Geology of Hong Kong
The geology of Hong Kong is dominated by igneous rocks formed during a major volcanic eruption period in the Mesozoic era. It made up 85% of Hong Kong's land surface and the remaining 15% are mostly sedimentary rocks located in the northeast New Territories. There are also a very small percentage of metamorphic rocks in New Territories. These are formed by deformation of pre-existing sedimentary rocks which changed its mineral assemblages.
The geological history of Hong Kong started as early as the Devonian period which is marked by the discovery of Placoderm fossils in northeast Hong Kong. While the youngest rocks in Hong Kong are formed during the Paleogene period. They are today exposed in Tung Ping Chau in northeast Hong Kong.
Each of the three types of rocks: igneous, sedimentary, and metamorphic rocks formed spectacular geological features in Hong Kong. Igneous rocks formed the hexagonal columns in Sai Kung. Sedimentary rocks formed various erosion features such as wave-cut platforms and sea stacks in Tung Ping Chau. Metamorphic rocks formed the iron ore deposits in Ma On Shan. Each of these will be introduced in later sections.
In terms of structural geology, faults in Hong Kong are mainly running from the northeast to the southwest. Deformation features such as sheared rocks, folds and faulted rocks can be found near major faults such as are the banks of the Tolo Channel. Past fault activities can be traced by some structures such as the Lantau dyke swarm and deformed caldera, etc. Faulting have shaped the landscape of Hong Kong.
File:HK_geological_map_revised_version_3.png|alt=|none|thumb|520x520px| Geological map of Hong Kong showing the distribution of faults and different rock types in Hong Kong. Modified from Civil Engineering Development Department, HKSAR.
Geological Evolution
The geological history of Hong Kong is mainly divided into three periods. From the Devonian to the early Jurassic is the pre-volcanic sedimentary period. Environment of Hong Kong alternated between a river plain and a shallow sea setting. Rocks of this period are characterized by a variety of fossils, heavily folded strata and steeply tilted beds. Later, from the middle Jurassic to the early Cretaceous period, Hong Kong experienced a volcanic period. It is marked by the massive coverage of volcanic lava, ash, and granitic rocks. From the middle Cretaceous onward, it is the post-volcanic sedimentary period. It is represented by reddish colored sedimentary rocks which as an indication of an arid tropical climate during deposition.Major rock units in Hong Kong are shown by chronological order in the table below.
| Period | Representative Formations | Dominant rock types | Deposition environment | Notes |
| Devonian | Bluff Head formation | Brownish folded sandstone | River channels | the oldest rock in Hong Kong, age determined by Placoderm fossils |
| Carboniferous | Yuen Long formation | white or greyish marble | marine | metamorphosed in Mesozoic volcanic period, formed iron ore in Ma On Shan mine |
| Carboniferous | Lok Ma Chau formation | metasandstones and siltstone graphite beds | deltaic swamps | metamorphosed in Mesozoic volcanic period |
| Permian | Tolo Harbour formation | siltstone, sandstone, conglomerate | tidal shore | oldest ammonoid fossils in Hong Kong |
| Triassic | missing | N/A | N/A | |
| Jurassic | Tolo Channel formation | black mudstone, grey siltstone | shallow marine | |
| Jurassic | Tuen Mun formation | Andesitic lava and crystal tuff breccia | volcanic arc | Volcanic period started here. |
| Jurassic | Tsuen Wan Volcanic Group | coarse ash crystal tuff | back-arc volcano | covered a large area in New Territories |
| Jurassic | Lantau Volcanic Group | rhyolite with larger crystals | back-arc volcano | related to Lantau caldera and dyke swarm, covered most of Lantau island |
| Cretaceous | Mount Davis formation | coarse ash crystal tuff | back-arc volcano | related to Kowloon granite |
| Cretaceous | High Island formation | fine ash tuff | back-arc volcano | formed hexagonal columnar joint |
| Cretaceous | Kau Sai Chau Volcanic Group | lapilli bearing tuff with rhyolitic bands | back-arc volcano | Volcanic period ended here |
| Cretaceous | Pat Sin Leng formation | reddish conglomerate and ash bearing sandstone | river plain | volcanic ashes mixed with depositions, the red colour showed an arid climate |
| Cretaceous | Port Island formation | reddish conglomerate and sandstone | river plain | |
| Paleogene | Ping Chau formation | calcium carbonate bearing siltstones | lake | Sea stacks and wave-cut platforms are found on Ping Chau formation |
Igneous rocks
The geology of Hong Kong is dominated by igneous rocks. They are rocks related to volcanic eruptions. During the middle Jurassic to the early Cretaceous period, Hong Kong was right at the convergent plate boundary where the Paleo-Pacific oceanic plate subducted beneath the Eurasian continental plate. The oceanic plate carried sea water into the hot lower crust, which lowered the melting point of the crust. The crust was therefore partially melted and magma was formed. The magma rose and formed a magma chamber beneath surface. Volcanoes were therefore formed above the magma chamber. When erupted, volcanic ash, pieces of rocks, and some magma were expelled. These materials then eventually cooled down and became volcanic rocks. These rocks cooled down quickly once they reached the Earth's surface. Mineral crystals in these rocks are therefore very small.Volcanic rocks are widely distributed in Hong Kong. They formed most of the highest mountains in Hong Kong, such as Tai Mo Shan and Lantau Peak. In the eastern part of Hong Kong, these volcanic rocks form hexagonal columnar cooling joints. They can be seen from the High Island reservoir and islands nearby. These areas are listed as part of the UNESCO Global Geopark of Hong Kong.
