Volcanic rocks are widely distributed in Hong Kong (green areas in Fig.1). They formed most of the highest mountains in Hong Kong, such as Tai Mo Shan (957 m, the highest mountain) and Lantau Peak (934 m, the second highest mountain). In the eastern part of Hong Kong, these volcanic rocks form hexagonal columnar cooling joints. They can be seen from the High Island reservoir and islands nearby. These areas are listed as part of the UNESCO Global Geopark of Hong Kong.