Exercise 7.3 shows how differential stress produces preferred mineral orientatio

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Exercise 7.3 shows how differential stress produces preferred mineral orientation in metamorphic rocks. Note Foliations are given different names depending on the intensity of metamorphism (see section 7.4.3), but all that is needed to name a rock is the presence or absence of any kind of foliation or lineation. EXERCISE 7.3 Visualizing Preferred Orientation Due to Reorientation Name: Section: Date: Course: We can gain insight into how preferred orientation develops with a simple laboratory experiment. (a) Randomly insert pennies (flat grains) and small sticks (elongate grains) into a ball of Play-Doh. Flatten the dough into a pancake by pressing on it with a book (i.e, apply differential stress). What happens to the alignment of the objects? Are the sticks parallel to each other in the plane of the pancake? (b) Clean off the pennies and sticks, and reinsert them into another ball of Play-Doh. This time, roll the dough into a cigar shape. What happens to the alignment of the objects now? Are the sticks parallel to each other? How are they oriented relative to the length of the cigar? Absence of Preferred Orientation Minerals in some metamorphic rocks are ori- ented randomly and therefore have neither foliation nor lineation. Such rocks are said to have a granoblastic texture (FIG77). Preferred orientation may be absent because (a) there was no differential stress during metamorphism, or (b) there were no platy or rod-shaped minerals to align, or (c) both (a) and (b). 7.4.3 Identifying Metamorphic Rocks TABLE 7.2 is a classification scheme for common metamorphic rocks based on ob- servable mineralogy and texture. Each column corresponds to one of the five major compositional classes and each row to textural variations. Different names are given to some textural varieties depending on whether they result from low, moderate- or high-grade metamorphism. For example, slate, phyllite, and schist are all foliated aluminous rocks, but they represent different metamorphic grades. To name a metamorphic rock, begin by identifying its minerals. This will place the rock in the correct compositional type and column in Table 7.2. Move down- ward through the table until you come to the row with the appropriate texture. In some cases there is only one choice; in others there may be a few possibilities. The following descriptions will help you choose. Slate: A low-grade metamorphosed shale or mudstone composed of strongly aligned

Explanation / Answer

a) NO the sticks are in the flat plane but not parallel to each other some of them are just in a flat plane but they are crossing and some of them are not crossing each other.

After pressing the sticks and pennis sit in a flattened plan.

b) The sticks and pennis are somewhat parallel. The sticks adjusted themselves along the length of cigar shape. The sticks are adjusted along the length of cigar and since its circular and the sticks which are apart are somewhat parallel to others but sticks which are slightly cross and not parallel to the cigar length are not parallel to others.

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