Tiankeng, unexpected fall

This is the fallen earth

It is God's "pit"

Cracks and joints, erosion and dissolution

Let these soluble rocks

Extremely deep and mysterious

▲Chongqing Fengjie Tiankeng, Photo by Bian Yupeng

In the karst area, there are two worlds: one on the surface and one underground.

The surface karst landscape is obvious, with peaks, peak clusters, stone forests and rock pillars, which are like landscapes growing out of the ground, steep and beautiful. The underground karst dragon is visible only from the tail, with caves, underground rivers and cave sediments, which are more like another world hidden deep in the earth, mysterious and unpredictable.

Between the two worlds, there is a karst landscape, which is between the surface and the underground, and is a channel connecting the surface world and the underground world, like the navel of the earth. We call this kind of karst landscape the negative landform of the surface, and depressions, funnels, and shafts all belong to this category.

Among them, a special form of negative landform is the most eye-catching. Its walls are like wells and its hole is like a basin. It is a sudden collapse of the earth and an unexpected wonder of nature. This is the tiankeng.

▲Typical karst landform (drawn by Yang Jinshan)

01

Tiankeng: The special contribution of Chinese scientists

Before 2001, "tiankeng" was just a special name for the Xiaozhai tiankeng in Fengjie County, Chongqing. Similar landforms had different names in different places, such as "longgang", "shiyuan", "shiwei", "yanwan", "tianpen", etc.

They are often regarded as a special form of shafts and funnels in karst landforms, that is, "huge funnels in the shape of shafts" or "huge shafts in the shape of funnels". But gradually, geologists found that it was inappropriate to call the same landform both a funnel and a shaft.

In 2001, karstologist Zhu Xuewen first proposed separating tiankeng from large funnels or shafts as a special karst landform term.

In 2005, after the International Karst Tiankeng Survey Team conducted a large-scale survey in Chongqing and Guangxi, the term "tiankeng" gained unanimous recognition in the international karst academic community and began to be used internationally in the Chinese pinyin "tiankeng".

This is the third karst landform term defined by the Chinese and named in Chinese and pinyin, following fenglin (peak forest) and fengcong (peak cluster).

▲The development stage of the tiankeng

02

What kind of pit is called a tiankeng?

According to the definition in the Tourism Geology Dictionary: Tiankeng refers to a special type of extra-large collapse landform formed by karst action. Its width and depth should be greater than 100 meters, and the width-to-depth ratio is very close.

According to the definition of Baidu Entry: Tiankeng refers to an extremely large karst negative landform with extraordinary spatial and morphological characteristics such as huge volume, steep and enclosed rock walls, deep well-shaped or barrel-shaped contours, etc., developed in a soluble rock layer with extremely thick thickness and extremely deep groundwater level, with an average width and depth of more than 100 meters, and the bottom is connected to an underground river (or there is evidence that the underground river has migrated).

Therefore, we can extract the following information about the tiankeng:

■ Tiankeng is a natural landscape formed by karst action. Therefore, giant pits formed by human action cannot be called tiankengs, such as mine pits. In addition, meteorite craters formed by meteorites hitting the earth cannot be called tiankengs because they are not formed by karst action.

■ Only a certain size of negative landform can be considered a tiankeng. This size generally means that the width and depth must be greater than 100 meters, otherwise it can only be called a shaft or funnel.

■In terms of shape, a tiankeng is called a tiankeng only if the pit walls are steep and it is in the shape of a deep well or barrel.

▲Three stages of the formation of the tiankeng (drawn by Yang Jinshan)

03

The Blue Hole is a sinkhole in the ocean

Blue holes are huge deep holes in the ocean.

From a bird's eye view, the blue hole is a circular area of ​​deep blue water that suddenly appears on the ocean surface. It looks like the pupil of the sea, deep, mysterious and strange.

Most of the blue holes on Earth are formed by limestone. They are formed when the cave systems on the surface were flooded by sea water. Therefore, blue holes should be considered sinkholes, sinkholes in the ocean.

▲The Great Blue Hole in Belize is actually a sinkhole in the sea (from the Internet)

04

How much do you know about tiankengs in my country?

