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Rock Classification

The famous Lianhuashan fault zone in Southern China has a decisive influence on Hong Kong's geological structure. This generally northeast-southwest trending zone is made up of more than 100 major faults. In Hong Kong, geological structure is also in this main orientation, as evidenced by large, continuous northeast-southwest trending faults. Among them the Tolo Channel-Shatin-Lai Chi Kok fault zone is a prominent example with distinctive geomorphologic features. The northwest-southeast trending faults, on the other hand, are smaller and mostly tensile. Most of these extend in a non-continuous pattern. This structural layout dictates the distribution of mountain ranges and topographic trends of present day Hong Kong.

The rocks of Hong Kong comprise about 50% volcanic rocks (tuff, lava) which are mainly distributed in the scarcely-populated countryside; 35% granitic rocks formed by lava intrusion, acidic and basic veins (For example, the granite on both sides of Victoria Harbour and in the southern part of Hong Kong Island like Shek O are typical outcrops); 15% sedimentary rocks (mudstone, siltstone, sandstone, conglomerate, breccia); and very small amount of metamorphic rocks (slate, schist, skarn, marble) which are very restricted in distribution and found mainly in the Northern New Territories with scattered specimens on both sides of Tolo Channel and western Lantau Island.

Intrusive rocks like granite and porphyry, and extrusive rocks (volcanic rocks) like tuff and volcanic breccia, are all igneous rocks. When they are partly eroded by the weathering process, remaining substances are transported, deposited, dehydrated and compacted to form sedimentary rocks like sandstone and conglomerate, as well as alluviums which are not solidified into rock. Under crustal high temperature and high pressure, intrusive rocks, extrusive rocks and sedimentary rocks may turn into metamorphic rocks, such as schist and quartzite.

Given varied chemical compositions and physical properties amongst rocks, such as hardness, resistance to erosion, contents of light and dark minerals, rock compositions and extent of fissure development, we have an extensive range of weathered terrains in Hong Kong.

Some ancient rocks contain fossils of marine and terrestrial organisms that date a long way back, from the Devonian 400 million years ago (Wong Chuk Kok Tsui) to the Tertiary more than 10 millions years ago (Tung Ping Chau). These are invaluable geological treasures.

Diverse and unique, Hong Kong rocks are truly captivating. A geological map of Hong Kong will help you understand the distribution of different rock types.

 

Map

The following is a brief introduction of rocks found in Hong Kong:

Extrusive igneous rocks (volcanic rocks)
Extrusive igneous rocks (volcanic rocks)Extrusive igneous rocks (volcanic rocks)

Volcanic rocks are made up of volcanic substances. They come in two major varieties. The first type is lava formed by solidification of extruded and flowing lava. The second type is pyroclastic rock formed by volcanic ash, volcanic bombs and lava substances that fall to the ground, get compacted and cool down. The latter tend to contain a significant amount of normal sediments or lava substances. Tuff is one example.


Intrusive igneous rocks (granitic rocks and various intrusive veins)
Intrusive igneous rocks (granitic rocks and various intrusive veins)

Granitic rocks are formed by magma that intrudes the crust from the mantle. Since the magma cools down over a long period deeply underground, the mineral crystals within are usually well developed. Grain size is relatively coarse.


Sedimentary rocks
Sedimentary rocks

Sedimentary rocks of Hong Kong are primarily products of eroded ancient rocks. After settling in seas, lakes and river estuaries, these efflorescences were dehydrated over millions of years to become rock. Sediments may contain grains of different sizes, including mud, silt, sand, pebbles or even boulders (forming mudstone, siltstone, sandstone, conglomerate and breccia). They may contain flora and fauna remains and debris.


Metamorphic rocks
Metamorphic rocks

When any kind of rock is under the pressure of great subterraneous force and/or baked under high temperature, structural changes or re-crystalisation will take place. metamorphic rock is formed.

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