The science behind giant sinkholes - What in the World podcast, BBC World Service

Key Concepts:

• Sinkholes: Depressions in the ground caused by the collapse of underlying rock.
• Limestone and Chalk: Types of rock susceptible to dissolution by acid.
• Carbonic Acid: A weak acid formed when carbon dioxide dissolves in rainwater.
• Abstraction: The removal of water from the ground, leading to a lowered water table.
• Tiankengs: Exceptionally large sinkholes, typically found in China, characterized by their size (over 100m across or deep) and rivers at the bottom.
• Karst: A landscape formed from the dissolution of soluble rocks such as limestone.

What is a Sinkhole?

A sinkhole is essentially a depression or dip in the ground surface, varying in size from shallow to very large. The underlying cause is the collapse of the rock beneath the surface. This often occurs due to geological reasons involving the dissolution of rocks like limestone or chalk by carbonic acid, which forms when carbon dioxide from the atmosphere dissolves in rainwater. This acidic water percolates through the rock, gradually dissolving it. "It's effectively a depression, a little dip in the surface."

Causes of Sinkholes

1. Natural Causes:
   • Dissolution of soluble rocks (limestone, chalk) by carbonic acid. The acid dissolves the rock over time, weakening it and eventually leading to collapse.
   • Floods and heavy rains can exacerbate the process by flushing out softer rock types like sand, causing a collapse.
2. Human Activities:
   • Abstraction: Excessive removal of water lowers the water table, reducing support for the overlying rock and causing it to collapse. This is a growing problem in drier regions.
   • Mining Activities: Historical mining operations, particularly in areas like the UK with old mineshafts, can create underground voids that eventually collapse, leading to sinkholes. Often, these old mineshafts are unknown, and building on top of them can cause collapses. "Sometimes what can happen is historically, if we've dug for a lot of things... we don't always know that they're there, and then we start building on top of them and then it eventually collapses."

Predicting Sinkholes

• Geological mapping is crucial, especially before construction. Civil engineers assess the risk of sinkholes by examining the underlying geology and looking for surface movements.
• However, sinkholes can be difficult to predict when limestone or chalk is covered by another layer of rock, as the dissolution process may be hidden until a sudden collapse occurs.

Worst-Case Scenarios and Impacts

• Sinkholes can be dangerous and even fatal, particularly when buildings collapse into them. "You know, we have had cases of people dying."
• Collapses in urban areas can damage underground infrastructure like electricity wires and water pipes, leading to floods and power outages.
• One example cited is a building collapse in Guatemala City over ten years ago, where the corner of a street disappeared as a building fell into a sinkhole.

Repairing Sinkholes

• In urban areas, sinkholes are often backfilled with sand and cement to solidify the area. However, surrounding areas need to be checked for further risks, especially if mining activity is suspected.
• In rural areas, a common method involves digging down to the bottom of the depression, reinforcing it with concrete, and then filling the hole.

Geographic Distribution of Sinkholes

• Sinkholes are more common in areas with sedimentary rocks susceptible to dissolution and in regions with historical mining activities.
• Regions with infrastructure, geology, and extreme weather are particularly at risk.
• Examples include Wales (UK), Central and South America, and China.

Tiankengs in China

• Tiankengs are massive sinkholes, exceeding 100 meters in depth or width, often with a river at the bottom.
• Devra Willingham from the National Cave and Karst Research Institute explains that tiankengs are distinct from typical sinkholes because of their size and the presence of unique ecosystems. These are found in very wet areas with monsoon seasons. "So you can have sinkholes that are small enough that your leg can fit in, but you can have sinkholes like the tiankengs, which are anything over 100m across or deep."
• Accessing tiankengs often requires technical caving skills and can involve hours of travel through caves.
• Tiankengs can harbor rare species and unique plant life, such as a rare type of Ginkgo.

Sinkholes and Climate Change

• Studies suggest that climate change may be increasing the prevalence of sinkholes.
• More intense rainfall can accelerate the dissolution process or flush out softer rock.
• Drier conditions can cause rocks to crack and weaken, increasing their susceptibility to dissolution.
• Increased demand for water in drought-stricken areas leads to greater abstraction, further destabilizing the ground. "So that means we're getting more intense rainfall over a shorter period of time. If we're getting more intense rain, then potentially we could be dissolving more of that rock or flushing more of it out."

The Scientific Value of Sinkholes

• Sinkholes offer a unique opportunity to study the subsurface of the Earth.
• They can reveal old mineshafts, fossils, and previously unknown ecosystems.
• Authorities may delay filling sinkholes to investigate potential discoveries.

Conclusion

Sinkholes are a complex geological phenomenon with both natural and human-induced causes. While they can be dangerous and destructive, they also offer valuable insights into the Earth's subsurface and can harbor unique ecosystems. Climate change appears to be exacerbating the problem, highlighting the need for careful geological mapping and sustainable water management practices. They’re almost a natural phenomenon that shows you what happens beneath the surface of the Earth.