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Name: Tina
Status: Student
Grade: 4-5
Location: AL
Country: United States
Date: Summer 2010


Question:
How do air-pockets form in underwater caves?



Replies:
In most cases, when the water level in the cave drops it can get so low that the roof of the cave is above water (it dries out). When the water rises again it can trap this air in pockets up against the ceiling of the cave.

Hope this helps.
Robert Avakian


Tina,

Most liquids can and do have gases dissolved in them. Carbon dioxide is readily dissolved in soda/pop, and even normal tap water has dissolved gases in them (you can see bubbles form in tap water). Think of fishes that live in the bottom of the ocean - they still have to take in oxygen, so the fish gills of deep ocean fishes must still be able to collect oxygen dissolved in deep ocean water.

Certain situations can force the dissolved gases to be released from the water. Opening a soda/pop bottle, because it was sealed under pressure results in the release of the carbon dioxide that was dissolved in the soda/pop under pressure. Stirring a glass of tap water or heating it (to a temperature well below the boiling point of water) can cause the dissolved gases to come out of the water.

So let us say there is a dome-shaped cavity on the top of a cave that is completely submerged in water. Small changes in temperature can result in some of the dissolved gases in the water to escape out of the liquid, accumulate, and form an air bubble. This air bubble would rise to the top of the dome. Over time, more air bubbles could accumulate in this dome until we have an air pocket. Of course, this air bubble could also redissolve into the water. So conditions such as a sustained elevated temperature, the influx of water that has more gas dissolved in it, a decrease in pressure such as if part of the dome were to collapse and form a bigger cavity, etc. all could lead to an air pocket.

Greg (Roberto Gregorius)
Canisius College


Tina

If you sat down and thought about it for a few minutes, you could probably think of some ways that air pockets form in underwater caves. Here are some thoughts that I came up with:

First of all, when we talk of air we mean a mixture of gas that is made up of 79% Nitrogen, 20% Oxygen, and 1% other gases.

Here is my first thought: The water level in tidal caves rises and falls with the tide. When the tide goes out and the water level falls, air fills the cave. Then when the tide comes back in and fills the cave, air sometimes gets caught in the top of the cave depending on the geometry of the space.

My second thought: there is a case where divers go down in underwater caves and their exhaled air floats to the roof of the cave and forms an air pocket there. It will be high in Carbon Dioxide from their exhaled breaths, but it still may have some air that people can breathe. In fact, some SCUBA (Self-Contained Underwater Breathing Apparatus) that divers use scrubs Carbon Dioxide out of used air and circulates that scrubbed air back for re-breathing allowing the divers to stay underwater longer.

My third thought: Sometimes, what looks like air-pockets in the roofs of caves, may be gases other than air that percolate out of water or come up from lower parts of the earth such as volcanoes. Sometimes oxygen will percolate out of water when it reaches lower pressures in less deep parts of the earth. But sometimes these gases may be poisonous and may consist of methane, sulfur, or carbon dioxide.

An example of a dissolved gas in a liquid is a soda pop. When you take off the lid you hear and see it fizz. This comes from dissolving carbon dioxide into the liquid under high pressure. Then when you release the pressure by opening the bottle, the gas comes out of solution.

Maybe you can think of other examples of how air pockets form in underwater caves.

Good luck to you.

Sincere regards,
Mike Stewart



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