Explain the operation of the rock cycle with reference to the geology in Hong Kong. To what extent are the rock types responsible for the deep weathering profile in the territory? Hong Kong is dominated by igneous rocks which granite takes up the largest proportion of 85% of the total land area. The remaining area is underlain by sedimentary rocks superficial deposits mainly in the North West area. Other than the above rock types, there still a kind of rock which covers only a small area of Hong Kong, its metamorphic rock, mainly can be found in the north-western part of the New Territories.
First, let’s talk about the igneous rocks in Hong Kong. Dated back to the late Jurassic age, Hong Kong was located near to the destructive plate boundary on the south- east coast of China, generating pressure during the subduction activity and causing vulcanicity. In the destructive plate boundary, magma currents beneath earth crust converge, dragging two plates together while the heavier plate sinks and being subducted under another plate, the subducted zone will then be melt back to magma again.Pressure accumulates while the subduction of the heavier plate is occurring, crack will develop on both plates near the boundary allowing magma to seep through them onto the earth surface.
Magma that rose through the cracks may soon be cooled down by the temperature of the earth surface and forming extrusive igneous rocks and building a volcano gradually at the same time. These extrusive igneous rocks can be found in western Lantau Island, the southern part of Hong Kong Island, and the central, northern and eastern parts of the Territories.Intrusive igneous rocks can also be found in both sides of the Victoria Harbour, eastern Lantau Island, Castle Peak, Tai Lam, Sha Tin and Tai Po. They are formed by exposed batholiths of the volcanoes in the Jurassic age; granite is the most abundant type of rock that can be found.
Many of the igneous rocks like tuff and granite we can find in Hong Kong have already over ten thousand years old! Other than igneous rocks, sedimentary rocks have also taken up a part in Hong Kong’s geology.The majority of these rocks are localized at the north-eastern part of the New territories, around Mirs Bay. Under the continuation of the rock cycle, the igneous rocks formed earlier will expose to external processes. They will undergo weathering, mass movement, erosion and transportation and deposition.
Rocks are broken down by denudation agents like wind, rain water or waves and gravity will break rocks into small fragments and removed downhill. When the energy of the moving agents has lost, deposition and sedimentation of the fragments happens.The layers of sediment pile up as sedimentation continues as the rock fragments and grains are squeezed tightly together, undergoing the process if compaction. The presence of water in between dissolved minerals has caused the effect of cementation of gluing the grains together, sedimentary rocks are formed. In Hong Kong we can mostly find clastic sedimentary rocks like conglomerate, sandstone, siltstone and mudstone, the above types of stones are formed from particles of other rocks.
It has come to the last type of rock which is also the smallest part of the Hong Kong geology.Metamorphic rock covers a small area in Hong Kong; mainly in the north-western part of the New Territories for example Yuen Long, Ma On Shan and Tung Chung. Marble and schists are indeed the main types of metamorphic rock in Hong Kong. Either sedimentary rocks or igneous rocks will change from their original form into a completely new rock under high temperature and pressure. Rising magma intruding into rock layers, high temperature can make the rock crystal melt and recrystallise into new rock, it’s the formation of the contact metamorphism which do not exhibit a foliated structure.
For the dynamic metamorphism, the internal stress accumulates along a fault or in a fold, the high pressure can make the mineral crystals in rocks realign to form metamorphic rocks with a foliated structure. The dominant rock type in Hong Kong is responsible to the deep weathering profile in the territory to a large extent. Granite is the dominant rock type in Hong Kong; it is well jointed and vulnerable to weathering and erosion. It contains minerals that are easily to be eroded by wind, water, wetting and drying.
It results in deep weathering profile due to its well-jointed structure. According to the geology of the Territories in Hong Kong there are sedimentary rocks and recent sediments located particularly in there, which means weathering and erosion process is very rapid since the granite there has been eroded into sediments by various denudation agents, resulting in deep weathering profile.