Download C. Igneous Rocks

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PLATE TECTONICS & MOUNTAIN BUILDING
Introduction
 Look at the following three figures, which shows the world distribution of fold mountains,
volcanoes and earthquake respectively.
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 Can you see any similarities in their distributions from these three figures?
 Why do they take place in such locations?
 How do they form?
 Before we answer these questions, we should first:
? have a general knowledge about the structure of the Earth and the concept of plate tectonics.
? know how plate tectonics results in the formation of fold mountains, volcanoes and other
tectonic features along the plate margins.
? understand how plate movement initiates mountain building processes, which result in several
landforms found on the Earth’s surface.
A. Structure of the Earth
-
The earth is a sphere composed of crust, mantle
and core.
Its radius is about 6,400 km.
-
1. Crust (Lithosphere)
-
The thinnest and the least dense solid layer of
rock
-
It can be divided into two layers:
Continental crust (sial)
Rocks rich in silica and alumina
Granitic rocks
(Thicker /Thinner)
(Lower / Higher) density
Discontinuous layer
Oceanic crust (sima)
Rocks rich in silica and magnesia
Basaltic rocks
(Thicker / Thinner)
(Lower / Higher) density
Continuous layer
2. Mantle
-
It consists of two layers (upper and lower mantle) in the form of semi-solid magma.
-
lies between the crust and core
The density, temperature and pressure in mantle is ________________than the crust.
3. Core
-

It consists of molten outer core and solid inner core.
The density, temperature and pressure in core is the ___________________ .
In conclusion, density, temperature and pressure of the earth
________________________from the interior.
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B. Plate Tectonics
-
The concept of plate tectonics suggests that the Earth’s surface layers are divided into several large
plates.
The plates are about 100km thick, and therefore consist of Earth’s crust and part of the upper
mantle.
The plates are not fixed but are moved by ______________________________________ of
magma.
The movement of plates results in _________________________________ drift and
________________________ spreading.
1. World distribution of plates
2. Kinds of plates
I.
Continental plate
a. It consists of both continental and oceanic crust, e.g., the ________________ plate.
II.
Oceanic plate
a. It consists of oceanic crust, e.g., the ____________________ plate.
3. Types of plate boundaries
-
The movement of the two plates determines the types of plate boundaries, which depends on
the direction of magma current under the plates.
Different tectonic processes will take place along the different types of plate margin, some
-
distinctive tectonic features will be formed.
There are three types of plate margin:
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I. Constructive plate margin (divergent boundary / zone of spreading)
 diverging magma
a. When two oceanic plates move
apart:

magma rises from mantle to
the ocean floor and
solidifies

divergence causes sea-floor
spreading

mid-oceanic ridges will be
formed, e.g., the
Mid-Atlantic Ridge

volcanoes are formed along
the ridges and may form islands
b. When two continental plates move apart:

magma rises, cools down and solidifies

rift valley and volcanoes will be formed

rift valley may be filled up by sea water and form new ocean

e.g. Red Sea and East African Rift Valley
II. Destructive plate margin (convergent boundary/ colliding boundary/ zone of
compression/ zone of subduction)
 converging magma
a. When two oceanic plates collide:

the denser plate normally plunges under the other and inserts into the mantle, where
is called
____________________________
zone

ocean trenches are then formed
along the subduction zone

plate material inside the subduction
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zone is melted because of the high ___________________________ in the mantle

melted material release pockets of magma in ______________ density

magma rises up through the overriding plate and creates
______________________________________________
b. When oceanic plate collides with continental plate:

denser _______________________ plate normally plunges under the less dense
_______________________________ plate, forming ____________________
zone

