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Igneous Rock Texture Homework
___1. If most of the crystals in a rock are so small that they cannot be distinguished
without a microscope, the rock is
a. pyroclastic b. glassy c. aphanitic d. phaneritic e. phenocrystic
___2. Which of the following textures was not formed slowly deep underground?
a. pegmatite b. porphyry c. coarse-grained d. phaneritic e. glassy
___3. A piece of granite with a few 1.5 cm feldspar crystals among a matrix of 5 mm
black and white visible crystals would be
a. a phaneritic porphyry b. an aphanitic porphyry c. a phaneritic pegmatite
d. a pyroclastic aphanitic rock
e. a fine-grained pyroclast
___4. Extrusive is to volcanic as intrusive is to
a. plutonic b. porphyritic c. pyroclastic d. phenocryst
e. aphanitic
___5. Volcanic tuff is made of ash ejected from a volcano. Tuff is
a. pyroclastic b. porphyritic c. pegmatic d. phaneritic e. phonyiferous
___6. Put the following into order from slowest cooled to fastest cooled
a. glassy, aphanitic, phaneritic
b. aphanitic, glassy, phaneritic
c. aphanitic, phaneritic, glassy
d. phaneritic, aphanitic, glassy
e. phaneritic, glassy, aphanitic
___7. A phenocryst can be
a. glassy
b. aphanitic
c. made of 1 cm hornblende crystals
d. also called the groundmass
e. a sedimentary inclusion
___8. Which of the following does not have a crystalline structure?
a. a coarse-grained rock
b. a fine-grained rock
c. a glassy rock
d. a pegmatite
e. an aphanitic porphyry
___9. Mafic is to basaltic as felsic is to
a. granitic b. andesitic c. coarse-grained
___10. What is true of all igneous rocks?
d. plutonic e. vesicular
a. they form underground b. they have crystals c. they are vesicular
d. they form from a volcano e. they were originally magma
11. Explain why cooling rate affects crystal size.
12. What substance is responsible for a magma becoming a pegmatite?