Activity 6: Metamorphic Rocks




Metamorphic rocks are normal rocks that have undergone incredible stresses. These stresses changed the previous rock into a new rock with some of the same properties and some new properties.

Instructional Method:



Explain to students that metamorphic rocks are new rocks made from old rocks and how this happens.

Students will be able to:

  • Draw the changes in rock as it is metamorphosed
  • List the three stresses that form metamorphic rocks


Preparation: 30 min
Demonstration: 45 min
Discussion: 15 min

Materials Needed:

  • Jar or glass pan
  • chocolate chips
  • peanut butter chips
  • white chocolate chips
  • M&M's
  • plastic wrap
  • piece of wood or something to push into the pan or jar
  • heat source
  • Printable rock cycle diagram




Metamorphic means 'changed'. When a rock has been affected by metamorphic processes it has changed from one kind of rock to another. Metamorphic rocks can begin as igneous, sedimentary or other metamorphic rocks. They form when any kind of rock is exposed to high heat and high pressure over a long period of time. Extreme conditions like these for long periods can develop deep within the crust or where tectonic plates collide.

All metamorphic rocks have one thing in common, time. The longer a rock is exposed to one or all of the metamorphic stresses the more metamorphosed it becomes.

The creation of metamorphic rocks never involves melting the original rock. If these stresses do melt the original rock, an igneous rock is created. Metamorphic rocks change in the solid state by replacement or rearrangement of molecules. These changes are brought about by squishing, folding, and heating without ever melting!

Classifying metamorphic rocks is sometimes more difficult than identifying igneous or sedimentary rocks because of the various results of the varying stresses that affect them. The important thing to remember is the different stresses that can affect any rock can turn it into a metamorphosed rock.

Two easy metamorphic classifications are: foliated and non-foliated. Foliation describes the texture (how the rock looks) of metamorphic rocks. It has to do with the way minerals are aligned in a rock. When rock is subject to extreme pressure grains will squish in the 'Y" plane of the pressure. Elongate or flat grains, such as mica, will align themselves parallel with each other in the 'Y" direction.

A small piece of Gneiss, with a quarter for size reference
A piece of gneiss, with a U.S. quarter for size reference


When minerals line up parallel to each other, the rock shows a tendency to split along a plane created by those parallel grains. This is called rock cleavage, or slate cleavage, named for the fine-grained metamorphic rock slate which demonstrates this phenomenon. Sometimes minerals will not only align themselves in parallel directions, but will also segregate into bands of differing composition. This happens when rocks are subjected to the most intense heat and pressure they can stand without melting. A common example of the result of this phenomenon is gneiss. Slate, schist and gneiss are all foliated metamorphic rocks.

You are now going to make a metamorphic chocolate rock. You will place different stresses on chocolate pieces to form a metamorphic chocolate rock. The chocolate pieces represent grains in a rock or more easily visualized, individual grains like sandstone. As different stresses are placed on pieces students can witness or imagine how real rock reacts to the same stresses.

Instructional Procedures:

  1. Place glass container on heat source with heat source off.
  2. Mix different kinds of chocolate pieces and M&M's and pour into glass container about 1 inch thick at bottom. Different colors allows students to choose a color and watch as it changes instead of loosing their 'grain' during the metamorphic process.
  3. Explain that the different pieces represent minerals in a rock or different grains in a sedimentary rock, i.e.. quartz, salt, feldspar, amphibole, sand.
  4. Turn heat source on a low setting (do not let the mixture boil or melt). Warm the pieces until they become slightly soft.
  5. Explain to kids that metamorphic rocks are rocks that have been changed from one rock type (sedimentary, metamorphic, igneous) into another rock type. The rock properties change like the chocolate pieces are changing before of them as they heat up. The minerals in real rock soften and are easily squished into different shapes when under pressure.
  6. Place plastic wrap around the object to be pushed into the jar. Now place pressure onto the chocolate pieces.
  7. This demonstration will take a few minutes to perform. That is fine, because time is the one factor affecting all metamorphic rock.
  8. Explain to students that the combined effects of heat and pressure over long periods of time are what change other rock into metamorphic rocks.
  9. Keep chocolate pieces on the heat source and under pressure until the pieces slowly squish together.
  10. Once the mass has become one "newly formed rock" remove from heat. After it cools cut up into pieces and give to kids to examine and eat.


As you perform the demonstration ask students to list the different stresses that form metamorphic rocks. On the same paper have students draw what is happening to the chocolate pieces. First, the pieces are normal. Second, placed on heat. Third, pressure applied, etc. Ask if one of the forces were removed would a metamorphic rock form? Try removing one of the forces (heat, pressure, or time). How much did the rock change from all stresses compared to only one? What changed from the first picture they drew to the last picture of the chocolate rock.


Instead of using chocolate based pieces you may want to try jellybeans or some type of jelly concoctions. Replace the condensed milk with a Karo syrup and water mixture. The demonstration can be performed the same way.


Have students collect possible metamorphic rocks from their neighborhood or surrounding area a few days before class. Bring them to class. Use their drawings of the chocolate rock and classify which of the real rocks are the most metamorphosed and which are least metamorphosed.

Included National Parks and other sites:

Big Bend National Park
Black Canyon of the Gunnison
Grand Canyon National Park
Northern Cascades National Park
Yellowstone National Park


Metamorphic Rock at the Bottom of the Grand Canyon

Utah Science Core:

2nd Grade Standard 6 Objective 1,2,3
4th Grade Standard 3 Objective 1,2


Last updated: February 24, 2015

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