Sinkhole
Also called: Doline, Swallow hole, Collapse sink · Topic: Landforms and their Evolution: Rivers, Groundwater, Glaciers, Waves and Wind · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 6 "Landforms and their Evolution"
Meaning
A sinkhole is a hollow in the ground in limestone areas. It is roughly circular at the top and narrows like a funnel towards the bottom. Rainwater picks up carbon dioxide and becomes weak carbonic acid. This acid slowly dissolves the limestone along cracks. Surface water often drains into the sinkhole and keeps flowing as an underground stream, so karst areas (limestone landscapes shaped by dissolving rock) have very few surface streams. Sinkholes range from a few square metres to a hectare in area, and from less than half a metre to more than 30 metres in depth.
There are two kinds:
- Solution sinks form when rock dissolves from the surface downwards. They are the more common kind.
- Collapse sinks, or dolines, form when the roof of an underground cave falls in.
Example
The Karst region of the Balkans, beside the Adriatic Sea, is full of sinkholes. The term "karst topography" comes from this region.
Don't confuse with
- Land subsidence: the ground sinks for other reasons, not because limestone dissolves. Joshimath (Uttarakhand) in 2023 was subsidence, not a karst sinkhole.
- Uvala: a long trench formed when several sinkholes join. A sinkhole is a single hollow.