Geology of Hong Kong

The geology of Hong Kong is dominated by igneous rocks (including granitic rocks and volcanic rocks) formed during a major volcanic eruption period in the Mesozoic era. It makes up 85% of Hong Kong's land surface, and the remaining 15% are mostly sedimentary rocks located in the northeastern New Territories. There is also a very small percentage (<1%) of metamorphic rocks within the New Territories, formed by deformation of pre-existing sedimentary rocks (metamorphism).

The geological history of Hong Kong started as early as the Devonian period (~420 Mya) which is marked by the discovery of fragmentary placoderm (a Devonian fish) fossils in northeast Hong Kong. While the youngest rocks in Hong Kong were formed during the Paleogene period(~50 Mya). They are today exposed in Tung Ping Chau in northeastern Hong Kong.

Each of the three types of rocks— igneous, sedimentary, and metamorphic— form spectacular geological features in Hong Kong. Igneous rock forms the hexagonal columns in Sai Kung, sedimentary rock forms various erosional landforms such as wave-cut platforms and sea stacks in Tung Ping Chau, and metamorphic rock forms the iron ore deposits in Ma On Shan.

In terms of structural geology, faults in Hong Kong mainly run from the northeast to the southwest. Deformation features such as sheared rocks, folds, and faulted rocks can be found near major faults, such as the banks of the Tolo Channel. Past fault activity can be observed in some structures such as the Lantau dyke swarm, deformed caldera, etc.

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