes are richer in silica than is basalt;
it is described as andesitic. Andesite is viscous and moves slowly
(acidic lava), forming thicker but generally less extensive lava sheets.
Moreover, andesitic eruptions are often accompanied by a great deal of
explosive activity, which shoots lava fragments (pyroclastic material) into
the air.
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v)
Folded Mountains
General Way to Form:
As the plate descends, it takes with it some of the sediments in the mantle. The rest
of the sediment seems to be scraped off, against the leading edge of the advancing
continental plate.
These sediments and the crust are deformed due to compression when the two plates
meet, resulting in the formation of an orogenic belt, which contains numerous
volcanoes.
-
Types of Fold Mountains
Fold Mountain
Coastal Mountain Range Type
Cordilleran Type
Eurasian Type Orogens
Himalayian Type
(i) Coastal Mountain Range Type / Cordilleran Type
Meaning
In short, coastal mountain range type happens when an oceanic plate and a continental
plate meet. The oceanic plate sinks into the subduction zone forming volcanic island
arc and resulting in vulcanicity.
Figure 9.16 A Cross Section through a Hypothetical, Single, Destructive Plate Margin
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Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
Example – The Andes
Let us consider a section through the earth’s crust extending from the South Atlantic
westwards to the East Pacific Rise along 20º latitude in the Southern Hemisphere.
Step 1:
New oceanic floor is generated at the Mid-Atlantic Ridge, so that the western
South Atlantic and South America, both part of the same plate – South American
plate, are moving westwards relative to the Mid-Atlantic Ridge.
At the same time,
new ocean floor generated at the East Pacific Rise is moving eastwards relative to
the Rise. The plate is called the Nazca plate.
Step 2:
On the western margin of the South American plate the oceanic crust of the Nazca
plate is heavier than the continental crust of the South American plate and is
subducted below it.
Step 3:
As the oceanic crust of the Nazca plate descends beneath the South American plate,
forms the Peru-Chile trenches.
Friction between the two plates triggers off
earthquakes along the fault systems of the South American coast. Materials of the
subducted block are partially melted as they are dragged deeper, and this melted
oceanic crust possibly acts as a source of magma for active volcanoes in the Andes.
Step 4:
It seems that initially subduction of the Nazca plate led to the creation of an arc of
volcanoes off the South American coast.
As subduction continued, volcanic
activity became more intense and huge bodies of magma were intruded into the
sedimentary rocks, raising the surface into what is now the Western Cordillera.
At the same time, the forces associated with this activity ripped eastwards, folding
the rocks and thrusting up the fold mountains of the Eastern Cordillera.
Figure 9.17 Simplified Section from East Pacific Rise across East Pacific, S.
America, S. Atlantic to the Mid-Atlantic Ridge
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(ii) Eurasian-type Orogens / Himalayian Type
Meaning:
It results from the collision of two full-sized masses of continental lithosphere.
Mountain chains are formed in this way such as the Alps and the Himalayas.
Formation of Fold Mountain
Through the processes of continental collision, fold mountains are formed as
following: (Please draw three successive annotated diagram to represent the
formation of fold mountain according to the description below.)
Stage 1 Subduction is in progress. There is a passive margin at the left, an active
subduction margin at the right. The lithospheric plate on the right is
moving toward the left, bringing the two continents closer together, while
the ocean basin is being reduced in width. Sediment is accumulating both
on the deep ocean floor and at the continental margin, under the continental
shelf.
Stage 2 Narrowing of the ocean basin continues. Sediment is being crumpled and
a succession of overthrust fault cuts through the oceanic crust.
Stage 3 The two continents have collided, squeezing the sediment mass and oceanic
floor strongly. They are folded and forced upward. Now the ocean basin
has disappeared entirely and a high mountain range has come into existence.
The oceanic crust has been eliminated entirely from the crust.
ocean
ocean
suture
Step 1
Step 2
Step 3
Example – Himalayas
Himalayas are formed by the collision between India (Indian-Austrlian plate), which
was separated from Gondwanaland and moving northwards, and continental Asia
(Eurasian plate). The ocean between the Indian shield and continental Asia has
completely disappeared now, and the Indus suture is left behind.
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Chapter 9 Plate Tectonics and the Distribution of Major Landform Features
The collision is thought to be still in progress, leading to the progressive increase in
the height of the mountain. Processes associated with such plate margins are
formation of fold mountains, active volcanic activities, earthquakes and faulting.
Figure 9.18
The Formation of Himalayas
3. Conservative Margins
These are margins along which plates slide past each other.
i) Transform Faults
Transform faults can also be found along the whole length of mid-oceanic rises, lying
parallel to the direction of movement.
ii) Earthquakes
Earthquakes, moderate to strong, are usually produced by sudden movement along
transform faults. Rock on both sides of an active fault are slowly bent over many
years as tectonic forces are applied. Energy accumulates in the bent rock, just as it
does in a bent crossbow. When a critical point is reached, the strain is relieved by
slippage on the fault and a large quantity of energy is instantaneously released in the
form of seismic waves.
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Example – San Andreas Fault
The San Andreas fault lies along the western seaboard of the USA. It extends into
southern California, passing about 60 km inland of the Los Angeles metropolitan area.
It is perhaps the best known fracture in the earth’s crust because of its size (900 km
long), and its closeness to centres of population like San Francisco and Los Angeles.
Nevada
Figure 9.19
The San Andreas Fault and associated fractures in the Western USA
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