Cave popcorn, or coralloids, are small nodes of calcite, aragonite or gypsum that form on surfaces in caves, especially limestone caves. [1] [2] They are a common type of speleothem. [1] [2]
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. [1] [2] The nodules tend to grow in clusters on bedrock or the sides of other speleothems. [1] These clusters may terminate suddenly in either an upward or downward direction, forming a stratographic layer. [1] When they terminate in a downward direction, they may appear as flat bottomed formations known as trays. [1]
Individual nodes of popcorn can assume a variety of shapes from round to flattened ear or button like shapes. [2]
The color of cave popcorn is usually white, but various other colors are possible depending on the composition. [2]
Formation
Cave popcorn can form by precipitation. [1] Water seeping through limestone walls or splashing onto them leaves deposits when CO2 loss causes its minerals to precipitate. [2] When formed in this way, the resultant nodules have the characteristics of small balls of flowstone. [1]
Cave popcorn can also form by evaporation in which case it is chalky and white like edible popcorn. [1] 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. [1]
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.[ citation needed]
Gallery
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Alisadr Cave, Hamedan, Iran -
Cave popcorn trays -
Diagram of dripstone cave structures (cave popcorn labelled Z) -
Button cave popcorn -
Calthemite coralloids under concrete, with soda straw
References
External links