After the concept of tiankeng was proposed, it triggered a wave of widespread tiankeng discovery and research. It is as thrilling and exciting as "Grave Robbers' Chronicles", making people unable to stop.

As of 2017, a total of 34 tiankeng groups have been discovered in the world, with a total of about 250 tiankengs. On the global tiankeng distribution map, China stands out for its large number, large scale and concentrated distribution.

More than 100 tiankengs have been discovered in China, especially in the karst development area in the south. Nine of the top ten tiankengs in the world are in China.

With the advancement of technology, people's means of finding sinkholes have also made great progress, especially with the popularization of Google Maps and drones, which have made it impossible to hide a large number of sinkholes hidden in the mountains and dense forests.

New tiankengs are constantly being discovered, such as the Hanzhong tiankeng group in 2016, the Napo tiankeng group in Guangxi in 2019, and the discovery of dozens of tiankengs in Hubei between 2019 and 2020, which has continuously refreshed the number of tiankengs in my country.

▲Distribution map of soluble rocks and provinces with large tiankengs in my country.

05

Typical tiankeng in China

Fengjie Xiaozhai Tiankeng

Before 2001, when people mentioned the word "tiankeng", they usually only referred to Xiaozhai tiankeng in Fengjie County, Chongqing.

The diameter of the Xiaozhai Tiankeng is 622 meters, the diameter of the bottom is 522 meters, and the depth is 666.2 meters. The volume is 120 million cubic meters. It is the world's largest karst tiankeng in terms of depth and volume, and ranks first in the world's tiankeng rankings. It is another wonder at 30 degrees north latitude.

Xiaozhai Tiankeng has a double-layer structure, with the first layer about 300 meters deep and the second layer about 400 meters deep, with a gently sloping terrace in between. It is obvious that Xiaozhai Tiankeng was formed after two periods of uplift and two large-scale collapses.

▲Fengjie Tiankeng and its plan and section drawings. Photo by Bian Yupeng

Leye Tiankeng Group

The Leye Tiankeng Group is a rare geological wonder in the world that was discovered by the Ministry of Land and Resources in 1998 during a land resources survey in Leye County, Baise City, Guangxi Zhuang Autonomous Region, China.

It consists of nearly 20 tiankengs, the deepest of which is over 600 meters deep and the shallowest is over 300 meters deep. The Leye tiankeng group includes almost all types of tiankengs and karst caves, and has a very high value for scientific research and exploration. It is called the "tiankeng museum" and "world karst holy land" by experts.

The largest of these, the Dashiwei Tiankeng, has a vertical depth of 613 meters, is 600 meters long from east to west, 420 meters wide from north to south, and has a volume of about 80 million cubic meters. Its underground forest area ranks first in the world, second in depth, and third in volume.

▲Leye Tiankeng. Image from the Internet

Wulong Houping Tiankeng Group

The Houping tiankeng group is located in Houping Township, Wulong County, and is an erosion-type tiankeng.

The formation of erosion-type tiankengs requires specific and special geological structures, rock distribution relationships, topographical and hydrogeological conditions, so current discoveries are very rare.

The occurrence of individual erosional tiankengs may not be uncommon, but the Houping tiankeng, which is a complete karst system composed of sinkholes, vertical shafts, tiankengs, and cave subsystems at different stages of tiankeng development, is extremely rare.

The Houping tiankeng group is located on the karst platform in the watershed area at an altitude of 1,300m. It consists of five tiankengs, namely Jingkou, Shiwangdong, Tianpingmiao, Daluotang and Niubidong. It is the only tiankeng group discovered in my country and even in the world that was caused by surface water erosion and is an extremely rare geological relic.

▲ Houping Tiankeng. Image from the Internet

Xiaoyanwan Tiankeng, Xingwen, Sichuan

Located 30 kilometers south of Xingwen County, Yibin City, southern Sichuan, Xiaoyanwan Tiankeng, commonly known as the "Heaven Basin", is 625 meters long from east to west and 475 meters wide from north to south. Its mouth area is 200,000 square meters and its maximum depth is 248 meters.