________________________________ is then formed along the subduction zone

____________________ is transported by rivers and waves and fill up the ocean
trench

the sedimentary layers will be _________________________, folded and uplifted
to form fold mountain, which is made up of sedimentary rocks
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
magma from the melted oceanic crust rises through the fold mountain and forms
_____________________________

e.g. Andes
c. When two continental plates collide:

ocean floor between the two continental blocks is being destroyed

none of the two continental crusts will plunge into the mantle, because both are in
_________________ density

the ocean in-between will finally ______________________________

the sediments accumulated in the continental margins will be compressed,
_____________________ and uplifted to form a fold mountain range

e.g. Himalayas
III. Conservative plate margin (Passive boundary/ Transform fault margin)
-
by ______________________ force, two
plates slide laterally along transform fault
(tear fault)
-
earthquakes often occur along the active
transform fault
-
e.g. ________________________ Fault in California (U.S.A)
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Summary of plate tectonic:
Constructive plate
margin
Destructive plate
margin
Conservative plate
margin
Plate movement
Force
Process
Sea-floor spreading

Associated
tectonic
features



Mid-oceanic ridges
(oceanic vs. oceanic)
Rift valleys
(continental vs.
continental)
Volcanoes
Volcanic island arc






Subduction
Folding
Ocean trenches
Fold mountains
(oceanic vs.
continental &
continental vs.
continental)
Volcanoes
Volcanic island arc
Faulting

Tear/transform fault
Natural
hazards
C. Mountain Building Processes
-
Plate tectonics provides a basic explanation for the formation of mountains, volcanoes and
several landforms on the earth’s surface.
There are three types of mountain building processes: folding, faulting and vulcanicity.
These processes can be identified as a result of the movement of plates.
1. Folding
-
Folding is the result of _______________________________ forces acting on rock layers.
Apart from large-scale folds (fold mountains), which are mainly found along the destructive
plate margins, small-scale folds can also be found within the plates, e.g. Hong Kong
-
based on the difference of __________________________ forces from two sides, different
resistance of rocks and the inclination of the resultant fold axis, folds may be classified into
five main types:
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Symmetrical /
simple fold
Equal
compressional
force from both
sides
Asymmetrical
fold
Overfold
Stronger compressional force from one side
Vertical axis
Limbs are of
equal steepness
Inclined axis
One limb is
steeper
One limb is
pushed over
the other
2. Faulting
-
Faulting results from the fracturing or
-
breaking of rock masses under tension,
compression or shearing.
According to the form of displacement,
which is determined by the different kinds of
forces, faults can be divided into three types.
I.
Recumbent fold Overthrust fold
Normal fault  tension
Rocks fracture
along a thrust
line
One limb is
Limbs are nearly pushed forward
parallel
overriding the
other
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II.
Reversed fault  compression
III.
Tear fault  shearing
-
Faulting can result in some landforms, either by tension or compression:

Block mountain (Horst)

Escarpment

Rift valley (Graben)

Inland basin

Plateau

Stepped faults
3. Vulcanicity
-
Vulcanicity is highly related to plate movement.
-
Some volcanoes form along the oceanic ridge, where magma is rising from the mantle.
Other volcanoes form near the subduction zones, where descending plates are being melted.
The magma released from the melted plates rises into the crust (intrusive vulcanicity), and
even comes out to the Earth’s surface in the form of lava (extrusive vulcanicity).
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I.
Intrusive volcanic landforms
- They occur within the crust.
a. Batholith

largest intrusive body occurs ____________ underground

magma cools ___________________
b. Dyke

inclined or vertical intrusive body

magma cools ___________________________
c. Sill
II.

lies parallel to horizontal rock layers

magma cools _________________________
Extrusive volcanic landforms
a. Lava plateau (lava platform)
formed by quiet outflow of ______________________
b. Volcano

formed by violent ____________________________

lava cools around crater

classified into active, dormant and extinct volcanoes

also classified into acid lava cones, basic lava cones, ash and cinder cones, and
composite cones, as described as follows:

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Basic Lava Cone

Formed by the solidification of basic lava
Basic lava is fluid, it solidifies (quickly /
slowly) and spreads far away from the vent
Broad base, low in height and dark colour
Gentle (concave / convex) slopes
e.g. Mauna Loa (Hawaii)

