LEGO® Magma Crystallization Activity

This exercise is designed for students who are not Geology majors, and who may have limited math and science experience. For a more complex and sophisticated method that is suitable for more advanced students or for freshman geology majors, see the M&M® magma chamber activity by Karl Wirth.

This exercise is taught during a lab section focused on igneous rocks in the GEOS 120 course at UAF. This course is for non-science majors. Students enrolled in the course come from very different backgrounds. Some students are traditional college students who recently graduated from high school. Other students are non-traditional, including retirees who may have been out of school for many years, employed adults returning to college for additional job training, or students taking courses for AA programs in technology or trade fields. The exercise requires basic math skills (calculating percentages and plotting data on a simple graph) and is accessible to a broad range of levels, from advanced middle school students, high school geology courses, two-year colleges, or core classes for non-science majors at four year colleges and universities.

Students should be familiar with the Bowen's Reaction Series and basic magma compositional series (basalt, andesite, dacite, rhyolite) from lecture material. Before this activity, students should be given lecture material covering the formation of igneous rocks from magmas, and the meaning of crystal textures like zoning patterns in plagioclase. Students must have mastered basic math skills involving calculation of percentages and plotting their results on a simple graph.

This activity is well-suited for courses that include igneous processes or volcanology as only one section within the larger course. At UAF, this exercise is paired with two other exercises focused on igneous rock textures and magma crystallization during a three hour lab section for the volcanoes section in the GEOS 120 Earthquakes, Glaciers, and Volcanoes course at UAF. This activity can be used as a standalone exercise in a course without a significant lab component. Including a short lecture (15 to 20 minutes), the activity can be completed within a 1 hour course time period. This activity can also be combined with other exercises to fit in a longer (e.g., 3 hour) lab section.

Concepts covered by this exercise include:

1. Magma composition and crystallization - how these processes are tied together, resulting in the zoned crystals that are like a magma chamber "barometer" (e.g., Plagioclase). When the crystals are removed from the system, the surrounding melt changes in composition as different molecules are taken up by the crystals. Bowen's reaction series describes how sequential crystal formation can also lead to changes in magma composition by fractional crystallization.
2. Bowen's Reaction Series - Magmas do not freeze through formation of a single crystal, like water to ice. Magmas freeze sequentially, according to the freezing points of each of the minerals. Since the crystals have different compositions, the melt composition changes in response to the types, amounts, and crystallization sequence of the different minerals forming in the cooling magma. Melt composition changes as a result. Crystals that have different growth "zones", like plagioclase, can form in response to the changes in melt composition.
3. Weight percent - Geologists use weight percent of the oxides as a way to catalog the compositions of different magmas. The proportion of SiO2 in a magma can vary widely between ~ 50 % to > 75 % SiO2 by weight. This exercise gives students a hands on model to explore the concept of weight percent oxides as a tool to track magma composition.
4. The exercise is set-up so that as the students build their LEGO® crystal, the remaining LEGO's match the compositions of the magmas in the most basic series: basalt, andesite, dacite, rhyolite. The hands on exercise can help students better grasp how different magmas can be created by crystallization of a "parent" magma, usually basalt.

Students will need to link the activity with the concepts of magma crystallization by synthesizing the concept of Bowen's Reaction Series with the Lego model to discuss how fractional crystallization can create a basic magma compositional series. Students should also be able to describe why some crystals in magmas have a pattern of zoning and what that says about crystal growth. An analogy to tree rings can be used, where the growth zones are linked with changes in the magma "environment" as each different molecule is used up from the melt. A pre-activity series of questions can be used to have students first develop a hypothesis about what will happen to the model melt composition as the crystal grows. Another hypothesis that can be developed is what the changes in the melt composition will do to the growing crystal.

Students will need to operate an electronic or mechanical balance to weigh the LEGO's for calculating weight percent. This activity can be done on an individual basis. However, at UAF, students work in groups because it helps save money on the supplies needed. Also, by working in small groups, students can often help each other at points during the exercise when one may have a question or encounter a difficult concept.

This exercise will require a supply of LEGO®, or any other brand, building blocks that can be snapped together on a mat or other substrate. There should be a sufficient quantity of each different color and choice of color that represents each oxide is up to you.

The exercise also involves looking at a thin section slide with a zoned plagioclase phenocryst in it. The slide image could be projected up onto a screen or the students could look into the microscope. If that is not available, the instructor can also show a picture of a zoned plagioclase from the web.

Student Handout for Lego magma exercise (Microsoft Word 2007 (.docx) 126kB Aug27 18)

This exercise works well with students working together in small teams (2 to 3 students, with 2 being ideal). It can also be done individually if you have enough supplies or the class size is small.

In order to get the correct melt composition after each crystallization step, the model LEGO® magma and crystal compositions are simplified. As you will see, Al₂O₃ is excluded from the activity. It is impossible to use stoichiometric plagioclase compositions for the model and still end up with the correct percentage of SiO₂ left over in the model melt to create the magma series!

During the lecture that accompanies this activity, it is therefore important to make sure students understand a few things. The activity is simplified. Magmas have more elements than represented, including MgO, Al₂O₃, and FeO. Magmas form different types of crystals, including olivine, pyroxene, and oxides in addition to plagioclase. This activity provides a simplified way of illustrating how magmas crystallize, how one type of crystal texture (zoned crystals) forms, and how different compositions of magmas can be created because of the sequential crystallization, as long as crystals are removed at each step.

Each color of Lego should be counted so that the resulting weight percentages of the model melt phase correspond with the SiO₂ contents of the major magma types basalt, andesite, dacite, and rhyolite. Having the exact numbers of each color building block in each magma "pool" is important for getting the model melt SiO₂ contents to scale properly with each step. Depending on your supply of building blocks, you can scale up the numbers of each color if you want to allow your students to build larger crystals.

Alternatively, a variation for this exercise could employ colored M&M® or Skittles® candy instead of building blocks. In that case, the students could arrange their candies into a zoned crystal-shaped "mosaic" on a piece of paper.

The activity itself and the questions before and after the exercise can be modified however you wish!

Start off the exercise with a minute paper or minute quiz to assess retention of the concepts from lecture or mini-lecture before the lab. The instructor could also do a think-pair-share activity before the lab where a critical thinking question is asked, the students discuss, and then report back. The minute paper/quiz or think-pair-share could be repeated at the end to gauge how the answers change after the students completed the hands on activity. The last question in the lab activity asks students to describe the "muddiest" and "clearest" points from the exercise. A follow up question asks for the most interesting thing the students learned from the hands-on activity.

Grading of the exercise would involve checking the resulting weight percent calculations in the table and ensuring that the students plot their results correctly on the graph. Each written answer is checked for accuracy and graded accordingly.

Using an M&M® magma Chamber to Illustrate Magmatic Differentiation: https://serc.carleton.edu/NAGTWorkshops/gsa03/activities/2028.html

Cashman, K.V. (1993) Relationship between plagioclase crystallization and cooling rate in basaltic melts. Contributions to Mineralogy and Petrology 113: pages 126 - 142, https://doi.org/10.1007/BF00320836