Cave and Spring Deposits in Karst Topography

• Cave and spring deposits are the depositional side of karst: calcium carbonate (or silica) that groundwater dissolved at one place is precipitated again inside caves or where water emerges at the surface.
  • Inside caves they form speleothems (stalactites, stalagmites, flowstone); at springs they form travertine, tufa and sinter; within soils the same chemistry builds kankar (calcrete).
• They are the constructive counterpart of the solutional karst landforms.
• Their layered growth makes them archives of past climate; a stalagmite from Mawmluh Cave, Meghalaya, defines the base of the current geological age.

Caves: The Setting for Deposition

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Solution and Speleogenesis

• Caves or caverns are large subsurface voids, the most significant landform of groundwater erosion (chiefly solution, aided by abrasion by underground streams) in soluble rock.
  • Largest in pure, massive, thickly bedded and well-jointed limestone and dolomite.
• Chemistry: rain and soil water absorb CO₂ to form carbonic acid, which converts calcium carbonate into soluble calcium bicarbonate: CaCO₃ + H₂O + CO₂ ⇌ Ca(HCO₃)₂.
  • Water widens bedding planes, joints and fractures into conduits; linked conduits grow into chambers.
  • Rate depends on CO₂ content of the water, water throughput and the purity and jointing of the rock.
• Speleogenesis is the overall process of cave origin and enlargement; horizontal passages linking chambers are galleries.
  • Vertical or inclined shafts linking sink holes to caves are ponors (Serbian) or avens (French).

Phreatic and Vadose Caves

• A cave often begins phreatic and becomes vadose when uplift or valley incision lowers the water table.
  • Only then is it air-filled, and only air-filled caves can grow dripstone.

Theories of Cave Origin

• Cave origin is the most debated question in karst geomorphology; theories differ over where (above, at or below the water table) and how (solution or abrasion) caves form.

• Two-cycle mechanism (Davis):
  • First cycle: slow, all-round phreatic solution below the water table.
  • Second cycle: uplift lowers the water table; the cave turns vadose and dry, and speleothems begin to form.
  • B. C. Moneymaker (1941) took a middle view: caves begin below the water table but develop above it.
• Current view: Derek Ford and Ralph Ewers (1978) reconciled the debate in a four-state model.
  • Fissure frequency decides the outcome: few fissures give deep-looping phreatic caves; many fissures give ideal water-table caves.
  • Most large caves develop at or near the water table, with vadose trenching later.
  • Some caves are hypogene, dissolved from below by sulphuric acid from rising H₂S-rich water; Carlsbad Caverns, USA, is the classic case.
• Kentucky’s Mammoth Cave is the world’s longest known cave system; Carlsbad Caverns is known for its vast chambers.

Caves of India

• Guptadham shows structural control: it is dissolved in Vindhyan (Rohtas) limestone beneath a protective capping of quartzitic sandstone.
  • Its horizontal galleries open into amphitheatre-like halls where tunnels cross.
• Meghalaya holds India’s densest cave systems: thick, well-jointed limestone meets extreme monsoon rainfall.
  • Krem Puri, also in Meghalaya, is among the world’s longest sandstone caves.
  • In August 2025, Mawmluh, Arwah, Mawsmai and Krem Puri were placed on UNESCO’s Tentative List as the Meghalayan Age Caves.

Cave Deposits (Speleothems)

The Chemistry of Deposition

• Speleothems (Greek spelaion, cave + thema, deposit) are secondary mineral deposits formed in air-filled caves, mostly of calcite, occasionally aragonite.
• Degassing, not evaporation, is the main trigger:
  • Seepage water carries soil CO₂ at high partial pressure; cave air holds far less.
  • On entering the cave, water loses CO₂; the reaction runs in reverse and calcite precipitates.
  • Evaporation matters only near cave entrances and in dry, ventilated passages.
• Deposition is thus the mirror image of the solution that made the cave.
  • All dripping-water deposits are collectively called dripstone.

Speleothems vs Speleogens

• Speleothems are added rock: new mineral precipitated in the cave.
• Speleogens are removed rock: forms sculpted into the bedrock by differential solution, with no new deposit.
  • Examples are scallops, wall flutes, pockets and pendants; they form more readily where bedrock is chemically non-uniform.
  • Scallop size is inversely related to past flow velocity, so speleogens record former cave streams.

Major Speleothem Types

Dripstones

• Stalactites: icicle-like pendants hanging from the ceiling, broad at the base and tapering downward.
  • Each drop leaves a thin ring of calcite around a central feeding canal, so they grow in length and girth.
• Soda straws: thin, hollow tubes, the first stage of a stalactite before the canal clogs and water runs down the outside.
• Stalagmites: mounds or columns rising from the floor beneath drips.
  • Drops splash on impact, so stalagmites are thicker, blunter and rounder-topped than stalactites.
  • Several closely spaced drip centres build compound stalagmites.
• Columns (pillars): a stalactite and stalagmite meet and fuse, linking floor and ceiling.
• Ceiling geometry controls form:
  • Flat or evenly arched ceilings give uniform, vertical stalactites.
  • Inclined ceilings let water run along the surface, giving elongated, inclined forms that grade into draperies.

