In June 2010, a sinkhole, estimated to be 60 feet wide and 300 feet deep, appears overnight at Guatemala city. Sinkholes cause thousands if not millions of dollars in damage each year to urban development and infrastructure. What is the exact science behind these hell holes?
A sinkhole is a natural depression or hole in the Earth’s surface. The surface of the earth is made up of different layers of soil, also known as overburden, on top of the bedrock. Sinkholes can be found worldwide but occur mostly in areas where the bedrock is porous and formed from soft materials and rocks. Soft bedrock is more susceptible to cracking, breaking and dissolving. There must be a opening in the bedrock surface. The sizes of sinkhole vary from 1 to 600 meters both in diameter and depth. Its formation could be gradual or sudden depending on the process, which could be categorised into three major types; dissolution, cover collapse and cover subsidence.
Dissolution sinkholes usually occur on land with a very thin layer of overburden or exposed bedrock. During the entire process of water falling from the atmosphere, percolating the ground and reacting with living and decaying plant matter, carbon dioxide is dissolved from the air and soil. This results in a weak carbonic acid. The acidic water will slowly dissolve the rock, usually at the surface. Joints, fractures or other openings in the bedrock retain the acidic water thus accelerating the dissolution.
Over time, the exposed limestone that is dissolved will be carried away and caused cavities. If there is a thin layer of overburden, sediments from the overburden will fill the cavities below them. This results in a gradual downward movement of the land surface causing usually a bowl-shaped depression.
Cover Collapse Sinkholes
Cover collapse sinkholes usually occur on land with a soft overburden. For a sinkhole to form, there must be an opening in the bedrock surface. Overburden will fill up the fracture. In the bedrock, there will be water passage, the underground basins, also known as recharge areas. Through the fracture, the water flow will remove the overburden, causing an empty area of space, known as void. Sediments from the overburden will starts spilling through the void – a process known as spalling, to the recharge area. Due to the movement of the water, the sediments in the recharge area are washed away. As this process continues, the void at the overburden will become bigger. Eventually, the overburden becomes so thin, it couldn’t support the weight of the overburden, so it collapses, creating a sinkhole.
Alternatively, water could be supporting the thin overburden. When the water level falls or there is a lack of water, creating a void, the overburden will has no support, causing it to collapse.
Cover Subsidence Sinkholes
Cover subsidence sinkholes usually occur on land with an overburden that is permeable, e.g. sand. Acidic water permeates through the overburden and dissolves the bedrock, leaving a void. Sediments from the overburden will start settling into the void – a process known as piping. This results in gradual downward erosion. Unlike the dissolution sinkholes, the overburden will cover the eroding rock, filling up the fracture.
Though sinkholes could be categorised into three distinct types, the actual formation can be a combination of different types or may form in several phases.
Human activities could also cause the formation of sinkholes. Examples are drilling, mining, changes in weight on the surface, heavy increase in water flow, formation of a pond or body of water, or broken pipes. Through these activities, the weight load of the overburden is increased. Mining, vibrations in the ground and increase in water bodies weaken the soil and bedrock.
Sinkholes create stunning beauty on Earth, yet at the same time it could cause great inconvenience. Through these acts of nature and human through their advance technology, movement is created, causing the formation of a sinkhole. We could only stand in awe at the magic of the Earth but no one will be able to combat it.