Magnificent sea stacks of Ung Kong Group–Bluff Island (Sha Tong Hau Shan)

Magnificent sea stacks of Ung Kong Group–Bluff Island (Sha Tong Hau Shan)

Background Information

Bluff Island (Sha Tong Hau Shan), or Ung Kong Chau, lies west of Basalt Island and south of Town Island (Fo Tau Fan Chau). In the southern corner of this small island you can find one of the four biggest sea caves in our eastern waters ¾ Sha Tong Hau Cave. Viewed from above, Bluff Island resembles a turtle with four legs stretched out wide. Lushly wooded slopes of the rounded hills roll gently down to scenic sandy bays. This is an ideal place to study rhyolite. Formations in clearly defined layers are prominent across the island, and along the wavy shores, huge blocks tower by the water. Fan Tap Pai in the south is one of the most breathtaking coastal rock scenes in Hong Kong. This huge sea cave cuts right through the island and it is just wide enough for a boat to sail past. Not a bad idea for a leisure sea expedition. Bluff Island is also a diving hot spot. An increasing number of divers are coming here to explore the mysterious ocean.

Geological Information

Most parts of the exposed stratum of Bluff Island belong to the Early Cretaceous Kau Sai Chau Volcanic Group. From the Late Jurassic to the Early Cretaceous, this region was the site of earth shaking changes. It began about 146 million years ago (Late Jurassic) when there were violent magma activities. Fiery underground magma intruded upwards, melting ancient rocks along its path. It forced its way through cracks and thrust along crustal faults or weak bands until it reached the surface to cause great volcanic eruptions. Sometimes magma was pressed inside underground voids. It created such an immense force that when it finally broke out it made catastrophic impact. Explosion shot volcanic ash and gravels high into the air. Lava was vented out from the crater from where it coursed down the slopes, igniting fires and generating great clouds of ash and fume. According to data for determining the absolute age of rocks, volcanic eruptions and magma intrusion activities took a cyclical pulse pattern. In every cycle, major eruptions were always followed by great magma intrusions. Such cyclical volcanic magma activities can be categorised into four stages, and consequently there are four volcanic rock groups in Hong Kong ranging from old to new : Tsuen Wan Group, Lantau Group, Repulse Bay Group and Kau Sai Chau Group. Judging by the exposed stratum of Bluff Island, we know that eruptions took place in the 4th stage.