A stalactite, "to drip", and meaning "that which drips") is a type of formation that
hangs from the ceiling of caves, hot springs, or manmade structures such as bridges and
mines. Any material which is soluble, can be deposited as a colloid, or is in suspension,
or is capable of being melted, may form a stalactite. Stalactites may be composed of
amberat, lava, minerals, mud, peat, pitch, sand, and sinter. A stalactite is not necessarily
a speleothem, though speleothems are the most common form of stalactite because of the abundance
of limestone caves. The corresponding formation on the floor of
the cave is known as a stalagmite.
Formation and type
Limestone stalactites The most common speleothems are stalactites
and occur in limestone caves. They form through deposition of calcium carbonate and other
minerals, which is precipitated from mineralized water solutions. Limestone is the chief form
of calcium carbonate rock which is dissolved by water that contains carbon dioxide, forming
a calcium bicarbonate solution in underground caverns. The chemical formula for this reaction
is:
CaCO(s) 3 + H
2O (l) + CO(aq)
2 → Ca(HCO 3)(aq)
2
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:
Ca(HCO 3)(aq)
2 → CaCO(s) 3 + H
2O (l) + CO(aq)
2
An average growth rate is 0.13 mm 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 per year. All limestone stalactites begin with a single
mineral-laden drop of water. When the drop falls, it deposits the thinnest ring of calcite.
Each subsequent drop that forms and falls deposits another calcite ring. Eventually,
these rings form a very narrow, 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 of calcium carbonate.
Lava stalactites Another type of stalactite is formed in lava
tubes while lava is still active inside. The mechanism of formation is similar to that
of limestone stalactites. Essentially, it is still the deposition of material on the
ceilings of caves, however with lava stalactites formation happens very quickly in only a matter
of hours, days, or weeks, whereas limestone stalactites may take up to thousands of years.
A key difference with lava stalactites is that once the lava has ceased flowing, so
too will the stalactites cease to grow. This means that if the stalactite were to be broken
it would never grow back. The generic term lavacicle has been applied
to lava stalactites and stalagmites indiscriminately and evolved from the word icicle.
Like limestone stalactites, they can leave lava drips on the floor that turn into lava
stalagmites and may eventually fuse with the corresponding stalactite to form a column.
Shark tooth stalactites The shark tooth stalactite is broad and tapering in appearance. It may
begin as a small driblet of lava from a semi-solid ceiling, but then grows by accreting layers
as successive flows of lava rise and fall in the lava tube, coating and recoating the
stalactite with more material. They can vary from a few millimeters to over a meter in
length.
Splash stalactites As lava flows through a tube, material will be splashed up on the
ceiling and ooze back down, hardening into a stalactite. This type of formation results
in a very irregularly shaped stalactite, looking somewhat like stretched taffy. Often they
may be of a different color than the original lava that formed the cave.
Tubular lava stalactites When the roof of a lava tube is cooling, a skin will form that
traps semi-molten material inside. Trapped gases force lava to extrude out through small
openings that result in hollow, tubular stalactites analogous to the soda straws formed as depositional
speleothems in solution caves, The longest known is almost 2 meters in length. These
are common in Hawaiian lava tubes and are often associated with a drip stalagmite that
forms below as material is carried through the tubular stalactite and piles up on the
floor beneath. Sometimes the tubular form collapses near the distal end, most likely
when the pressure of escaping gases decreased and still-molten portions of the stalactites
deflated and cooled. Often these tubular stalactites will acquire a twisted, vermiform appearance
as bits of lava crystallize and force the flow in different directions. These tubular
lava helictites may also be influenced by air currents through a tube and point downwind.
Ice stalactites A common stalactite found seasonally or year
round in many caves is the ice stalactite, commonly referred to as icicles, especially
on the surface. Water seepage from the surface will penetrate into a cave and if temperatures
are below freezing the water will form stalactites. Creation may also be done by the freezing
of water vapor. Similar to lava stalactites, ice stalactites form very quickly within hours
or days. Unlike lava stalactites however, they may grow back as long as water and temperatures
are suitable. Ice stalactites can also form under sea ice
when saline water is introduced to ocean water. These specific stalactites are referred to
as brinicles. Ice stalactites may also form corresponding
stalagmites below them and given time may grow together to form an ice column.
Concrete stalactites
Stalactites can also form on concrete, and on plumbing where there is a slow leak and
limestone in the water supply, although they form much more rapidly there than in the natural
cave environment. 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:
CaO (s) + H
2O (l) → Ca(OH)(aq)
2
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.
Ca(OH)(aq) 2 + CO(g)
2 → CaCO(s) 3 + H
2O (l)
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 5 mm. Records
The White Chamber in the Jeita Grotto's upper cavern in Lebanon contains an 8.2 m limestone
stalactite which is accessible to visitors and is claimed to be the longest stalactite
in the world. Another such claim is made for a 20 m limestone stalactite that hangs in
the Chamber of Rarities in the Gruta Rei do Mato. However, vertical cavers have often
encountered longer stalactites while exploring. One of the longest stalactites viewable by
the general public is in Pol an Ionain, 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 m2. Origin of the term
Stalactites are first mentioned 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 coined the Latin word from the Greek word σταλακτός and the Greek suffix -ίτης
Photo gallery
See also Stalagmite
Lavacicle Rusticle
Karst Icicle
Bottlebrush - Stalactite coated with pool spar.
Brinicle References
Dripstone in time-lapse - Schmidkonz, B.; Wittke, G.; Chemie Unserer Zeit, 2006, 40,
246. doi:10.1002/ciuz.200600370 External links
The Virtual Cave's page on stalactites "Stalactites" by Enrique Zeleny, Wolfram Demonstrations
Project.