Formed by the solidification of acid lava

Acid lava is viscous, it solidifies (quickly /




Acid Lava Cone
slowly) before it can flow far away from the
vent
Ash and Cinder Cone

Narrow base, high in height and light colour

Steep (concave / convex) slopes

e.g. Mt. Mayon (Philippines)


Formed by successive eruptions and
accumulations of ash and cinder
During eruptions, lava and rock fragments are
ejected and broken into ash and cinder
Steep slopes and high in height
e.g. Vulcaro (Italy)

Formed by alternate quiet outflows of lava


Composite Cone
and violent eruptions

____________________structure with
alternate layers of ash and lava

_______________ slopes and
______________ in height


Sometimes parasitic cones are found on the
sides of the cone
e.g. Fujiyama (Japan)
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Summary of mountain building processes:
Processes
Forces
Types
Compression Small-scale fold
Folding
Large-scale fold
Compression Reversed fault
Faulting
Associated Landforms




Tension
Normal fault



Shearing
Vulcanicity
III.
Tear fault
Intrusive:
Extrusive:


Fold mountains
Block mountains
Rift valleys
Plateaux
Escarpments
Inland basins
Stepped faults
Batholith, dyke, sill, laccolith
Volcano, lava plateau, plug
dome, caldera, crater lake,
volcanic neck, hot spring,
geyser, fumarole
Effects of volcanic eruption
a. Positive effects:

Weathered basic lava produces ______________________________for farming.

___________________________________ can be utilized for heating, or
developed into health resorts.

Volcanoes attract tourists and thus increase national income.

Volcanicity sometimes lead to the formation of precious minerals, e.g.
_________________________
b. Negative effects:

Loss of ________________ and ______________________.

Lava flows causes ___________ hazards and destruction of
_______________________________.

During violent eruptions, large amount of hot gases and ash are ejected, they
usually form dense cloud and cause heavy rain.

Volcanic eruption causes pollution and thus brings about extreme climate.
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Rocks
Extrusive rocks
By solidification and
cooling of magma or
lava
Igneous
rocks
Volcanic rocks e.g. Rhyolite
By depth of
occurrence
Hypabyssal e.g. Porphyry
rocks
Intrusive rocks
Plutonic rocks e.g. Granite
Mechanically-formed e.g. conglomerate, breccia, sandstone, shale
Rocks
By sedimentation
of rocks
By
metamorphism
of rocks
Sedimentary
rocks
Metamorphic
rocks
Chemically-formed e.g. gypsum, rock salt, calcite, chert
Organically-formed e.g. limestone, chalk, coal, oil
Thermal metamorphism
Dynamic metamorphism
e.g. gneiss from granite
quartzite from sandstone
slate from shale
marble from limestone
Regional metamorphism
A. Introduction
 The crust of the earth is made up of different types of rocks.
 Rocks are made up of minerals.
 According to mode of formation, rocks can be classified into three groups:
1. ______________________ rocks
2. ______________________ rocks
3. ______________________ rocks
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B. Distribution of Rocks in Hong Kong
__________________
__________________
____________________________
____________________________

