Hong Kong itself does not have much volcanic activity at the current time. However, throughout the land of Hong Kong, volcanic rocks and plutonic rocks are dominant. Geologists have confirmed that the very existence of Hong Kong is because of a huge volcanic eruption that occurred 140 million years ago. The eruption came from a very rare supervolcano, one of only about 50 supervolcanoes that have erupted since man walked the earth. So although it may be dormant, the main city of Hong Kong sits atop a very old ancient volcano.
Here are some images of a volcanic rock formation in Hong Kong resulting from the supervolcano. The rocks formed hexagonal shapes.
Sources:
https://www.scmp.com/yp/discover/lifestyle/features/article/3067002/how-was-hong-kong-made-really-big-volcanic-eruption
https://hkss.cedd.gov.hk/hkss/eng/education/gs/eng/hkg/chapter6.htm
