Geol 375 Petrology – Spring 2015
Study Guide for Exam 1
Progress in completing these questions will be checked weekly.
Introduction
1. List the major elements and minerals of the Earth’s crust
2. List the most common minerals in each of the major categories of rocks: ig, sed, & met.
3. Summarize the relative abundances of the major rock types (ig, sed, & met) in terms of crustal volume
and surface outcrop area.
4. What is the geothermal gradient?
5. How and why does the geothermal gradient differ between thick (continental) lithosphere and thin
(oceanic lithosphere)? How and why does the gradient differ between the lithosphere and the deeper
mantle? Include the difference in efficiency/speed between convective vs. conductive heat transfer in your
explanations. Diagrams would also be useful.
6. Explain how and why the geothermal gradient can differ tectonic setting by comparing ocean ridges and
subduction zones.
Textures
1. What are nucleation & growth?
2. Relate textures (e.g. aphanitic, phaneritic, porphyritic, glassy) to cooling rates (undercooling) and how
that affects the relative rates of nucleation and growth. What conditions are needed to produce a rock with
fewer larger crystals? What conditions result in many smaller crystals? A diagram/sketch would be helpful.
3. What is the importance of diffusion to crystal growth? How would the liquid composition vary (or not)
with distance from a growing crystal depending on whether the diffusion rate is high or low? Explain why.
A diagram/sketch would be helpful.
4. What is meant by skeletal growth of minerals?
5. How and why do crystals with pointed projections from their ends (e.g. “swallowtail plagioclase”) form?
What does it mean if you find them in a rock?
6. What is zoning? What does zoning in plagioclase look like on a petrographic microscope?
7. What are some of the major causes of zoning in minerals?
8. Why might continuous zoning from more (in core) to less (in rim) Ca occur in plagioclase?
9. What might cause oscillatory zoning or repeated sharp increases in Ca content followed by more gradual
declines in plagioclase?
10. How can you use textures in plutonic rocks to figure out the sequence of crystallization? Include both
sequential crystallization (e.g. mineral A then mineral B, etc.) and simultaneous crystallization (both A and
B). Include relevant igneous rock textures as examples (interstitial, ophitic, poikilitic, cumulate).
11. What is a “graphic granite” texture? What does this texture tell us about crystal nucleation/growth in
pegmatites, including the importance of bulk magma composition and diffusion? Explain.
12. Discuss reaction textures in volcanic rocks. What does it mean for a crystal to be resorbed? What does a
partially-resorbed crystal look like? What could cause a crystal (e.g. quartz) to become resorbed? What is
oxyhornblende? What causes it to form?
Classification
1. What four minerals (technically mineral families or solid-solution series) are found on corners the IUGS
classification diagrams for igneous rocks (this is the one that has two ternary diagrams stuck together for
classifying granite, syenite, etc.)? – i.e. What four minerals are considered “essential” minerals for
classification? On which corner is each mineral (can you sketch the chart outline and label it)?
2. What additional minerals do you need to consider when classifying a rock that has a high proportion of
mafic minerals and that plots near the rightmost corner of the IUGS diagram?
3. Describe the relative proportions of essential minerals in the following:
 Granite
 Granodiorite
 Syenite
 Tonalite
 Gabbro
 Anorthosite
 Dunite
 Harzburgite
4. Describe the general layout of the total alkalis vs. silica classification plot for volcanic rocks. Where do
basalt, andesite, dacite, and rhyolite fall on the diagram? How do phonolite and trachyte compare to the
others? Be able to label these on a blank copy of the diagram.
5. Which of the following has the highest MgO content? Which of the following has the highest K2O
content? Which of the following has the highest FeO content? Which of the following has the highest Al2O3
content?
 Peridotite/Komatiite
 Gabbro/Basalt
 Granite/Rhyolite
 Andesite
6. What is the CIPW Norm? What is it used for?
Magma, Volcanism
1. What are the main components of magma?
2.What are the main dissolved gasses in magmas? Which is typically the most abundant?
3. What are the typical liquidus temperatures for basalt and rhyolite?
4. What is polymerization? What components of magmas polymerize?
5. How and why is polymerization a major control on viscosity in magmas? How do silica content, water
content, and temperature each affect polymerization and viscosity?
6. How does the presence of crystals and bubbles affect viscosity?
7. What causes magma to rise toward the surface?
8. What happens to the gasses in magma as it rises? How might the ascent rate affect the amount of gas that
remains when the magma gets close to the surface?
9. How does the formation of bubbles affect magma density? How can this affect the likelihood that the
magma will reach the surface?
10. What factors determine whether a magma may erupt effusively or explosively?
11. What causes levees to form along the edges of some lava flows?
12. What causes jointing to form in lava flows?
13. Compare/contrast shield and stratovolcanoes. What causes the difference in shape? Be sure to relate this
to magma properties and eruption types.
14. Why do felsic magmas often form lava domes whereas mafic magmas tend to form thinner and more
extensive flows?
15. What is a phreatomagmatic eruption?
16. How do calderas form? What causes the floor of a caldera to be resurgent?
17. What are the fundamental differences between lava fountains, strombolian, and vulcanian eruptions.
Include both eruption character and mechanisms.
18. What is a plinian eruption? What does it look like? What does it do to the surrounding landscape?
19. What are the major ways that pyroclastic flows form? Which might produce a lapilli tuff? Which might
produce a block and ash flow?
20. What controls the amount of welding in pyroclastic flow deposits?
21. What is a lateral blast? Is it easy to tell if a volcano had a volcanic blast in the distant past? Explain.