Exploring Rocks: You Figure It Out


Clue: This mylonite was strongly deformed in a fault zone separating moving blocks of rock. The two prominent muscovite flakes were originally rectangular. Can you tell which way the rock at the right was moving relative to that at the left?

Hint: Imagine modeling clay containing flakes of aluminum foil being squeezed between two blocks moving in opposite directions. The clay near each block moves with the block. Foil flakes are bent and dragged by the movement. The mylonite is like the clay and the muscovite is like the foil.

Answer: The rock at the right was moving up relative to the rock at the left. Clues like this one help to determine which way rocks moved along ancient faults.


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Mylonite, Pima County, Arizona
Crossed polarizers with gypsum plate

Clue: Stringers of garnet (red) are enclosed in twinned plagioclase feldspar, the striped mineral. Can you tell which mineral formed first?

Answer: The feldspar formed first, because garnet cuts the twinning in the feldspar.


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Olivine Metagabbro, Essex County, New York
Crossed polarizers with gypsum plate

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