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Laboratory Manual for Introductory Geology 4e

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FIGURE 7.1 Examples of changes from protolith (left) to metamorphic rock (right) (continued ).

(ii) Dolomite and quartz in micrite react to produce actinolite (green; an amphibole) in marble.

(c) Multiple changes.

(i) Tiny, randomly oriented clay grains and quartz in mudstone react to form coarse-grained foliated muscovite flakes with large

garnet crystals in schist.

(ii) Randomly oriented plagioclase gray and pyroxene crystals in basalt react to form aligned amphibole crystals (hornblende) and a

different plagioclase in amphibolite. This alignment of rod-shaped crystals is called lineation.

Diffusion is much slower in solid rock than in magma or water, but over thousands or

millions of years, new metamorphic minerals can grow to impressive sizes (FIG. 7.2).

7.3.2 The Effect of Pressure

If you pull an inflated balloon underwater, it is affected by the pressure caused by

the surrounding water. The balloon responds to this pressure by getting smaller

while retaining its shape. Rocks behave the same way when they are buried deeper

and deeper in the Earth. Pressure causes the ions in minerals to be jammed

closer together (the opposite of the effect of heat). Minerals formed close to the

7.3 AGENTS OF METAMORPHISM

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