Students are expected to identify the distribution of different types of rocks in Hong Kong from
the map.
C. Igneous Rocks
-
formed by cooling and solidification of magma within the Earth’s _______________ (intrusive
igneous rocks) or lava on the Earth’s ______________ (extrusive igneous rocks)
-
contains interlocking ____________________compact, non-layered (non-stratified) and (with
/ without) fossilsaccording to the depth of occurrence and the speed of cooling, igneous rocks
can be classified into three sub-types:
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Depth of
occurrence
1. Volcanic
Extrusive
2. Hypabyssal
Intrusive
3. Plutonic
Intrusive
Rocks
formed in
Lava flow,
Volcano
Sill,
Dyke
Rate of
cooling
Examples
Acid
Basic
Grain size
Rhyolite
Basalt
Porphyry
Dolerite
Granite
Gabbro
Batholith
Volcanic rocks
-
When ____________ cools and solidifies on the earth’s _______________, volcanic rocks are
-
formed.  Extrusive igneous rock
The most common type of volcanic rock in Hong Kong is _______________ and _________.
-
Formation and characteristics of rhyolite
Rhyolite has some distinctive characteristics, which can be explained by its formation.
Formation
Characteristics
 ______________ in colour with high
Formed from acid lava
silica content
 ______________ crystals
Formed on Earth’s surface, so it is
 ___________-grained texture, so it is
cooled rapidly and has little time for
more resistant than granite against
crystallization
weathering and erosion
Formed when lava flow on ground
surface
 ____________ structure
Near vertical extrusion and rapid
cooling of acid lava
 Hexagonal ________________ joints
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Associated landscapes in Hong Kong:
a. Vertical columns form steep cliffs.
b.Where marine erosion has attacked the rhyolite columns along their joints and worn away
some columnar blocks, sea caves, geos and sea arches are formed.
c. Found in Sai Kung Peninsula, central New Territories, southern Hong Kong Island, and
central and western Lantau Island
1.
Hypabyssal rocks
-
When magma cools and solidifies at _________________ depth, i.e. in ___________,
___________ or laccolith, hypabyssal rocks are formed. Intrusive igneous rock
-
The most common type of hypabyssal rock in Hong Kong is _________________.
I. Formation and characteristics of porphyry:
Formation
Characteristics
 Has well-developed large crystals
(called ____________________)
Formed from acid magma within the
Earth’s crust at intermediate depth, so it
has slow to intermediate rate of cooling
embedded in _________ and
_____________ groundmass which
gives it a mixed-grained texture
 Hard and more resistant than granite
against weathering and erosion
The phenocrysts are usually of quartz
(milky) or feldspar (pink or white)
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II. Associated landscape in Hong Kong:
a. It forms high peaks (e.g. Tai Mo Shan) because it is resistant.
b. The resistant outcrop of dykes and sills forms cliffs, waterfalls and rapids (e.g. at Silver
Mine Bay).
c. Also found in N.E. Lantau Island and Lamma Island
2. Plutonic rocks
-
When magma cools and solidifies at _________________ depth, for example, in
____________________, plutonic rocks are formed. Intrusive igneous rock
-
The most common type of plutonic rock in Hong Kong is __________________.
-
Granite occupies about one third of the total land area of Hong Kong.
I.
Formation and characteristics of granite:
Formation
Formed from acid magma
Formed within the Earth’s crust at great
depth, so it has slow rate of cooling and
crystallization
Characteristics
 _______________ in colour with
high silica content
 Has well-developed,
well-_______________ and
__________ crystals which give it a
coarse-grained texture
II.
Contraction of magma upon cooling
 ____________________with vertical,
horizontal and oblique joints
Consists of quartz, feldspar and mica, of
which feldspar and mica can be easily
decomposed
 Hard but not _________________
against weathering and erosion
Associated landscape in Hong Kong
a. Deep, weathered profile is commonly found.
b. Spheroidal weathering and removal of weathered materials leave tors on ground surface.
c. Mass movement usually forms barren area with gully erosion and badland.
d. Found in northern Hong Kong Island and Kowloon. Peninsula, Castle Peak area, eastern
and northern Lantau.
e. Also form gentle sea cliff which is deeply dissected (e.g. in Tai Tau Chau)
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Sedimentary rocks
-
Formed by _____________________ and ______________________ of elements, such as
weathered material or remains of living organisms, under weight and pressure
-
non-crystalline and less compact
-
layered, with _______________________ between strata
-
may contain ________________
-
according to the mode of formation, sedimentary rocks can be classified into:
1. mechanically-formed sedimentary rocks
2. chemically-formed sedimentary rocks
3. organically-formed sedimentary rocks
1. Mechanically-formed sedimentary rocks
I.









