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Internationally rare acidic polygonal volcanic rock columns - High Island

Background Information

Sea Cave

 

So close and yet so far – the ancient Maritime Silk Road revisited. Surprisingly, the staggered hexagonal rock formation, formed about 140 million years ago, stands not far from the city fringe. From Sai Kung Town, Hong Kong’s holiday hot spot, you can reach the East Dam of High Island Reservoir in about 40 minutes. This is the best place to see the world-class hexagonal rock columns. Towering columns rise in the distance, and the High Island Geo Trail takes you to the shore. There is a sea cave at the water’s edge. From there, a wooden boardwalk leads to a lookout where you can feel the force of wave erosion up close.

The East Dam was completed in the 1970’s. It is set on the east side of Kwun Mun Channel. Your tour guide will lead you on a journey back to the Ming dynasty several hundred years ago when many merchant ships sailed between mainland China and Southeast Asia. High Island Reservoir, which supplies potable water to Hong Kong today, was once a small channel and stopover point on the Maritime Silk Road.

All roads were built by people. Today, High Island Reservoir is a scenic location surrounded by sea and hills, with no sign of human settlement, but it was once the site of a thriving fishing village. The East Dam Half-day tour takes you to a forgotten fishing village that now lies submerged.

Hong Kong is more than a financial centre, and there is much more to see than dramatic rock formations and verdant hills. There are also colourful stories of Hong Kong’s heritage and struggle for success.


Rock Columns
 

Geological Information

Graphic
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The volcanic rock with columnar joints found in the High Island region is a type of acidic rhyolite ¾ dacite porphyroclastic lava. It is rich in potash feldspar and quartz phenocrysts. About 400 metres thick, these rocks are known as High Island Formation. They are part of the Early Cretaceous Kau Sai Chau Volcanic Group.

These grotesque volcanic rock columns stand as high as 30 metres. They first appeared some 140 million years ago when subterranean magma and volcanoes were active. Whenever there was an eruption, great lava flows gushed out along with scorching volcanic ash. They spread across the ground surface and formed lava layers. During the cooling period, the rock contracted very uniformly and gave rise to the marvelous hexagonal columnar joints seen today.

Columnar joints are common in basalt as well as other igneous rocks like obsidian and welded tuff. After consolidating in temperature as high as several hundred degrees Centigrade, igneous rock continues to cool down until it reaches air temperature. During such time it contracts and eventually develops columnar joints. Similar columnar joints can be seen in mud that has dried and contracted.

Judging by its distribution characteristics, High Island Formation seems to have been formed inside a huge caldera. When the lava flow ran rapidly into this recess, just like porridge being poured into a bowl, it created a semi-enclosed environment which was beneficial to slow cooling. When the lava cooled down slowly, it gradually developed contraction joints.

 

Landform Video

High Island

Po Pin Chau

How to get there

1. You can book the new East Dam Half-day Tour at the Volcano Discovery Centre. Both round-trip and one-way itineraries are available. For enquiries, please contact the Volcano Discovery Centre (tel: 2394 1538); or

2. Take bus no. 94 departing from Sai Kung town centre or bus no. 96R departing from Diamond Hill MTR Station (weekends and public holidays only) and get off just after Pak Tam Chung. Walk along Tai Mong Tsai Road to the junction ahead. Turn right on Sai Kung Man Yee Road and walk about 9 km; or

3. Take a taxi from Sai Kung town centre or Pak Tam Chung to the High Island Geo Trail.

 

 

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