Lab 20 IGNEOUS ROCK MINERAL SIZE Part 1: Forming Minerals The size of minerals i

Question

Lab 20 IGNEOUS ROCK MINERAL SIZE Part 1: Forming Minerals The size of minerals in an igneous rock is determined by how long the mogma takes to cool To illustrate, everyone should stand up and scotter throughout the room. 1. You have two seconds to form groups as big as possible. How many per group? 2. Scotter ogain. Now you have 10 seconds 3. Two students are deboting about how this activity relates to mineral size in rocks How many per group Student 1: Ir seems to me that with a longer amount of time, it is possible for oll the atoms to form really large minerals Student 2: I don't know, I would think that more time means that more minerals will form, and only a little bit of time means only a few big minerals will form With which student do you agree? Why? Part 2: Mineral Formation Location Two bodies of magma are shown in a cross section below, One is above ground and the other is deep within the crust. The length of the arrows represents the rate at which heat is escoping from the molten rock as it cools. Magma deep In the crust Lava flow lava erupted onto the surface mogma deep underground 4. Which will cool faster? 5. The igneous rocks granite and gobbro have large minerols. In which location would they have formed? on the surface deep in the crust 6. The igneous rocks rhyolite and basalt have minerals so small it is difficult to distinguish them with the naked eye. In which location would they have formed? on the surface deep in the crust Lecture Tutorials for Eorth Science © W. H. Freeman and Company 49

Explanation / Answer

Part 1: I agree with student 1. The more the time magma gets to cool, more and more crystals forms and joins together to form larger crystals. If the magma gets less time then the crystals dont get the time to crystallise with any minerals and nucleates here and there to produce a glassy texture.

Part 2:

4. The one in which lava has come up to the surface encounters a temperature which is much lower than that of itself. So it will cool rapidly,compared to the one which is deep underground.

5. As granite and gabbro have larger crystals, its justified that they got more time to cool down and form large crystals. Its only possible when they form deep inside the crust.

6. Rhyolite and basalt on the other hand have got small grained minerals which are not even visible to the naked eye. They didnot get time to nucleate and form large crystals. They must have been formed on the surface.

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