Start with a mafic magma composition (slide bar to the far right) and select a very slow cooling rate ( 1-2 on the cooling rate slidebar). Nothing will happen until the temperature drops below 1400 C. As soon as the temperature drops below 1400, stop the cooling (slidebar to the left) until the crystals have a chance to grow, then resume cooling at a rate of 1-2. The first crystals to form are Olivine. Soon, a Ca Plagioclase also appears. Crystals from both these minerals will continue to grow until 1200 C, when Pyroxene begins to crystallize. What happens to the Olivine? Why?
At about 1000 C all the crystallization is complete. When the temperature reaches 600 C, any remaining liquid quenches to form glass. Assuming that the minerals average at least several millimeters in size, what rock have you just formed?
Now restart the simulation by closing its window and pressing
. This time, try a faster cooling rate (7-8 on the slidebar). What was the result? Where the crystals larger or smaller?
If the crystal size is substantially smaller than 1 mm, what is the rock? And how (where) would it likely
Restart the simulation
and push the button marked "quench". What do you get?
Let's try a magma of intermediate composition. Restart the simulation
and move the magma composition bar to "intermediate" and then start cooling at a moderate rate (3-4). What is the first mineral to form?
The brown mineral which forms below 1050 C is Amphibole and the black mineral below 800 is Biotite. When the rock finishes cooling (~600 C) what is it?
Now try a magma with felsic composition. Restart the simulation
and move the magma composition bar to "felsic" and then start cooling at a moderate rate (3-4). What is the first mineral to form?
When the cooling process is finished, what is it?
One final experiment involves fractionation. Restart the simulation
and move the magma composition bar to "mafic" and then start cooling at a moderate rate (3-4). Wait until the temperature is 1300 C and push the "fractionation" button. The crystalized minerals will settle out and the magma composition will be re-calculated to sum to 100% - then the crystalization process will continue. How is the rock that forms this way different from the rock you formed without fractionation?
The processes of magma crystallization is much more complex than this simple exercise can simulate. Many more minerals, with more variable chemical compositions, occur in nature. This becomes especially true when you remove more than 25% of the original melt through fractionation. Under these conditions the simulation is not able to produce a reasonable crystalization sequence with the limited minerals in its memory and very
strange rocks may result. We are currently working on a better (more realistic) simulation, but don't hold your breath!