(Though several people have suggested I try doing this lab with something other than M&Ms (e.g. plastic counters, colored beads), I think its the fact you can actually eat the magma chamber at the end that causes this lab to be a favorite.)
So, what is the process that we’re trying to demonstrate to the students? Once a melt forms, several things start to happen to the liquid:
- it rises due to a lower density than the surrounding rock (buoyancy)
- it interacts with the surrounding rock & starts to cool down
- in some cases, the surrounding rock melts & adds into the existing liquid (assimilation)
- sometimes the first melt meets a second melt and they mix together to form some form of intermediate melt (magma mixing)
- in all cases, the melt cools to a point and starts forming crystals which can either continue reacting with the melt (equilibrium crystallization) or separate themselves chemically from what’s going on in the melt (fractional crystallization), which changes the overall composition of the melt over time
- in more traditional models of fractional crystallization, the crystals would form & then sink to the bottom of the pool of melt (magma chamber) to collect and each successive layer of crystals would have formed from a later & later point in the melts history
- how the melt during fractional crystallization changes depends on what the 1st then 2nd then 3rd crystals are made of: if you first take out X from a collection of WXYZ, then the rock will proportionally become more enriched in WYZ & depleted in X; the 2nd mineral removed has a different proportion to pull from, so maybe it takes XY, making WZ be more enriched & XY depleted
For this lab, we’re using the the more traditional sinking model and the M&Ms represent various elements removed to make up a variety of minerals. The students are given a list of minerals that crystallize out at each stage, so they can figure out how many of each color M&M to move out of the magma chamber. At the end, you have a “fully crystallized” layered magma chamber:
My students divided into two groups and went about this in very different ways. The seniors (upper picture) kept all of the “minerals” that crystallized out at every step together, so their colors look mixed together. The juniors (lower picture) chose instead to group by element, so you can’t see each “mineral” that crystallized, but you have a better idea of how much “Si”, “Al”, or “Fe” was in a given layer. If you look at the pictures, you should see some trends about how many “crystals” formed at each stage; how the chemical composition changed from layer to layer; and, if you’re really good, how the names of the rocks varied from layer to layer.
There are a few M&M magma chamber activities out there, but I tend to use the more complex version of Wirth’s (intro level also exists). There are some limitations to what we can model & how the system has to be modified in a delicious candy form, so I tend to follow this lab with the not-as-beloved MELTS lab I adapted from Jim Brophy at Indiana.