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.