Caves are found throughout the world on every continent. There are a number of different types of caves, composed of different geological materials, but caves formed from limestone are some of the most interesting and commonly found.
How do Caves Form?
While caves can form in many different geologies, most known caves form from the erosion of limestone by water, known as karst. As the crust slowly shifts, limestone fractures and cracks. Rainwater, which is naturally acidic, seeps into these fissures and slowly erodes away solid rock. The speed at which this erosion of limestone takes place varies greatly by: the volume and frequency of rain, the temperature, and variations in geology.
Underground Rivers
Borehole, a relatively uniform and smooth cylindrical passage through the rock, is formed by large underground rivers that once flowed in the cave. Lesser volumes of water result in more sinuous canyon passages that weave their way underground with sharp and jagged walls. Water also may reach a layer of limestone that is more easily dissolved and form plunging pits that can extend downward into the abyss below.
Living Caves
Many caves are still actively forming, with water flowing throughout, enlarging existing passage while also forming new passages. Caves in which the water source has long since dried up, or has shifted course, are far from being dead. Many caves are home to numerous species of bats, reptiles, fish, invertebrates, and microscopic organisms. Cutoff from the outside world, cave organisms take a vastly different evolutionary track, and many lie genetically dormant for thousands of years.
Formations Found in Caves
While water forms borehole or canyon passages, there are many formations that line these passages that are created with and without the direct force of water:
- Breakdown occurs when parts of the ceiling or wall collapse. These jumbled piles of rock can be ten to a hundred feet high, but are usually stable, since thousand pound slabs of rock take a lot of force to move.
- Speleothem is a catch-all term minerals deposited by gently flowing or dripping water. Stalactites hang from the ceiling gathering mass as following water leaves behind minerals. Stalagmites typically form underneath stalactites, or another source of dripping water, and grow by the same process from the ground up. If this keeps up for long enough, the two can meet to form a column.
- Rimstone is thousands of years of ring-around-the-bathtub, where pooled water creates ridges a few inches deep, or less.
Many other formations exist, even beautiful and delicate crystals such as gypsum.
How Far Does the Cave Passage Go?
Caves are described as having a horizontal and vertical extent. While a cave may lie under only a square mile of land, above it twisting passages can double-back and encircle each other to result in tens of miles of passage. Mammoth Cave in Kentucky has over 360 miles of surveyed passage in 82 square miles.
When the geology is right, caves can have a significant vertical extent. Caves can extend from level with the valley floor to high inside a mountain. Some caves in the Southeast United States have drops of several hundred feet in a single pit, and even greater total vertical extents. Sótano de las Golondrinas in Mexico is a karst sinkhole almost 1,200 feet straight down. Voronya, in the Asian country of Georgia, is the deepest known cave, has been explored 7,188 feet from top to bottom!
Cave researchers and explorers are constantly discovering new and important facts about the underground. Understanding how caves form and the unique and delicate organisms that live there is an important step in learning about the Earth and protecting its resources.
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