HKDSE Geography/E1/Rock Type and the Landscape

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The effects of rock type on Hong Kong's landscape are great. We will look at the three rock types here.

Although all three rock types are found in Hong Kong, igneous rocks are by far the most common, taking up nearly 85% of Hong Kong's rock outcrop. That means the layer of rock on the surface of Hong Kong is mainly igneous rock. Sedimentary rocks take up most of the remaining 15%, with a very small percentage for metamorphic rocks. This is not to count the parts of Hong Kong covered by superficial deposits (discussed below).


Volcanic Rocks[edit]

Distribution[edit]

Although it was previously believed that granite was the dominant rock type in Hong Kong, it is now known that volcanic rocks are the most important, taking up half of Hong Kong's outcrop. Important places with volcanic rocks include the northern, eastern and central portions of the New Territories, including Tai Mo Shan and the Sai Kung Peninsula. The western portion of Lantau Island and the southern part of Hong Kong Island, excluding the southernmost part, are also made of volcanic rocks.

Major Types and Properties[edit]

Tuff is one of the most important volcanic rocks in Hong Kong. Hong Kong's tuff was formed during the Middle and Late Jurassic and Early Cretaceous. Ash and pyroclasts from violent volcanic eruptions of the era were compacted and solidified. Tuff varies in composition, ranging from fine ash to small rock fragments. There are usually various crystals, mainly quartz, feldspar, biotite and hornblende, embedded in fine, dark groundmass. The last major volcanic eruption in Hong Kong was one of extreme magnitude, and produced the spectacular hexagonal tuff columns at High Island Reservoir. The columnar joints were bent during solidification as weight caused a sliding effect on the upper part of the rock.

Rhyolite is another important rock in Hong Kong, formed from lava flows and ash flows. The main mineral in feldspar and the rhyolite often exhibits flow bands, which indicate that it was formed from lava.

Effects on the Landscape[edit]

Volcanic rocks shape Hong Kong's landscape. As a volcanic rock, tuff has very high resistance - that is to say, it is very tough. Therefore, tuff hills in Hong Kong are less prone to denudation. Tai Mo Shan and Lantau Peak are notable examples. Also, volcanic rocks are responsible for various sea cliffs near the coast like those found in Port Shelter.

Plutonic Rocks[edit]

Plutonic rocks were formed deep in the earth's crust and were exposed onto the earth's surface after the overlying rocks were removed. About 35% of Hong Kong's outcrop are plutonic rocks. Granite is the most important.

Distribution and Properties[edit]

Granite is the most common, and is found in the southwestern part of the New Territories (most notably Tuen Mun and Tai Lam), othe southern part of the New Territories (such as Sha Tin), Tai Po, the northern part and southernmost tips of Hong Kong Island. It is also found on the outlying islands: the eastern part of Lantau Island, most of Lamma Island, Po Toi Island, and the Soko Islands. Most notably, the two sides of Victoria Harbour are a granite batholith.

Hong Kong's granite has either pink or grey feldspar minerals. These are the dominant colours of granite.

Effects on the Landscape[edit]

Plutonic rocks are less resistant than their volcanic relative, and have produced shorter hills which have gone through more denudation, such as Castle Peak and Beacon Hill. (The badland landscape in granitic slopes will be discussed later.)

Granite produces a specific weathering profile in Hong Kong by spheroidal weathering. On the bottom sits the unweathered bedrock. Above the bedrock lies corestones of decreasing size and angularity until the surface of the weathering profile, where fine regolith is found. The layer of regolith is very thick and provides support for lush vegetation. The weathering profile is also thick, and may extend up to 60 metres below the earth surface. As rainfall continues to infiltrate and percolate into the bedrock, the bedrock will be continuously removed unless the layer of regolith is too thick for infiltrating water to progress. Erosion by rain and wind, especially the former, will wash away the layer of topsoil to reveal the corestones below. Tors are thus commonly found in Hong Kong. Amah Rock and Lion Rock are two notable examples.

(This is only a description of the weathering profile. For its formation, you should write out the processes involved in spheroidal weathering, which is provided in the Chemical Weathering chapter.)

Sedimentary Rocks[edit]

Distribution[edit]

Bluff Head formation

Sedimentary rock outcrops are sparse and scattered over the city, taking up 15% of the land area. They appear in the northereastern the New Territories, including Pat Sin Leng, Port Island and Tung Ping Chau. The very first rocks in Hong Kong are sedimentary. They were developed along Tolo Channel before its submergence, and includes Bluff Head and Ma On Shan. The youngest rocks in Hong Kong are also sedimentary. They were developed on Tung Ping Chau.

Major Types and Properties[edit]

Various types of sedimentary rocks are found in Hong Kong, including the full range of grain sizes: conglomerate, sandstone, siltstone, mudstone, shale. We will discuss conglomerate and siltstone in detail.

Layers of conglomerate generally overlie volcanic rock layers or are sandwiched by other other sedimentary rocks. They are formed from cemented pebbles, cobbles and boulders. The colour varies. White conglomerate was formed from lithified rounded pebbles along Tolo Channel and on Ma On Shan during the Devonian period. They were formed in a wet delta environment, hence reduction took place, discolouring the rocks. Red conglomerate was formed from lithified colluvium, composed of volcanic rock and other sedimentary rocks. They were formed in an arid environment during the Cretaceous period, hence oxidation took place, colouring the rocks.

Siltstone was also found. Surprisingly, siltstone was formed from fine silt. Found in Tung Ping Chau and contains minerals like quartz. The bedding plates are very thin and fossils are commonly found. The colour ranges from dark grey to reddish-brown. It was formed in a lake that originally housed gypsum from salt evaporation. The gypsum later dissolved and the lake was filled with sediments, forming siltstone.

Effects on the Landscape[edit]

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Metamorphic Rocks[edit]

Metamorphic rocks rarely occur as outcrop in Hong Kong. Marble and schist are sometimes found here. Marble is found in Yuen Long and Ma On Shan. Metamorphosed sedimentary rocks are also found in Lo Wu and Lok Ma Chau.