Chapter 4 Igneous Rocks
Earth: An Introduction to Physical Geology, 9e (Tarbuck/Lutgens)
4.1 Multiple-Choice Questions
1) Lava flows are typically finer grained than intrusive igneous rocks. Why?
A) Intrusive magma is cooler because it is well insulated by the surrounding rock.
B) Intrusive magma flows onto the Earth's surface and cools very slowly, allowing many
small mineral grains to grow.
C) The extrusive magma cools quickly so the mineral grains do not have time to grow.
D) The extrusive magma, because it is deep below the surface, cools very slowly
producing very small mineral grains.
Answer: C
Diff: 1
2) Which magma is most likely to quench (congeal) to a natural glass?
A) highly viscous; cools quickly
B) highly viscous; cools slowly
C) highly fluid; cools slowly
D) highly fluid; cools quickly
Answer: A
Diff: 1
3) The sizes, shapes, and arrangements of mineral grains in an igneous rock is known as
________.
A) silica content
B) texture
C) mineral content
D) Bowen's reaction series
Answer: B
Diff: 1
Match the description with the appropriate texture.
A. aphanitic
B. porphyritic
C. phaneritic
D. glassy
4) ________ magma cools and consolidates without growth of mineral grains
Answer: D
Diff: 1
5) ________ mineral grains are of roughly equal size and coarse enough to be seen without a
microscope or magnifying glass
Answer: C
Diff: 1
1
6) ________ rock has two or more, distinctly different-sized populations of mineral grains
Answer: B
Diff: 1
7) ________ a magnifying glass or microscope is needed to see individual mineral grains
Answer: A
Diff: 1
8) A ________ is an open cavity in a volcanic rock that was filled by a gas bubble when the lava
was still mainly liquid.
A) porphyrocryst
B) vesicle
C) phenocryst
D) xenocryst
Answer: B
Diff: 1
9) Consider the Bowen's reaction series. Which mineral would you expect to see as a
phenocryst in a porphyritic basalt?
A) olivine
B) quartz
C) orthoclase
D) sodium-rich plagioclase
Answer: A
Diff: 1
Match the following rocks to their equivalent aphanitic or phaneritic igneous rocks.
A. andesite
B. gabbro
C. rhyolite
10) ________ granite
Answer: C
Diff: 1
11) ________ basalt
Answer: B
Diff: 1
12) ________ diorite
Answer: A
Diff: 1
13) Which of the following igneous rocks exhibit aphanitic texture?
A) granite; gabbro
B) andesite; rhyolite
C) andesite; diorite
D) rhyolite; gabbro
Answer: B
Diff: 1
2
14) In a porphyritic volcanic rock, which mineral grains are the last to crystallize?
A) phenocrysts
B) vesicles
C) pegmatites
D) matrix or groundmass
Answer: D
Diff: 1
15) Visible quartz and potassium feldspar grains are the main constituents in a ________.
A) granite
B) gabbro
C) basalt
D) rhyolite
Answer: A
Diff: 1
16) Which of the following igneous rocks has a pyroclastic texture?
A) rhyolitic tuff
B) porphyritic basalt
C) intrusive granite
D) andesitic lava
Answer: A
Diff: 1
17) ________ is a volcanic rock that is extremely vesicular and glassy.
A) Obsidian
B) Pegmatite
C) Tuff
D) Pumice
Answer: D
Diff: 1
18) ________ is composed mainly of ferromagnesian minerals.
A) Peridotite
B) Rhyolite
C) Andesite
D) Granite
Answer: A
Diff: 1
19) Which of the following minerals crystallize early in Bowen's reaction series?
A) biotite
B) quartz
C) olivine
D) muscovite
Answer: C
Diff: 1
3
20) ________ is the dominant feldspar in basalt.
A) Plagioclase
B) Microcline
C) Orthoclase
D) Pyroxene
Answer: A
Diff: 1
21) Which of the following are used for studying rocks with a polarizing microscope?
A) polished cubes
B) broken chips
C) thin sections
D) grain karats
Answer: C
Diff: 1
22) ________ is characterized by very coarse mineral grains?
A) Obsidian
B) Pumice
C) Pegmatite
D) Granite
Answer: C
Diff: 1
23) In which of the following igneous rocks and environments would you expect to find
unusually high concentrations of rare elements such as lithium, beryllium, and boron?
