The Primrose Terrace and the Wai-O-Tapu Geothermal Area

There are many places in the world where near-surface geothermal activity causes silicates to bubble up with hot water and then to be precipitated as the hot water trickles away or evaporates. Where these siliceous precipitates are deposited, impressive structures of delicate miniature pools and terraces are formed. Perhaps the most famous is the Mammoth Hot Spring terrace of Yellowstone National Park. In the southern hemisphere, New Zealand’s North Island has some of the most impressive geothermal features, particularly around Rotorua and Wai-O-Tapu.

The Wai-O-Tapu area includes a variety of natural features such as hot spring pools, bubbling mud pots, collapsed craters, and a sinter terrace known as the Primrose Terrace. This is the largest known sinter terrace (1.5 ha) in the Southern Hemisphere, since the Pink and White Terraces were destroyed in 1886 by the eruption of Mount Tarawera. The siliceous sinter flows in hot water from the Champagne Pool, which gets its name from the carbonic gases that bubbling out of the 73-degree water. Microbiolites and microstromatolites play an important role in the silicate precipitation by providing templates for silica precipitation. Around the Champagne Pool, various elements provide the colours of the rich pallet, including colloidal sulphur and ferrous salts (green), antimony (orange), manganese oxide (purple), silica (white), sulphur (yellow), iron oxide (red), and sulphur and carbon (black).

The Wai-O-Tapu area is associated with volcanic activity dating back to 160,000 years ago. The Champagne Pool is much younger, only about 900 years old, and the Primrose Terrace is believed to have been forming only for the last 700 years. 

Huangshan - Yellow Mountain

The granite towers of Huangshan in Anhui Province in China have attracted visitors for centuries, perhaps ever since their name was changed from Yishan to Huangshan in 747 AD by imperial decree. The name, which means “Yellow Mountain”, may have come from Huang Di, commonly thought to be the name of a legendary Chinese emperor. The towers, spires, and needles of granite and their crooked pines (pinus hwangshanensis) gazing over seas of clouds have inspired artists for ages. A well-developed tourist area, the mountain is etched by stone steps, sometimes augmented with concrete; the original steps carved out of the granite are said to be over 1,500 years old.

Huangshan formed over several periods of development. The first stage was a magmatic intrusion of granite that occurred about 143 Ma, followed by the formation of a batholith and stock formation of granite 96 Ma. The surface was covered by sedimentary deposits and was near a sea. About 65 Ma, the batholith experienced upheaval and the first granite mountains formed. The first Himalayan episode of uplifting took place about 54 Ma and the middle mountain and canyon were formed. This was followed by a long relatively stable period were shallow depressions of confluence basins formed on the paleo-gradational surface. Then came the long second movement of Himalayan uplifting followed by another stable stage. By the Quaternary, the granite was uplifted further as a fault block and as the granite cracked and split, streams cut their way deep into the rock. Exfoliation, sheeting, and frost weathering have also helped shape the fantastic metropolis of soaring stone.

I was very interested to read that ice age glaciation had contributed to the sculpting of the mountain; however, when I visited in December of 2004 I could not see any of the familiar tell-tale signs of former glacial occupation. In a paper called “Structural and Geomorphological Evolution of Huangshan (Yellow Mountain), Anhui Province, China” by Pei-Hua Huang, Robert F. Diffendal, and Ming-Qing Yang for the University of Nebraska, the authors point out that there is no determining evidence to support the claim that glaciers once existed on the mountain. This theory was originally proposed by J.S. Lee in 1936 but since the 1960’s P.H. Huang has studied the area and found non-glacial explanations for most of the formations previously believed to have been of glacial origin. You can read the paper by Huang et-al here. Interestingly, the article on Wikipedia adheres to Lee’s obsolete theories that glaciers once scoured the mountains. 

One question I personally have concerns the 7-ton boulder called Feilaishi. It rests atop a pedestal of granite which reputedly is of a different age or type from the boulder. It is said to be a glacial erratic, and usually this would be a credible deduction. But if not an erratic then how did this enormous boulder end up on its lofty precipice?