FAIRLY SIMPLE GEOLOGY EXERCISES
STUDENTS AND THEIR TEACHERS
John J. Thomas
To demonstrate the difference between how igneous and metamorphic rocks crystallize. To show how you can get an ordered crystallization pattern both by crystallization in the liquid state and in the solid state.
Ions are the basic building blocks of crystalline material. A solid consists only of round ion spheres stacked up in an ordered pattern. Ions in the solid are arranged much like billiard balls in a rack, marbles in a box, cannon balls in a pyramid in the town square, or round stones in a field stonewall. Since all of these are stacked spheres there are lots of gaps between the spheres, but like a stonewall the gaps do not matter, the wall is solid in spite of the voids. Also, like a chipmunk in a stonewall, things (in our case other ions) can pass through the openings in the solid without destroying the solid.
Igneous rocks are crystallized from a liquid, molten rock. Because the medium from which the igneous rock crystallizes is a fluid, the ions that will form the minerals are able to move freely. The hotter the fluid the greater the movement and freedom, and the quicker the crystallization takes place. (Did you know that hot water freezes faster than cold? It does! The ice-resurfacing machine at the ice-skating uses hot water!). As the melt cools down the freedom of movement decreases and the ions must fit together into a solid. The ions are of different sizes and there are only certain ways that they can fit together depending on their chemistry. But the ions are still in a fluid so they can still easily get to where they ought to be and form a strong solid. Have you ever run out of ice cubes and been impatient for new cubes to freeze? When you check the ice cube tray, you find that you have a hollow ice cube with water in the center. Crystallization starts at a point, the water ions move to where crystallization started and add to the already formed crystals until the whole cube is solid.
In Metamorphic rocks there never is a melt. The recrystallization takes place in the solid. The metamorphic process forms new minerals from the ions already in the rock. Just like igneous rocks, the new crystals start at a point and bring to them the ions they need to grow. The ions are not freely moving as they were in the fluid igneous rock. The only way they can reach the new mineral is through the voids (spaces) in the ions that are already there, just like the chipmunk in the stonewall. In this way, new minerals form by bring to them the material they need and sending away the material they do not need, all in the solid, until you have a new rock made up of new minerals consisting of new arrangements of the same old ions.
The free movement of material in a fluid can easily be demonstrated. Have the students observe the dust particles floating in the sunbeam shining through the window. Their pattern is completely free and very close to random.
Voids in an arrangement of spheres can be demonstrated with marbles. If you pour marbles into a container and shake them around, they will eventually form an arrangement like ions in a mineral. You can then see the voids by putting a container with one layer of marbles on an overhead projector. With another experiment you can measure the voids. Fill a jar to the top with marbles. Pour water into the container until it is full. It will take a lot, so there is a lot of openness in the arrangement. Pour the water into an empty jar of the same size and compare the two jars.
You can demonstrate the process of crystallizing an igneous and a metamorphic rock with the following exercise. It takes a lot of guts to try this, everything could go wrong, but if it works it is a lot of fun.
Two different colors of paper - I use 3X5 cards of two different colors, but you could use construction paper or have the students color one side of a 3X3 inch square of paper. You need exactly the number of squares as the students in your class. Half of the squares are one color and half the other.
For example, you are using red and blue cards and have 33 students in your class the day you are going to do this experiment. You choose 17 red and 16 blue cards. Shuffle them. On one side, mark the first card with a plus (+), the second with a minus (-), the third with a plus (+), etc. so that every other card has a plus (+) or a minus (-). Now arrange the cards so that your deck alternates red and blue, starting with red on top. Do this the day of the exercise and you will end up with the right number of reds and blues and pluses and minuses.
Start with all the students sitting at their desks, the desks arranged in a regular square or rectangular classroom pattern. Tell them that they are going to demonstrate the difference between the crystallization of igneous and metamorphic rocks. Hand out the cards, blank colored side up starting in one corner of the classroom. Hand them out along the first row then go back one row handing them out back across that row, etc. (see fig. 1). This will arrange your cards so that the blue cards are surrounded by red and red by blue (see fig. 1). They will all have noticed that there is a plus or minus on the other side of their card, tell them that they will use that later.
Tell them that they are ions (or particles) that are going to form a crystal. Explain that igneous rocks crystallize from a liquid and, therefore, they (the particles) are completely free moving, until they start to crystallize. Have them take their card with the blank colored side up and start to wander around the room. Let their wandering pattern be really random. They are ions so they do not make any noise (the first time I did this the principle heard us from 100 yards away). Tell them that they are cooling so they are slowing down. Have the student in desk #1 (see fig. 1) sit down. Then have the students next to that desk sit (desks 2 and 14). In an orderly fashion have each adjacent desk sit down (desks 3, 13, 15, etc). It is alright if they all sit down at once. When they are all sitting they are no longer free moving, they are in a fixed position, and their pattern is orderly, each one of them is surrounded by cards of a different color (figure 1). Have them check that this is true. They are now an igneous rock!
Now the second part. Tell the students that they are going to become a metamorphic rock. They are going to be recrystallized in the solid. This means that they cannot move. Have them flip their cards over to the plus or minus. Now they have to make an orderly pattern of pluses and minuses only by passing the cards (ions) between them. Start with one of the corners, for example desk #1 (see figure 2). If this desk is a plus (or a minus), desks #2 and 14 must be minus (plus). If desks #2 and 14 are a plus (minus), they must trade with the nearest minus (plus) and become a minus (plus). They cannot get up to do the trade. They must trade by passing the cards from one student to another. In this way they are getting what they need and rejecting what they do not need. You will have to be watching each trade and checking to be sure that the right trades are being made so that each plus is surrounded by minuses and vice versa. Have the trading proceed slowly or chaos will reign. You must watch each of the trades to make sure it is going correctly. When it is all done, all the pluses should be surrounded by minuses and the minuses by pluses (figure 3), have the students check this pattern. Once again you have an orderly pattern (see figure 3). They are now a metamorphic rock! When I do this I usual miss a trade in the middle of a pattern so I have a break in the order. Usually another trade will cure the disorder. You may want to keep a couple of plus (+) and minus (-) cards hidden in a pocket to correct the pattern if no swap can make the pattern come out right. If all of this falls apart and the structure is not an orderly pattern, everyone has had a good time and you can tell them that disorder is frequently the pattern in nature.
Figure 1. Pattern for handing out cards.
Figure 2. Metamorphic card swapping pattern.
Figure 3. Final pattern of metamorphic cards.
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