Flowing-Water Forms

• Flowstone: sheet-like, layered coatings on walls and floors where water flows as a thin film, including seepage from stalagmites.
• Draperies (curtains): thin, wavy sheets hanging from inclined ceilings; colour banding gives “cave bacon”.
• Rimstone dams (gours): calcite builds up on the lip of flowing water, damming stepped pools.

Erratic and Pool Forms

• Helictites: twisted, branching growths that defy gravity.
  • Water moves by capillary action through very fine canals, and crystal growth, not dripping, sets direction.
  • Heligmites are their floor-based equivalents; globulites are globular helictites.
• Cave pearls: concentrically layered spheres in shallow, agitated pools.
  • Dripping water keeps a nucleus (often a sand grain) rolling, so calcite coats it evenly.
• Moonmilk: a soft, white, paste-like deposit, unlike hard crystalline forms, linked to fine precipitation and microbial activity.

Speleothems as Climate Archives

• Stalagmites grow in annual to decadal layers and can be precisely dated by uranium–thorium (U–Th) methods.
  • Their oxygen isotope ratios track changes in rainfall and monsoon strength; carbon isotopes track vegetation.
• India’s key contribution: a Mawmluh Cave stalagmite records a 200-year megadrought about 4,200 years ago.
  • In June 2018 it was ratified as the GSSP of the Meghalayan Age, the youngest of three ages of the Holocene (after the Greenlandian and Northgrippian).
  • The same drought is linked to the decline of Indus (Harappan), Mesopotamian and Yangtze valley societies.
  • The International Union of Geological Sciences listed Mawmluh as a geological heritage site in 2022.
• Nearby limestone quarrying for cement is a conservation concern for Meghalaya’s caves.

Spring Deposits

• Water emerging at springs is often saturated with dissolved minerals, mostly calcium carbonate, sometimes silica.
• At the surface it loses CO₂, cools or evaporates, and can no longer hold its load, so minerals precipitate around and downstream of the outlet.

Travertine, Tufa and Sinter

• Travertine: precipitates rapidly where water degasses fast, at hot springs or over cascades.
  • Successive layers build stepped terraces; banded travertine also forms in caves.
• Tufa: forms around plant stems, mosses and algal mats, which act as nuclei, leaving a vuggy, cellular texture.
  • Softer tufa also accumulates at cave mouths, where evaporation helps.
  • Mono Lake towers grew underwater at spring inlets and were exposed as the lake fell.
• Sinter: the silica counterpart of travertine, from geothermal water circulating through volcanic rock.
  • It lines and seals geyser plumbing, helping pressure build.

Kankar (Calcrete)

• Hard, nodular to massive CaCO₃ that forms within soil or loose sediment by the same chemistry.
  • Carbonate-rich water rises and evaporates in dry seasons, cementing grains into nodules and later a continuous hardpan.
• Best developed in semi-arid climates with a marked dry season.
  • Widespread in the older alluvium (bhangar) of the Indo-Gangetic plain and in Rajasthan; traditionally quarried for lime and road metal.

Geodes

• Hollow or partly hollow rock masses whose cavities are lined with inward-growing crystals (quartz, chalcedony, calcite).
  • The cavity is a gas bubble in lava or a solution void in limestone or shale; mineral-rich groundwater lines it slowly.

Hot Springs and Geysers

• Hot springs discharge heated groundwater continuously; geysers are hot springs that spout water and steam intermittently.
  • The word comes from Geysir, a spring in Iceland; the Icelandic word means “gusher”.
  • They belong to the waning, minor phase of vulcanicity, with fumaroles.

Mechanism

• A geyser needs three conditions:
  • Magmatic heat close to the surface.
  • Abundant groundwater.
  • Constricted plumbing (the geyser tube), often sealed by sinter.
• Water deep in the tube is superheated under the weight of the column above.
  • When some flashes to steam, pressure drops, the rest boils explosively, and the column erupts; the tube then refills.

Types

• Intermittent geysers are also grouped by eruption length: long-period geysers play for up to an hour; feeble geysers only briefly.

Distribution

• Yellowstone (USA) holds about half the world’s geysers across several basins, such as Norris and the Upper and Lower Geyser Basins.
• Other major fields:
  • Valley of Geysers, Kamchatka (Russia), the second-largest concentration.
  • El Tatio (Chile).
  • Taupō Volcanic Zone, New Zealand, with Whakarewarewa near Rotorua.
  • Haukadalur, Iceland, home of Geysir and Strokkur.
• India has hot springs rather than major geyser fields:
  • Manikaran (Parvati valley, Himachal Pradesh), Tattapani (Chhattisgarh), Bakreshwar (West Bengal) and Rajgir (Bihar).
  • Puga valley (Ladakh) carries borax and sulphur encrustations around its vents.
  • At Puga, ONGC has completed two wells for India’s first demonstration geothermal power project; the ONGC–Ladakh collaboration was renewed for five years in 2026.