Image showing the six most common
speleothems with labels.
A
stalactite (from the
Greek stalasso, (σταλάσσω), "to
drip" and meaning "that which drips") is a type of
speleothem (secondary mineral) that hangs from
the ceiling of
limestone
caves. It is a type of
dripstone.
Formation and type
Experiment modeling drip stone formation in the lab.
Stalactites are formed by the
deposition of
calcium carbonate and other minerals,
which is
precipitated from mineralized
water
solutions.
Limestone is calcium carbonate
rock which is
dissolve by
water that contains
carbon
dioxide, forming a
calcium
bicarbonate solution. The
chemical formula for this reaction
is:
- : + + →
This solution travels through the rock until it reaches an edge and
if this is on the roof of a
cave it will drip
down. When the solution comes into contact with
air the
chemical
reaction that created it is reversed and particles of calcium
carbonate are deposited. The reversed reaction is:
- : → + +
An average growth rate is 0.13
mm (0.005
inches) a year. The quickest growing stalactites are
those formed by fast-flowing
water rich in
calcium carbonate and carbon dioxide, these can grow at 3
mm (0.12
inches) per year.
Every stalactite begins with a single mineral-laden drop of water.
When the drop falls, it leaves behind the thinnest ring of calcite.
Each subsequent drop that forms and falls deposits another calcite
ring. Eventually, these rings form a very narrow (0.5 mm), hollow
tube commonly known as a "
soda straw"
stalactite. Soda straws can grow quite long, but are very fragile.
If they become plugged by debris, water begins flowing over the
outside, depositing more calcite and creating the more familiar
cone-shaped stalactite. The same water drops that fall from the tip
of a stalactite deposit more calcite on the floor below, eventually
resulting in a rounded or cone-shaped
stalagmite. Unlike stalactites, stalagmites never
start out as hollow "soda straws." Given enough time, these
formations can meet and fuse to create
columns.
Stalactites can also form in
lava tubes,
although the mechanism of formation is much different.
Concrete
Stalactites can also form on
concrete, and
on plumbing where there is a slow leak and limestone (or other
minerals) is in the water supply, although they form much more
rapidly there than in the natural cave environment (description and
experiments see literature).
The way stalactites form on concrete is due to different chemistry
than those that form naturally in limestone caves and is the result
of the presence of
calcium oxide in
concrete. This calcium oxide reacts with any rainwater that
penetrates the concrete and forms a
solution of
calcium
hydroxide. The
chemical formula
for this is:
- : + →
Over time this calcium hydroxide solution reaches the edge of the
concrete and, if the concrete is suspended in the air, for example,
in a ceiling or a beam, then this will drip down from the edge.
When this happens the solution comes into contact with
air and another
chemical
reaction takes place. The solution reacts with
carbon dioxide in the air and
precipitates calcium carbonate.
- : + → +
When this solution drops down it leaves behind particles of calcium
carbonate and over time these form into a stalactite. They are
normally a few centimeters long and with a
diameter of approximately half a centimeter.
Records
The White
Chamber in the Jeita
Grotto
's upper cavern in Lebanon
holds an 8.2
meters stalactite which is also accessible to visitors and is
claimed to be the longest stalactite in the world.
While it
has been claimed that the longest stalactite known hangs in the
Chamber of Rarities in the Gruta Rei
do Mato (Sete
Lagoas
, Minas
Gerais
, Brazil
) and is 20
metres long, vertical cavers have often
encountered longer stalactites while exploring. One of the longest
stalactites viewable by the general public is in Doolin Cave,
County
Clare
, Ireland
, in a
karst region known as The Burren
, what makes it more impressive is the fact that the
stalactite is held on by a section of calcite less than 0.3 square
meters.
Origin of the term
Stalactites are first mentioned (though not by name) by the
Roman natural historian
Pliny in a text which also mentions
stalagmites and columns and refers to their creation by the
dripping of water. The term "stalactite" was coined in the 17th
century by the Danish Physician
Ole Worm
who created the word from the
Greek
root
stalasso, (σταλάσσω), "to drip".
Photo gallery
Image:Mosses around Stalactite with Waterdrop.jpg|Water droplet
coming out of the central canal of a stalactite.Image:Doolin Cave
Stalactite.jpg|Stalactite in Doolin Cave, Ireland measuring 20ft
(6.54m) long.Image:Stalactite-drop.jpg|Mineralized water drop
forming at bottom of stalactites.
Image:1.p1430419.jpg|Stalactites of the
type called "soda straws" from the Choranche caves in the Vercors, France
See also
References
- Dripstone in time-lapse ("Tropfsteine im Zeitraffer")
- Schmidkonz, B.; Wittke, G.; Chemie Unserer Zeit, 2006,
40, 246.
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