In the early Cretaceous period, volcanic activities ceased. The hot magma in the magma chamber eventually cooled down and became granitic rocks. These magma cooled slowly below the ground surface. Mineral crystals are therefore large enough to be seen.
Granitic rocks cover about 35% of Hong Kong's land surface. They are mainly distributed in Kowloon, north Hong Kong Island, east Lantau, and Tuen Mun. Granitic rocks formed the Victoria Harbour where Hong Kong is given its name as the "scentful harbour".
Sai Kung hexagonal columnar cooling joints
Hexagona columar joints are parallel vertical cracks that are formed when homogeneous volcanic materials cool down and evenly contract inward towards a contraction center point. In the early Cretaceous period, there was a volcano centred east of the Sai Kung peninsula. The final eruption of the volcano was explosive and the magma chamber was emptied. It lost support in its core and collapsed. The remains became a caldera with a diameter of about 20 km. The large amount of volcanic ash produced in this eruption eventually settled in the caldera and formed a thick layer of hot viscous ash. The hot ash eventually cooled down. Then each column started to contract inwards. When each side of the hexagon shrinks evenly towards the centre, it formed regular hexagonal shaped cracks. Starting from the top part, the cracks developed downwards. Finally the pillars were created.The total number of hexagonal columns in Hong Kong are estimated to be 200 000, covering 100 square kilometers. Diameters of the columns ranges from 1 to 3 meters where most of the columns are tilted and are dipping towards the northwest at about 80 degrees. Some columns, such as those on the east dam of High Island reservoir, are curved by tectonic force, showing the ductile nature of the columns
The hexagonal columns in Hong Kong are light brown colour because of its silica-rich chemistry. It contains about 76% silica. Comparing with columns in the rest of the world which are mostly basaltic or andesitic, such a large group of well-preserved silica-rich hexagonal columns is very rare. The hexagonal columns are therefore the most important feature of the UNESCO Global Geopark of Hong Kong.
Lion Rock and Kowloon granite
is located on the north of Kowloon Peninsula. Its appearance resembles a laid down lion which is often used as a city symbol and landmark. of Hong Kong. The Lion Rock is part of the Kowloon granite that covers Kowloon, Victoria Harbour and northern Hong Kong Island. The middle part of the Kowloon granite was subjected to heavier weathering, forming the Victoria Harbour, where Hong Kong started its development. To the north of Kowloon, granite formed the Lion Rock, and hills lining up along the northern boundary of Kowloon. Most of the buildings on the two sides of Victoria Harbour are sitting on the Kowloon granite.The Kowloon granite exhibits a circular shape surrounding Victoria Harbour, and is surrounded by volcanic rocks. The volcanic rocks are oriented in a way such that they are surrounding the circular Kowloon granite. During early Cretaceous period, a ball shaped magma rose. It pushed and deformed the surrounding volcanic rocks outwards and formed the interesting orientations of its surrounding volcanic rocks.
