Earth: An Introduction to Physical Geology, 11e (Tarbuck)
Chapter 4 Magma, Igneous Rocks, and Intrusive Activity
4.1 Multiple-Choice Questions
1) Igneous rocks and metamorphic rocks derived from igneous “parents” make up what
percentage of the Earth’s crust?
A) 50%
B) 95%
C) 82%
D) 75%
2) What three components make up most magmas?
A) Silicon, Potassium Feldspar, and Muscovite
B) Water, Melted Rock, and Solid Crystals
C) Lava, Ash, and Gas
D) Liquid Portion, Solid Portion, Gaseous Portion
3) What is a volatile?
A) A hostile person
B) Solid mineral crystals in a melt
C) Gases dissolved in a magma
D) The liquid portion of a magma
4) Which of the following substances is not a volatile?
A) O2
B) CO2
C) H2O
D) SO2
5) Select the choice below that best describes the role of volatiles in an ascending magma.
A) Volatiles increase the rate of crystallization in intrusive structures
B) Volatiles will force the magma to contract as it rises
C) Volatiles will shatter surrounding rocks, making it easier for the magma to rise
D) Volatiles will expand with decreasing pressure, carrying the melt upward
6) What was the rate of cooling and the environment of formation for an extrusive igneous rock?
A) Slow because the magma was insulated below the surface
B) Fast because it was erupted from a volcano
C) Slow because it cooled in a volcanic lava flow far from the vent
D) Fast because it cooled six miles below the surface
7) Which mineral class makes up most igneous rocks?
A) Carbonates
B) Sulfides
C) Silicates
D) Halides
8) Which of the following minerals is not a ferromagnesian mineral?
A) Olivine
B) Potassium Feldspar
C) Pyroxene
D) Biotite Mica
9) Peridotite is the main constituent of which part of the Earth?
A) Crust
B) Magma chambers
C) Rift zones
D) Upper mantle
10) Which two minerals define a felsic composition?
A) Pyroxene and Amphibole
B) Muscovite and Biotite Micas
C) Quartz and Potassium Feldspar
D) Plagioclase Feldspar and Biotite Mica
11) What is the geological definition of texture?
A) How the sample feels to the touch
B) The mineral content of the sample
C) The percentage of silica in a sample
D) Size and shape of mineral grains in the sample
12) Which igneous texture has visible crystals that are only a few millimeters across?
A) Phaneritic
B) Aphanitic
C) Pegmatitic
D) Porphyritic
13) Which of the following textures would be the most common texture found in a volcanic lava
flow?
A) Pegmatitic
B) Aphanitic
C) Porphyritic
D) Phaneritic
14) Which of the following textures would be the most common texture found in a batholith?
A) Aphanitic
B) Pyroclastic
C) Porphyritic
D) Phaneritic
15) What creates the small holes found in a vesicular texture?
A) Liquid water
B) Volatiles
C) Soluble minerals
D) Weathering
16) What substance enhances ion migration in the later stages of magma crystallization, which
allows abnormally large mineral crystals to form and create pegmatites?
A) Water
B) Oxygen
C) Silica
D) Iron
17) ________ is a felsic igneous rock with a meringue-like vesicular texture created by small
shards of volcanic glass.
A) Scoria
B) Obsidian
C) Granite
D) Pumice
18) Which is the following statements best describes the relationship between pressure and
melting point in the Earth’s Interior?
A) Higher pressure leads to lower melting points
B) Melting points of rocks are not dependent on changes in pressure
C) Higher melting points are determined by higher pressures
19) What is the term used to describe increased temperature with depth in the Earth?
A) Decompression melting
B) Convection cells
C) Geothermal gradient
D) Hydrothermal alteration
20) What role will water play in generating magma?
A) Water will lubricate the path for subducting plates, allowing them to subduct deeper
B) Water will lower the density of the of rock, allowing it to melt
C) Water will cool the rock, preventing melting
D) Water will lower the melting temperature of the rock, allowing it to melt
21) ________ describes how an entire suite of silicate minerals can form from a single basaltic
magma as it cools and crystallizes.
A) Theory of Plate Tectonics
B) Mohs Hardness Scale
C) Bowen’s Reaction Series
D) Geologic Rock Cycle
22) ________ describes the formation of one or more secondary magmas from a single parent
magma.
