ALL STRESSED OUT!
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.
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
- Jar or glass pan
- chocolate chips
- peanut butter chips
- white chocolate chips
- 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.