Key concepts

Have you ever heard the expression, "solid as a rock"? As it turns out, rocks are not entirely solid. Rocks actually have tiny pockets of air inside them. This is obvious when you look at a piece of volcanic rock (often called basalt), which is full of visible holes. But dense rocks, such as granite, have tiny air pockets inside them, too. These pockets of air are just much smaller.

If you picked up one volcanic rock as well as one granite rock of the same size, you would notice they don't weigh the same. The granite is heavier than the volcanic rock. The many large holes of air in the latter make it less dense—and more porous—than the granite, which also makes it lighter. Something that has more holes in it is more porous. So "porosity" is one characteristic that can help tell you what kind of rock you have.

Rocks—and most other objects, for that matter—are made up of particles of varying sizes that are packed together. In between the particles are spaces that are filled with gas, air or liquid. Particles' shapes and sizes affect how they aggregate, including how tightly they can pack together, which affects a rock's porosity—a property that is the ratio of the volume of a rock's empty spaces to its total volume. 

In general, larger particles cannot pack together as well as smaller particles can, which means that packing larger particles together leaves more space for air to fill between the particles. You can imagine this if you have one cup full of marbles and another cup full of sand. You'll be able to see many more spaces between the marbles than between the grains of sand.

    Three clear plastic cups
    Measuring cup
    Rocks that can be sorted into one of three size groups (ideally all of the same type of rock, such as granite)
    Screen (optional)

    Make sure that the rocks are sorted into three different groups by size. The greater the difference in size between the rocks is, the easier it'll be to interpret your results. There should be enough of each group of rocks to completely fill a plastic cup.
    Fill each clear plastic cup to the top with one of the groups of rocks. How much space do you see between the rocks in the different cups?
    Fill the measuring cup with one cup of water.
    Pour the water into one of the cups of rocks, filling the cup to the top.
    How much water is left in the measuring cup? Subtracting the amount left in the measuring cup from one cup will tell you how much volume the air between the rocks took up. How much volume did the air take up?
    To each of the two other cups of rocks, again measure one cup of water, fill each cup of rocks with water, and determine how much volume the air took up.
    How much air did the cup with the largest rocks have compared with the cup with the smallest rocks? How did the volume of air in those cups compare with the volume of air in the cup with the medium-size rocks?
    Extra: You can calculate the porosity of each of the cups of different size rocks you used in this activity. To do this, divide the volume of air taken up by each cup of rocks by the total volume of water the cup could hold (without rocks in it). For example, if the air took up one half cup and the cup could hold one cup total, the porosity would be 50 percent. What is the porosity of each of the cups of the different-size rocks?
    Extra: Soil is a mixture of rocks, minerals and organic matter. Porosity is also a property of soil. Try the same activity using different types of soil: clay, loam, sandy, silty, potting soil, compost, etcetera, but put a screen on top of the cup to keep organic matter from floating out as you pour the water into the cup. Do different types of soils have different porosities?

Observations and results
Could you see the spaces between the rocks in the cups? Did the cup with the largest rocks have more air than the cup with the smallest rocks? Did the cup with the smallest rocks have less air than the cup with the medium-size rocks?

This activity modeled the inside of rocks on a much larger scale. Because, in general, larger particles cannot pack together as tightly as smaller particles can, a rock made out of larger particles will usually be more porous than a rock made out of smaller particles. But rocks are not static; like everything, rocks change over time. When enough pressure, or force, is applied to a rock, the pressure can make the rock more efficiently pack its particles. Usually pressure builds up on rock over a long period of time, as dirt and other rocks end up on top of it. You can see how this process, known as compaction, makes the rock's porosity decrease with time.

More to explore
" Rock Properties: Porosity and Density" from Wisconsin Geological and Natural History Survey
"The Rock Cycle: Compaction and Cementation" from The Geological Society of London
"Porosity and Permeability" from eNotes
"Porosity and Particle Size" from Science Buddies

This activity brought to you in partnership with Science Buddies