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Laboratory 4 : Volcanoes, Plutons, and Igneous Rocks: ESS 101 B Wi 22: Introduction To Geology And Societal Impacts
Laboratory 4 : Volcanoes, Plutons, and Igneous Rocks
Due Feb 6 at 11:30pm
Points 15
Questions 42
Available Jan 28 at 9am - Feb 6 at 11:30pm 10 days
Allowed Attempts 2
Time Limit None
Instructions
The Prelab Video serves as an introduction to the topics covered in this lab. See below.
Igneous Processes pre lab line here
(https://youtu.be/m24UfUP60lo)
(https://youtu.be/m24UfUP60lo)
A PDF version of this lab can be found below:
Lab4_IgneousRocks_Sum20.pdf
This quiz was locked Feb 6 at 11:30pm.
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Laboratory 4 : Volcanoes, Plutons, and Igneous Rocks: ESS 101 B Wi 22: Introduction To Geology And Societal Impacts
Learning Objectives
By completing this lab, students will learn how:
Igneous rocks form when magma cools, either slowly underground or
rapidly at earth’s surface (i.e. near volcanoes).
Magma cooling processes affect the size of crystals in igneous rocks.
Bigger crystals form in slow-cooling, underground magma chambers,
and smaller crystals form in fast-cooling lava flows/eruptions.
To recognize eight common igneous minerals, and nine common
igneous rocks.
Mafic rocks are usually dark-colored and felsic rocks are usually lightcolored.
The viscosity of lavas relates to their silica content. Felsic (or silicarich) lavas are more viscous than mafic (or silica-poor) lavas that flow
like rivers. Lava viscosity affects the shape of volcanoes.
Introduction
Igneous rocks form when liquid rock cools and crystallizes. Liquid rock is
called magma when underground and lava when on the surface. Igneous
rocks have different chemical and physical characteristics depending on
what they are made out of and how or where they formed. Like using
paleomagnetic stripes to understand tectonic movement, geologists can
use other information recorded in igneous rocks to study earth’s history. In
this lab, we will examine several different igneous rocks, and we will
explore the various conditions under which they formed. Igneous rocks
are classified by mineral composition and by mineral grain texture. This
classification system is interpretive, because these characteristics imply
something about the source of the magma and the conditions under which
the rock formed.
Chemical composition. The mineralogy of an igneous rock is
related to the chemical composition of the cooling parent magma
from which it forms. For example, a magma that is mostly Si, Fe,
Mg, Al, Ca, and O, a common composition for mantle melts, is
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likely to form olivine, a mineral made out of Si, Fe, Mg, and O and
plagioclase, a mineral made out of Si, Al, Ca, and O. The chemical
ingredients in the melt turn into the minerals in the rock.
Texture. The sizes and arrangement of the minerals in an
igneous rock is known as texture. In this case, texture is not
how the rock feels but how it looks. Igneous rock texture
relates to the cooling environment in which the rock formed.
The same melt composition can form very different rocks
depending on the cooling rate.
Slow cooling inside the earth forms large (greater than
about a quarter of a millimeter) crystals, which slowly, over
thousands of years grow together to form intrusive, or
plutonic, rocks.
Volcanic eruptions result in rapid magma cooling (seconds
to days), and the formation of igneous rocks with small
grains (often too small to see with your eyes). During a
volcanic eruption, magma does not have enough time to
solidify before it reaches the surface. The rocks that form
at the surface during a volcanic eruption cool rapidly and
are called extrusive or volcanic rocks.
A. Igneous Rock Composition
99% of the total bulk of most igneous rocks is made up of only eight
elements:
silicon (Si)
sodium (Na)
oxygen (O)
aluminum (Al)
magnesium (Mg)
calcium (Ca)
iron (Fe)
potassium (K)
Most of these elements occur in the crystal structures of eight minerals,
which constitute over 95% of the volume of all common igneous rocks.
