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Geology of Hong Kong

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Geology of Hong Kong is dominated by igneous rocks (including granitic rocks and volcanic rocks). formed during a major volcanic eruption period in Mesozoic. It made up 85% of Hong Kong's land surface. The remaining 15% are mostly sedimentary rocks formed by deposition of sediments located in northeast New Territories of Hong Kong. There are also a very small percentage (less than 1%) of metamorphic rocks found in New Territories. These are formed by deformation of pre-existing sedimentary rocks which changed the

mineral assemblages in the rock (metamorphism).[1]

Geological history of Hong Kong started as early as Devonian period (~420 million years old) which was determined by the discovery of Placoderm (a Devonian fish) fossils in northeast Hong Kong.[2][3] The youngest rock in Hong Kong from Paleogene (~50 million years old) is exposed in Tung Ping Chau[4][5], an island in the northeastern corner of Hong Kong.

The three type of rocks: igneous, sedimentary, and metamorphic rocks all formed spectecular geological landscapes in Hong Kong. Igneous rocks formed the famous hexagonal columns in Sai Kung and most of the hills in Hong Kong. Sediementary rocks formed various erosional features such as wave-cut platforms and sea stacks in Tung Ping Chau.[4] Metamorphic rocks fromed the iron ore site in Ma On Shan which was an active mining site. Each of these will be introduced in later sections.

In terms of sturctural geology, faults in Hong Kong are mainly running from the northeast to the southwest of Hong Kong. Deformation features such as sheared rocks, folds and faulted rocks can be found near the fault. These faults can be identified by some igneous features formed due to faulting such as the Lantau dyke swarm, defromed caldera, etc.[6] (See more on "Faulting" section)

[Fig.1] Geological map of Hong Kong showing the distribution of faults and different rock types in Hong Kong. Modified from Civil Engineering Development Department, HKSAR.[7]

Igneous rocks

Geology of Hong Kong is dominated by igneous rocks. They are rocks that are related to volcanic eruption. During middle Jurassic to early Cretaceous period, Hong Kong was right at the convergent plate boundary where the Paleo-Pacific oceanic plate subducted beneath the Eurasian continental plate.[8] The oceanic plate sank and brought sea water into the hotter lower crust, and lowered the melting point of the crust. The crust was partially melted and created magma. As the less dense magma rose, it formed a magma chamber beneath the crust and volcanos formed around the magma. When these volcanoes erupted, ashes, pieces of rocks, and some magma were expelled out of the volcano. Magma became lava when they reached the earth surface. These materials eventually cooled down and became volcanic rocks.They cooled down quickly once they reached the earth surface. Therefore, mineral crystal are very small and are hardly seen.

Volcanic rocks are widely distributed in Hong Kong (green areas in Fig.1). They formed most of the highest mountains in Hong Kong, such as Tai Mo Shan (957m) and Lantau Peak (934m).[9] In the eastern part of Hong Kong, these volcanic rocks formed the spectacular hexagonal columns, which can be seen on the coast near High Island reservoir and islands nearby such as the ninepin group and Ung Kong group. These areas were listed in the UNESCO Global Geopark of Hong Kong.

Until early Cretaceous (about 140 million years ago), as the subduction zone moved away from Hong Kong, volcanic activities stopped. The hot magma eventually cooled down and became granitic rocks found in Hong Kong. As magma was cooled slowly below ground surface, mineral crystals are large enough to be seen.

Granitic rock comparises of about 35% of Hong Kong land surface (red area in fig.1). They are mainly distributed in Kowloon, north Hong Kong Island, east Lantau, and Tuen Mun.[10]

[add cross secion on subduction]

Sai Kung Hexagonal columnar cooling joints

Curved hexagonal columns on East dam of High Island Reservoir, Hong Kong

Hexagonal columns, also called hexagonal columar joints, are parallel vertical cracks that are formed when volcanic materials cool down and contract. In early Cretaceous period, there was a volcano centred right at the sea east of the Sai Kung peninsula. During the final eruption of the volcano, the magma chamber was emptied and the eruption was explosive. The volcano lost support in its inner, collapsed and became a caldera with diameter of about 20-km. The large amount of volcanic ash produced in this eruption settled in the caldera and formed a thick layer of hot ash.[11] As the hot ash eventually cooled down, each column contracted inward towards a centre, forming hexagonal shaped cracks. The top part of the columns cooled down first and formed the cracks first, then the cracks developed downwards, creating the prismatic hexagonal pillars.[12]

Hexagonal columns in Hong Kong are distributed along the coast near High Island in Sai Kung, and on nearby dispersed islands. The total number of columns are estimated to be 200 000, covering 100 square kilometers. Diameter of the columns ranges from 1 to 3 meters. Most of the columns are tilted and are dipping towards northwest at about 80 degrees. Some columns such as those on the East dam of High Island reservoir, are curved by tectonic force. [13]

The hexagonal columns in Hong Kong are light brown to white in colour, reflecting its silica-rich chemistry. It contains about 76% silica.[11] Most of the columns in the world are basaltic or andesitic (low silica), such large, regular in shape and well-preserved silica-rich columns are rare.[13] The hexagonal columns in Hong Kong are therefore very important for geological studies and were chosen to be the logo representing the UNESCO Global Geopark of Hong Kong in 2011.

