Geology of Hong Kong
It made up 85% of Hong Kong's land surface and the remaining 15% are mostly sedimentary rocks located in the northeast New Territories.
Sedimentary rocks formed various erosion features such as wave-cut platforms and sea stacks in Tung Ping Chau.
In terms of structural geology, faults in Hong Kong are mainly running from the northeast to the southwest.
Past fault activities can be traced by some structures such as the Lantau dyke swarm and deformed caldera, etc.
Rocks of this period are characterized by a variety of fossils, heavily folded strata and steeply tilted beds.
It is represented by reddish colored sedimentary rocks which as an indication of an arid tropical climate during deposition.
[1] In the eastern part of Hong Kong, these volcanic rocks form hexagonal columnar cooling joints.
They are mainly distributed in Kowloon, north Hong Kong Island, east Lantau, and Tuen Mun.
[12] The total number of hexagonal columns in Hong Kong is estimated to be 200,000, covering 100 square kilometers.
[13] In respect of it being 'one of the most spectacular rhyolitic columnar rock formations in the world', the International Union of Geological Sciences (IUGS) included the 'Early Cretaceous rhyolitic columnar rock formation of Hong Kong' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022.
The middle part of the Kowloon granite was subjected to heavier weathering, forming the Victoria Harbour, where Hong Kong started its development.
[1] The sedimentary rock that makes up around 15% of Hong Kong land surface[7] formed by deposition of alluvial sediments, such as sand, mud, the exoskeletons of marine plankton, and pebbles.
Since sediments always deposit in horizontal layers, any observable deformations, such as folds, record tectonic activities.
The youngest sedimentary formations date to the Paleogene (~50 Mya) in Tung Ping Chau in northeast Hong Kong.
[16] Lung Lok Shui, which means 'dragon diving into water,' is a well-known geological formation that looks like a dragon's back descending into the sea.
The structure's most prominent feature, the grey layer of chert, which is more resistant to erosion than the surrounding sandstone, is said to resemble a dragon's spine.
[16] Rocks on Tung Ping Chau are fine-grained and reddish-brown from increased iron oxidation during the hot and humid Paleogene period and weak, shallow currents depositing fine sediments.
Fossils of terrestrial plants and evaporites in rocks on Tung Ping Chau indicate that it may have been a saline lake during the Paleogene.
Various deformed structures such as folds, kink bands, microfaults and sheared rocks can be observed on Ma Shi Chau.
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.
Metal ores are concentrated into hot fluids as it forced itself into cracks of the Ma On Shan limestone.
This process finally produced skarn, which an altered rock that carried the concentrated metal ores.
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 structures recorded the active strike-slip motion of the northeast trending faults in Lantau Island during Late Jurassic.
[21] 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.
These structure are all found in the middle Jurassic Tolo Channel formation sedimentary rocks and are traces of shearing events.
