Section 8 Dynamic Earth, Study notes of Geography

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NSS Geography Revision Notes

Section 8: Dynamic Earth

NSS Geography Revision Notes (Section 8)

1. What is the earth system? What is the overall physical landscape of Hong Kong?

A. What is the earth system?

⇨ A system is a group of things that interact within a boundary and perform specific functions. It

has inputs, processes and outputs. It is driven by the flow of matter and energy.

⇨ The earth can be considered to be functioning as a system. The earth exists as it does due to the

interaction between the four spheres : atmosphere , lithosphere , hydrosphere and biosphere.

⇨ Changes in any inputs and processes of one sphere may affect the functions of the other spheres

or even the whole system. This explains why the earth is dynamic and always changing.

NSS Geography Revision Notes (Section 8)

2. How do different types of rock affect the physical landscape of Hong Kong?

A. What is rock? How are rocks formed?

⇨ Rock is the solid material that makes up the lithosphere. Basically, rock is made up of minerals.

Different rocks have different combinations of minerals. This forms rocks with different shapes,

colours, weights, hardness and textures.

⇨ Rocks can be broadly classified into three types: igneous rock , sedimentary rock , volcanic rock.

(1) Igneous rock

⇨ Igneous rock is formed during the process of volcanism. They can be divided into two types: extrusive

igneous rock and intrusive igneous rock.

Characteristic Extrusive (Volcanic) Intrusive (Plutonic / Granitic)

Location of formation On the earth’s surface Within the crust

Forming material Lava or volcanic ash Magma

Structure/Texture (^) ⇨ Crystalline /Crystals interlocking with one another

⇨ Usually non-stratified/ No layered structure

⇨ Compact/ Hard

⇨ Well-jointed

They are fine-grained as the cooling

rate of lava is fast on the earth’s

surface

They are coarse-grained as the

cooling rate of lava is slow deep

within the crust

Colour Can be both light and dark in colour, depending on the silica content.

The higher the silica content, the lighter the colour of the rock.

Prescence of fossil Normally contains no fossils as the high temperature of magma or lava

would have destroyed any dead plants or animals

Examples ⇨ Rhyolite

⇨ Tuff

⇨ Basalt

⇨ Granite

⇨ Gabbro

NSS Geography Revision Notes (Section 8)

(2) Sedimentary rock

I. Formation and types

⇨ Sedimentary rock is made up of minerals which have been eroded from pre-existing rocks and the

remains of living organisms.

⇨ Sedimentary rock usually has a definite layers or strata. The surface where one stratum meets another

is known as a bedding plane.

⇨ Due to the earth movement, sedimentary rock may incline in a certain direction. On inclined

sedimentary rock, we can identify dip and strike.

a. Clastic sedimentary rock

⇨ Sediment that forms sedimentary rock may come from many sources. When sedimentary rock is

formed from particles of pre-existing rocks , it is called clastic sedimentary rocks.

⇨ These pre-existing rocks are broken down and removed downhill either by moving agents or under

gravity. When the energy of the moving agent is lost, such as when rivers reach the sea, the fragments

are deposited as sediment and then accumulate. Sedimentation or deposition occurs.

⇨ The sediment will undergo compaction and cementation. It then lithifies into hard sedimentary rock.

⇨ Generalized steps from source to sedimentary rock:

1. weathering 2. erosion

2. transportation 4. deposition

3. compaction 6. cementation

4. lithification 8. sedimentary rock

NSS Geography Revision Notes (Section 8)

b. Non-clastic sedimentary rock

⇨ Non-clastic sedimentary rock is formed by the chemical processes of evaporation or precipitation. It

can also be formed by an organic process through the accumulation of dead plants or animals.

i. Chemically-formed

⇨ Some minerals such as saly, gypsum and calcite can easily dissolve in water. When these dissolved

minerals precipitate from water as evaporation continues, large beds of sediment result. Under

continuous accumulation and compaction, chemical sedimentary rock forms.

◼ Example 1: rock salt is formed when water evaporates from a salt water body and leaves behind

mineral salts.

◼ Example 2: gypsum is produced by the chemical precipitation of CaSO 4.

ii. Organically-formed

⇨ Organic sedimentary rock is formed from the accumulation of any animal or plant debris such as

shells and bones. For example, coal is formed by burying and compacting dead vegetation in

swamps. The buried vegetation will develop into different types of coal, depending on how long it is

buried.

Stage I (Peat) Stage II (Lignite) Stage III (Bituminous)

⇨ Limestone , on the other hand, is made up of

tiny particles of calcium carbonate from the

remaining of shells or skeletons of sea-

dwelling organisms. Chalk is a kind of

limestone.

