# HKDSE Geography/E1/Internal Processes and the Landscape

We learnt from E1 that the three main internal processes are folding, faulting and vulcanicity. How do they shape Hong Kong's landscape?

## Folding

Large-scale fold mountains are absent in Hong Kong. Only small folds bent by internal forces are found. We can find all five fold types in Hong Kong (symmetrical, assymetric, overfold, recunbent, overthrust) because of differences in the strength of compressional forces.
Most of the folds are found around Tolo Channel, including Lai Chi Chong, Bluff HHead and Ma Shi Chau. A larger fold is found in Yuen Long. They are found in older sedimentary rocks deformed by gravity during sedimentation and lithification.

## Faulting

Faults are common in Hong Kong. They were oriented according to the general trend of Guangdong's fault system, i.e. the Lianhuashan Fault System, and are thus aligned from northeast to southwest. Others are perpendicular to this direction, and go from northwest to southeast. The three main faults are:
* Tolo Channel Fault, which runs from Tolo Channel through Sha Tin to Western Kowloon
* Sha Tau Kok Fault, which runs from Sha Tau Kok through Tsing Lung Tau to Pui O on Lantau Island
* Tai Lam Fault, which runs from Sha Tau Kok to southern Lantau
In addition to the above faults, there are also numerous smaller-scale faults in Hong Kong, such as normal faults found on Lai Chi Chong, Tung Ping Chau and Crooked Island. There are also reverse faults on Port Island and Tai Mo Shan, amongst others.
Faults shape the landscape of Hong Kong as they affect external processes. Faults are more vulnerable to weathering and erosion than neighbouring rocks. Therefore, denudation is concentrated along faults. Major spurs, valleys, ridges, headlands, bays and water inlets are positioned and oriented according to faults.
* Tolo Channel is an important example. It is a water inlet formed along the Tolo Channel Fault, with ridges on both sides. It was originally a valley formed by weathering and erosion before submergence.
* The valley at Tai Lam Chung is controlled by the Tai Lam Fault. Like Tolo Channel, it was created by weathering and erosion along the fault. The rectangular fault pattern of the area created a rectangular drainage pattern.

## Vulcanicity

Both intrusive and extrusive vulcanicity have shaped Hong Kong's landscape in the past. Vulcanicity is roughly divided into four periods (y.a. stands for years ago):
* Phase 1 (162-159 million y.a.): Vulcanicity was found on southeastern Lantau, northern Lamma and most of the New Territories (such as Tai Lam Chung and Ma Wan)
* Phase 2 (146 million y.a.): Vulcanicity was found along the Tolo Channel Fault, including at Tsing Yi
* Phase 3 (142 million y.a.): Vulcanicity was found on Cheung Chau and southeastern Lantau, southern Hong Kong Island, Sai Kung and eastern Tai Po
* Phase 4 (140 million y.a.): Vulcanicity was found around Victoria Harbour and in Sai Kung (Po Pin Chau, East Dam of High Island Reservoir); this includes the hexagonal columns

### Intrusive
#### Batholiths

Exposed batholiths cover a third of Hong Kong's land area, and include the eastern part of Lantau Island, Castle Peak, Tai Lam Chung, the two sides of Victoria Harbour, including the Stonecutter's. The overlying rocks were removed by denudation to expose the batholiths. As we have discussed in the previous chapter, these rocks produce lower hills because of their lower resistance.

#### Dykes

Dykes are also common in Hong Kong. The range from a few centimetres to metres in width, and up to several kilometres in length. They are easy to identify as these intrusions are always darker than the surrounding rocks. They intrude into batholiths, lava flows and older sedimentary rocks.
Dykes appear in swarms in the southwest. They cover and extended area and are roughly parallel. They are found in eastern Lantau and Lamma Island, as well as Tsing Yi and Ma Wan.
Dykes affect the landscape. Resistant dykes form spurs and waterfalls on Lantau Island. Less resistant dykes form narrow coastal inlets like on Cheung Chau.

### Extrusive

Volcanic eruptions produce large amounts of solidified volcanic ash and lava. They survive to this day and are evidence of prehistoric vulcanicity.
* Extrusive vulcanicity in Hong Kong
* Solidified ash layers on Lantau Island
* The hexagonal columns at Po Pin Chau near East Dam of High Island Reservoir
* Flow bands in Clear Water Bay Peninsula
* For the effects of extrusive vulcanicity on the landscape, see the previous chapter.