You may have heard that Hong Kong is a tiny but busy city somewhere in southeast China (well, some people do mistakenly think that Hong Kong is a city in Japan…). Some of you may have visited Hong Kong as a tourist, or have briefly transited via the international airport.
So, where exactly is Hong Kong? Geologically speaking?
Hong Kong is located at the month of the Pearl River, which is the third longest river system in China and crosses much area of Guangdong and Guangxi provinces. The Pearl River drains into South China Sea, which is to our south and southeast. We are now located at the passive continental margin in the southeast side of the Eurasian landmass. Pretty stable and free of active volcanoes or strong earthquakes. However, in the geological past, we did underwent a period of time when things were much more…violent.
Geologically speaking, the southeast China consists of a large crustal block, known as the South China Block (some people call it South China Sub-plate). The South China Block has been present for a long period of time. Some of the basement rocks are Archean in age, i.e. over 2.5 billion years old! Before the start of Palaeozoic era (some 550 million years ago), the South China Block was part of the Gondwana supercontinent. It broke up from the great landmass and started drifting northward in the early Palaeozoic, together with many other crustal blocks. Finally, the South China Block collided with its brother, the North China Block in the late Palaeozoic, and joined the family of the Eurasian Plate.
We do not have any early Palaeozoic rocks exposed in Hong Kong. The oldest rocks that are now present here is Devonian in age (about 410 million years old). The tectonic environment here was relatively stable in southeast China for most of the late Palaeozoic.
At the start of Jurassic Period (or perhaps earlier), an active plate boundary was formed in southeastern margin of the South China Block, where the palaeo-Pacific Plate converged and dived under the Eurasian Plate. Many active volcanoes were present at that time. Probably because of some unusual configurations of this subduction zone, a 1200 km wide igneous province (mostly granitoids and associated volcanic rocks) was formed in southeast China. The age of these rocks span from some 200 to 90 million years old.
About 80% of the exposed rocks in Hong Kong are igneous rocks that formed during Middle Jurassic to Early Cretaceous (between 180 to 140 million years ago). For examples, the prominent landmark of Hong Kong — the Lion Rock, is made of the 140 million year old granite, which have exhumed from the deep ground. The highest Peak, Tai Mo Shan (957 m), consists volcanic rocks of 164 million years old.
After the cessation of volcanic activities, the land began to subside, and basins were formed. It has been said that there are more than 100 pull apart basins identified in Guangdong Province. These basins were then filled up by Cretaceous or younger sediments, some have preserved numerous dinosaur eggs and some are oil-bearing.
I have given here a very brief description of the geological history of Hong Kong. Stay tuned, more is coming.