Climbing to the top of the cliff on the eastern edge of the basin, the shape of the tiankeng is clearly visible. It is like a deep well sunk into the earth, with the upper rock wall exposed and smooth like a wall, and the middle and lower parts are covered with deposits and covered with dark blue vegetation, as if facing the abyss.

Walking along the path on the inner wall of the cenote, you will see steep cliffs all around, and your vision is limited by the reddish-brown rock walls, with the blue sky like a basin. Going down to the bottom of the cenote is like looking at the sky from the bottom of a well, with the blue sky like a hole, which is even more amazing.

▲Sketch of Xiaoyanwan Tiankeng and its structure. Photo by Yang Jian

Yunyang Longgang Tiankeng

It is located in Qingshui Town, Yunyang County, on the northwest wing of the Qiyaoshan anticline, with exposed strata being limestone and dolomite of the Jialingjiang Formation of the Lower Triassic.

The Longgang Tiankeng is an irregular ellipse in plane, with a major axis distance of 304-326 meters, a minor axis distance of 178-183 meters, and a depth of 335 meters.

Its depth ranks third in my country, fifth in the world, its diameter ranks eighth in the world, and its volume ranks ninth in the world. The wall of Longgang Tiankeng is nearly 90 degrees, and this straight up and down shape is even more "astonishing to the world". It is known as "the best tiankeng in the world".

▲Cross-section of Longgang Tiankeng.

Hanzhong Tiankeng Group

The Hanzhong tiankeng group is located in the Qinba Mountain area, mainly distributed in four areas: Chanjiayan Town, Ningqiang County, Xiaonanhai Town, Nanzheng County, Luojiaba Town, Xixiang County, and Sanyuan Town, Zhenba County.

Within 5,019 square kilometers, 54 tiankengs, 316 karst caves, 23 shafts, 25 funnels, and 109 other geological relics were discovered. Among them, the Nanzheng and Zhenba tiankeng groups are the most densely populated, reaching 23 and 19 respectively; the largest single tiankeng is the Zhenba Sanyuanquanziya tiankeng, with a maximum diameter of 520 meters and a maximum depth of 320 meters.

Although most of the dozens of tiankengs in the Hanzhong tiankeng group do not meet the conditions of tiankengs in terms of scale and morphological characteristics, and it is difficult for all 54 tiankengs to be recognized by experts, considering that the tiankeng group is located near 33 degrees north latitude, the northernmost boundary of the humid tropical and subtropical karst landform area, its discovery still surprised many geologists and its value should not be underestimated.

▲Hanzhong Tiankeng. Photo by Li Zhongdong

Qijiehe Tiankeng Group in Zhijindong, Guizhou

Zhijin Cave is not only a famous karst cave, but also has a cenote. The cenote is located in the Jiehe Gorge in the upper reaches of the Wujiang River. It was formed by the uplift of the land mass and the river cutting down the dissolved rock mass due to the influence of the neotectonic movement.

Among them, the largest and most complete ones are the Daluoquan Tiankeng and the Xiaoluoquan Tiankeng. Both Tiankengs are surrounded by vertical cliffs, with a flat bottom and clay accumulation of varying thickness.

▲Qijiehe Tiankeng in Zhijin Cave. Photo by Yang Jian

Sichuan Leshan Yandouyan Tiankeng

Located in Shawan District, Leshan City, Sichuan Province, adjacent to the Dadu River, this tiankeng was developed in the carbonate rock strata of the middle and upper Permian system more than 200 million years ago. It is generally distributed in the northwest-southeast direction. The north wall of the tiankeng is very steep and almost vertical, and the south end is mainly colluvial accumulation. Because the cliff is steep and high, the locals vividly call it the xiaodou (steep) rock.

In terms of appearance, the mouth of the Xiaodouyan Tiankeng is oval in shape, with a long axis of about 110 meters and a short axis of about 65 meters. The deepest part of the Tiankeng is 400 meters deep.

▲Xiaodouyan Tiankeng. Photo by Zhou Jianjun