Monday, 7 July 2008

The porosity of rocks

The Space within - the porosity of rocks. For this Earthlearningidea, the pupils make a sedimentary rock by packing any roughly sphere-shaped things of the same size, e.g. oranges or marbles, into a suitable container. The oranges or marbles are like the grains which make up a sedimentary rock, like sand grains in a sandstone.
You can get the pupils more involved in this demonstration as follows: when a container is full of spheres, ask ‘Is it full’? After they answer ‘Yes’, you add water to show that there was still lots of space.
Similarly, before pouring in the water, ask them to predict how much water could be added. Most will be surprised at how much water can be poured in – and that apparently solid materials can be more than a third space.
Natural sandstones have porosities that range up to around 50%, so they may have plenty of space for water or oil/gas. Natural clays can have porosities up to more than 80% - but they are often impermeable, since the pore spaces are so small that water cannot flow through.
Try this activity with your pupils and let us know how you get on.

1 comment:

Anonymous said...

A photo of the Triassic sand from Budleigh Salterton, Devon, U.K. can be seen on the website,, in the extension ideas in the 'Earth materials' category. Sands like this are useful for this activity; they have a high porosity.