Sinkholes form when the ground surface collapses into an empty space below. They matter because they can damage roads, buildings, water pipes, and entire neighborhoods with little warning. Many sinkholes occur in areas where soluble rock, such as limestone, gypsum, or salt, lies beneath soil and sediment.
Water is the main driver because it can slowly dissolve underground rock and weaken the support holding the surface up.
The process often begins when slightly acidic rainwater seeps into cracks and pores in the ground. Over time, this water enlarges underground openings, forming caves or cavities that may be hidden beneath a stable-looking surface. If the roof of a cavity becomes too thin or heavy loads press down from above, the surface can sag or suddenly collapse.
Human activities such as leaking pipes, groundwater pumping, construction, and poor drainage can speed up sinkhole formation.
Key Facts
- Sinkholes commonly form in karst landscapes where limestone, gypsum, or salt can dissolve in groundwater.
- Carbonic acid forms when CO2 mixes with rainwater: CO2 + H2O = H2CO3.
- Limestone dissolves when acidic water reacts with calcium carbonate: CaCO3 + H2CO3 = Ca2+ + 2HCO3-.
- A collapse sinkhole forms when the roof of an underground cavity can no longer support the weight above it.
- Risk increases when groundwater level drops because water pressure no longer helps support the cavity roof.
- Average subsidence rate can be estimated by rate = change in height / time.
Vocabulary
- Sinkhole
- A sinkhole is a depression or hole in the ground caused by the collapse or sinking of surface material into an underground void.
- Karst
- Karst is a type of landscape formed when groundwater dissolves soluble rock, creating caves, springs, and sinkholes.
- Groundwater
- Groundwater is water stored beneath Earth's surface in soil pores, sediment, and cracks in rock.
- Subsidence
- Subsidence is the gradual sinking or settling of land over time.
- Soluble rock
- Soluble rock is rock that can dissolve in water or weak acid, such as limestone, gypsum, or salt.
Common Mistakes to Avoid
- Assuming all sinkholes are sudden collapses is wrong because many begin as slow subsidence that may show warning signs like cracks, sagging soil, or tilted pavement.
- Blaming only earthquakes for sinkholes is wrong because most sinkholes are caused by groundwater dissolving rock or by changes in underground water and soil support.
- Ignoring leaking pipes is wrong because leaking water can wash away soil and enlarge underground cavities, especially beneath roads and buildings.
- Thinking a small surface hole shows the full danger is wrong because the underground cavity may be much larger than the opening visible at the surface.
Practice Questions
- 1 A section of ground sinks 18 cm over 6 years. What is the average subsidence rate in cm per year?
- 2 A circular sinkhole has a diameter of 12 m. Estimate its surface area using A = pi r^2 and pi = 3.14.
- 3 A neighborhood is built on limestone and has several broken stormwater pipes. Explain why this combination increases the chance of sinkholes forming.