Rock cycle in chocolate lab

Pete Stelling
Western Washington University
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This lab activity simulates the rock cycle with a piece of chocolate instead of actual rock. Students melt, crystallize, erode, lithify and metamorphose a single small block of chocolate. Along the way students complete questions and compare the products of each step with actual rocks.

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High school or undergraduate introductory Physical Geology lab students with little previous exposure to geology.

Skills and concepts that students must have mastered

Few; ideally students will have experience with the rock cycle, but this lab also serves as a comprehensive introduction.

How the activity is situated in the course

This is a stand-alone lab exercise designed to be completed in two hours. Some students will take a little more time to complete the final write up on the last page.


Content/concepts goals for this activity

The rock cycle, and many components thereof:
  • melting temperatures vs. composition
  • melting and magma mixing
  • plutonic vs. volcanic textures
  • erosion
  • lithification
  • metamorphism
  • distinguishing between igneous, metamorphic and sedimentary rocks

Higher order thinking skills goals for this activity

Formulation and testing of hypotheses
Synthesis of ideas and concepts

Other skills goals for this activity

Working in groups
Setting up experiments
Evaluating experimental limitations

Description of the activity/assignment

In this lab students receive two small blocks (1 cm3) of chocolate (white and dark), and follow it through the entire rock cycle. The chocolate blocks are melted on a hot plate, with different melting temperatures and rheologies due to compositional differences. The "magma" is then cooled either slowly or quickly, and the resulting textures are examined and compared to granite and basalt hand samples. The "igneous" chocolate is then ground and abraded to show erosion, and the eroded material is pressure-lithified to form "sedimentary" chocolate. The sedimentary chocolate then undergoes greater pressure to mimic metamorphism, and additional heat re-melts the chocolate back into magma. Students compare the chocolate "rocks" in each of these stages with real rock samples. The final assignment is to describe the "life story" of complex conglomerate rock sample. The lab is a bit messy and takes a bit of preparation, but students come away with a significantly better understanding of the rock cycle as a whole and each of its component parts.

Determining whether students have met the goals

The lab has been written with a series of questions, some of which are fairly simple (drawing, observations, etc.) and others are open-ended for students to explain their understanding of each step. The final question, the discussion of the "life story" of a conglomerate, is an easy way to determine if students understand the rock cycle relationships as an overall concept.

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