The Earth Story

Original caption:

Moon Hill

Moon Hill is a mountain with a natural arch just outside of Yangshuo in southern China. It is named for a wide almost perfect semicircular hole in the hill. The hole is approximately 50 meters wide and 50 meters tall. It is all that remains of what was once a limestone cave.

The area is known for its karst formations and is made up of compact carbonate rocks which became uplifted and transformed by erosion. The rocks started out as sea floor sediments and were pushed up during the collision of India with Asia. Afterwards, the warm and wet Monsoon climate of southern China created the well-known karst formations we can see today. The naturally-occurring acid rains of Yangshuo’s subtropical climate caused the dissolution of limestone and caused caves and sinkholes to form.

Moon Hill has a concrete path passing through the arch and several rock climbing routes. It also offers great panoramic views of the surrounding knobby karst hills found throughout the region.

Xandi Image Credits: http://bit.ly/2mXUOxS http://bit.ly/2mFLe0A Sources: http://bit.ly/2n4xVdJ http://bit.ly/2n4iRN9

Kayaking through the amazing Karst Landscapes in Yangshuo, Gulin, Southeastern China

Boat ride up the Li River in Yangshuo China, where erosion of a multi-kilometer thick unit of limestone has created one of the world’s best Karst Landscapes, with limestone pillars sticking out and collapsed cave structures in-between.

Li River

The landscape of China’s Yangshuo County, in the southeastern part of the country, is perhaps the most spectacular example of a karst terrain anywhere in the world.

In the Devonian and Carboniferous, about 400-300 million years ago, the pieces of crust that would eventually become China were detached from the larger continental blocks that were assembling to form the supercontinent Pangaea. Much of the Earth’s landmass sat in the southern hemisphere, including much of the Gondwana supercontinent, allowing the formation of large glaciers on that continent (http://tmblr.co/Zyv2Js1_8Ry38).

Laurentia, which would eventually form the core of North America, and the blocks that would eventually become China, sat north of the Gondwana supercontinent, bounding a large seaway in-between (the proto-Tethys seaway). These landmasses sat in the tropics, allowing formation of large limestone and coal deposits.

The Southern China block formed some of the largest sequences of carbonate rocks found anywhere on Earth. Nearly 10,000 meters of limestone cover this block of crust, recording over 100 million years of Earth’s history, from the middle of the Paleozoic into the Mesozoic. Because this crustal block was detached from the major continental collisions that occurred during this time, these sedimentary units are thick and formed with limited faulting and folding.

Eventually, these detached crustal blocks were attached to the growing landmass that would become Asia and, after the collision with India and other landmasses to the south, the limestones were pushed up above the ocean waters, exposing them to rain.

Limestone is made of calcium carbonate minerals, with other elements occasionally included. When these minerals are exposed to acidic water, they begin to slowly dissolve. Because the Earth’s atmosphere has carbon dioxide in it, most fresh rainwater is slightly acidic; the carbon dioxide reacts with water in rain droplets to form carbonic acid.

When limestone is pushed upwards in rainy areas, this chemical reaction causes the rock to begin dissolving. Water flowing across the surface concentrates in low areas and those low areas become the focus of additional dissolution, lowering them even more. These low areas first form dolines (sinkholes) or caves, which in turn allows even more water to flow through them, dissolving even more limestone.

Typically, as the flowing water widens caves and channels, it undercuts the surrounding hills and they start to thin or wash away. However, in this area of southern China, slow uplift associated with the gigantic continental collisions has continued gradually pushing the land upwards.

As the land is pushed upwards, the high ground is gradually cut off from the chemical weathering happening at its base. The flowing water shapes the hills into sharp spires as it flows around the edges, but little dissolution happens at the hilltops. Over time, the landscape thus develops into this spectacular marvel of high hills surrounded by deeply incised lowlands. Because the limestones are so thick and contain few structures that would otherwise influence the erosion, the landscape just cuts down due to the interaction with water.

This area, home to the Li River (seen here) and several other similarly spectacular landscapes, is a national park and has been listed as a tentative UNESCO World Heritage site. China is currently considering proposals from nearby communities to expand the preserved national park to cover larger areas of this spectacular karst landscape.

-JBB

Image credit: Tyler Sprague https://flic.kr/p/BSdkvm

References: http://whc.unesco.org/en/tentativelists/108/ http://www.scotese.com/newpage3.htm http://bit.ly/1U0a7iu http://karst.iah.org/karst_hydrogeology.html http://nyti.ms/1FcRXYl http://www.lunaguava.com/painted-veils-yangshuo-county/ http://bit.ly/1QCiiDP http://bit.ly/1mwpyEA