Igneous Rocks and Volcanoes

Practice exam questions written by Timothy H. Heaton,
Professor of Earth Sciences, University of South Dakota

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  1. What is a rock with mineral grains too small to see with the unaided eye called?
    Aphanitic.
    Glass.
    Phaneritic.
    Porphyritic.
    Tuff.

  2. What is a rock with both large and microscopic mineral grains called?
    Aphanitic.
    Glass.
    Phaneritic.
    Porphyritic.
    Tuff.

  3. What is a rock with mineral grains that are all big enough to see with the unaided eye called?
    Aphanitic.
    Glass.
    Phaneritic.
    Porphyritic.
    Tuff.

  4. What is a rock made of unordered atoms called?
    Aphanitic.
    Glass.
    Phaneritic.
    Porphyritic.
    Tuff.

  5. Which term describes the composition of the earth's mantle?
    Felsic.
    Intermediate.
    Mafic.
    Ultramafic.

  6. Which term describes the composition of a basalt?
    Felsic.
    Intermediate.
    Mafic.
    Ultramafic.

  7. Which term describes the composition of a granite?
    Felsic.
    Intermediate.
    Mafic.
    Ultramafic.

  8. Which of the following rock types is most derived (most different) than the earth's mantle due to repeated magmatic differentiation (partial melting, crystal settling, etc.)?
    Andesite.
    Basalt.
    Diorite.
    Granite.
    Peridotite.

  9. When a basalt undergoes partial melting, what kind of rock generally forms from the resulting magma?
    Andesite.
    Basalt.
    Granite.
    Peridotite.
    Rhyolite.

  10. Which rock is the fine-grained equivalent of granite (same composition but aphanitic)?
    Andesite.
    Basalt.
    Komatiite.
    Obsidian.
    Rhyolite.

  11. Which rock is the coarse-grained equivalent of basalt (same composition but phaneritic)?
    Diorite.
    Gabbro.
    Granite.
    Peridotite.
    Pumice.

  12. Which rock is the coarse-grained equivalent of andesite (same composition but phaneritic)?
    Diorite.
    Gabbro.
    Granite.
    Peridotite.
    Pumice.

  13. Which of the following rocks contains the most quartz (the lowest temperature mineral on Bowen's Reaction Series)?
    Andesite.
    Basalt.
    Komatiite.
    Obsidian.
    Rhyolite.

  14. Which of the following rocks contains the most olivine (the highest temperature mineral on Bowen's Reaction Series)?
    Diorite.
    Gabbro.
    Granite.
    Peridotite.
    Pumice.

  15. Which of the following rocks is pyroclastic?
    Diorite.
    Gabbro.
    Granite.
    Peridotite.
    Tuff.

  16. Which type of pluton is the largest?
    Batholith.
    Dike.
    Laccolith.
    Sill.
    Volcanic neck.

  17. Which type of pluton is planar (sheet-like) and discordant (cuts across existing geologic structures such as rock layers)?
    Batholith.
    Dike.
    Laccolith.
    Sill.
    Volcanic neck.

  18. Which type of pluton is lens-shaped and arches up the overlying rock?
    Batholith.
    Dike.
    Laccolith.
    Sill.
    Volcanic neck.

  19. Which type of pluton is concordant and is most likely to develop columnar jointing?
    Batholith.
    Dike.
    Laccolith.
    Sill.
    Volcanic neck.

  20. Which type of pluton generally contains granite?
    Batholith.
    Dike.
    Laccolith.
    Sill.
    Volcanic neck.

  21. Which igneous rock is composed of unusually large crystals?
    Granite.
    Pegmatite.
    Volcanic breccia.
    Welded tuff.
    Xenolith.

  22. What factor primarily controls the viscosity, and therefore the explosiveness, of a magma?
    Dissolved gasses.
    Pressure.
    Silica content.
    Temperature.

  23. What is the most viscous type of lava flow?
    Aa.
    Pahoehoe.
    Pillow lava.
    Pyroclastic.

  24. What term is given to underwater lava flows?
    Aa.
    Pahoehoe.
    Pillow lava.
    Pyroclastic.

  25. Which type of volcano is large and has a low slope, such as Hawaiian volcanoes?
    Caldera.
    Cinder cone.
    Composite volcano.
    Pluton.
    Shield volcano.

  26. Which type of volcano is high and steep, such as Mount Rainier and Mount St. Helens?
    Caldera.
    Cinder cone.
    Composite volcano.
    Pluton.
    Shield volcano.

  27. Which type of volcano is composed of andesite lava flows and pyroclastic deposits?
    Caldera.
    Cinder cone.
    Composite volcano.
    Pluton.
    Shield volcano.

  28. What is a volcanic summit depression such as Crater Lake in Oregon called?
    Caldera.
    Cinder cone.
    Composite volcano.
    Pluton.
    Shield volcano.

  29. Which term refers to a glowing cloud of hot air-cushioned ash?
    Fumarole.
    Hot spot.
    Nuée ardente.
    Pipe.
    Vent.

  30. Which volcanic feature connects a volcano to a nearby magma chamber below?
    Fumarole.
    Hot spot.
    Nuée ardente.
    Pipe.
    Vent.

  31. Which volcanic feature emits only gas?
    Fumarole.
    Hot spot.
    Nuée ardente.
    Pipe.
    Vent.

  32. What type of volcanoes are associated with subduction zones?
    Cinder cones.
    Composite volcanoes.
    Shield volcanoes.

  33. What rock type is produced by spreading center vulcanism?
    Andesite.
    Basalt.
    Granite.
    Peridotite.
    Tuff.

  34. Which state is largely covered by basalt flows (flood basalts)?
    Arizona.
    Oregon.
    South Carolina.
    South Dakota.
    Virginia.

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