Sedimentary rocks
s cover around 15% of Hong Kong land surface. They are formed by deposition of sediments such as sand, mud, skeletons of marine species and pebbles, etc. As more and more sediments were deposited in layers, older layers are compressed by weight of younger layers above, eventually hardened and become sedimentary rocks. Undeformed sedimentary rocks always form horizontal layers. However, if deformed, sedimentary rocks can form deformation structures such as folds that record tectonic activities. Fossils are often better preserved in sedimentary rocks.In Hong Kong, the oldest sedimentary rocks come from Devonian period, dated by Placoderm fossils discovered in Bluff Head formation in northeast New Territories. The youngest sedimentary rocks come from Paleogene in Tung Ping Chau at the very northeast of Hong Kong.
Tung Ping Chau erosion features
, in Cantonese, means eastern flat island. It is a crescent shaped island lying at the very northeast of Hong Kong. Its 'flat' is caused by the flat lying layers of sedimentary rocks. The island is famous for its spectacular erosion features, such as sea stacks and wave-cut platforms. "Lung Lok Shui", which means dragon going into water, is a famous structure that looks like a dragon's back extending towards the sea. The structure contains a layer of chert which is more resistant to erosion than the surrounding rocks. This formed an outstanding layer of greyish chert that looks like a dragon's spine.Rocks on Tung Ping Chau are reddish brown in color and fine-grained. This reflects a hot and humid climate during Paleogene that increase oxidation of iron in the rock and a quiet water setting which deposits fine sediments. Fossils of terrestrial plants and evaporites in rocks on Tung Ping Chau indicates that it might be a saline lake during Paleogene.
Ma Shi Chau
is a tidal island in the Tolo Harbour in northeast New Territories. It is an important special area for geological studies. It contains rocks from three different formations: Permian sedimentary rocks, Early Cretaceous volcanic rocks and middle Cretaceous sedimentary rocks. Fossils of ammonoids, corals and bivalves were found in the black Permian sedimentary rocks. Layers of fine volcanic ash deposits formed the light grey colored tuffaceous layers interbedded with the brownish Cretaceous sediments. Ma Shi Chau is very close to a major fault. Rocks on Ma Shi Chau are therefore subjected to deformation by fault activities. Various deformed structures such as folds, kink bands, microfaults and sheared rocks can be observed on Ma Shi Chau.Metamorphic rocks
s made up less than 1% of Hong Kong land surface. They are found in Lok Ma Chau near the border with Shenzhen, Ma On Shan and Yuen Long. However, metamorphic rocks in Ma On Shan and Yuen Long were only seen in boreholes. Metamorphic rocks are sedimentary rocks or igneous rocks that are altered under high temperature and pressure but are not melted. Atoms are re-arranged and new minerals are formed. Metamorphic rocks in Hong Kong are all altered sedimentary rocks formed in Carboniferous period. Then until the middle Jurassic volcanic activity, magma chambers were formed and they intruded into older rocks. The heat of the magma together with active movements along major faults in Hong Kong, created a high temperature and pressure environment, causing the relatively older Carboniferous sedimentary layers to alter. Rocks in Lok Ma Chau became meta-sedimentary rocks and phyllites, which were low-grade metamorphic rocks. This indicates that Lok Ma Chau rocks were not much altered. However, rocks in Ma On Shan and Yuen Long, which were originally limestones, became a high-grade marble. These rocks were significantly altered by the high temperature of magma intrusions.Ma On Shan Iron ore
Iron ore ore bodies were found in Ma On Shan. They are both located near a granitic body, where hot magma intrusions existed during late Jurassic. The hot magma carried metal ores to the crust from the mantle as it rose. Metal ores are concentrated into hot fluids as it forced itself into cracks of the Ma On Shan limestone. The hot concentrated fluid triggered chemical reactions. This process finally produced skarn, which an altered rock that carried the concentrated metal ores.Mining in Ma On Shan first started in 1906 and became very active during the second World War for weapon production. Later, in 1976, the mine was closed down due to dropping metal prices. Today, the mining tunnels and the mining pit can still be seen in Ma On Shan.
Faulting
The main faults in Hong Kong are oriented northeast–southwest, and northwest–southeast. They are generally of the same orientation as those in neighboring Guangdong Province. They are part of the Lianhuashan fault zone that contains faults of similar orientations extended along the southeast China coast to Shanghai.Although faults are recorded throughout the known geological history of Hong Kong, they are considered to have been most active during the Jurassic to Cretaceous periods when strike-slip and thrust faulting was dominant. Some faults represent structures that were active during the period of Late Jurassic to Early Cretaceous volcanic activity and facilitated the rise of magma to the surface. Faults in Hong Kong formed interesting features that can be traced to understand their activities.