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The term Karst refers to geographic areas that display landforms produced by the dissolution
of surface rocks.
Typically these regions consist predominantly of limestone, a sedimentary rock made of calcite.
In limestone, calcite is usually in the form of microscopic crystals that are recrystallized
shells or exoskeletons of organisms such as corals that lived in the area at some time
in the past.
Often we will see fossils of these ancient organisms in the limestone, as the calcite
simply replaces the organic material and maintains the same shape.
The mineral calcite has the chemical formula CaCO3 and is also called calcium carbonate.
Calcite dissolves when in contact with acids.
And most groundwater is slightly acidic, and will dissolve limestone when in contact with
it over hundreds to thousands of years.
Limestone has no pore space as it’s made of intergrown crystals, but it does form cracks.
Water makes its way down those cracks and sits in them, slowly dissolving the calcite
over time and making the cracks bigger and bigger.
After tens of thousands of years, these cracks can turn into giant caverns and tunnels filled
with water.
When later the water table drops because the land lifted up or water was diverted elsewhere,
these caverns can empty.
Water that now drops down from the surface through holes or cracks in the roofs of the
caverns will precipitate calcite in icicle-like structures called stalactites.
Where the drops of water hit the floor below the stalactites, more calcite precipitates
and grows upward in cone-shaped stalagmites.
Stalactities grow from the ceiling.
Stalagmites grow from the floor.
And when the two meet in the middle we call it a column.
To learn more about these chemical reactions that precipitate the calcite, continue to
the end of this video.
Areas of the world that have limestone layers near the surface and lots of ground and surface
water will display an array of landforms associated with the chemical dissolution of the limestone
by the water, including caverns, sinkholes where the top of a cavern has collapsed, and
tunnels.
Often these structures are buried under thick layers of soil, and we can’t even see them.
Where we do see the limestone from below just peeking up on the surface with no top soil,
we call that the limestone pavement.
Where the water table has dropped and the caves underground are empty, we can enter
these caves and explore.
Sometimes the caves and tunnels contain streams that have sunk down from the surface and continued
their transit through the underground cave system until they spring out later from the
rock.
When the water table has dropped completely, and enough of the limestone has dissolved,
and all the cavern roofs collapsed, the landscape takes on a different look.
Steep-sided hills of limestone are scattered across the landscape as erosional remnants
of the dissolution of the thick limestone layers that once blanketed the surface.
We can call them erosional remnants.
Imagine this region was once a solid 100-meter thick layer of limestone, and now these hills
are all that’s left.
Karst regions can be found all over the world, on almost every continent, including Florida
and the southeast of the U.S. Other top locations include the Yucatan in Mexico, the Rock Islands
in Palau, the Burren in County Clare in Ireland, Madagascar, Chile, and many locations in China
and Southeast Asia including Vietnam, Indonesia, and Thailand.
The rest of this video takes a closer look at the chemistry of the dissolution and precipitation
of calcite and how it forms stalactites and stalagmites.
Natural surface waters are all slightly acidic because of the reaction of water and carbon
dioxide that produces carbonic acid: water + carbon dioxide produces carbonic acid
The double arrows between the ingredients on the left and the ingredients on the right
means that this equation finds an equilibrium, and that any changes to the relative amount
of the ingredients on either side of this reaction will cause the reaction to move in
the direction that will undo that change and re-establish equilibrium.
For example, the more carbon dioxide in the environment, the more that drives the reaction
to the right to remove the carbon dioxide and produce more carbonic acid.
When the solid mineral calcite gets in contact with carbonic acid, the bonds in the calcite
mineral are broken, turning it into calcium and bicarbonate ions that are now as dissolved
ions in water.
Water molecules are polar and have slightly positive and slightly negative sides or poles
that allows them to stick to ions.
An ion dissolved in water is actually surrounded by water molecules stuck to it on all sides
creating a hydration sphere.
These hydration spheres keep the ions separated from each other.
Here’s the chemical reaction for the dissolution of calcite:
calcite + carbonic acid produces calcium ion + bicarbonate ion
As long as there’s enough water to surround these ions by hydration spheres, the ions
will not get back together to form calcite.
When water first comes in contact with calcite, dissolution occurs, but eventually the water
becomes saturated with the calcium and bicarbonate ions.
The water molecules are surrounding to many of them and can’t hold anymore.
We say the water is saturated with dissolved calcium and bicarbonate.
Again, remember that we have a system in equilibrium.
So, when we’re at saturation point, if we later change any of the ingredients or conditions,
we can push the reaction in the opposite direction to undo that change.
Instead of dissolving calcite, we move to the left and the calcite mineral precipitates.
How does that happen?
When carbon dioxide gas decreases in water, going back to the first reaction, that means
there is less carbonic acid.
Less carbonic acid in this reaction will drive the equation to the left to produce more.
The ions will recombine to produce more calcite, and it will precipitate.
So the primary cause of calcite precipitation is actually the reduction of carbon dioxide
in the water that has the dissolved ions in it.
What makes carbon dioxide leave water?
What can you do to get bubbles of carbon dioxide coming out of your soda?
First thing you can do is open the cap and reduce the pressure on it.
Immediately you will see the carbon dioxide gas bubbling out.
This is exacty what happens when water exits a crack through a hole in the roof of an empty
cavern.
That water is coming from an environment where it was trapped in a crack in a rock under
high pressure.
Now it enters a cavern where the pressure’s much less.
You’ve just opened the soda can.
The carbon dioxide gas comes out of the water that’s come out of the crack, and that causes
calcite to precipitate.
Small crystals will form around the water droplet and add to the thickness and length
of the stalactite that’s forming.
When a drop of that water later hits the ground under the stalactite, more carbon dioxide
gas will leave the water and more calcite will precipitate and form the stalagmite.
It’s just like when you shake your soda can when it’s open.
That shake, or the agitation that’s produced when the water drops on the floor, will allow
more of carbon dioxide to leave and a little bit of calcite to precipitate.
Pause now.
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