Therefore, these minerals are of paramount importance in the study of
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igneous rocks. Below is a list of the common igneous rock-forming
minerals, along with their diagnostic properties and chemical formulas:
Mineral
Properties
Chemical Formula
olivine
Green to yellow-green;
vitreous; small,
(Mg,Fe)2SiO4
equidimensional grains
Usually White or Gray; 2
cleavages at 90°; elongate
grains; striations
plagioclase
Black, greenish black, or
brownish black; rather dull
luster; blocky grains; poor
pyroxene
Calcium Plagioclase:
CaAl2Si2O8
Sodium Plagioclase:
NaAlSi3O8
Complex Ca-Mg-Fe-Al
silicates
cleavage, 2 planes at 90°
amphibole
Black with shiny, splintery
appearance; two cleavages at
60° and 120°; elongate grains
biotite
Shiny, black flexible sheets;
one perfect cleavage
Usually white or pink; 2
cleavages at 90°;
orthoclase
Complex hydrous NaCa-Mg-Fe-Al silicates
K(Mg,Fe)3AlSi3O10(OH)2
KAlSi3O8
equidimensional grains
muscovite
Shiny, silvery flexible sheets;
one perfect cleavage
KAl3AlSi3O10(OH)2
Colorless to gray; glassy with
quartz
conchoidal fracture; irregular
grains in intrusive rocks;
SiO2
equidimensional phenocrysts
in extrusive rocks
Table 4-1 Eight most common minerals found in igneous rocks, their diagnostic
properties, and chemical composition.
Most igneous rocks can be classified into three compositional groups,
based on a particular assemblage of minerals:
1. Mafic magma/lava cools to produce dark-colored (or green) rocks that
are composed of dense dark (Fe-Mg rich) minerals that crystallize at
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high (e.g., 1000°C) temperatures.
2. Intermediate magma/lava cools to produce intermediate-color (or
gray) rocks that are composed of the minerals that crystallize at midrange (e.g., 800°C) temperatures.
3. Felsic magma cools to produce light-colored (or orange) rocks that
are composed of the light-colored, lower-density (silica) minerals that
crystallize at comparatively low (e.g., 600°C) temperatures.
You used the following tables to identify common minerals in Lab 3. You'll
need them again to identify minerals in this lab.
Non-metallic Minerals
Metallic Minerals
Light Colored Minerals
Dark Colored Minerals
Table A-1
Table A-2
Table A-3
Question 1
0.25 / 0.25 pts
Laboratory Honor Statement
Cheating or plagiarism of any kind will not be tolerated in ESS 101. This
includes copying answers from a friend or classmate, copying answers
verbatim found on the internet or other literary sources, or copying any
work that may answer the question being asked. Make sure you always
use your own words when answering the questions in the homework and
cite appropriate references if you use them to help you answer the
question. Anyone caught violating the academic code of conduct
(https://www.washington.edu/cssc/for-students/academic-misconduct/) will
receive a “0” grade on the assignment, and if the conduct is deemed
egregious, reported to the UW Academic Misconduct representative.
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I acknowledge that I have carefully read and understand the above
statement regarding the consequences of cheating and plagiarism, and
promise to complete my work in this class with honesty and
integrity. Answer "True" below supporting your acknowledgement.
True
False
Question 2
0.5 / 0.5 pts
Use the following diagnostic properites to identify Mineral 6, a mineral
commonly found in igneous rocks.
Luster
This mineral reflects light like glass. What is the luster of this
mineral? vitreous
Streak
The streak of this mineral is white or gray.
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Hardness
This mineral can scratch glass. What is the minimum hardness of
this mineral? 6
Cleavage vs Fracture
This mineral has 2 cleavage planes that meet at ~90 degrees, but
these cleavage planes are usually difficult to see.
Color & Other Diagnostic Properties
How would you describe the color of this mineral based on the
above photo? dark green to black
Now use these characteristics and Tables A1-3 to identify this
mineral.
This mineral is pyroxene .
Answer 1:
vitreous
Answer 2:
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6
Answer 3:
dark green to black
Answer 4:
pyroxene
Question 3
1.5 / 1.5 pts
Identify the other seven common igneous rock-forming minerals displayed
on the following image. You identified all of these minerals last week in
Lab 3B. Refer to your mineral identification charts (Appendices A-1, A-2,
and A-3) and the diagnostic properties of igneous rock-forming minerals
(Table 4-1). Note that you identified Mineral 6 above.
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Mineral 1
biotite
Mineral 2
orthoclase
Mineral 3
olivine
Mineral 4
amphibole
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Mineral 5
muscovite
Mineral 7
plagioclase
Mineral 8
quartz
0.5 / 0.5 pts
Question 4
Examine the figure below with Minerals 1-8.
a. The minerals on the top row all
have relatively dark colors. What
type of minerals are these?
b. The minerals on the bottom row
all have relatively light colors, what
mafic
felsic
type of minerals are these?
