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Breaking Rocks

3.1 based on 20 ratings

Updated on Mar 20, 2013

Grade Level: 4th - 7th; Type: Geology


To discover whether certain types of rocks are more difficult to break than other types of rocks.

The purpose of this experiment is to test various sedimentary, igneous and metamorphic rocks to find out whether one of these rock families is stronger than the others.


There are many different types of rocks on Earth. Rocks are broken down into three families based on how they are formed. Igneous rocks are formed in the heat of a volcano, sedimentary rocks are formed when layers of soil and organic matter are pressed together for a long period of time and metamorphic rocks are formed when other types of rocks are exposed to high levels of heat, pressure or chemical changes below the surface of the earth. The processes used to make different types of rocks have an effect on their strength and resistance to breakage once they are fully formed. By knowing the strength of different types of rocks, engineers can choose stones that will best stand the test of time when using them for tools or structures.

Research Questions

  • How are igneous rocks formed?
  • How are sedimentary rocks formed?
  • How are metamorphic rocks formed?
  • What types of rocks are known for their strength?
  • What types of rocks are known for their malleability?
  • What are some common uses for different types of rocks?

Terms to Know

  • Igneous rock
  • Metamorphic rock
  • Sedimentary rock
  • Magma
  • Volcano
  • Pressure
  • Detritus


  • Various rocks in the sedimentary family (such as sandstone, limestone or coal)
  • Various rocks in the metamorphic family (such as marble, slate or phyllite)
  • Various rocks in the igneous family (such as granite, obsidian or pumice
  • Tongs with a rubber grip on the end where they hold the object
  • Protective glasses
  • Protective gloves
  • A book to help identify rocks
  • (optional) A scale
  • (optional) A tape measure

Experimental Procedure

  1. You can collect rocks from the area around your home or school, or you may be able to purchase rocks using the internet or a science catalogue. It may also be possible to borrow rocks from your school’s science lab, but make sure your teacher knows that you intend to break them.
  2. Gather together a number of different samples of rocks. You can find them in the area around your house or school, borrow them from a friend or teacher, or purchase rocks through the internet or a science catalogue.
  3. Record the types of rocks on a chart such as the one below.
  4. Record the classification of each rock as igneous, sedimentary or metamorphic.
  5. Choose two rocks to start with.
  6. Examine the rocks using your sense of sight and touch.
  7. (optional) Make measurements of the rocks including size and mass.
  8. Record information about these two rocks on the chart.
  9. Make a prediction about which rock you think will break first and record this prediction on the chart.
  10. Put on the safety goggles and gloves.
  11. Place one rock on the ground outside.
  12. Hold the other rock in the tongs.
  13. Using the tongs, hit the rocks together until one of them breaks, cracks, or fractures. It is possible that neither rock will break or that both will break at the same time.
  14. Record the results on the chart.
  15. Choose another rock to break against the harder of the two rocks.
  16. Repeat steps 5-13.
  17. Continue in this manner until all of the rocks have been tested against at least one other rock. It is not necessary to test every rock against every other rock. If, for example, you discover that limestone is harder than sandstone, and that marble is harder than limestone, you can assume that marble is also harder than sandstone. Use your logical skills to determine which rocks should be tested against which other rocks until you feel you can list the rocks in order of softest to hardest.
  18. Create a list of the results of your experiment, from the softest rock to the hardest rock.
  19. Record the family of each rock next to its name.
  20. Determine if there is a correlation between the type of rock and the relative hardness of it.

Name of rock

Family of rock







P 2

R 2



hard; heavy; flecks of different colors

granite v pumice granite is harder

granite v pumice granite is harder



light; porous

pumice v granite granite is harder

pumice v granite granite is harder



smooth; heavy; hard

marble v sandstone marble is harder



rough; flakes away





Writer and educator Crystal Beran is rarely seen without a pen. Her adventures have brought her to four continents and her quest for answers has led her to discover more questions than she could fill all the pages with. She currently resides in Northern California, where she can be found sipping tea and writing books.