[evolution diagram showing formation of columnar joints]

Tsing Shan (Castle Peak) granite and landslides

Several major landslides happened in Tsing Shan in past decades. From 1990 to 2000, there were at least 3 major landslides (1990, 1992, 2000) occured that caused destructions.[14] For example, in 2000, debris flow from Tsing Shan reached a housing estate (Leung King estate) and blocked road access to the estate.[15] Tsing Shan is covered by heavily weathered granitic rocks. Weathered granitic rock often form loose surfaces. This makes the surface slippery and vulnerable to landslides.

Lion rock viewed from Kowloon

Lion Rock and Kowloon granite

Lion rock is located on the north of Kowloon peninsula. Its appearance resembles a laid down lion looking over major urban areas around Victouria Harbour. It is often used as a city symbol and a distinctive landmark representing Hong Kong. The Lion Rock is part of the Kowloon granite that covers Kowloon, Victoria Harbour and northern Hong Kong Island. The Kowloon granite south of Lion rock is already weathered, forming flat land on both sides of the Victoria Harbour, where today's major urban area of Hong Kong sits on. To the north of Kowloon, granite formed the Lion Rock, and various shallow hills lining up along the northern boundary of Kowloon[16]. Since 1956, Anderson Road quarry haven been used to take the Kowloon granite from these hills. The quarry closed down in 2013.[17]

Sedimentary rocks

Sedimentary rocks cover around 15% of Hong Kong land surface. [7]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 sediemenstary rocks. Undeformed sedimentary rocks always form horizontal layers. However, if deformed, sedimentary rocks can form deformation stuctures 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 priod (~416million years ago), dated by Placoderm (a Devonian fish) fossils discovered in Bluff Head formation in northeast New Territories. The youngest sedimentary rocks come from Paleogene (~50million years ago) in Tung Ping Chau at the very northeast of Hong Kong.[1]

Photo from Lung Lok Shui, Tung Ping Chau. A grey chert layer (the dragon's spine) on the brownish siltstones layers.

Tung Ping Chau erosion features

Tung Ping Chau, in Cantonese, means eastern flat island. It is a cresent 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 flatforms. "Long 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 whch deposits fine sediements. Fossils of terrestrial plants and evaporites in rocks on Tung Ping Chau indicates that it might be a saline lake during Paleogene.[5]

A fold on Ma Shi Chau, Hong Kong. Red lines show the limbs,the blue line shows the axis

Ma Shi Chau

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 sediementry rocks, Early Cretaceous volcanic rocks and middle Cretaceous sedimentary rocks. Fossils of ammonoids, corals and bivalves were found in the black Permian sedimentary rocks.[18] Layers of fine volcanic ash depositions formed the light grey colored tuffaceous layers interbedded with the brownish Cretaceous sediments. Ma Shi Chau is very close to a major fault (Tolo channel fault). [7]Rocks on Ma Shi Chau are therfore 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

Metamorphic rocks 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.[1] 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 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 (hyrothermal fluid) triggered chemical reactions. This process finally produced skarn, which an altered rock that carried the concentrated metal ores.[19]

Mining in Ma On Shan started in earlist in 1906 and became very active during the second World War for weapon production. Later, until 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.   [17]

Faulting

A simplified geological map showing the Lantau dyke swarm and faults.Modified from Civil Engineering Development Department.
A drag fold formed by upper layer moving right, lower layer moving left. The middle layer is dragged and rolled over, forming a drag fold. Red arrows showed the direction of motion of the upper layer and the lower layer. (Photo took in Nai Chung)

The main faults in Hong Kong are oriented northeast-southwest, and northwest-southeast (see fig.1).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.

Lantau dyke swarm

The Lantau dyke swarm is located on east Lantau Island. It is a group of vertical sheet of rocks formed by magma and lava flowing into a group of northeast trending cracks on pre-existing rocks. Those cracks were probably created by lateral movements (strike-slip) along northeast trending faults. The Lantau caldera, which was the volcanic centre of the magma, is also bounded by faults and exhibits an elongated shape towards the northeast. These strcutures recorded the active strike-slip motion of the northeast trending faults in Lantau during Late Jurassic. (~148 million years ago)[20]

Tolo Chanel Fault system

The Tolo Channel fault system is the longest fault system in Hong Kong running from Tolo Channel in the northeast, cutting through Shing Mun river in Sha Tin and extending to southeast Lantau Island. It is approxiately 60-km long. Traces of displacements and shearing are well-preserved in rock units on the both side of the Tolo Channel. Examples are kink bands, microfaults, veins at Ma Chi Chau on the north coast and en echelon veins, drag folds, sigma structures at Nai Chung on the south coast. These structure are all found on the middle Jurassic Tolo Channel formation sedimentary rocks and are traces of shearing events. They represented the most active period of the Tolo Channel fault systems during the middle Jurassic volcanic activities.

