Ch. 4: Magma & Igneous Rocks

Question 1

The three most common volatiles in magma are:

Answer 1

carbon dioxide, sulfur dioxide, and water vapor

Question 2

Considering a felsic magma, what was the likely composition of the source of that magma, knowing that most magma forms by partial melting of a source rock?

Answer 2

intermediate

Question 3

determine the composition of the granite and basalt below

Answer 3

granite = felsic

basalt = mafic

Question 4

What does grain size generally indicate about the cooling of a melt?

Answer 4

larger crystals indicate that the melt cooled very slowly at depth

Question 5

Based on what we learned about igneous composition and texture, which of the following statements is TRUE?

Answer 5

any composition of magma/lava can cool to form any of the discussed textures, because the two properties are independent from one another

Question 6

Which of the following is a chemical equivalent of granite?

Answer 6

rhyolite

Question 7

If a body of magma becomes more felsic (richer in Si), its viscosity will:

Answer 7

increase

Question 8

The hotter the magma…

Answer 8

the less viscous it becomes

Question 9

Match the following magma producing processes with their definitions:

1. the melting temperature of hot material is lowered as the surrounding pressure decreases
2. water is added to a region with rocks at a temperature where they will melt if water is added
3. hot material moves into a region with higher temperatures. than its melting temperature
4. hot material moves into a region with water or carbon dioxide, which lowers its melting temperature

Answer 9

1. decompression melting
2. volatile melting
3. heat transfer melting
4. not a form of melting

Question 10

Cooled magma chambers from _____ whereas cooled volcanic feeders form ____, which are both ____ igneous features.

Answer 10

1. plutons
2. dikes
3. intrusive

Question 11

What is the geothermal gradient?

Answer 11

the increases in temperature of the Earth with increased depth

Question 12

Why does water induce melting in a subduction zone?

Answer 12

the addition of water lowers the melting of rock

Question 13

If you had the ability to travel anywhere on Earth, inside or out, where would be the best place to go find peridotite?

Answer 13

the mantle

Question 14

Most igneous rocks are composed of ____ minerals.

Answer 14

silicate

Question 15

Why does a higher temperature cause the viscosity of a magma to decrease?

Answer 15

heat breaks bonds between atoms and allows them to move easily past one another

Question 16

What does pyroclastic texture in an igneous rock suggest about its formation?

Answer 16

the rock likely formed by a violent volcanic eruption

Question 17

Magmas are classified based on their SiO2 content. Match the following composition terms with the amount of SiO2 contained in a rock of the composition.

1. felsic
2. intermediate
3. mafic
4. ultramafic

Answer 17

1. 66-76%
2. 52-66%
3. 45-52%
4. 38-45%

Question 18

Based on what you learned about minerals and igneous rocks, and based on the following mineral compositions, are following mafic or felsic minerals?

1. olivine - Mg2SiO4
2. quartz - SiO2
3. biotite mica - KMg3AlSi3O10(OH)2
4. feldspar - KAlSi3O8

Answer 18

1. mafic
2. felsic
3. mafic
4. felsic

Question 19

Water induced melting at subduction zones comes from where?

Answer 19

water contained within minerals in the subducting plate is released during metamorphism

Question 20

This is a glassy textured igneous rock that is black in color. What rock is this?

Answer 20

obsidian

Question 21

What is the general temperature range that most silicate magmas exist?

Answer 21

650-1100 degrees Celsius

Question 22

Magmas are composed of which of the following components:

Answer 22

liquid melt, solid minerals, gasses (either dissolved in the melt or as gas bubbles)

Question 23

Which of the following could cause rock to melt?

Answer 23

increase in temperature, change in composition, decrease in pressure

Question 24

For each of the tectonic settings listed below, choose the most likely form of melting you would expect to see:

1. mid ocean ridge
2. felsic magma in a continental rift
3. oceanic hot spot
4. subduction zone
5. continent continent collision

Answer 24

1. decompression melting
2. heat transfer melting
3. decompression melting
4. volatile melting
5. no melting

Question 25

What type of texture occurs in igneous rocks that cool so rapidly that unordered ions are “frozen” in place?

Answer 25

glassy

Question 26

When will magma rise to Earth’s surface?

Answer 26

when the magma is less dense than adjacent rock

Question 27

In general, when will melted rock in the mantle rise?

Answer 27

almost always

Question 28

How are felsic magmas formed?

Answer 28

Heat from the mantle melts part of the lower crust.

Question 29

In general, why does Earth’s crust sit on top of the mantle?

