Memories of volcanic lava flows － Sharp Island (Kiu Tsui Chau)
Sharp Island (Kiu Tsui Chau) is just 2,000 metres off Sai Kung Pier. Set in Port Shelter, it lies southeast of Sai Kung town centre and east of Kau Sai Chau which is about 1 km away. This south-north trending island is long and narrow, extending some 2,500 metres long and about 500 metres wide from east to west. The highest point is 136 metres above sea level. Kiu Tsui Country Park is the smallest of its kind in Hong Kong. Covering only 100 hectares of island territories, its boundaries take in several islets, including Cham Pin Chau, Pak Sha Chau, Tai Tsan Chau, Cham Tau Chau and Kiu Tau. This popular holiday destination offers lucid water, silvery sand and dreamy vistas of island reefs and pristine woodlands. Visitors can explore the main island along a pleasant hiking trail, or walk across a spectacular natural sand levee that connects Sharp Island with neighbouring Kiu Tau. A sand levee is a coastal sedimentary landform. In geology, it is known as a tombolo. The Sharp Island tombolo is about 250 metres in length. It is flanked on both watersides by medium to fine grain sand and seashell debris, while the centre is coarse sand mixed with gravel. This natural tombolo is one of several tombolos found in Hong Kong. Similar terrain features can be seen in Pui O of Lantau and between Ma Shi Chau and Yim Tin Tsai. The Sharp Island tombolo is not always visible. Emerging only when low tide , it is a popular place for a seaside stroll or a dip in the sea.
Sharp Island is an iconic example of wave erosion landscape. Relentless erosive action has sculpted an eclectic array of bays and headlands : Hoi Sing Wan, Ha Mun Bay, Long Mong Wan, Shek Kwu Wan. Among them Kiu Tsui Beach and Ha Mun Bay are the best beaches. The former, situated near Sharp Island on the western shore of the island, is a favourite destination for seaside recreation. There are lifeguards on duty to ensure safety and visitor facilities include changing rooms, showers and a beachside barbecue area. The latter, situated in southern Sharp Island, is a crescent-shaped beach. Also known as Half Moon Bay, it is an excellent beach backing on to expansive grassland. Facilities include a barbecue area and campsite, changing rooms, showers and kiosk. It is one of the hottest swimming and barbecue destinations of the Sai Kung islands.
Tombolo is accessible when tidal level is lower than 1.4 meter. Visitors should note the tidal levels before crossing the tombolo. Please refer to Hong Kong Observatory for tidal information.
Most outcropped strata of Sharp Island belong to Mesozoic Cretaceous volcanic Clearwater Bay Formation, a formation composed primarily of flow-banded porphyritic rhyolite lava, rhyolite breccia and eutaxitic vitric tuff. Clearwater Bay Formation is part of the Kau Sai Chau Volcanic Group, the youngest of its kind in the territory. It is marked by features of volcanic activities in Hong Kong during the final stage of the Mesozoic Era. Also found on Sharp Island, just below Clearwater Bay Formation, are Mang Kung Uk Formation volcanic rocks. The rock here is essentially tuffaceous siltstone mixed with crystal bearing fine ash vitric tuff and tuff breccia.
All the above are extrusive volcanic rocks. Rhyolite is an acidic extrusive rock formed by solidification of cooled granitic magma extruded upon the ground surface. It is so named for the iconic flowing lines. Observed under a microscope, you can often find rhyolitic and porphyritic vitreous properties, pellets, felstone and micrographic textures. Phenocrysts are usually quartz and alkali feldspar, and occasionally containing a little plagioclase. The groundmass is usually tight cryptocrystalline or vitreous substances. In most cases, distribution of rhyolite formed by lava flow is limited, as this type of rock is usually outpoured by relatively small volcanic domes and lava flows. Hong Kong is no exception. Apart from Sharp Island, similar flow-banded lava can only be found in Yim Tin Tsai. The rhyolite lava of Sharp Island is living record of ancient volcanic eruptions and lava flows. While characterised by flow lines, it is also highly porphyritic. The phenocrysts are mainly quartz and sanidine, with the occasional presence of plagioclase in varied concentrations. This rhyolitic and porphyritic structure is very often the product of devitrification. Rhyolitic lava is widely distributed in southeast coastal provinces of China, including Liaoning, Inner Mongolia, Hebei, Shanxi, Shandong, Jilin and Heilongjiang.
Significant distributions of current acidic volcanic rocks are generally made of welded tuff. This rock is outpoured by sheets, volcanic domes and dykes. In Hong Kong, most outcropped volcanic rocks are such tuff emerging in island arcs and active margins or created by continental intraplate activities. The tectonic setting of Hong Kong lies within a continental margin. In the Late Jurassic and Early Cretaceous massive volcanic eruptions took place. Most volcanic rocks seen today were formed during such times.
At the northern tip and west side of Sharp Island, you can find porphyritic fine to medium grained quartz monzonite. This is an acidic intrusive rock whose chemical composition is between granite and granodiorite. Granite and granodiorite are both acidic rocks with abundant quartz and feldspar substances. Major differences being that granodiorite is slightly less acidic than granite; granite contains more potash feldspar than plagioclase; and granodiorite contains more plagioclase (acidic plagioclase) than potash feldspar. When the plagioclase and potash feldspar contents are level, it is known as quartz monzonite. Given that the Sharp Island rock mass has the same property as the quartz monzonite in Tai Mong Tsai, Sai Kung, it is known as the Tai Mong Tsai Quartz Monzonite.