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c. The double arrow next to mineral
M7 indicates that it has a spectrum
of possible compositions and can be
grouped with either the dark or the
intermediate
light minerals. What type of mineral
falls between mafic and felsic?
Answer 1:
mafic
Answer 2:
felsic
Answer 3:
intermediate
Question 5
0.1 / 0.1 pts
Examine Table 4-1. What two elements are found in all eight of the
minerals?
silicon
oxygen
Answer 1:
silicon
Answer 2:
oxygen
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Bowen’s Reaction Series
At this point, you understand how to differentiate a mafic mineral from a
felsic one, and have a sense of the differences in chemical composition
between those groups. The next clear question, then, is how do these
differences arise?
The answer to this question lies in how igneous rocks form: through the
cooling of magma. As a magma cools, it crystallizes to solid rock by the
nucleation (initial formation of a crystal from a solution, or liquid) and
growth of various mineral crystals. Importantly, the order in which
minerals crystallize out of a magma directly depends on
temperature.
The figure below illustrates the order in which minerals crystallize as a
magma cools, starting with high temperature at the top. As you can see,
different minerals crystalize at different temperatures. This relationship is
termed Bowen’s Reaction Series. We can then classify the rock they
form as mafic, intermediate, and felsic based on the final mineral
composition, as shown on the right side of the figure. Note that
“ultramafic” (composition of upper mantle rock) is just a sub-category of
mafic rocks.
Figure 4-1. Bowen's Reaction Series, which describes the
crystallization temperatures of minerals.
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The next question you could ask is won’t a fully cooled magma have all
of these minerals? After all, the magma will start at a high temperature
and cool to a very low temperature. Simply put, however, the answer is
no. But why? The answer lies in the elements that you found were shared
amongst all of these minerals. If we combine those elements, we get the
compound silica (SiO2). The concentration of silica in the magma melt will
affect whether a newly crystalized mineral is chemically compatible or
incompatible. A chemically compatible mineral will continue to grow,
while a chemically incompatible mineral will react with the melt.
Higher concentrations of silica in the melt will cause the minerals that
crystalize first (i.e., at higher temperatures) to react and form a new
mineral. For example, olivine in a melt with high silica concentrations
would react to form pyroxene, which would react to form amphibole, and
so on. Lower concentrations of silica in the melt, however, would limit this
cascading reaction. Therefore, the concentration of felsic minerals in
an igneous rock is proportional to the amount of silica in the magma
melt.
If we focus on the upper part of Bowen’s Reaction Series, it has two
reaction paths: continuous and discontinuous. If a magma cools at a slow
rate, some minerals follow a series of discontinuous reactions to form the
next lower-temperature mineral. For example, olivine crystallizes first, but
as the temperature decreases, the olivine can dissolve back into the melt
and form pyroxene crystals. At these same temperatures, plagioclase
crystals also grow; however, they continuously react with the melt to form
more sodium-rich compositions of plagioclase. A whole range of
plagioclase minerals exist, ranging from those with 100% calcium to those
with 100% sodium.
Using what you've learned about Bowen's Reaction Series,
answer the following questions.
Question 6
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An igneous rock is examined and found to have a very low silica content.
Which minerals are most likely to be found in this rock?
Amphibole, Muscovite, Quartz
Olivine, Biotite, Muscovite
Olivine, Pyroxene, Ca-rich Plagioclase Feldspar
Pyroxene, Potassium Feldspar, Quartz
Amphibole, Na-rich Plagioclase Feldspar, Biotite
Question 7
0.1 / 0.1 pts
A rock is found to be composed almost entirely of quartz, but with a small
amount of a flaky mineral that breaks into thin sheets when you scratch it.
What is the mineral composition of this rock?
Quartz, Potassium Feldspar
Quartz, Pyroxene
Quartz, Muscovite
Quartz, Na-rich Plagioclase Feldspar
Muscovite, Biotite
Question 8
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A rock has quartz, orthoclase, and a black mineral. What is the black
mineral most likely, according to Bowen’s Reaction Series?