Steps of formation
weathering and erosion of rocks
transportation of weathered materials by river, wind or wave
deposition
sorting according to their size and weight
cementation and consolidation under pressure
sedimentary rocks formed
uplifting of sedimentary rocks above sea level by tectonic process or Earth’s movement
resultant landforms on Earth’s surface
resultant landforms being weathered and eroded
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II. Important mechanically-formed sedimentary rocks in Hong Kong
a. Conglomerate and Breccia
- largest and heaviest rock material deposit, nearest to the coast
- fine materials act as groundmass (cementing materials)
- formed through the processes of compaction, cementation, desiccation and
hardening
-
relative resistant
coarse-grained sedimentary rock with little sorting of rock materials, representing
immature sedimentation
absence of layered structure
reddish in colour
conglomerate contains rounded pebbles while breccia contains angular fragments
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-
Associated landscape in Hong Kong:
1. It forms high mountain and steep slopes as they are resistant, e.g. Pat Sin Range
(conglomerate).
2. Their outcrop across river bed may form rapids and waterfalls, e.g. Bride’s Pool
(conglomerate).
3. Breccia can be found in Ap Chau.
b. Sandstone
- medium-grained sedimentary rock
- sand grains (mainly quartz) with uniform size deposit, farther away from the coast
- formed through the processes of compaction, cementation, desiccation and
hardening
-
has porous inside so is permeable
less resistant
yellow or red-coloured
-
Associated landscape in Hong Kong:
1. Produces a gentle landscape because it is
less resistant.
2. Sandstone outcrops are found in N.W. Lantau Island, Port Island and north of
Tolo Harbour.
c. Shale
- _____________-grained sedimentary rock
- silt and mud deposit, farther away from the coast
- formed through the processes of compaction, cementation, desiccation and
hardening
- very compact so is ________________________
- not resistant, can split into layers easily
- _____________________ structure clearly shown
- grey or black-coloured
- contains _____________________
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-
Associated landscape in Hong Kong:
1. Produce low landscape because it is less resistant.
2. Along the coast, shale is often eroded by waves to form sea caves, geos, stack,
sea cliff and wave-cut platform.
3. Found in Ping Chau in Mirs Bay.
2. Chemically-formed sedimentary rock
- After water in ancient salt lakes or shallow sea is evaporated, some minerals are left, and
cemented together to form sedimentary rock chemically.
- e.g. gypsum, rock salt, calcite and chert
3. Organically-formed sedimentary rock
- When remains of sea organisms like _____________________ and _________________ or
remains of plant matters compress and cement together, sedimentary rocks are formed
organically
- For example:
1. skeletons and shells  ______________________ and chalk
2. plant matters  coal and _________________
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D. Metamorphic rocks
-
formed from either igneous, sedimentary or other metamorphic rocks, which have changed
their form completely into new rocks through metamorphism because of
_____________
and _____________________ .
-
rocks develop new minerals and are greatly changed in appearance and composition
generally harder and more compact than before
there are three main types of metamorphism:
1. Thermal (contact / local) metamorphism
-happens when rocks contact _________________ or ________________ directly
-caused by high ________________________
-small affected area
2. Dynamic metamorphism
-happens when there are Earth movements such as _______________ or _________________
-caused by great ______________________
-large affected area
3. Regional metamorphism
-caused by high temperature and pressure at great depth
4. Examples of metamorphic rocks
From
Shale
Sandstone
Limestone
Granite




Changed to
Slate
Quartzite
Marble
Gneiss
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E. Rock cycle
Rocks change from one type to another in rock cycle:
(1) Cooling and solidification of magma or lava form igneous rocks.
(2) Weathered and eroded materials from igneous, sedimentary or metamorphic rocks form
sedimentary rocks through sedimentation.
(3) Metamorphic rocks are formed from igneous, sedimentary or other metamorphic rocks through
metamorphism.
(4) Igneous, sedimentary and metamorphic rocks can be turned back to magma again by heating
and melting.