Take a peek as water works its way into gaps between sand and rocks. (Image: Thinkstock)

# Thirsty rocks

Follow these instructions to find out more about geology and the rocks beneath our feet.

• 7 May 2007 | Updated 3 April 2013

### You will need

To do this activity you will need:

• three tall, thin jars of the same size and shape
• sand
• gravel
• soil
• water
• food colouring
• permanent marker.

### What to do

To find out more about geology and the rocks beneath our feet you will need to:

1. use the marker to label all three jars at the same height, around halfway
2. fill each jar with either sand, gravel or soil up to the mark, so that each jar has the same amount of material in it
3. use the measuring jug to pour the same amount of water into each jar; make sure you have more water than the amount of material but not enough to fill the jar; add a few drops of food colouring to the jug before pouring the water into the jar.
4. wait to ensure the water has filtered all the way through the materials
5. compare the water levels in the jars.

### What's happening?

When you fill the jars, the water works into the gaps between the sand and rocks, even the ones that are too small for us to see. The water is heavier than air so it pushes the air up and out as bubbles.

The different sized materials let more or less water through. If more water gets into the gaps, there will be less left up the top.

The jar with the lowest water level has the most porous material and the highest water level has the least porous material.

### Applications

Tiny holes inside rocks are called pores, so a measure of how much water, gas or oil can be in a rock is called porosity. Underground oil and gas deposits can be found in the pores of rocks, and it is where carbon dioxide could be stored.

Over millions of years oil and gas filter through porous rocks that can hold them, but they need a cap of less porous rock above that will stop them escaping.

This could also be a way of storing extra carbon dioxide, replacing oil or gas that is taken out and burned.

By Deb Hodgkin

This activity was featured in Science by Email.