A) basalt dike; fills a vertical fracture at shallow depth
B) pumice lump; crystallized at depth in a mass of intrusive granite
C) peridotite; crystallized at depth in the upper mantle
D) pegmatite; crystallized from a water-rich, highly differentiated, residual magma
Answer: D
Diff: 1
24) A (an) ________ texture represents a single, long period of cooling and crystallization.
A) glassy
B) pyroclastic
C) aphanitic
D) phaneritic
Answer: D
Diff: 1
25) ________ has the same mineral composition as andesite?
A) Basalt
B) Granite
C) Gabbro
D) Diorite
Answer: D
Diff: 1
4
26) What do pumice and obsidian have in common?
A) basaltic composition
B) glassy texture
C) ultramafic composition
D) phaneritic texture
Answer: B
Diff: 1
27) Which of the following best describes an aphanitic texture?
A) The rock is crystalline; mineral grains are too small to be visible without a magnifying
lens or microscope.
B) The mineral grains have glassy textures.
C) The rock consists of broken, volcanic-rock and mineral fragments.
D) The rock is crystalline; mineral grains are of distinctly different sizes.
Answer: A
Diff: 1
28) A ________ texture would be most unlikely to occur in an extrusive igneous rock.
A) pyroclastic
B) glassy
C) aphanitic
D) phaneritic
Answer: C
Diff: 1
29) ________ is named for a prominent, volcanic mountain range in western South America.
A) Basalt
B) Andesite
C) Pegmatite
D) Peridotite
Answer: B
Diff: 1
30) ________ is the dominant lava erupted from volcanoes on Hawaii and Iceland.
A) Rhyolite
B) Andesite
C) Peridotite
D) Basalt
Answer: C
Diff: 1
31) Which igneous rock or magma has the lowest silica (Si
A) granite
B) basalt
C) andesite
D) peridotite
Answer: B
Diff: 1
5
) content?
32) ________ is thought to be common in the Earth's mantle but rare in the crust?
A) Pumice
B) Granite
C) Pegmatite
D) Peridotite
Answer: D
Diff: 1
33) ________ often contain gem-quality crystals of minerals such as beryl and tourmaline and
high concentrations of relatively rare elements such as lithium, boron, and beryllium.
A) Welded tuff sheets
B) Basaltic lavas
C) Granitic pegmatites
D) Diorite plutons
Answer: C
Diff: 1
34) The last minerals to crystallize on Bowen's Reaction Series result in igneous rocks with a
________ composition.
A) felsic
B) intermediate
C) mafic
D) ultramafic
Answer: A
Diff: 1
35) Changing the composition of magma by incorporating surrounding host rock is known as
________.
A) magma mixing
B) partial melting
C) differentiation
D) assimilation
Answer: D
Diff: 1
36) All of the following are factors that affect the generation of magma except for ________.
A) heat
B) pressure
C) crystal size
D) volatiles
Answer: C
Diff: 1
6
4.2 Word Analysis Questions
Examine the words and/or phrases for each question below and determine the relationship among the
majority of words/phrases. Choose the option which does not fit the pattern.
1) A) granite
B) basalt
C) diorite
D) gabbro
Answer: basalt
Diff: 1
2) A) aphanitic
Answer: glassy
B) phaneritic
C) porphyritic
D) glassy
B) hornblende
C) plagioclase
D) augite
B) rhyolite
C) pumice
D) welded tuff
B) crystals
C) groundmass
D) vesicles
Diff: 2
3) A) biotite
Answer: plagioclase
Diff: 1
4) A) obsidian
Answer: B
Diff: 2
5) A) phenocrysts
Answer: D
Diff: 1
4.3 True/False Questions
1) Bowen's reaction series predicts the sizes of the different mineral grains that grow from
crystallizing magmas.
Answer: FALSE
Diff: 1
2) In an igneous rock with a phaneritic texture, the mineral grains are visible to the unaided
eye.
Answer: TRUE
Diff: 1
3) Olivine and quartz commonly crystallize together from mafic or basaltic magmas.
Answer: FALSE
Diff: 1
4) Most lava crystallizes to form igneous rocks with phaneritic textures.
Answer: FALSE
Diff: 1
5) Pegmatites are smaller volume, intrusive bodies with glassy textures.
