E1 book 1

The three main types of rocks based on their formation are igneous, sedimentary, and metamorphic rocks.

Intrusive igneous rocks are formed through the release of pressure that allows magma to rise through weaknesses in the crust. This magma is trapped at great depths, leading to a slow rate of cooling and solidification, resulting in a long crystallization process and the formation of larger crystals.

Examples of intrusive igneous rocks include:

1. Granite
2. Gabbro
3. Diorite

Extrusive igneous rocks are formed through the aggregation of minerals that crystallize from lava and ash ejected during volcanic eruptions. The rapid cooling and solidification occur due to contact with air, resulting in smaller crystals or finer particles.

Examples of extrusive igneous rocks include:

• Tuff
• Rhyolite
• Basalt

The rapid cooling of lava and ash during volcanic eruptions leads to a short crystallization process, resulting in smaller crystals or finer particles in extrusive igneous rocks.

• Hard and compact
• Non-stratified / with no layered structure
• Crystalline / with crystals
• Contain no fossils

Granite is composed of:

• Quartz: colorless
• Feldspar: white or pink in color
• Mica: black / dark in color

Granite forms deep below the earth's surface (batholith) where:

• The temperature is high
• There is a very slow rate of cooling and solidification of magma This results in a long crystallization process, allowing crystals to grow larger, leading to a coarse-grained texture.

The color of granite is determined by the acidity of magma and the mineral content. It is formed by magma with high silica content, making it a felsic rock with high acidity and light color due to low amounts of iron and magnesium.

The major minerals found in granite include Quartz, Potassium feldspar, and Sodium feldspar (plagioclase).

Granite is formed under great depth in the crust within a batholith. When the granitic batholith is exposed to the atmosphere due to denudation of overlying materials, the release of pressure leads to the development of well-jointed structures with horizontal, vertical, and oblique joints.

The most common color of granite is light colored due to its felsic composition.

Granite is less resistant due to:

1. Composition of easily weathered minerals:
   • Contains mica and feldspar which are prone to chemical weathering in hot and wet climates.
   • Mica turns to clay materials after hydrolysis and to iron oxide after oxidation.
   • Feldspar also turns to clay materials after hydrolysis.
2. Well-jointed structure:
   • Allows weathering agents like water to seep along joints, promoting both physical and chemical weathering.
   • Physical weathering example: salt crystallization.
   • Chemical weathering example: spheroidal weathering.
3. Formation conditions:
   • Formed under high temperature and pressure at great depths, becoming unstable when exposed to the atmosphere due to denudation.

Tuff is composed of volcanic ash particles with crystals of quartz and feldspar embedded in a greyish fine groundmass. It is formed above the earth's surface in contact with air, leading to low temperatures, rapid cooling, and solidification of lava and ash, resulting in a fine-grained texture due to a short crystallization process.

The color of tuff is determined by the acidity of the lava and its mineral content. Tuff is formed by lava with high silica content, making it a rhyolitic (felsic) rock with high acidity and very few amounts of iron and magnesium, resulting in a mainly light color in places like Hong Kong.

Tuff in SE Sai Kung features hexagonal columnar joints, which are formed due to rapid cooling and the development of tensional forces during the cooling process of the volcanic ash and lava mixture.

About 140 million years ago, during the Jurassic Period, an extremely violent volcanic eruption occurred, resulting in a large-scale collapse of the crater and forming a caldera. This eruption ejected a large volume of ash blended with lava, which was then filled with hot volcanic ash and lava, leading to rapid cooling and the formation of tuff.

Hexagonal rock columns in tuff form due to contraction during rapid cooling of the hot mixture in contact with the atmosphere. Tensional forces act towards evenly distributed cooling centers, pulling open joints that ideally assume a hexagonal pattern as cooling proceeds towards the center of the mixture.

Tuff is more resistant due to its crystalline structure and fine-grained minerals, which contribute to its compact structure that enhances resistance to weathering.

The distribution of intrusive rocks in Hong Kong includes:

• Western New Territories
• Kowloon Peninsula
• Northern part of Hong Kong Island
• Eastern part of Lantau Island
• Occupies 35% of outcrop

The distribution of extrusive rocks in Hong Kong includes:

• Central, northern and eastern New Territories
• Southern part of Hong Kong Island
• Central and western Lantau Island
• Occupies 50% of outcrop

Granite is characterized as less resistant due to:

• Easily weathered minerals
• Well-jointed structure
• Unstable when exposed to the earth's surface Intensive weathering leads to rapid denudation, forming lower hills with gentle slopes and rounded topography.

Granite weathering in Hong Kong leads to the formation of lower and rounded topography, exemplified by hills such as:

• Beacon Hill (457m)
• Lion Rock (495m)
• Castle Peak (583m)

Tuff's fine-grained texture and compact structure make it more resistant to weathering, which slows down denudation and results in less loose weathered materials available for erosion and mass wasting.

Tuff forms the highest summits with steep slopes, leading to rugged and angular topography due to its resistance to weathering and erosion.

Examples of high peaks formed by tuff include Sunset Peak (869m), Lantau Peak (934m), and Tai Mo Shan (957m).

Hexagonal rock columns can be found in areas like the Ninepin Group and East Dam of High Island Reservoir.

Steep cliffs are formed due to the presence of columnar joints, which create lines of weakness in the rock. Weathering, both physical and chemical, occurs along these joints, leading to their expansion. As weathering continues, the joints enlarge until the rock breaks, resulting in steep walls after the surface rock is removed.