NSS Geography Revision Notes (Section 8)

⇨ Sometimes, traces of the remains of plants,

shells or skeletons of dead organisms from

millions of years ago are preserved in

sedimentary rock. These are called fossils.

They are like a record of the environments that

existed during the formation of this rock. By

studying sedimentary rock of different ages,

scientists can find evidence showing how

climate and environments have changed

over time.

II. Characteristics

i. Clastic sedimentary rock

Forming material Particles of pre-existing rock

Structure/Texture ➔ Layered structure

➔ Less compact

➔ Prescence of bedding plane

➔ Non-crystalline

➔ Grain size is affected by the strength of water currents/winds in the

environment where deposition takes place

Colour Varies, depending on the rock fragments that comprised it

Prescence of fossil May have fossils, especially the fine-grained ones

ii. Non-clastic sedimentary rock

⇨ Fossils are common in non-clastic sedimentary rock, especially the organically-formed ones.

Compared to igneous rock, non-clastic sedimentary rock is less compact. Its colour varies ,

depending on the colour of the materials that formed the rock and the environment and climate in

which it formed.

(3) Metamorphic rock

I. Formation

⇨ Metamorphic rock is transformed from existing rock which has undergone changes as a result of

extreme pressure and/or heat. Therefore, the texture and chemical composition of metamorphic rock

is different from that of the parent rock.

⇨ Metamorphism is the term given to the process of transformation of existing rock.

NSS Geography Revision Notes (Section 8)

II. Characteristics

Foliated Non-foliated

Condition leading

to formation

High pressure and temperature High temperature

Structure/Texture (^) ⇨ More compact/ Harder than the parent rock

⇨ Generally crystalline

⇨ Fine to coarse-grained

⇨ Mineral grains show elongated alignment.

Some common foliated structures include:

◼ very thin layers

◼ shiny appearance

◼ alternate bands of light and dark

minerals

⇨ Fine to medium-grained

⇨ Mineral grains show no

visible alignment

Colour Varies, depending on the colour of the minerals

Prescence of fossil Absent, as high pressure and temperature will destroy fossils

III. Examples

NSS Geography Revision Notes (Section 8)

B. What is a rock cycle?

⇨ The formation, transformation and breaking down of rocks occur continuously.

◼ When magma is produced from the melting of part of the asthenosphere, this provides a new

source of material for volcanism. Cooling and solidification of magma form new igneous rock.

◼ When rock on the surface is subject to weathering and erosion, the loose materials will be carried

away and deposited, forming sedimentary rock.

◼ When rock is subject to high temperature and pressure, metamorphism changes the texture and

mineral composition of the rock, forming metamorphic rock.

⇨ The cycle of transformation which affects all rock is known as the rock cycle. Under the influence

of geological processes driven by plate tectonics, all rocks can be transformed to different rock

types in the rock cycle. The rock cycle can begin with any one of the three rock types.

NSS Geography Revision Notes (Section 8)

⇨ Characteristics of major rocks in Hong Kong

Rock Major characteristics Formation processes

Granite

(intrusive igneous rock)

⇨ Light colour

⇨ Interlocking crystals

⇨ Composed of:

◼ quartz

◼ felspar

◼ mica

⇨ Coarse- to medium-grained

⇨ Joints are common

⇨ Formed from slow cooling of

magma

⇨ Forming batholiths

⇨ Exposed to the surface when

overlying materials above are

eroded and carried away

gradually

Tuff

(extrusive igneous rock)

⇨ Crystals of quartz and feldspar

embedded in dark fine

groundmass

⇨ Composed of crystals ranging

from fine ash to small volcanic

rock fragments

⇨ Columnar jointing occurs in

some areas

⇨ Formed from cooling and

solidification of volcanic ash

and rock fragments

Conglomerate

(clastic sedimentary rock)

⇨ Some are cemented together in

thick layers

⇨ Some rest on top of extrusive

rock; some others are

sandwiched by other

sedimentary rocks

⇨ Red or white in colour

⇨ White conglomerate formed

by round pebbles from rivers

at around Tolo Channel and

Ma On Shan

⇨ Red conglomerate formed in

an arid climate, when rocks

fell from hill slopes in the

north-east of Hong Kong, then

compacted and cemented

Siltstone

(clastic sedimentary rock)

⇨ Dark grey to reddish brown in

colour

⇨ With thin strata

⇨ With fossils of plants and

insects

⇨ Formed from lithification of

silt in a quiet environment,

probably in a lake under

semi-arid conditions

NSS Geography Revision Notes (Section 8)

D. How does the type of rock affect the physical landscape of Hong Kong?

⇨ Some rocks are more resistant to weathering and erosion than others when exposed to the same

environment. Among the major types of rock in Hong Kong, conglomerate is the most resistant,

followed by tuff and then granite. In general, sedimentary rock (except conglomerate) is less resistant

to weathering and erosion.

