Cave popcorn

File:Popcorn-with-Frostwork.jpg

Cave popcorn with frostwork

Cave popcorn, or coralloids, are small nodes of calcite, aragonite, or gypsum that form on surfaces in caves, especially limestone caves.<ref name=palmer>Template:Cite book</ref><ref name=hill>Template:Cite book</ref> They are a common type of speleothem.<ref name=palmer/><ref name=hill/>

Appearance

The individual nodules of cave popcorn range in size from 5 to 20 mm and may be decorated by other speleothems, especially aragonite needles or frostwork.<ref name=palmer/><ref name=hill/> The nodules tend to grow in clusters on bedrock or the sides of other speleothems.<ref name=palmer/> These clusters may terminate suddenly in either an upward or downward direction, forming a stratographic layer.<ref name=palmer/> When they terminate in a downward direction, they may appear as flat-bottomed formations known as trays.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=palmer/>

Individual nodes of popcorn can assume a variety of shapes from round to flattened ear-like or button-like shapes.<ref name=hill/>

The color of cave popcorn is usually white, but various other colors are possible depending on the composition.<ref name=hill/>

Formation

Cave popcorn can form by precipitation.<ref name=palmer/> Water seeping through limestone walls or splashing onto them leaves deposits when CO₂ loss causes its minerals to precipitate.<ref name=hill/> When formed in this way, the resultant nodules have the characteristics of small balls of flowstone.<ref name=palmer/>

Cave popcorn can also form by evaporation in which case it is chalky and white like edible popcorn.<ref name=palmer/> In the right conditions, evaporative cave popcorn may grow on the windward side of the surface to which it is attached or appear on the edges of projecting surfaces.<ref name=palmer/>

On manmade structures (outside the cave environment)

Popcorn can also occur on concrete structures outside the cave environment; these are classified as calthemite coralloids. Calthemite coralloids also occur in "artificial caves", such as mines, railways, or vehicle tunnels where there is a source of lime, mortar, or cement from which the calcium ions can be leached.

Coralloids can form by a number of different methods in caves; however, the most common form on concrete is created when a hyperalkaline solution seeps from fine cracks. Due to solution evaporation, deposition of calcium carbonate occurs before any drop can form. The resulting coralloids are small and chalky with a cauliflower appearance.Template:Citation needed

Gallery

• Alisadr Cave, Hamedan, Iran.JPG

  Alisadr Cave, Hamedan, Iran

• Cave popcorn trays.jpg

  Cave popcorn trays

• Tropfsteine.svg

  Diagram of dripstone cave structures (cave popcorn labelled AH)

• Button cave popcorn.jpg

  Button cave popcorn

• Calthemite coralloids.jpg

  Calthemite coralloids under concrete, with soda straw

External links

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• Kids' page on popcorn, Underground Adventures (archived)
• National Park Service page on popcorn