Landforms by Groundwater: Karst, Sinkholes, Stalactites

Introduction

Groundwater plays a significant role in eroding landmasses and shaping landforms, especially in regions with calcium carbonate-rich rocks like limestone and dolomite. Key groundwater processes involve solution and deposition. Groundwater activity forms a distinctive landform in limestone regions known as Karst topography. Let’s discover how the actions of groundwater erosion and deposition mould the land

Erosional Landforms Formed by Groundwater

• Agents of Rock Dissolution: The process involves rainwater mixed with atmospheric carbon dioxide (CO2), acting as a solvent agent that disintegrates and dissolves carbonate rocks both on the surface and below. 
• Rate of Dissolution: It is influenced by groundwater temperature, the presence of joints in rocks, and the contact time of the solvent with carbonate rocks.
• Features of Karst Landscapes: Karst landscapes are characterised by rugged terrains with features like solution holes, ravines, gullies, clefts, narrow valleys, caves, stalagmites, and stalactites.
• Swallow Holes: small to medium-sized round to sub-rounded shallow depressions formed on the surface of limestones through solution.
• Sinkholes: These are depressions that are circular at the top and funnel-shaped at the bottom. Formed in 2 ways:
• • Solution Sinks are formed solely through the solution process, where limestone dissolves over time, creating depressions.
  • Collapse Sinks or Dolines: occur when the roof of an underground void or cave collapses, leaving a noticeable depression or hole on the surface.
• Uvalas or Valley Sinks: When sinkholes and dolines merge due to the slumping of materials or the collapse of cave roofs, they form long, narrow to wide trenches known as valley sinks or uvalas.
• Lapies or Ridges: These are irregular limestone surfaces characterised by a maze of points, grooves, and ridges. 
  • They develop due to differential solution activity along parallel or subparallel joints in the limestone.
• Caves or Caverns: Caves form in areas with alternating rock layers (like limestone sandwiched between other rock types). 
  • Some caves even have openings at both ends, earning them the name “tunnels.”
• Ponores: vertical pipe-like chasms or passages connecting caves and swallow holes formed through the downward extension of sinkholes via the continuous solution of carbonate rocks.
• Natural Bridges: These form in limestone areas due to cave roof collapses or the transformation of surface streams into subterranean streams, creating valleys below the ground surface.

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Depositional Landforms Formed by Groundwater

• Causes of Deposition: Deposition happens when there’s an obstruction in the groundwater’s flow path, water evaporates due to temperature changes, or there’s reduced solution capacity.
• Stalactites: They hang from the cave ceiling; broad at their base, and taper towards the free-hanging ends.
• Stalagmites: These formations rise from the cave floor, typically originating from dripping water from the cave ceiling.
• Pillar: Form when Stalactites and stalagmites fuse.
• Dripstones: Calcite deposits formed from dripping water in dry caves.
• Drapes or Curtains: Needle-shaped dripstones that hang from the cave ceiling.
• Helictites and Heligmites: Dripstones grow sideways from stalactites and stalagmites, respectively, with globular helictites called ‘globulites.’
• Flowstones: Deposits on the cave floor formed by seepage water and water flowing out of stalagmites.

Formation of Limestone Pavement and Grikes

• Limestones are well joined and it is through these joints and cracks that rain-water finds its way into the underlying rock. 
  • Progressive widening by solution enlarges these cracks into trenches and a most intriguing feature called limestone pavement is developed. 
  • The enlarged joints are called grikes and the isolated , rectangular blocks are termed clints.
• Unique Landforms of Chalk: The landforms of chalk are rather different from those of other limestones. 
  • There is little or no surface drainage and valleys which once contained rivers are now dry. These are often called coombes.

Conclusion

• Groundwater shapes the land in remarkable ways, especially in regions with calcium carbonate-rich rocks like limestone and dolomite. 
• Through processes of erosion and deposition, unique landforms such as karst landscapes, caves, sinkholes, and stalactites/stalagmites are formed. These formations offer valuable insights into the active processes happening beneath the surface.

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