Cave Chemistry
The Making of a Cave Formation
Cave Minerals
Caves are first created through processes of weathering and erosion. After a cave forms, it often has a variety of minerals deposited, or dropped off, throughout the cave. These mineral deposits become speleothems, also known as cave formations. Most of these deposits are made of calcium carbonate that precipitates from mildly acidic water (carbonic acid). This calcium carbonate makes up the limestone bedrock in which the cave is located.
When the acidic water (carbonic acid) seeps downward from the surface, it dissolves the limestone bedrock, becoming saturated (containing the largest amount possible) with calcium. As the acidic water enters the cave on the ceiling or along the walls, it enters an air-filled empty space. Since the space in the cave has much less carbon dioxide than the acid, the droplet of carbonic acid will begin to lose its carbon dioxide into the air just like soda pop loses carbon dioxide once it is opened. Just as an open soda pop will become “flat” over time from lost carbon dioxide, the liquid in the cave loses more and more of its acidity (acid). This loss of acid reduces the amount of dissolved calcium the droplet can hold in solution. Because the liquid cannot hold as much calcium, the calcium precipitates as crystals of calcite. In other words, the calcium solution creates solid calcite crystals. As more and more crystals are made, a speleothem is formed. Over time, the speleothem grows larger as more calcite crystals precipitate and are deposited on the formation. (see flow chart below)
Passage Terminology
* Mineral - a natural solid substance that has a definite chemical composition and physical structure
* Weathering - the process that breaks rocks into smaller pieces, weathering can be mechanical or chemical
* Erosion - removes and moves the weathered materials
* Bedrock – the bottom layer of soil, made mostly of solid rock
* Precipitates - a precipitate is the insoluble solid created as a result of a liquid solution. (Insoluble means it will not dissolve) Explanation: When two soluble substances react in a solution, the reaction produces an insoluble product. The insoluble product that separates from the liquid is called the "precipitate".
* Limestone -a sedimentary rock made from the mineral calcite, which came from the beds of evaporated seas and lakes and from sea animal shells.
FLOWCHART
How a Speleothem is Formed
1.
carbonic acid drips into the ground
2.
the acid dissolves the limestone bedrock
3.
bedrock becomes full of the calcium solution
4.
carbonic acid condenses on the cave ceiling and wall
5.
the acidic water loses carbon dioxide in the empty air
space of the cave
6.
As the calcium solution drips along walls and ceilings,
solids (precipitates) are left behind
7.
the precipitates (calcite
crystals) come together to form cave formations called speleothems
8.
the process continues over many years causing the
speleothem to grow
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