How are Speleothems formed?
< Speleothems, sometimes referred to as formations or decorations, are
cave features formed by mineral deposit. >
<
<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 cracks in dolomite.
The mineral calcite is dissolved from the 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 (re-deposited) on
cave walls, ceilings and floors.
As the re-deposited 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. >
>
<
>
Other Interesting Formations
< Most scientists believe that the
colour of speleothems is determined
by mineral content.
Pure calcite is white and almost
colourless.
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 colour of speleothems.
Others believe that acids in the soil
may also contribute to the colouring
of the speleothems. >
< Straw formations:
Remember that all the water moving into
the cave carries carbonic acid. Just like
sugar dissolves in water, the mineral
calcite is dissolved out of the rock by the
acidic water and carried down into the
cave.
<
When the water drips down into the cave,
it deposits calcite on the ceiling. Each
drop deposits a little ring of calcite,
building a long hollow tube.
These are called soda straws, because
they look like the straws through which you
sip a soda. >
>
< This happens very slowly, often taking 100 years for a straw to grow only
25 mm . Once one is broken, it takes a long time to grow back.
A soda straw is the beginning of a stalactite. "Stalactite" has a "c" in it, and a stalactite is a
calcite deposit that sticks tightly to the ceiling.
A soda straw becomes a stalactite when the inside of the tube becomes blocked. Unable to
run down the inside, water runs down the outside, depositing calcite and thickening the
soda straw.
The stalactite continues to grow as long as the water keeps dripping. >
< Curtains/Draperies:
A calcite formation in the form of a wavy
or folded sheet hanging from the roof or
wall of a cave.
If the ceiling is nearly horizontal, the water
will form a stalactite but if the angle of the
ceiling is about 45 degrees, the water will
not fall down, but will run along the wall. It
will always use the same path. >
< While the water runs down the wall, it loses carbon dioxide to the atmosphere of the cave
and as a result deposits calcite in a thin vertical line on the wall. This line grows and very
soon there is a small rim.
Once there is a small rim, the water has no possibility to leave this path. It always follows this
path like a train follows its tracks.
As the curtain growth is a very thin line-as thin as a drop running down the wall-it normally is
very thin and since it consists of small calcite crystals, it is translucent >
<
>
Rimstone Pools:
These tend to form when there is a flow of water down a slope. A very thin layer of water on a
big area has a large surface.
Because of this large surface, the water loses carbon dioxide to the atmosphere of the cave
and as a result, calcite is
re-deposited.
The flowing water has waves on its surface, caused by irregularities of the rock surface.
These little waves of water form the calcite into arcs or curves that look like sand dunes
caused by wind.
If a rim is formed in this way, the rim holds a pool of water.
>