06 Mystery of air hidden in strange rocks

What kind of scenery conjures up in your mind when you hear "Great history of China"? Is it something like a Chinese-styled landscape painting with a great river running in between strange rock mountains, with some small fishing boats floating on misty water surface? You can find such a scenery in the real world if you visit Shilin or Guilin in China. Also, much smaller scale of such a landscape can be found in many other places in the world.

Strange shaped rocks in Shilin and Guilin are made of limestone, a type of rock ubiquitous throughout Japan. It is often used to make stone products. Characteristic landscape is formed in places abundant in limestone, such as a limestone plateau and a limestone cave, which attract people as tour spots with its mysterious feature.

Under what conditions had the landscapes in Shilin and Guilin been formed? There is no doubt that they have been formed over a long period of time.

Even though the time it took for the formation may be quite long in the perspective of human history, it must be an instant event for the history of earth from the formation of strange rocks until they disappear from earth sometime in the future because much wider span of time had been spent in the background of strange rock formation.

The background in this context refers to the time until original rocks of limestone had been formed and moved to the location where strange rocks are formed. Specifically, hundreds of millions of years had been spent from the formation of limestone rocks at sea bottom, slowly pushed above water due to plate-tectonics and until they go through various transformations before finally reaching the present location. Landscapes in Shilin and Guilin have been formed due to erosion by rainfalls and flow of rivers after that.

Now, let's take a look at how limestone rocks had been formed. Geologic time it takes for the formation varies widely. Fossils can be found in limestone rocks formed in Paleozoic or later. Such limestones are composed of remains of organisms, especially ones that form reefs such as a coral reef. It is not unusual that the original shape of fossils disappeared over time while going through various changes.

The main ingredient of limestone is a mineral called calcite. Calcite is made of calcium carbonate (a chemical formula is CaCO₃). The organism used hard calcium carbonate as shell or bone (in the case of coral, it is an outside frame). The material is the carbonic acid ion and calcium ion in seawater.

Carbon dioxide in air dissolves in seawater and taken in by organisms and turns into shells and bones. Thus, quite a huge volume of carbon dioxide gas is required to form even a small piece of solid substance.

The fact is easily understood from a scientific experiment that carbon dioxide gas bubbles out fiercely by pouring hydrochloric acid over a piece of limestone.

The abundant existence of limestone rocks on earth refers to the fact that extremely huge amount of carbon dioxide stock is fixed on earth, effectively compressed in the form of solid, by the medium of organisms. Based on the estimated gross deposit of limestone rocks on earth, the total stock of carbon dioxide is estimated to range from 50 to 100 Atm. The carbon dioxide of that amount had been fixed to organisms' body over a long time. Without the existence of organisms, the earth could have been a very hot planet by the greenhouse effect of carbon dioxide.

Originally, the earth had been a place where carbon dioxide is dominant in air, and as I have mentioned in the previous essay, photosynthetic organisms represented by stromatolite started to generate oxygen in air. Another important work of organisms had been to lower the concentration of carbon dioxide in the air.

It is nothing but coexistence of organisms and earth over enormous amount of time that created the present earth environment. There is no doubt that a large scaled extinction happened in the course of the creation and we are the descendants of winners that survived such drastic changes.

1st June 2003
Yoshiyuki Koide

Image - 1 Satellite picture of Shilin (ASTER/VNIR observed on 9th February 2002)
For enlargement (jpeg file 1.5MB)

This is the false color image of ASTER. ASTER band 3, 2 and 1 are allocated to red, green and blue, respectively. A big lake and a small lake are found on the westen side of the picture. Limestone rocks in Shilin are located to the east side of these lakes. Areas appearing in blight red and surrounding light blue indicate agricultural land and cities, respectively. Areas appearing in light blue except cities indicate bare limestone rocks.

Image - 2 Bird's eye view image of karst topography (ASTER/VNIR and DEM data observed on 9th February 2002)
For enlargement (jpeg file 0.2MB)

This is the bird's eye view image of karst topography in southeast area. Even if limestone rocks are covered with vegetation and others, the distribution of limestone rocks is identifiable due to its characteristic topography.

Image - 3 Satellite image of Guilin (ASTER/VNIR observed on 18th November 2001)
For enlargement (jpeg file 1.9MB)

This is the ASTER false color image of 60km x 180km. ASTER band 3, 2 and 1 are allocated to red, green and blue, respectively. Meandering line from south to north at the center of the image is Lijiang River. Areas appealing in light blue at the center is Guilin and at the southernmost is Yangshuo. Since vegetation appears in red, other areas in light blue along the river are considered to have reflected limestone characteristics.

Figure-1 Location map

Picture - 1 Shilin in Yunnan at south China

Sharp edges of thin and tall strange rocks shoot up toward the air. The strange appearance was formed by the act of rain and/or rivers. In the large karst of 350 square kilometers, Shilin Scenic Area 44 square kilometers exists. Nearly all of the area (43.9 square kilometers) has become a sightseeing spot with Stone Forests and lakes.

Picture - 2 Strange rocks in Shilin

Limestone rocks in Shilin was formed approximately 270 million years ago at the sea bottom. The rocks were pushed above ground due to the collision of Indian Continent and Eurasia Continent (Himalaya mountain making) started 4 million years ago, then moved to the present location approximately 2 million years ago. The present steepletop shaped cluster of strange rocks were formed from 200,000 to 10,000years ago by the erosion due to rains and rivers.

Picture - 3 Jiuxiang limestone cave

Land of limestone rocks exist in the east of Kunming, north of Shilin. Limestone rocks here in Jiuxiang Scenic Area exists underground where they were eroded and turned into intricate shapes. This picture shows a large-scaled one, with an elevator installed to go down to see the cave, where even an underground shop is built. Limestone caves in China is vividly illuminated.

Picture - 4 Guilin in Guangxi at China

This may be one of the typical images of China, commonly held by people all over the world. A boat sails down Lijiang River for 83km from Guilin to Yangshuo. The scenery may represent the image of "Great history of China", where nature and people used to cohabitate, and indeed, the scenery was formed over great span of time.

Picture - 5 Guilin

Beautiful scenery of Guilin, renowned worldwide is in fact made of limestone rocks. In Paleozoic (400 ~ 350million years ago), the rocks precipitated in sea bottom and rose above water due to upheavals in Jurassic (approx. 180million years ago). The present landscape had been formed by erosion since then.