Limestone caves form due to the chemical weathering of limestone bedrock caused by natural acid present in groundwater and rainwater.
As rain falls to the ground, it dissolves carbon dioxide in the air to form weak carbonic acid. This acidity causes solution or chemical weathering to limestone bedrock which eventually creates a limestone cave over a long time of gradual weathering.
There is also carbon dioxide in the soil, which the groundwater dissolves to form weak carbonic acid. To weather limestone bedrock, the weak carbonic acid water has to seep through layers of the earth along cracks and other weak points to reach the limestone bedrock.
Once the acidic solution reaches the limestone, it dissolves it and forms holes that grow over many years to form full-fledged limestone caves.
How Are Underground Limestone Caves Formed?
Underground limestone caves form through the natural chemical weathering process described above. Most caves form in karst, a landscape composed of limestone, gypsum, and dolomite rocks that dissolve gradually in slightly acidic water.
The water eats away the bedrock, forming underground passages that take as many as 100,000 years to become large enough for human entry.
How Are Stalactites and Stalagmites Formed in Limestone Caves?
As acidic water seeps through weak points in limestone bedrock, it dissolves the limestone and continues to flow down until it reaches the ceiling of an existing cave, where it drips onto the surface below.
Stalactites form as the mineral-containing water drips off the thin projections hanging from the cave’s ceiling towards the floor.
Over decades and centuries to millions of years, the stalactites grow in length as the water drips off or evaporates at the end of the projection, leaving a little mineral deposit behind at the tip.
When the water drips off the stalactites, projections also form on the floor, facing upwards towards the ceiling. These projections are called stalagmites.
Both stalagmites and stalactites are speleothems – geological formations from mineral deposition in caves. Speleothems are mostly a feature of calcareous caves because of the carbonate reactions between limestone (calcium carbonate) and acidic water.
Most of the time, each stalactite has a complementary stalagmite. But this is not always the case.
Sometimes, a stalagmite and stalactite grow in length until they touch each other, and the formation goes from floor to ceiling and is called a pillar, column, or stalagnate.
Stalagmites usually have a wider base and are often thicker or wider than stalactites, a characteristic that makes the joined speleothems look like a pillar.
Why Do Caves Form in Limestone?
Rocks such as limestone contain calcium carbonate with minerals like aragonite or calcite. These carbonate minerals dissolve well in water when there are lower temperatures and a high concentration of carbon dioxide.
As the water falls through the air and seeps through the soil, it dissolves carbon dioxide to form weak carbonic acid. This carbonic acid dissolves the limestone rocks until holes or hollows curve out beneath the surface.
Where Are Limestone Caves Found?
Limestone caves are the most common type of cave on Earth. You find them in areas with high concentrations of limestone formed from the remains of marine life, usually in shallow seas.
Limestone caves form in karst topographies characterized by landforms that appear due to the water-solubility of the underlying rocks.
Karst landscapes also feature sinkholes, disappearing streams, caves, springs, and enclosed depressions.
Some countries where you will find limestone caves include the U.S., Iran, Egypt, Russia, Portugal, Canada, Vietnam, India, China, Turkey, and Indonesia.
Are Caves Formed by Chemical Weathering?
Some are, but natural caves form in many other ways besides chemical weathering.
While limestone caves form through chemical or solution weathering, other types form through erosion, wind, waves, and other natural causes.
For example, talus caves form from rocks and boulders piling up atop a mountain slope and leaving a small opening. Sea caves form from waves hitting against a cliff, causing it to erode.
An ice cave forms due to glacial melting when temperatures rise or the sun shines on a glacier and melts it away. The melted water flows down and leaves a large opening along its path.
Warm water may also flow under a glacier, melt its ice, and form a cave along the path.
Sand caves are erosional caves that form when the wind blows and moves the sand, forming a carved groove that becomes a large cave passage.
An ice cave – Vatnajokull Glacier, National Park, Iceland
What Creates Caverns in Limestone Rock?
A cavern forms mostly underground and is large enough for human entry and exploration. A cavern usually contains speleothems, such as stalagmites and stalactites. Not all caves are caverns.
Limestone caverns form because the calcium carbonate is soluble in water and worn down by the slightly acidic water that seeps through the bedrock, forming a passage that widens with time as the acidic water continues to break it down.
Limestone Caves Interesting Facts
What are some interesting world facts about limestone caves? Let’s find out!
- Exploring caves as a sport is called spelunking, and the people who do it are called spelunkers.
- Limestone caves usually have projections from the ceiling called stalactites or cave floor called stalagmites.
- Limestone caves are made of organic sedimentary rock because limestone forms from the remains of dead marine organisms.
- Mammoth Cave in the U.S. is the world’s most extensive limestone cave system with over 400 explored miles and about 600 more miles believed to be held within the labyrinth. (source)
- Krubera Cave, a limestone cave discovered in 1960, is the second deepest discovered cave on Earth at 7,208 feet (2,197m) deep. (source)
- The Hranice Abyss in the Czech Republic is the deepest freshwater cave in the world in a limestone landscape. (source)
- The Skocjan Caves in Slovenia are a limestone cave system considered one of the world’s largest discovered underground water canyons. The Reka River cut into the limestone rock and even disappears underground into the karst caves at some point. (source)
Skocjan Caves in Slovenia
Now that you know what limestone caves are and how they form, why not look around, grab your outdoor gear, and head off to explore a limestone cave near you?
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