Page 165 - Laboratory Manual in Physical Geology

A CTIVIT Y 5.4 Glassy and Vesicular Textures of Igneous Rocks

Name: ______________________________________ Course/Section: ______________________ Date: ___________

Place equal parts of sugar (sucrose, C H O ) and water in the pan/beaker and heat on medium high. Do not touch the hot
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plate, beaker/pan, or boiling sugar, because it is very hot! Notice that steam is given off after the sugar dissolves and the solution
boils. After a few minutes there will be no more steam, and the remaining molten sugar will be have a very thick (viscous)
consistency. At this point (before the sugar begins to burn), pour the thick molten sugar onto a piece of aluminum foil on a flat
table. DO NOT TOUCH the molten sugar, but lift a corner of the foil to observe how it flows and behaves until it hardens
(2–3 minutes).

A. Viscosity is a measure of how much a fluid resists flow. Water has low viscosity. Honey is more viscous than water. How did
the viscosity of the sugar solution change as the water boiled off?

B. What happened to the viscosity of the molten sugar as it cooled on the aluminum foil?

C. When the molten sugar has cooled to a solid state, break it in half and observe its texture. Look about the room where you are

now seated and name two objects that have this same texture.

D. Now observe the texture of the cooled solid mass of sugar with a hand lens. Notice that there are some tiny bubbles of gas
within it. Geoscientists call these “vesicles,” and rocks containing vesicles are said to have a “vesicular texture.” What prevented
the gas bubbles from escaping to the atmosphere?

E. REFLECT & DISCUSS When a sugar solution is permitted to slowly evaporate, sugar crystals form. The process of
crystallization depends on the ability of atoms to move about in the solution and bond together in an orderly array. What two
things may have prevented crystals from forming in the molten sugar as it cooled on the aluminum foil in this experiment?

F. In your collection of numbered igneous rock samples, do any of the samples have the texture that you just observed in Part C?
If yes, which one(s)?

G. In your collection of numbered igneous rock samples, do any of the samples have the texture that you just observed in Part D?
If yes, which one(s)?

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