Final Chapter 3

Question 1

What are the eight common minerals that form igneous rocks? Which four are mafic (black or dark green),
and which four are felsic?

Answer 1

Mafic: Olivine, Amphibole, Pyroxene, biotite
Felsic: Na-rich plagioclase, quartz, k-feldspar, muscovite

Question 2

According to Bowen’s reaction series, which three minerals tend to dominate mafic rocks, and which one of these minerals is not a mafic mineral? (‘Mafic’ means rich in Mg and/or Fe.)

Answer 2

-olivine, pyroxene, and ca-rich plagioclase feldspar
-plagioclase feldspar is not mafic

Question 3

Which four minerals tend to occur in intermediate rocks?

Answer 3

feldspars, minor quartz, hornblende, and biotite

Question 4

Which six minerals tend to occur in felsic rocks?

Answer 4

k-feldspar, na-plagioclase, quartz, and slight muscovite, biotite, amphibole

Question 5

Which textures indicate intrusive rocks and which extrusive? Which indicate a two-stage cooling history?

Answer 5

• coarse-grained textures indicate intrusive rocks
• fine-grained texture indicate extrusive rocks
• porphyritic textures indicate a two-stage cooling

Question 6

Which of the following rocks form from lava flows and which are pyroclastic: Basalt, andesite, rhyolite, scoria, tuff, obsidian, pumice?

Answer 6

Lava flows: scoria, obsidian, and pumice
Pyroclastic: rhyolite, basalt, tuff, and andesite

Question 7

Is obsidian glassy because it cools much faster than other lava flows? Explain.

Answer 7

yes, a glassy texture suggests rapid cooling that’s so quick that crystals did not have the time to grow

Question 8

Are pegmatites extra coarse-grained because they cool much slower than other intrusive rocks? Explain.

Answer 8

pegmatites cool slowly and have extra water in the magma; the water promotes rapid crystal growth by speeding migration of chemical elements to the growing crystals

Question 9

Along which types of plate boundaries do igneous rocks form?

Answer 9

igneous rocks form at convergent and divergent plate boundaries

Question 10

What causes the partial melting that occurs along subduction zones? What causes the partial melting that
occurs along ocean ridges and continental rifts?

Answer 10

-partial melting from subduction zones is caused by the subducting plate releasing its water-rich fluids into the rocks of the overlying mantle
–partial melting from ocean ridges and continental rifts is caused by magma rising up the mantle and reaching a pressure decrease until eventually the hot peridotite partially melts

Question 11

As part of the lab exercises you graphed the compositional trends formed by mafic to felsic rocks. Did this
trends suggest a continuum of rock compositions, or do igneous rocks naturally fall in to discrete and separate
compositions, with no intermediates?

Answer 11

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