Why Can't a Rock Float in Salt Water?

Most individuals think that salt water is more dense than fresh water, and they are correct. However, this could lead to the assumption that rocks can easily float in salt water. That isn't necessarily true.

Density and Bouyancy

There are two key terms to know when discussing the ability of objects to float in water. The first is density, which is an object's mass per unit of volume. Bouyancy is the second; it's the ability of an object to remain afloat.

Misconceptions

Many people believe that, because some large objects--such as floating piers or boats--have no problem staying afloat in salt water, that any rock of a certain weight will be able to float. That's not true because of the concepts of bouyancy and water displacement.

Salt Water vs. Freshwater

Salt water has an average density of 1.022. So, although it is more dense than fresh water, it doesn't give objects such as rocks the ability to float.

Expert Insight

Most rocks have a density that is greater than both freshwater and salt water. This is the main reason why a rock can't float in salt water.

Considerations

Pumice is know to have a greater buoyancy in water because of its low density. At a density of .0023, it's one of the least dense rocks.

References

• SEED: Shcumberger Excellence in Educational
• Education.com
• Geotechnical, Rock and Water Resources Library - University of Arizona

Resources

• Geotechnical, Rock and Water Resources Library - University of Arizona