A) Magmatic Differentiation
B) Hydrothermal Alteration
C) Decompression Melting
D) Partial Melting
23) The partial melting of a mafic magma would produce what composition of igneous rocks?
A) Ultramafic
B) Mafic
C) Intermediate
D) Felsic
24) What magma would have formed from the partial melting of mantle material?
A) Felsic Magma
B) Primary Magma
C) Intermediate Magma
D) Secondary Magma
25) Which mineral on the Bowen’s Reaction Series will be the last to melt if an igneous rock is
heated?
A) Quartz
B) Pyroxene
C) Potassium Feldspar
D) Olivine
26) Which mineral on the Bowen’s Reaction Series will be the first to melt if an igneous rock is
heated?
A) Quartz
B) Pyroxene
C) Potassium Feldspar
D) Olivine
27) Which of the following statements best describes the formation of secondary magmas?
A) Decompression melting creates a mafic magma from mantle material
B) Iron-rich minerals will form in a cooling magma, which leaves the remaining melt with a
more andesitic composition
C) Partial melting of felsic rocks, creating a silica-rich magma
D) Water sweating from a subducted plate will melt mantle material
28) Which process is thought the generate most felsic magmas?
A) Decompression melting in a continental rift zone
B) Crystal settling during magmatic differentiation
C) Heat from basaltic magma partially melting overlying crust
D) Compression of the crust in a convergent plate boundary
29) What is the name of the pre-existing rock that is displaced by intrusive igneous bodies?
A) Acid Rock
B) Parent Rock
C) Stock
D) Country Rock
30) A ________ is a tabular, concordant pluton that is nearly horizontal and forms when magma
exploits a weakness between sedimentary layers.
A) Dike
B) Sill
C) Batholith
D) Laccolith
31) A ________ is a mushroom-shaped pluton that forms by injecting magma between
sedimentary strata, forcing the upper layers to arch upward.
A) Dike
B) Sill
C) Batholith
D) Laccolith
4.2 True/False Questions
1) Over 82% of the volume of the Earth is in the form of metamorphic rocks.
2) Mineral crystals that form early in the crystallization process will have better-developed
crystal faces than later crystals.
3) The igneous rocks that make up the Sierra Nevada Mountains in California and Nevada are
extrusive.
4) Ultramafic rocks are commonly found forming at the Earth’s surface.
5) Igneous rocks with an andesitic composition are found along volcanic island arcs.
6) A porphyritic texture can include coarse phenocrysts surrounded by a phaneritic groundmass.
7) Pegmatitic textures represent a two-stage cooling process.
8) Pumice has a large percentage of voids and therefore has the ability to float in water.
9) The large pieces of material in a welded tuff are mineral crystals that formed during the
cooling process.
10) Temperatures will decrease with increasing depth into the Earth.
11) Partial melting will occur when continental collisions occur, thickening the crust and forcing
some rocks to a depth where melting can occur.
12) Magma mixing occurs when rising magma incorporates some of the surrounding country
rock, melting it.
13) Gabbro that undergoes partial melting will create a melt enriched in iron and magnesium.
14) Silica-rich lavas mainly erupt along continental margins.
15) Dikes tend to form as solitary structures.
16) Concordant plutons cut across existing structures while discordant plutons are parallel to
existing features.
17) Sills are intrusive igneous structures that display a fine-grained texture.
18) Plutonic bodies must have a surface exposure greater than 100 km2 to be called stocks.
4.3 Short Answer Questions
1) Define magma. Explain how magma differs from lava.
2) Magma can form up to 250 km deep in the Earth. If magma forms at such a depth, explain
how magma at that depth is able to rise to the surface.
3) Below is a list of elements. Eight of these elements represent the most common elements on
Earth. Circle the eight most common elements found in the Earth’s crust.
Silicon, Rubidium, Nitrogen, Carbon, Sodium, Manganese, Hydrogen, Oxygen, Potassium,
Calcium, Phosphorous, Aluminum, Iron, Chlorine, Magnesium, Uranium, Helium, Gold
4) Will density increase or decrease with the crystallization of a magma?
5) How does the presence of silica affect the temperature and viscosity of magma?
6) Slow cooling produces ________ crystals whereas fast cooling produces ________ crystals.