Answer: FALSE
Diff: 1
7
6) Basalt is the aphanitic or fine-grained equivalent of gabbro.
Answer: TRUE
Diff: 1
7) Plutonic rocks are intrusive and generally consist of mineral grains coarse enough to be
readily visible in a hand sample.
Answer: TRUE
Diff: 1
8) A porphyritic texture includes two different sizes of mineral grains, the phenocrysts and the
vesicles.
Answer: FALSE
Diff: 1
9) Olivine is an important mineral in peridotites and other ultramafic rocks.
Answer: TRUE
Diff: 1
10) The process of magmatic differentiation can generate residual, more felsic magmas from
mafic magmas such as basalt.
Answer: TRUE
Diff: 2
11) Glassy igneous rocks form when magma cools too fast for mineral grains to grow.
Answer: TRUE
Diff: 1
12) Minerals, such as plagioclase feldspar, on the continuous branch of Bowen's Reaction Series
react with the magma to form lower temperature, more stable minerals such as hornblende
or biotite.
Answer: FALSE
Diff: 1
4.4 Short Answer Questions
1) What term denotes the larger mineral grains in a porphyritic igneous rock?
Answer: phenocryst
Diff: 1
2) ________ is the name given to molten rock below the Earth's surface.
Answer: Magma
Diff: 1
3) Melting of rocks to form magma as a result of decreases pressure is known as ________.
Answer: decompression melting
Diff: 1
8
4) What type of magma, commonly erupted along oceanic ridge systems, originates by partial
melting of mantle peridotite?
Answer: basalt
Diff: 1
5) Pegmatites consist of what size mineral grains?
Answer: coarse to very coarse
Diff: 1
6) A ________ texture refers to rocks composed mainly of mineral and volcanic fragments.
Answer: pyroclastic
Diff: 1
7) Pyroxene, hornblende, and biotite are all minerals found on the ________ series of Bowen's
Reaction Series.
Answer: discontinuous
Diff: 1
8) Name two igneous rocks with glassy textures.
Answer: pumice, obsidian
Diff: 1
9) Technically, the process whereby ions arrange themselves into orderly patterns during the
cooling of a liquid is called ________.
Answer: crystallization
Diff: 1
10) ________ refers to the removal and isolation of early-formed mineral grains that can cause
the composition of remaining magma to change.
Answer: Crystal settling
Diff: 1
11) Igneous rocks are classified on the basis of what two main characteristics?
Answer: texture and mineral composition
Diff: 1
4.5 Critical Thinking Questions
Use complete sentences, correct spelling, and the information presented in Chapter 4 to answer the
questions below
1) The size of crystals in igneous rocks is generally a function of the rate of cooling. However,
within the same rock crystal size does not necessarily reflect the order of crystallization for
individual minerals. Without using Bowen's Reaction Series, how might you determine the
order of crystallization of minerals in an individual rock sample? (Hint: Think about the
texture)
Diff: 3
9
2) All other factors being equal (geothermal gradient, pressure, composition, and heat from
other sources such as friction), why is magma generally produced in association with
subduction zones as opposed to areas away from subduction zones?
Diff: 2
3) In a series of basaltic lava flows on Hawaii, the top and bottom of each flow is difficult to
distinguish just by looking at the layers of rock. What are some rock characteristics (texture,
mineralogy, etc.) and other features that you might use to recognize individual lava flows?
Diff: 3
4.6 Visualization Questions
1) Fill in the missing rock names on the chart below.
Answer: See figure 4.11 in Earth, 9e.
Diff: 1
10
2) What process is exhibited by the diagram below? When does this process take place?
Answer: Decompression melting. This occurs when confining pressure drops enough to lower
melting points. Such drops in pressure occur when rock ascends due to convective
upwelling like that at divergent plate boundaries.
Diff: 1
11
3) What is the role of water and other volatiles that are driven from the descending plate in the
diagram below?
Answer: These volatiles lower the melting temperature of rocks in the mantle to generate
magma.
Diff: 2
4) In general, what happens in terms of composition as crystallization proceeds down the
Bowen's reaction series?
Answer: The early-formed silicates are enriched in iron and magnesium, resulting in such rocks
as peridotite and basalt. As crystallization proceeds, the later-formed silicates are
enriched in sodium, potassium, and silicon, resulting in andesite and granite.
Diff: 2
12
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