Caverns & Sinkholes: Definition, Formation & Effects

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  • 0:05 Caverns and Sinkholes
  • 0:49 Sinkhole Formation and Effects
  • 2:48 Cavern Formation and Effects
  • 4:23 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Groundwater can have dramatic effects on Earth's landscape, both above and below ground. In this video lesson you will learn about caverns and sinkholes, both of which are directly related to groundwater movement.

Caverns and Sinkholes

As we learned in another lesson, groundwater is water below Earth's surface. Groundwater is an important part of the water cycle because it is where rainwater and snow end up after they hit the ground, soaking down into the soil. Once in the ground, this water either stays there for a very long time or it discharges into streams and rivers, keeping them flowing smoothly.

Though water may look simple, do not be fooled! Water is a very powerful substance. When water moves underground it can drastically change the landscape, like when it creates sinkholes and caverns. Sinkholes are funnel-shaped holes in the ground, and caverns are large open spaces underground.

Sinkhole Formation and Effects

Groundwater is important not only to the water cycle, but also to us. We pump groundwater for a variety of uses, like crop irrigation and drinking water. However, groundwater also acts like a support system for the ground, similar to how the bones in your body provide you with support. If you suddenly removed all of your bones, you'd fall to the ground like a big pile of mush.

The same happens when too much groundwater is removed too quickly - the ground collapses into the open space below and the result is a sinkhole. Sinkholes come in all shapes and sizes. Some are small and simply look like dips in the land, while others are so large that entire city blocks collapse into the ground. Water is not the only important structure underground, though. The soil and sediment also play an important role. Sometimes groundwater dissolves these sediments, leaving little left to support the ground above. Just like with removing too much groundwater, the ground sinks down into the empty space (which is exactly why they're called sinkholes) and fills the void left where the sediment stood before.

When the land surface lowers into the ground like this, we say that the land has subsided. Subsidence not only creates quickly falling sinkholes, but can also cause large-scale land sinking over long periods of time. For example, the Leaning Tower of Pisa leans because it was built on unstable land and as the soil underneath it compacted and sank into the ground the tower began to tilt. In the U.S., extreme groundwater pumping in the San Joaquin Valley of California caused the water table, or the area below ground where it's saturated with groundwater, to sink 245 feet over 50 years! The land on top simply couldn't help but sink with it, and over that same period of time dropped 30 feet.

Cavern Formation and Effects

Caverns form in much the same way that sinkholes do. Sometimes when groundwater dissolves sediment underground it simply leaves a big hole and the ground above doesn't fall in. These holes are caverns.

Groundwater is stored in underground reservoirs called aquifers, which get dissolved by rainwater as it soaks into the ground. Cracks develop in the aquifer and as time goes on those cracks get larger and larger, forming channels underground. Eventually they may get so large that they form underground rivers, connecting caves and caverns like a giant highway beneath the surface of the Earth.

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