Answer 29

Earth’s crust is less dense than the mantle.

Question 30

How do mafic volcanic rocks get to Earth’s surface?

Answer 30

Pressure from overlying crust forces mantle magmas through cracks to Earth’s surface. These magmas erupt out of volcanoes, cool, and solidify into rock.

Question 31

Which of the choices below lists the four basic compositional groups of igneous rocks in order from highest silica content to lowest silica content?

Answer 31

felsic, intermediate, mafic, ultramafic

Question 32

Based on the diagram shown in the video, name two minerals that are unlikely to coexist in the same igneous rock.

Answer 32

Muscovite and pyroxene

Question 33

Igneous rocks of felsic composition have __________ and __________ relative to mafic igneous rocks.

Answer 33

greater silica; lower melting temperature

Question 34

Which of the following is true about mafic igneous rocks?

Answer 34

Mafic igneous rocks are rich in Fe, Mg, and Ca

Question 35

You have found a mafic rock with a phaneritic texture. What is a potential rock name?

Answer 35

Gabbro

Question 36

Based on what you learned in this video, what minerals make up rhyolite?

Answer 36

Quartz, K-feldspar, and plagioclase feldspars, with relatively fewer biotite and amphiboles

Question 37

What is one way a glassy texture forms?

Answer 37

Lava cools rapidly.

Question 38

Which of the following describes a texture containing bubble-shaped cavities?

Answer 38

Vesicular

Question 39

Which of the following textures describes an igneous rock with anomalously large crystals?

Answer 39

Pegmatitic

Question 40

What does an igneous rock with a pyroclastic texture tell a geologist?

Answer 40

The rock was likely formed by a violent volcanic eruption.

Question 41

Which of the following textures indicates two stages of cooling and crystallization?

Answer 41

Porphyritic

Question 42

As a result of partial melting, magma originates in __________.

Answer 42

the mantle and crust

• Tectonic forces act in the mantle and crust to cause rocks to melt and form magma.

Question 43

When lava erupts at Earth’s surface, what type of rock is produced?

Answer 43

extrusive

• Magma that reaches the surface cools quickly, has fine-grained texture, and produces extrusive rocks.

Question 44

Pyroclastic debris from a volcanic eruption can include _____.

Answer 44

ash and cinders

• Along with bombs and lapilli, ash and cinders are common components of pyroclastic debris ejected from volcanoes.

Question 45

Why does magma have a tendency to rise after its formation?

Answer 45

It is less dense than the surrounding rock.

• As rock melts, the density decreases. This process causes the magma to rise.

Question 46

Most igneous rocks never reach the surface. However, igneous rocks other than those formed in volcanoes are found exposed on many parts of Earth. How do you account for this phenomenon?

Answer 46

They formed at depth and have been exposed by uplift and erosion.

Question 47

Which two processes commonly generate magma?

Answer 47

decompression melting and wet melting (the addition of volatiles)

Question 48

Where does the water involved in melting at subduction zones come from?

Answer 48

Water contained within minerals in the subducting plate is released during metamorphism.

Question 49

What is peridotite?

Answer 49

the rock making up the mantle

Question 50

Why is water a necessary component of the melting process in subduction zones?

Answer 50

The addition of water lowers the melting temperature of rock.

Question 51

Why is decompression melting common at mid-ocean ridges but not at subduction zones?

Answer 51

Tectonic plates are moving apart at mid-ocean ridges, resulting in a lowering of pressure beneath the ridge. Tectonic plates are moving together at subduction zones, resulting in an increase of pressure under new mountains.

Question 52

Which of the following could cause rock to melt?

Answer 52

change in composition, decrease in pressure, increase in temperature

Question 53

What experiments did Tuttle and Bowen perform?

Answer 53

tests of the temperature and pressure conditions required to melt granite and form magma

Question 54

What material was added to powdered rock during Tuttle and Bowen’s experiments?

Answer 54

water

Question 55

Which of the following statements about rock melting is most accurate?

Answer 55

Rock can remain solid, become partially melted, or melt completely at the same depth within Earth, depending on the temperature.

Question 56

How does water content of melt change with increasing pressure?

Answer 56

Water content increases with increasing pressure.

Question 57

Why is Tuttle and Bowen’s experiment useful to geologists?

Answer 57

This experiment allows geologists to interpret where granitic rock solidifies beneath Earth’s surface.

Question 58

How does water content of granitic magmas change as the magmas move towards Earth’s surface?

Answer 58

Water content decreases.

Question 59

How are igneous rocks formed?

