Grades K–2 · Materials & Matter
How can a rock with gas-filled spaces float?
A piece of pumice can float even though it is rock. Pumice forms from frothy volcanic material that hardens with many spaces inside. The close-up shows gas-filled spaces in this particular piece.
In the diagramGas-filled spaces can help a rock floatEnlarged interiorWhole pieceWaterGas-filled spacesFloats in this recordGas adds room but little mass to the whole piece.
The solid rock around the spaces is heavier than water for the same size. But the whole piece includes lots of gas, making it light for its size. Water can push up enough to hold this piece at the surface.
Not every pumice piece floats in the same way. If water fills enough spaces, a piece can become heavier and sink. Floating does not mean that all rocks are light.
About this explanation
Starting knowledge: Recognizes rock, water, gas-filled spaces, and objects floating or sinking.
Sources
- U.S. Geological Survey — Mineral resource of the month: Pumice and pumicite
- U.S. Geological Survey — Volcano Watch — Rocks float briefly where lava meets the sea