Underground precipitation features

Precipitation features

There are many underground features that are created by precipitation of minerals:

Stalactites

Sword of Damocles stalactite

Stalactites grow where water droplets do not drop from exactly the same place all the time. They are formed of precipitated calcite and hang down from the ceiling of limestone caves. Although some may have originated as straws, stalactites are much thicker and lack the hollow tube.

Examples of stalactites can be seen in:

  • South Wales — Dan-yr-Ogof
  • Derbyshire — Peak, Speedwell, Treak Cliff and Blue John caverns (all near Castleton) and Poole's Cavern (Buxton)
  • Somerset — Gough's Cave (Cheddar) and Wookey Hole (near Wells)
  • Yorkshire — Stump Cross Cavern (near Grassington), White Scar Caves (Ingleton) and Ingleborough Cave (near Clapham)

How do stalactites form?

  1. Percolating water arrives on the ceiling of a cave at the end of a joint.
  2. The water flows across the cave roof for a short distance and calcite is deposited in a wide area around the end of the joint.
  3. A droplet forms on the cave ceiling and drops to the floor.
  4. More water seeps into the cave, flows over the ceiling precipitating a thin layer of calcite on top of that previously deposited. Again, it forms a droplet that falls to the floor.
  5. The calcite gradually accumulates in a solid mass (rather than as a long, thin straw).
  6. The percolating water flows down the sides of the developing stalactite, precipitating calcite all the time. The stalactite forms a conical structure, widest where it is attached to the ceiling where most calcite is precipitated, and tapering downwards.

Very large stalactites can build up in this way (some in France are 30 m long). In time they may become so heavy that they fall from the ceiling. Each layer of calcite covers the previous one so that when you look at the end of a broken stalactite, it is made of concentric rings — a little like tree rings.

Stalagmites

Stalagmites

Stalagmites are columns of calcite growing up from the cave floor, often immediately below a stalactite.

Like stalactites, stalagmites can be seen in:

  • South Wales — Dan-yr-Ogof
  • Derbyshire — Peak, Speedwell, Treak Cliff and Blue John caverns (all near Castleton) and Poole's Cavern (Buxton)
  • Somerset — Gough's Cave (Cheddar) and Wookey Hole (near Wells)
  • Yorkshire — Stump Cross Cavern (near Grassington), White Scar Caves (Ingleton) and Ingleborough Cave (near Clapham)

How do stalagmites form?

  1. The water drips down from the end of a straw or larger stalactite and splashes on the cave floor.
  2. The droplet still contains calcium carbonate, which is then precipitated onto the cave floor, below the stalactite.
  3. The next droplet falls to the cave floor and lands in approximately the same place as the previous one.
  4. A little more calcite is released on top of that previous precipitated.
  5. An irregular mound starts to form on the cave floor.
  6. The mound gradually grows vertically upwards as each droplet lands on its top.

Straws

Straws

Straws are a type of stalactite, hanging down from the ceiling of a cave. They form where water drips from exactly the same place all the time, in some cases for thousands of years, perhaps at the end of a joint. Examples of straws can be seen in the Easter Grotto of the Ease Gill system.

How do straws form?

  1. Mineralised water seeps down through a joint and forms a droplet.
  2. This droplet partially evaporates, depositing a thin ring of calcite perhaps about four to six millimetres in diameter.
  3. The next water droplet then forms in exactly the same place and a thin ring of calcite is deposited on top of the first.
  4. Gradually a hollow tube develops with a wall about one millimetre thick.
  5. The water moves down the hollow centre of the tube and forms a droplet at the end. Only here does the water comes into contact with the air in the cave, and so only here is the thin ring of calcite precipitated.
  6. It takes about five years to grow one millimetre and some of the longest known are about six metres long (at least 30 000 years in the making). However, they are extremely fragile and rarely reach that length.
  7. Straws usually block up with calcite and the water then flows down the outside where it deposits more calcite. This builds up year upon year, thickening to form a stalactite.

Pillars

Column in Crackpot Cave

Pillars are columns of calcite that extend from the cave floor to the ceiling.

How do pillars form?

  1. The water that drips from the end of a stalactite splashes onto the top of the stalagmite growing below.
  2. Calcite is deposited both at the end of the stalactite and on the top of the stalagmite.
  3. Gradually the space between the two is narrowed.
  4. The stalactite and stalagmite eventually unite and grow together.
  5. If the water drips rapidly, more calcite will be deposited on the cave floor than the ceiling, so that the stalagmite will be longer than the stalactite. Where the water drips slowly, more calcite is precipitated on the ceiling than the floor and the stalactite will be the longer than the stalagmite.

Flowstone, curtains and others

Little Man helictite

There are a number of different other features that develop by precipitation, but they are essentially variations of the stalactite and stalagmite.

  • curtains or shawls are created when water trickles down a wall
  • flowstone forms when thin sheets of water deposit calcite over wider areas and on the cave floor
  • rimstone dams form on the floor of the cave and hold back pools of water
  • shields comprise two parallel plates of calcite growing out from the cave wall where water is forced out of joints or the bedding planes under hydrostatic pressure
  • helictites appear to defy gravity, forming elongate growths of calcite with a smooth surface and sometimes growing spirally or changing direction of growth. This is apparently the a result of changes in the direction the calcite crystal grows. This is may be due to water being subjected to hydrostatic pressure or drafts of air blowing across a speleothem causing preferential deposition of calcite