Answer 59

Igneous rocks are formed by the cooling and crystallization of molten rock.

Question 60

Intrusive and extrusive igneous rocks are classified based on what criteria?

Answer 60

How quickly the molten rock cools and crystallizes, and where this occurs with respect to Earth’s surface

Question 61

What is required to generate an igneous rock with a fine-grained texture?

Answer 61

A fine-grained texture is the product of rapid cooling and crystallization of lava.

Question 62

A phaneritic texture is characteristic of a(n) __________ igneous rock that cooled __________.

Answer 62

intrusive; below the surface

Question 63

What does it mean if an igneous rock has an aphanitic texture?

Answer 63

The igneous rock exhibits mineral crystals too small to see with the naked eye.

Question 64

What type of igneous rock consists of very coarse crystals?

Answer 64

Pegmatite

• Pegmatites, exceptionally coarse-grained igneous rocks, may form under special conditions. Composed of interlocking crystals all larger than a centimeter in diameter, (pegmatitic texture), pegmatites form late in the crystallization of a magma, when water and other volatiles, such as carbon dioxide, chlorine, and fluorine, make up an unusually high percentage of the melt. Because ion migration is enhanced in these fluid-rich environments, the crystals that form are abnormally large. Thus, the large crystals in pegmatites are not the result of inordinately long cooling histories; rather, they are the consequence of a fluid-rich environment that enhances crystallization.

Obsidian is a dark-colored glassy rock that usually forms when silica-rich lava is quenched quickly. Pumice is a volcanic rock with a glassy texture that forms when large amounts of gas escape through silica-rich lava to generate a gray, frothy mass. Granite is a coarse-grained rock composed of about 25 percent quartz and roughly 65 percent feldspar, mostly potassium and sodium-rich varieties. Hexagonal quartz crystals are often glassy and clear to light gray in color. By contrast, feldspar crystals are generally white to gray or salmon pink in color, and exhibit a rectangular rather than hexagonal shape.

Question 65

Which of the following minerals crystallize early in Bowen’s reaction series?

Answer 65

olivine

• Continued cooling crystallizes the mineral biotite. During the crystallization process, the composition of the remaining liquid portion of the magma continually changes. The removal of iron, magnesium, and calcium from the magma, because these elements are the major constituents of the earliest-formed minerals, results in a melt that is enriched in sodium, potassium, and silica. The last minerals to crystallize are potassium feldspar, muscovite mica, and quartz.

Question 66

Which rock would be most likely to contain visible quartz and potassium feldspar crystals?

Answer 66

Granite

• Granite is a coarse-grained rock composed of about 25 percent quartz and roughly 65 percent feldspar—mostly potassium and sodium-rich varieties. Six-sided quartz crystals are often glassy and clear to light gray in color. By contrast, feldspar crystals are generally white to gray or salmon pink in color, and exhibit a rectangular rather than hexagonal shape.

Rhyolite is the extrusive equivalent of granite and, like granite, is composed essentially of the light-colored silicates.

Basalt is a very dark green to black, aphanitic rock composed primarily of pyroxene and calcium-rich plagioclase feldspar, with lesser amounts of olivine and amphibole. Gabbro is the intrusive equivalent of basalt. Like basalt, it tends to be dark green to black in color and composed primarily of pyroxene and calcium-rich plagioclase feldspar.

Question 67

Which of the following pairs of igneous rocks exhibit aphanitic texture?

Answer 67

Andesite; rhyolite

• Igneous rocks that form at the surface, or as small intrusive masses within the upper crust where cooling is relatively rapid, exhibit a fine-grained texture termed aphanitic. By definition, the crystals that make up aphanitic rocks are so small that individual minerals can only be distinguished with the aid of a polarizing microscope or other sophisticated techniques. Common igneous rocks that have an aphanitic texture are rhyolite, andesite, and basalt.

When large masses of magma slowly crystallize at great depth, they form igneous rocks that exhibit a coarse-grained texture described as phaneritic. Coarse-grained rocks consist of a mass of intergrown crystals that are roughly equal in size and large enough so that the individual minerals can be identified without the aid of a microscope. The common igneous rocks with a phaneritic texture are granite, diorite, and gabbro.

Question 68

Which texture listed below would be most unlikely to occur in an extrusive igneous rock?

Answer 68

Phaneritic

• When large masses of magma slowly crystallize at great depth, they form igneous rocks that exhibit a coarse-grained texture described as phaneritic.