Wave action contributes to the erosion of tuff with columnar joints through hydraulic action. When waves splash against the rock, water and air are pushed into the cracks and joints. The air gets compressed, creating high pressure, and when the waves retreat, the pressure is released, causing the rocks to break.

Hexagonal rock columns of tuff are prone to wave erosion because they have columnar joints that serve as lines of weakness. Additionally, their location on the east coast of Hong Kong, facing the South China Sea with a long fetch of up to 600 km, exposes them to high wave energy, which enhances erosion.

Hexagonal rock columns of tuff are predominantly found on the east coast of Hong Kong, particularly in areas like Sai Kung, where they face the South China Sea.

The formation of clastic sedimentary rocks involves three main processes:

1. Denudation and Transportation:

   • Weathering breaks down pre-existing rocks in situ.
   • Erosion by agents like water and mass wasting under gravity transport the loose weathered materials.

2. Sedimentation / Deposition:

   • Occurs when the energy of moving agents declines, leading to the sorting and depositing of sediments in layers in various environments like lakes and coastal waters.

3. Lithification:

   • This process turns loose sediments into sedimentary rocks through compaction (reducing pore space) and cementation (filling pore space with minerals like calcite and silica).

The types of clastic sedimentary rocks formed are determined by the sorting of sediments during transportation and the depositional environment. Larger and coarser particles are deposited first, while smaller and finer particles are deposited later as energy decreases.

Shale and siltstone are more likely to form in calm environments such as lakes and the deep seabed.

In swift-flowing rivers or coasts with strong waves, conglomerate is more likely to form due to the high energy environment.

• Less compact / soft
• Stratified with layered structure and bedding planes
• Non-crystalline
• May contain fossils from embedded remains of plants or marine species
• Less resistant to denudation (weathering, erosion, and mass movement)
• Ripple marks or imprints may be found

Conglomerate is coarse-grained with rounded pebbles, while breccia contains angular fragments. Both are cemented by fine materials like silt and clay.

Red conglomerate can be found in Northeast New Territories and is formed under arid climate conditions.

• Fine-grained with sand, silt, or clay
• Reddish brown or grey in color
• Well-sorted sediments leading to a stratified structure with bedding planes
• Less resistant to denudation due to loose cementation and bedding planes being lines of weakness

Sedimentary rocks are mainly found in northeastern New Territories, including areas like Pat Sin Range, Port Island, and Ping Chau. They also appear in smaller portions around Ma On Shan, Yuen Long, and along the coast of Tai O in Lantau Island, occupying 13% of the outcrop.

Conglomerate is a more resistant rock that retards denudation due to its strong pebbles and cementing materials. It supplies less loose weathered materials for erosion and mass wasting, leading to the formation of higher grounds.

In the Pat Sin Range, differential denudation leads to the formation of an escarpment. The northern slope, protected by resistant conglomerate, experiences slower denudation, resulting in a gently-falling slope. In contrast, the southern slope, where tuff is exposed, undergoes faster denudation, creating a steeper, cliff-ridden slope.

Pat Sin Leng is characterized by an east-west-trending ridge formed by younger sedimentary rocks, specifically conglomerate at the top layer. It showcases distinct layering and rugged terrain, influenced by the processes of denudation and the protective role of conglomerate.

Sedimentary rocks such as sandstone, siltstone, and shale are less resistant to denudation because:

1. Sediments are less compact and are loosely cemented together.
2. Bedding planes and strata act as lines of weakness, making them more susceptible to active weathering and erosion.

The vertical erosion of less resistant sedimentary rocks like shale can lead to the formation of a waterfall. As the river flows over these rocks, continuous vertical erosion results in a vertical drop of river water, increasing the river's velocity suddenly.

The highest elevation of Tung Ping Chau is only 48 meters above sea level. The major sedimentary rocks found on the island are siltstone and mudstone, which are prone to denudation.

When a river flows over sedimentary rocks, differential erosion occurs, where vertical erosion or downcutting is more active on less resistant rocks like shale. This can lead to significant geological features such as waterfalls.

Bedding planes and stratas serve as lines of weakness in sedimentary rocks, making them susceptible to active wave erosion, particularly through hydraulic action.

Active wave erosion leads to the formation of various coastal erosional landforms, including:

1. Wave cut platforms
2. Sea arches

These features develop at weak spots in the rock structure, such as bedding planes.

The 'Duck's Eye' at Ap Chau is a sea arch formed by continuous wave erosion that concentrates along the horizontal bedding of breccia layers.

Lan Kwo Shui is a wave-cut platform formed by the continuous wave erosion of siltstone and the retreat of the sea cliff.

Sedimentary rocks in Tung Ping Chau were formed under a calm shallow lake setting during the Early Jurassic Period, which favored the process of sorting and resulted in clear horizontal layers.

Features indicating the depositional environment include:

1. Ripple marks formed by water surface ripples disturbing deposits of sand and mud.
2. Mud cracks formed when the lake dried up and the shallow silt deposits were exposed and contracted.

The greyish white, massive chert found at Lung Lun Tsui is more resistant to denudation than the surrounding siltstone. This differential rate of weathering and erosion has led to the formation of a hard and narrow ridge about 1m thick.

The rocks at Kang Lau Shek are characterized by numerous cracks and fractures formed under compression, making them easier to remove due to these lines of weakness. In contrast, two columns of rocks are protected by hard rock blocks on top, resulting in two remaining huge rock stacks.