Chapter 5 Answer Keys
Bowen’s Reaction Series
1. The right branch is characterized by a continuous,
gradual change of feldspar minerals; the left branch is
characterized by an abrupt change of minerals rich in iron
and magnesium.
2. The first feldspars to crystallize are calcium-rich
feldspars.
3. A zoned crystal has sodium-rich outer layers and a
calcium-rich core. It can form when magma cools rapidly
and the calcium-rich crystal core is unable to react
completely with the magma.
4. olivine
5. pyroxene
6. A new mineral, biotite mica, is formed.
7. The remaining melt contains silica and oxygen, and
forms quartz.
Classification of Igneous Rocks
1. Granitic (felsic), intermediate, basaltic (mafic), ultrabasic
(ultramafic)
2. Extrusive rocks are glassy (non-crystalline) or finegrained, while intrusive rocks are coarse-grained to very
coarse-grained.
3. Basaltic glass is glassy in texture and extrusive, while
gabbro is coarse-grained and intrusive. They are both
basaltic, meaning their mineral compositions are similar.
4. the ultrabasic rocks peridotite and dunite
5. Granitic rocks are made up of light-colored minerals, and
basaltic rocks are made up of dark-colored minerals.
6. andesite
7. 0–10 percent
8. granite
Study Guide – Chapter 5 – Igneous Rocks
Section 5.1 What are igneous rocks?
1. magma
2. igneous rock
3. lava
4. basaltic
5. rhyolitic
6. true
7. false
8. true
9. true
10. false
11. true
12. As the pressure on a rock increases, its melting point
increases.
13. No; different minerals have different melting points.
14. Temperature increases with depth in Earth’s crust.
15. Pressure increases with depth from the weight of
overlying rock.
16. melting points
17. partial melting
18. elements
19. reverse
20. fractional crystallization
21. magma
22. discontinuous reaction series
23. continuous reaction series
24. calcium
25. sodium
26. When magma cools rapidly, the calcium-rich cores are
unable to react completely with the magma, resulting in a
zoned crystal with sodium-rich outer layers and calciumrich cores.
27. At the end of magma crystallization, the remaining
melt, which is enriched with silica and oxygen, finally
crystallizes, forming quartz.
Section 5.2 Classification of Igneous Rock
1. ultrabasic
2. intermediate
3. granitic
4. basaltic
5. ultrabasic
6. basaltic
7. intermediate
8. basaltic
9. granitic
10. ultrabasic
11. Obsidian has a very small grain size.
12. Obsidian is extrusive; its glassy texture without obvious
grains probably formed as lava cooled quickly on Earth’s
surface.
13. Gabbro has large crystals, in contrast to obsidian’s
crystals, which are very small.
14. Gabbro is intrusive; its large grains probably formed as
magma cooled slowly below Earth’s surface.
15. c.
16. d
17. b
18. e
19. f
20. a
21. to observe the shapes of mineral grains
22. Intrusive igneous rock cools slowly beneath
Earth’s surface, while extrusive igneous rock cools rapidly
at or near Earth’s surface.
23. They have space in which to grow freely.
24. Porphyritic rocks are characterized by large, wellformed crystals surrounded by finer-grained crystals.
25. Porphyritic textures can form if a slowly cooling
magma suddenly begins cooling rapidly.
26. d
27. a
28. c
29. b
30. a
31. c
32. a
33. d
34. b