Define ‘speleothem’. Discuss the various forms and features of speleothems. (2022)
- A speleothem (Greek spēlaion, “cave”, and thema, “deposit”) is a secondary mineral deposit formed in a cave by precipitation from percolating water — all the deposits of a cavern taken together. Most are calcite; some are aragonite, gypsum or opal.
- They are the depositional counterpart of karst solution: the same groundwater that dissolves limestone above the cave rebuilds it, grain by grain, inside the cave.
How Speleothems Form
- Dissolution above: rain takes up CO₂ from the air and, far more, from soil, forming weak carbonic acid that dissolves limestone along joints and bedding planes.
- Precipitation below: the cave air holds less CO₂ than the seepage water, so the water degasses on reaching the cave and calcite comes out of solution: CaCO₃ + H₂O + CO₂ ⇌ Ca²⁺ + 2HCO₃⁻, run in reverse. Evaporation helps only near cave mouths and in dry caves.
- The master control is how the water arrives — dripping, seeping, flowing or ponding — so the forms are classified by water supply.
Forms of Speleothems
Dripstones (Fed by Dripping Water)
- Soda straws — thin, hollow calcite tubes, one drop wide, the first stage of a stalactite.
- Stalactites — icicle-shaped pendants hanging from the ceiling; they form when a straw’s central canal blocks and water runs down the outside.
- Stalagmites — the floor counterparts beneath a drip; broader and blunter, often flat-topped, because the falling drop splashes and spreads.
- Columns (cave pillars) — formed where a stalactite and stalagmite meet.
Flowstones (Fed by Flowing Films of Water)
- Flowstone — sheet-like, layered calcite coating walls and floors where water flows as a film.
- Draperies or curtains — thin, wavy sheets hanging from a sloping ceiling, built by water trickling along a line; banded ones are called cave bacon.
Pool Deposits (Formed in Standing Water)
- Rimstone dams (gours) — terraced rims that build up where water overflows the edge of a pool, damming a staircase of pools.
- Cave pearls — spheres of concentric calcite around a grain kept turning by drips falling into a shallow pool.
Erratic Forms
- Helictites — twisted, branching growths that sprout sideways or upward from ceilings and walls; water seeps through a tiny central canal, and capillary force and crystal growth, not gravity, set their direction.
- Heligmites — the corresponding sideways growths rising from the floor.
- Anthodites and cave popcorn — needle clusters of aragonite and knobbly calcite crusts.
Related Carbonate Deposits
- Travertine — dense, banded calcium carbonate; tufa — the softer, porous deposit at cave mouths and springs.
Features of Speleothems
- Growth layers: calcite is laid down in thin laminae, sometimes annual, like tree rings.
- Slow growth: an average stalactite grows about 0.13 mm a year, rarely up to about 3 mm — a 1 m column can represent many thousands of years.
- Colour: pure calcite is white or translucent; iron oxides, manganese and organic acids from the soil tint it orange, brown or black.
- Shape follows water: drip rate, flow direction, air currents and humidity decide whether a deposit becomes a straw, a cone, a sheet or a helictite.
- Fragility: growth stops if drips are diverted, and even skin oils alter surfaces — the reason show-caves restrict touching.
- Datable archive: uranium–thorium dating fixes ages over several hundred thousand years, and oxygen isotopes record past rainfall, making speleothems prime palaeoclimate archives.
Indian Occurrences and Current Research
- Borra Caves, Ananthagiri hills of the Araku Valley in Andhra Pradesh, hold stalactites up to about 3.5 m long and a 6 m column.
- Belum Caves (Nandyal district, formerly Kurnool, Andhra Pradesh) and Kotumsar Cave (Kanger Valley, Chhattisgarh) are well-known decorated caves; Krem Liat Prah in Meghalaya is the longest natural cave in South Asia.
- In 2018 the International Commission on Stratigraphy ratified the Meghalayan Age (4,200 years ago to the present), with its boundary defined in a stalagmite from Mawmluh Cave, Meghalaya — the youngest unit of the geological time scale rests on an Indian speleothem.
- Ashish Sinha, Gayatri Kathayat and Hai Cheng (2015) reconstructed two millennia of monsoon rainfall from a Sahiya Cave stalagmite (Uttarakhand): the worst drought interval centred near AD 1620, and the recent drying trend lies within, though at the lower edge of, the monsoon’s natural variability — a finding that tempers claims that today’s monsoon decline is unprecedented.
- Judgement: speleothems are more than cave ornaments — each form records exactly how water reached that spot, and their dated layers now settle questions about past monsoons and even about how geological time itself is divided.