Igneous rocks that form at the surface, or as small intrusive masses within the upper crust where cooling is relatively rapid, exhibit a fine-grained texture termed aphanitic. During some volcanic eruptions, molten rock is ejected into the atmosphere, where it is quenched quickly. Rapid cooling of this type may generate rocks having a glassy texture.

Igneous rocks form from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption. The ejected particles might be very fine ash, molten blobs, or large angular blocks torn from the walls of the vent during the eruption. Igneous rocks composed of these rock fragments are said to have a pyroclastic or fragmental texture.

Question 69

What do pumice and obsidian have in common?

Answer 69

Glassy texture

• Pumice is a volcanic rock with a glassy texture that forms when large amounts of gas escape through silica-rich lava to generate a gray, frothy mass.

Question 70

Which volcanic rock is extremely vesicular and glassy?

Answer 70

pumice

• Pumice is a volcanic rock with a glassy texture that forms when large amounts of gas escape through silica-rich lava to generate a gray, frothy mass. Vesicles are small holes left by escaping gas bubbles.

Pegmatites, exceptionally coarse-grained igneous rocks, form under special conditions in a magma chamber. Obsidian is a dark-colored glassy rock that usually forms when silica-rich lava is quenched quickly. Tuff is one of the most common pyroclastic rocks composed mainly of tiny, ash-size fragments that were later cemented together.

Question 71

What term describes the sizes, shapes, and arrangements of mineral grains in an igneous rock?

Answer 71

texture

• The term texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of its mineral grains. Texture is an important property because it reveals a great deal about the environment in which the rock formed.

An important aspect of the chemical composition of igneous rocks is silica content. Typically, the silica content of crustal rocks ranges from a low of about 40 percent in ultramafic rocks to a high of more than 70 percent in granitic rocks. The percentage of silica in igneous rocks correlates with the abundance of other elements. Mineral content is the types of minerals found in the rock. Bowen’s reaction series lists the sequence of mineral crystallization from magma.

Question 72

Which term describes the changing of the composition of magma by incorporating surrounding host rock?

Answer 72

assimilation

• In a process called assimilation, as magma migrates through the crust it may incorporate some of the surrounding host rock through melting. The magma’s composition can change through the incorporation of foreign rock material into the melt.

Magma mixing occurs when one magma body intrudes another having a different composition. The incomplete melting of rocks is known as partial melting, a process that produces most magma. The process of developing more than one magma type from a common magma is called magmatic differentiation.

Question 73

Which of the following are used for studying rocks with a polarizing or petrographic microscope?

Answer 73

thin sections

• Thin sections are very useful in identifying the mineral constituents in rocks. A slice of rock is cut from a hand sample using a diamond saw. This slice is cemented to a microscope slide and ground until it is transparent to light (about 0.03 millimeter thick). This very thin slice of rock, called a thin section, is examined under a petrographic microscope.

Polished cubes are sometimes used to examine rocks of all types, especially if they are being tested for strength. Broken chips are sometimes used in studies of rocks as use for aggregate. Hand specimens are typically used for a preliminary examination of most rock.

Question 74

The last minerals to crystallize on Bowen’s Reaction Series result in igneous rocks with a __________ composition.

Answer 74

felsic

• The last minerals to crystallize on Bowen’s Reaction Series result in igneous rocks with a felsic composition.

Bowen’s reaction series illustrates the sequence of mineral formation within magma. During the crystallization process, the composition of the remaining liquid portion of the magma also continually changes.

At the stage when approximately a third of the magma has solidified, the melt will be nearly depleted of iron, magnesium, and calcium because these elements are major constituents of the earliest-formed minerals that result in a mafic composition. The removal of these elements causes the melt to become enriched in sodium and potassium.

Because a basaltic magma contains about 50 percent silica, the crystallization of the earliest-formed mineral, olivine, which is only about 40 percent silica, leaves the remaining melt rich in silica, called silica enrichment.

Question 75

Which term describes a texture of igneous rocks that crystallized over a single, long period?

Answer 75

Phaneritic

• When large masses of magma slowly crystallize at great depth, they form igneous rocks that exhibit a coarse-grained texture described as phaneritic.

During some volcanic eruptions, molten rock is ejected into the atmosphere, where it is quenched quickly. Rapid cooling of this type may generate rocks having a glassy texture.

Igneous rocks form from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption. The ejected particles might be very fine ash, molten blobs, or large angular blocks torn from the walls of the vent during the eruption. Igneous rocks composed of these rock fragments are said to have a pyroclastic or fragmental texture.

Igneous rocks that form at the surface, or as small intrusive masses within the upper crust where cooling is relatively rapid, exhibit a fine-grained texture termed aphanitic.

