Speleothem

Image showing the six most common speleothems with labels. Enlarge to view labels.

A speleothem (from the Greek for "cave deposit") is a secondary mineral deposit formed in caves. It is the formal term for what is also known as a cave formation, or amongst cavers sometimes known as decorations or pretties.

1 Speleothems in limestone and dolostone caves
- 1.1 Chemistry
- 1.2 As climate proxies
2 Other speleothems
3 Gallery
4 External links
5 Categories

Speleothems in limestone and dolostone caves

Water seeping through cracks in a cave's surrounding bedrock may dissolve certain compounds, usually calcite and aragonite (both calcium carbonate), or gypsum (calcium sulfate), depending on the amount of carbon dioxide held in solution, temperature, and other factors. Upon reaching an air-filled cave, a discharge of carbon dioxide may alter the water's ability to hold these minerals in solution, causing its solutes to precipitate. Over time, which may span tens of thousands of years, the accumulation of these precipitates may form speleothems.

Speleothems take various forms, depending on whether the water drips, seeps, condenses, flows, or ponds. Many speleothems are named for their resemblance to man-made or natural objects. Types of speleothems include:

stalactites are sharply or bluntly pointed pendants hanging from the cave ceiling, sometimes resembling carrots;
stalagmites are bluntly pointed mounds on the cave floor, often beneath stalactites;
columns result when stalactites and stalagmites meet;
flowstone is found on cave floors and walls;
soda straws;
helictites are stalactites that have a central canal with twig-like or spiral projections that appear to defy gravity;
rimstone dams, or gours, occur at stream ripples and form barriers that contain water;
chandeliers
popcorn;
cave pearls are the result of water dripping from high above, causing small "seed" crystals to turn over so often that they form into near-perfect spheres of calcium carbonate;
dogtooth spar are large calcite crystals often found near seasonal pools;
and many more.

Speleothems made of pure calcite are a translucent white color, but often speleothems are colored by minerals such as iron, copper or manganese, or may be brown because of mud and silt particulate inclusions.

Chemistry

Most cave chemistry revolves around calcite; CaCO₃, the primary mineral in limestone. It is a slightly soluble mineral whose solubility increases with the introduction of carbon dioxide, CO₂. It is paradoxical in that its solubility decreases as the temperature increases, unlike the vast majority of dissolved solids. This decrease is due to interactions with the carbon dioxide, whose solubility is diminished by elevated temperatures; as the carbon dioxide is released, the calcium carbonate is precipitated.

Most other solution caves that are not composed of limestone or dolostone are composed of gypsum (calcium sulfate), the solubility of which is positively correlated with temperature.

As climate proxies

Samples can be taken from speleothems to be used like ice cores as a proxy record of past climate changes [1]. A particular strength of speleothems in this regard is their unique ability to be accurately dated over much of the late Quaternary period using the uranium-thorium dating technique. Stalagmites are particularly useful for palaeoclimate applictions because of their relatively simple geometry and because they contain several different climate records, such as oxygen and carbon isotopes and trace cations. These can provide clues to past precipitation, temperature, and vegetation changes over the last ~ 500,000 years.

Other speleothems

Speleothems may also occur in lava tubes. Although sometimes similar in appearance to speleothems in solutional caves, these are formed by the cooling of residual lava within the lava tube.

Speleothems formed from salt, sulphur and other minerals are also known.

Formations within caves that are created from the removal of bedrock (rather than as secondary deposits) are called speleogens. These include pillars, scallops and boxwork.