See also

References

  1. ^ a b c Sewell, R. J. (2000). The Pre-Quaternary Geology of Hong Kong. Geotechnical Engineering Office. Hong Kong: Hong Kong Geological Survey. ISBN 9620202996.
  2. ^ Lee, Cho Min (May 1983). "THE OCCURRENCE OF A DEVONIAN PLACODERMI FISH FOSSIL IN HONG KONG" (PDF). Geological Society Hong Kong Newsletter. 1 (4): 5–6.
  3. ^ Lee, C. M.; Chen, J. H.; Atherton, M. J.; He, G. H.; Wu, S. Q.; Lai, K. W.; Nau, P. S. (June 1990). "Supplementary report on the discovery of lower and middle Devonian fossils in Hong Kong" (PDF). Geological Society Hong Kong Newsletter. 8 (2): 16–24.
  4. ^ a b Wang, Lulin (7 February 2015). "Discussion on the sedimentary structure, geochemical characteristics and sedimentary environment of Ping Chau formation at Tung Ping Chau, Hong Kong". Journal of Environmental Biology. 36: 777–788.
  5. ^ a b Lee, C. M.; Chen, J. H.; He, G. X; Atherton, M. J.; Lai, K. W. (March 1991). "On the age of the Ping Chau Formation" (PDF). Geological Society Hong Kong Newsletter. 9 (1): 34–49.
  6. ^ Lai, K. W.; Langford, R. L. (January 1996). "Spatial and temporal characteristics of major faults of Hong Kong". Geological Society of Hong Kong Bulletin. 5 – via Research Gate.
  7. ^ a b c "The Geology of Hong Kong (Interactive On-line)". www.cedd.gov.hk. Retrieved 2018-11-16.
  8. ^ Campbell, S. D.; Sewell, R. J (November 1997). "Structural control and tectonic setting of Mesozoic volcanism in Hong Kong". Journal of the Geological Society. 154 (6): 1039–1052. ISSN 0016-7649 – via Researchgate.
  9. ^ Sewell, R. J. (2000). The Pre-Quaternary Geology of Hong Kong. Geotechnical Engineering Office. Hong Kong: Hong Kong Geological Survey. ISBN 9620202996.
  10. ^ "The Geology of Hong Kong (Interactive On-line)". www.cedd.gov.hk. Retrieved 2018-11-16.
  11. ^ a b Sewell, Roderick J.; Tang, Denise L. K.; Campbell, S. Diarmad G. (2012-01). "Volcanic-plutonic connections in a tilted nested caldera complex in Hong Kong". Geochemistry, Geophysics, Geosystems. 13 (1): n/a–n/a. doi:10.1029/2011gc003865. ISSN 1525-2027. {{cite journal}}: Check date values in: |date= (help)
  12. ^ Fang, Shi-ming; Li, Jiang-feng; Ng, Sai-Leung; Guo, Xu (2 November 2009). "Large six-party columnar joints of acidic volcanic rocks and its geological causes and significance in Hong Kong China [in Chinese]". Maine Science. 35 (5): 89–94.
  13. ^ a b Shum, C. [岑宗陽]. (2017). Columnar joints of high island formation in Hong Kong : comparison with overseas examples. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR.
  14. ^ King, J.P. June 2013. GEO REPORT No. 281: TSING SHAN DEBRIS FLOW AND DEBRIS FLOOD. Geotecnical Engineering Office, Civil Engineering Development Department.
  15. ^ H., Department of Civil and Structural Engineering ; The Hong Kong Polytechnic University Chau, K. T. Lo, K. Hazard assessment of debris flows for Leung King Estateof Hong Kong by incorporating GIS with numericalsimulations. HAL CCSD. OCLC 893032729.{{cite book}}: CS1 maint: multiple names: authors list (link)
  16. ^ "CEDD - Kowloon Granite - Klk". www.cedd.gov.hk. Retrieved 2018-11-17.
  17. ^ a b "CEDD - 10 Economic Geology". www.cedd.gov.hk. Retrieved 2018-11-17.
  18. ^ Yim, W. S., Nau, P. S., & Rosen, B. R. (1981). Permian Corals in the Tolo Habour Formation, Ma Shi Chau, Hong Kong. Journal of Paleontology. 55(6). 1298-1300
  19. ^ Strange, P. J.; Woods, P. W. (March 1991). "THE GEOLOGY AND EXPLOITATION OF THE MA ON SHAN MAGNETITE DEPOSIT" (PDF). Geological Society of Hong Kong Newsletter. 9 (1): 3–15.
  20. ^ Davis, D. W.; Sewell, R. J.; Campbell, S. D. G. (1 December 1997). "U-Pb dating of Mesozoic igneous rocks from Hong Kong". Journal of the Geological Society. 154 (6): 1067–1076 – via GeoScienceWorld.
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