Question 76

Which of the following igneous rocks has a pyroclastic texture, meaning it was formed from a glowing avalanche?

Answer 76

welded tuff

• Tuff is one of the most common pyroclastic rocks composed mainly of tiny, ash-size fragments that were welded together as they cooled. Igneous rocks composed of the consolidation of individual rock fragments that are ejected during a violent volcanic eruption are said to have a pyroclastic or fragmental texture. The ejected particles might be very fine ash, molten blobs, or large angular blocks from the walls of the vent during the eruption.

A porphyry forms when molten rock containing some large crystals move to a different environment, for example, by erupting at the surface; the remaining liquid portion of the lava would cool more quickly. The resulting rock has large crystals embedded in a matrix of smaller crystals. Basalt is a common igneous rock that forms at the surface, or as small intrusive masses within the upper crust where cooling is relatively rapid; it exhibits a fine-grained aphanitic texture.

Granite is a common igneous rock that forms when large masses of magma slowly crystallize at great depth; granites exhibit a coarse-grained phaneritic texture. When magma loses its mobility before reaching the surface it eventually crystallizes to form intrusive igneous rocks. Extrusive or volcanic igneous rocks result when molten rock reaches Earth’s surface, where it is called lava, and quickly cools.

Rocks with a composition between granitic and basaltic rocks are said to have an intermediate or andesitic composition.

Question 77

Which of the following is a factor that does not affect the generation of magma?

Answer 77

crystal size

• Crystal size indicates the rate of cooling, not the generation of magma. Slow cooling promotes the growth of fewer, but larger crystals. When cooling occurs rapidly, the result is a solid mass of tiny intergrown crystals producing a fine grained (aphanitic) texture.

Heat is an increase in temperature. Most magma originates when essentially solid rock, located in the crust and upper mantle, melts. The most obvious way to generate magma from solid rock is to raise the temperature above the rock’s melting point. Pressure change may involve melting, which is accompanied by an increase in volume. Changes in pressure may occur at higher temperatures at depth because of greater confining pressure. Reducing confining pressure lowers a rock’s melting temperature. When confining pressure drops sufficiently, decompression melting is triggered. Volatiles, including water content, are another important factor that affect the melting temperature of rock. Volatiles cause rock to melt at lower temperatures. Further, the effect of volatiles is magnified by increased pressure. Deeply buried “wet” rock has a much lower melting temperature than “dry” rock of the same composition. Therefore, in addition to a rock’s composition, its temperature, depth (confining pressure), and water content determine whether it exists as a solid or liquid.

Question 78

Why are lava flows typically finer-grained than intrusive igneous rocks?

Answer 78

Because the extrusive magma cools quickly and mineral grains do not have time to grow

• When cooling occurs rapidly—for example, in a thin lava flow—the ions quickly lose their mobility and quickly combine to form crystals. This results in the development of numerous embryonic nuclei, all of which compete for the available ions. The result is a solid mass of tiny intergrown crystals that produces a fine-grained igneous rock.

A very large magma body located many kilometers beneath Earth’s surface will cool over a period of perhaps tens to hundreds of thousands of years. Initially, relatively few crystal nuclei form. Slow cooling permits ions to migrate freely until they eventually join one of the existing crystalline structures. Consequently, slow cooling promotes the growth of fewer, but larger crystals.

Question 79

What is the dominant lava erupted from volcanoes on Hawaii and Iceland?

Answer 79

basalt

• Rhyolite is less common than basalt, and generally less voluminous. You can find extensive rhyolite lava flows and thick ash deposits at Yellowstone National Park. Andesite is a medium-gray, fine-grained rock of volcanic origin. In addition to the volcanoes of the Andes and the Cascade Range, many of the volcanic structures occupying the continental margins that surround the Pacific Ocean are of andesitic composition. Peridotite is the main constituent of Earth’s upper mantle.

Question 80

A __________ is an open cavity in a volcanic rock that was filled by a gas bubble when the lava was still mainly liquid.

Answer 80

vesicle

• Vesicles are small holes left by escaping gas bubbles. The rocks that contain them are said to have a vesicular texture. Rocks that exhibit a vesicular texture usually form in the upper zone of a lava flow, where cooling occurs rapidly enough to preserve the openings produced by the expanding gas bubbles.

A “vug” is a hole in rocks, which is sometimes filled with large crystals. A phenocryst is the large crystal embedded in a rock with a porphyritic texture. A xenolith is a piece of the country rock in an intrusive igneous rock.