I live in Hong Kong. As a nature lover, I often spend time visiting the south-eastern coasts of Sai Kung Peninsular - a local nature reserve.

The area's famous feature is the coastal erosion landscape. Sea arches and caves are present on almost every island off the coast. See the map below for the locations of major sea arches in the area.


A point to note is that the entire area was once a super volcano 140 millions years ago, and most of the rocks found in the area are basalt. There are several locations where basalt columns reaches over 50 meter tall, and over 2 meters in diameter. One example is Po Pin Island - technically a sea stack - with a basalt column cliff cut so deep that it almost look like someone cut the island in half with a knife.

Po Pin Island

All the sea arches, sea caves and cliffs are semi-submerged. One can row a boat through some of these sea arches. Below are some images of these sea arches for your better understanding of their scale and the adjacent environment.

Sea Arch 1 Sea Arch 2 Sea Arch 3 Sea Arch 4

Note that despite the erosion of coastline, the mountain above the affected area are quite smooth and have retained enough soil to support plant growth. It appears that wind erosion has much less effect on these rocks compare to oceanic erosion.

It is only reasonable to assume that these sea arches and caves are formed under the strong erosion forces from the ocean. However, to form such a large opening in basalt rocks takes time. While I cannot find any study on the estimated time required for such landscapes to form, I assume it will take tens of thousands of years.

Now comes the problem - the sea level was no where near its current level in the past. Currently, even at several kilometres away from the coastline, the sea floor is merely 30-40 meters deep, where it could have been land ten thousand years ago. In fact, there are evidence that there existed some stone age primitive villages within Mirs Bay (see the location on the map above). It is then washed away or submerged when the sea level rises to current level.

Given this record on sea level changes, the current coastline, including the sea arches and caves, has only around 10000 years to evolve from a relatively smooth mountain ridge into the state we see today. This is a very short time in geological scale. It is baffling that such significant erosion can manifest within just 10000 years.

Grateful if any professionals can look into this question and explain how could this happen.

  • $\begingroup$ You're looking at two very different timescales. There was indeed a minimum in sea level 20 ka ago, allowing for humans to settle before it started to rise to the current level, but there were also long periods of high sea level (higher than today) millions of years ago. Look at this curve: en.wikipedia.org/wiki/Past_sea_level#/media/… So if your basalts were formed 140 Ma ago, they had plenty of time to get eroded by the sea. $\endgroup$ Jun 26, 2020 at 12:09
  • $\begingroup$ It would be interesting to know how much weathering the basalt has undergone, this will affect the competency of the basalt. You third photo, the close up of the arch, shows the columns to be semi vertical, broken, rough & fractured. It doesn't look as competent as fresh basalt. What I suspect has happened is he sea has worn away a small hole & under gravity chunks of basalt have just dropped into the water because of the hole. This has then lead to a cascade of material just dropping out, more so than just erosion by water & wind. $\endgroup$
    – Fred
    Jun 26, 2020 at 20:18
  • $\begingroup$ Also the wave action will not just move water to erode the cliffs, but water containing anything from sand up to fairly large rocks, which should make a fairly good abrasive. $\endgroup$
    – jamesqf
    Jun 29, 2020 at 0:55


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