⇨ Very often, different types of rock can be found at the same site. Weathering and erosion processes

occur at faster rates in less resistant rock. This is known as differential weathering and differential

erosion. It results in uneven surfaces, with the more resistant rock forming outcrops and the less

resistant rock forming depressions.

⇨ Differential weathering and differential erosion occur on a variety of scales in Hong Kong. On a small

scale , more resistant layers of rock stand above the softer rock to form small ridges. On a large scale ,

harder rock that can better resist weathering and erosion form mountain peaks. They are often much

higher than the surroundings foothills and valleys that are comprised of softer rock.

⇨ As igneous rock is the dominant type of rock in Hong Kong, its effect on the landscape is significant.

Sedimentary rock, although covers only around 13% of the land area in Hong Kong, still formed

distinctive landscapes. Metamorphic rock, on the other hand, has little influence on the landscape

because most of it is buried underground.

NSS Geography Revision Notes (Section 8)

3. How is the physical landscape of Hong Kong shaped by internal processes?

A. Faulting

⇨ Faulting occurs when rock cracks and is displaced along a fracture (a fault).

⇨ In Hong Kong, faults are common. The orientation of the faults in Hong Kong is in line with the trend

of the faulting system of south China. The scale of faults varies from several meters to hundreds of

kilometers in length.

NSS Geography Revision Notes (Section 8)

⇨ Faults in Hong Kong are usually oriented in two directions: north-east/south-west and

northwest/southeast. The two largest faults in Hong Kong are:

◼ the Tolo Channel Fault – from Tolo Channel through Shatin to the west of Kowloon

◼ the Sha Tau Kok Fault – from Sha Tau Kok through Tsing Lung Tau to Pui O on Lantau Island

⇨ Weathered and erosion tend to concentrate along faults. This is due to the rock along faults is often

crushed into rock fragments due to the force associated with rock displacement. As rock is fragmented,

its surface area increases. This increases the area in contact with air and water, which favours chemical

weathering, hence, faults are lines of weaknesses which allow water to penetrate into the bedrock

easily and carry out chemical weathering.

⇨ Weathering turns rock along faults into loose weathered materials. Erosion then removes these loose

materials and lowers the land along faults. This forms valleys and water inlets , with headland areas

and ridges on the two sides.

⇨ On a smaller scale, normal faults can be seen in Lai Chi Chong, on Ping Chau and Crooked Island.

Reverse faults can also be found on Tai Mo Shan.

NSS Geography Revision Notes (Section 8)

⇨ Four phases of volcanism in Hong Kong:

a. Extrusive volcanism

⇨ Between 140 and 164 million years ago, violent volcanic eruptions created several large calderas in

Hong Kong. Although most of these calderas have been already eroded, evidence of volcanic eruptions

can be found in some rocks formed from the solidification of volcanic ash and lava.

⇨ In the eastern part of Sai Kung, violent volcanic eruptions ejected very large volumes of volcanic ash.

As the hot volcanic ash cooled and solidified, tuff formed. In the cooling processes, tuff contracted and

hexagonal columnar joints developed. Spectacular hexagonal columns developed in the rock.

⇨ At the coast, wave erosion is concentrated along these columnar joints to form numerous sea caves and

sea arches. Rock columns collapse and form step to vertical sea cliffs, such as those found on the

offshore islands in Port Shelter off Sai Kung.

⇨ Other than in spectacular columns, ash can solidify in layers, while flow bands can be found on

solidified lava.

NSS Geography Revision Notes (Section 8)

b. Intrusive volcanism

i. Batholiths

⇨ In Hong Kong, exposed batholiths cover over one-third of the area. The batholiths became exposed

after the rocks on top were removed due to erosion.

⇨ Since the rocks produced by intrusive volcanism are formed under very high temperatures and

pressure deep below the earth’s surface , they become unstable and vulnerable to weathering when

they are unloaded and exposed to the atmosphere. Therefore, hills containing these rocks (such as

Castle Peak) are low.

ii. Dykes

⇨ Dykes are common in Hong Kong. They vary in width from a few centimeters to a few meters. Some

can be traced for several kilometers. They are usually dark in colour, and intrude into batholiths , lava

flow and in some places, older sedimentary rocks.

⇨ In south-west Hong Kong, dykes occur in the form of swarms and extended over a large area. They

are more or less parallel, such as those on eastern Lantau Island, Ma Wan, Tsing Yi and Lamma Island.

⇨ The impact of dykes on relief varies. Resistant dykes form spurs and waterfalls, , while less

resistant dykes form narrow inlets along the coasts.