I know, or think I do, the basic idea behind these natural structures, as from what I know they form due to water dripping from the top leaving a salt deposit behind. But started to wonder about the principles involved here.
My question is mainly in stalactites, what makes the salt left behind in the process to remain attached to the overlaying rock and keep growing instead of falling down? is there some physical-chemical explanation for this?
Stalactites do fall, which I would guess maybe due to the weight of the column surpassing a certain limit, which makes me believe there maybe something related to the electrostatic attraction of the hydration sphere of the salts/minerals left behind and the rock minerals. Am I even close?
Dear Dr. Denis Rodrigues ,
speleothems, sometimes referred to as formations or decorations, are cave features formed by the deposition of minerals. The word speleothem is derived from the Greek words spelaion meaning "cave" and thema meaning "deposit". The speleothems with which most people are familiar are stalactites and stalagmites. Stalactites grow down from the cave ceiling, while stalagmites grow up from the cave floor. It’s easy to remember which is which: Stalactites have a "T" for top and stalagmites have a "G" for ground.
Speleothems actually form because of water. Rainwater seeps through cracks in the rock. As it passes through organic material, it picks up carbon dioxide gas, creating carbonic acid. This weak acid passes through joints and cracks in limestone. The mineral calcite is dissolved from the limestone rock in which a cave is formed. When this water that now holds the dissolved rock is exposed to the air in the cave, it releases the carbon dioxide gas, much like when a bottle of soda is opened. As the carbon dioxide is released, calcite is precipitated (redeposited) on cave walls, ceilings and floors. As the redeposited minerals build up after countless water drops, a stalactite is formed. If the water that drops to the floor of the cave still has some dissolved calcite in it, it can deposit more dissolved calcite there, forming a stalagmite.
Speleothems form at varying rates as calcite crystals build up. Several factors can determine the rate of growth. Two important factors are the temperature outside (which affects the rate of decay of plants and animals, hence the amount of carbon dioxide in the soil), and the amount of rainfall. The shapes of speleothems are determined by how the acidic water enters the cave (by dripping, seeping, or splashing) and how the water stands or flows after entering the cave.
Most scientists believe that the color of speleothems are determined by the mineral content. Pure calcite is white and almost colorless. Iron and other minerals, as well as acids from surface vegetation, combine with calcite crystals to add shades of red, orange and black to the color of speleothems. Others believe that humic and fulvic acids in the soil may also contribute to speleothem coloration.
For more details, please see the source:
-How Stalactites and Stalagmites Form – Ozark National Scenic Riveways, Missouri
Available at: https://www.nps.gov/ozar/learn/education/speleothems.htm
My best regards, Pierluigi Traverso.
Water percolating through the calcareous hanging walls of a cave or generally spoken an open space in the host rock get loaden with bivalent Ca and carbon dioxide while disolving the carbonate minerals on moving downward according to the laws of gravity. When crossing the boundary between the hanging wall and the open space a release in pressure and a slight increase in temperature may cause the value of the solubility product to get exceeded and the solution becomes oversaturated so that stalactites come into existence by the dripping water flow from above. At the opposite side stalagmites begin growing from the footwall of the cave following the same physical laws and the same water flow in the same direction top down.
This process can be applied to all kinds of Ca carbonate (aragonite, Mg clalcite and calcite) as well as some Fe carbonates) all of which are poorly crystallized in the statu nascendi.
In (abandoned) underground mines you will also find similar features made up of sulfates of the melanterite-goslarite s.s.s. (Fe-Zn) which as a function of the wetness or dryness in the adits accommodate more or less water molecules into their lattice. They are man-made and called post-mining mineralization. The uptake of water is reversible and the stalactites and stalagmites become unattractive as to their outward appearrance due to the change in the blue, green or bown mineral shades when the water flow diminishes for whatever reason.
H.G.Dill
Dear Dr. Denis Rodrigues ,
speleothems, sometimes referred to as formations or decorations, are cave features formed by the deposition of minerals. The word speleothem is derived from the Greek words spelaion meaning "cave" and thema meaning "deposit". The speleothems with which most people are familiar are stalactites and stalagmites. Stalactites grow down from the cave ceiling, while stalagmites grow up from the cave floor. It’s easy to remember which is which: Stalactites have a "T" for top and stalagmites have a "G" for ground.
Speleothems actually form because of water. Rainwater seeps through cracks in the rock. As it passes through organic material, it picks up carbon dioxide gas, creating carbonic acid. This weak acid passes through joints and cracks in limestone. The mineral calcite is dissolved from the limestone rock in which a cave is formed. When this water that now holds the dissolved rock is exposed to the air in the cave, it releases the carbon dioxide gas, much like when a bottle of soda is opened. As the carbon dioxide is released, calcite is precipitated (redeposited) on cave walls, ceilings and floors. As the redeposited minerals build up after countless water drops, a stalactite is formed. If the water that drops to the floor of the cave still has some dissolved calcite in it, it can deposit more dissolved calcite there, forming a stalagmite.
Speleothems form at varying rates as calcite crystals build up. Several factors can determine the rate of growth. Two important factors are the temperature outside (which affects the rate of decay of plants and animals, hence the amount of carbon dioxide in the soil), and the amount of rainfall. The shapes of speleothems are determined by how the acidic water enters the cave (by dripping, seeping, or splashing) and how the water stands or flows after entering the cave.
Most scientists believe that the color of speleothems are determined by the mineral content. Pure calcite is white and almost colorless. Iron and other minerals, as well as acids from surface vegetation, combine with calcite crystals to add shades of red, orange and black to the color of speleothems. Others believe that humic and fulvic acids in the soil may also contribute to speleothem coloration.
For more details, please see the source:
-How Stalactites and Stalagmites Form – Ozark National Scenic Riveways, Missouri
Available at: https://www.nps.gov/ozar/learn/education/speleothems.htm
My best regards, Pierluigi Traverso.
Basically, rain water dissolves some of the limestone, then flows down inside the rock and when it reaches a small opening some of the water evaporates living the dissolved material behind to "grow" on the rock around it.
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