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Answers for Chapter 2:
Classification and Nomenclature of Igneous Rocks
- What are the three principal categories of igneous rocks? What characterizes each?
Plutonic (intrusive) rocks crystallize beneath the surface of the Earth. They tend to have coarser grain size as a result.
Volcanic (extrusive) flow rocks crystallize at the surface of the Earth. They tend to have very fine grain size or glass as a result.
Pyroclastic volcanics result from explosions, collapse, or spray and accumulate much like a sediment. They exhibit fragmental textures.
Field expression also distinguishes each type, as will be described in Chapter 4.
- What is the difference between aphanitic and fine-grained?
Aphanitic means that the individual crystals are too fine to see with the naked eye. Fine-grained means that you can see the grains, but they are small (< 1 mm diameter).
- How does a felsic mineral differ from a mafic mineral? Which minerals on Bowen’s Series are mafic? Which are felsic?
Felsic generally refers to the light-colored silicates (feldspars, quartz, feldspathoids, muscovite) whereas mafic refers to the darker ones (biotite, hornblende, pyroxene, olivine …).
Composition, however, has precedence (e.g., smoky quartz and dark feldspars are considered felsic).
- What does the term acidic mean, and how does it differ from felsic? How does basic differ from mafic?
Acidic is a purely chemical term that refers to the SiO2 content of a rock. Felsic minerals tend to be acidic as well, but the terms are not synonyms. Some feldspathoid-rich rocks, for example, may be quite felsic but low in SiO2 and thus not very acidic. The opposite of acidic is basic (low SiO2 and generally higher in Fe and Mg). Basic rocks tend to contain abundant mafic minerals.
- Does the IUGS system consider pure albite to be an alkali feldspar or a plagioclase feldspar? Why?
Pure albite is considered an alkali feldspar, because it contains Na, an alkali metal.
- Why does the phaneritic QAPF diagram present problems for P-rich rocks? What are the three possible phaneritic P-rich rock names and on what basis may they be distinguished?
Gabbro, diorite, and anorthosite all contain much more plagioclase than quartz or alkali feldspar and thus all three plot near P and cannot be distinguished on the basis of QAPF ratios alone. Anorthosite has greater than 90% plagioclase in the un-normalized mode, and is thus
easily distinguished. Diorite and gabbro have more than 10% mafics and are distinguished on the basis of the average composition of the plagioclase, which can be estimated by observing the extinction angle in thin section. The plagioclase in gabbros is defined as more anorthite-rich than An50 whereas the An-content of plagioclase in diorite is less than An50. In hand sample this generally correlates with color. Plagioclase more calcic than An50 is usually dark gray to black, whereas it is whitish when more sodic. Gabbros are thus typically very dark (and the mafic mineral is usually a pyroxene and/or olivine) whereas diorites have a black-and-white (“salt-and-pepper”) appearance (and the mafic is usually hornblende and/or biotite).
- What are the two contrasting P-rich aphanitic rocks and how they are distinguished?
Andesite and basalt are common P-rich volcanics and cannot be distinguished using Figure 2.3. The IUGS recommends a distinction based on color index or silica content, and not on plagioclase composition. Andesite is defined as a plagioclase-rich rock with either a color index below 35% or with greater than 52% SiO2. Basalt has a color index greater than 35%, and has less than 52% SiO2. Many andesites defined on color index or silica content have plagioclases of composition An65 or greater.
- What would you name a volcanic rock in which the weight percentage of Na2O = 2%, K2O = 4%, and SiO2 = 49%?
According to Figure 2.4 it would be a trachybasalt.
- What would you name a volcanic rock in which the weight percentage of Na2O = 5.5%, K2O = 3%, and SiO2 = 58%?
According to Figure 2.4 it would be a benmoreite.
- What would you name a volcanic rock in which the weight percentage of Na2O = 8%, K2O = 5%, and SiO2 = 55%?
According to Figure 2.4 it would be a shoshonite.
- What would you name a rock containing 60% ash, 30% fragments between 3 and 15 mm, and 10% fragments over 64 mm if it were a light pink color?
Figure 2.5 indicates a lapilli tuff and the pink color implies a rhyolitic composition. I’d call it a rhyolitic lapilli tuff.