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Internationally rare acidic polygonal volcanic rock columns - High Island

Background Information

Background Information

 

High Island Reservoir is one of Hong Kong's most popular geological sites, and home to our most spectacular rock wonder. Here, volcanic rock composes neat arrays of giant polygonal joint columns along the shore. Amongst these the hexagonal rock columns are most typical examples. Catching your eyes with striking angular facets, these giants are rare natural prodigies and invaluable geo-tourism resources that draw great interest.

Around High Island Reservoir, especially the East Dam, you can observe the most typical hexagonal columnar joints from different angles. There is also a chance to examine the rock composition and structure at close range.

Looking down from the East Dam rock cliff, you have a clear view of buckles in the hexagonal columnar joints. Judging by the s-shape section, we know the rock columns buckled under gravity before it could cool down completely and solidify, and thus the scene we see today. With such evidence, it is easy to trace the formation of volcanic extrusive rock.

At the observation point for the hexagonal columnar joints, you can also find outcropped basic lamprophyre veins. Lamprophyre is a dark dyke comprising mainly biotite, amphibolite, augite and light minerals like plagioclase and orthoclase. It is usually in vein attitude and wide distribution.

Po Pin Chau just off the East Dam is another interesting place to see extraordinary formations. This is where you find giant stacks that emerged when a complete hill was slashed into two by natural forces. It is undoubtedly a fine example amongst similar geo attractions. Geology enthusiasts will certainly cherish the opportunity to see towering rock stacks and fascinating hexagonal joints. Maritime explorers will find Po Pin Chau intriguing and unforgettable.

After admiring nature's exquisite works, don't forget to see High Island Reservoir, the impressive man-made wonder. This water storage facility is the biggest in Hong Kong. It is also a water project of key importance. The dramatic East Dam and West Dam are great attractions in their own right. The seafront is protected by concrete dolosse units. Functional as it is visually appealing, this cofferdam attracts many visitors.

Background Information
 

Geological Information

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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

Take bus no. 94 at Sai Kung Market or no. 96R at Diamond Hill MTR station (service on weekends and public holidays only). Get off after Pak Tam Chung and walk along Tai Mong Tsai Road to the junction ahead. Turn into Man Yee Road of Sai Kung on the right and walk on for about 9 km for the High Island Reservoir East Dam, the starting point of High Island Geo Trail.

 

 

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