Biotite
Amphibole
Pyroxene
Muscovite
Quartz
Now that we better understand how a rock gets its mineral
composition, we can start to classify rocks based on that
mineralogy. Fill in the blanks in the following questions.
Question 9
0.1 / 0.1 pts
A rock that is composed of amphibole, biotite, and orthoclase can best be
described as ____ .
mafic
intermediate
felsic
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Question 10
0.1 / 0.1 pts
A mafic rock is found to contain 3 minerals in equal proportion. Two of
these are olivine and amphibole. The third is most likely ____.
pyroxene
muscovite
quartz
Question 11
0.1 / 0.1 pts
An igneous rock that formed from a magma with a very low silica
concentration is most likely ____ .
mafic
intermediate
felsic
Question 12
0.1 / 0.1 pts
In contrast, an igneous rock that formed from a magma with a very high
silica concentration is most likely ____ .
mafic
intermediate
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felsic
B. Igneous Rock Texture
Texture refers to the size, shape, and arrangement of the crystals or
grains composing a rock. It has nothing to do with how a rock feels,
just how it looks. Texture is a consequence of the physical and chemical
conditions under which it formed.
Crystalline Textures
As magma cools within the earth’s crust, minerals within the magma grow.
Eventually the magma body completely solidifies with interlocking crystals,
which results in a crystalline texture. The temperature of surrounding
rock is higher at greater depths, resulting in a slower rate of cooling of the
magma chamber. In general, the more slowly a magma cools, the larger
the mineral crystals will be, because slow cooling provides more time for
the chemical constituents to migrate to the growing mineral. When magma
reaches the surface as lava, the melt solidifies extremely rapidly to form
small, invisible crystals.
Since the mechanism of formation has a distinct impact on crystal size,
two main categories are used to classify crystalline texture. By identifying
grain size, we can tell whether a rock is intrusive or extrusive.
(1) Coarse-grained (phaneritic): those that contain individual
mineral grains that may be observed without the aid of a microscope
(larger than 0.062 mm).
(2) Fine-grained (aphanitic): those that contain individual mineral
grains too small to be observed without the aid of a microscope
(smaller than 0.062 mm).
Question 13
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Identify the crystal size and mode of origin of the rocks below.
sample
crystal size
mode of origin
A
fine-grained
extrusive
B
coarse-grained
intrustive
Answer 1:
fine-grained
Answer 2:
extrusive
Answer 3:
coarse-grained
Answer 4:
intrustive
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The examples above are described as equigranular rocks, where all
crystals are roughly the same size. This indicates that the rocks formed
from cooling at a constant rate. However, this is rare in nature. Most
magma bodies move through the crust at some time, which changes the
rate of cooling. Often, a magma body that begins to cool intrusively will
breach the surface, bringing with it any coarse-grained crystals already
formed. Complex cooling histories, where cooling rate changes during
formation, are referred to as two-stage cooling.
Two-stage cooling results in porphyritic texture, where larger grains
formed early in the cooling process (phenocrysts) are surrounded by a
matrix or groundmass of smaller grains. Depending on whether the
magma ultimately cooled in the crust or at the surface, the groundmass
can be coarse or fine-grained (Note in Appendix B-1 that the groundmass
is used to identify rock names).
Take a look at the following rocks which exhibit porphyritic
texture representative of two-stage cooling histories.
Question 14
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a) This rock above contains many large (25cm across) phenocrysts that are salmon
pink. Which mineral is this?
orthoclase
(options: amphibole, biotite, muscovite,
olivine, orthoclase, plagioclase, pyroxene,
quartz)
b) Given Bowen’s reaction series, what are
3 minerals that most likely comprise the
matrix?
muscovite
orthoclase
(options: amphibole, biotite, muscovite,
olivine, orthoclase, plagioclase, pyroxene,
quartz)
c) Is the grain size of the groundmass finegrained or coarse-grained?
d) Is this an intrusive or extrusive rock?
quartz
coarse grained
intrusive
Answer 1:
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orthoclase
Answer 2:
Muscovite
Answer 3:
orthoclase
Answer 4:
quartz
Answer 5:
coarse grained
Answer 6:
Intrusive
Question 15
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Identify the phenocryst (A) and groundmass (B) in the rock above
(shown in the figure on the left). The phenocryst has been magnified on
the figure on the right. Note the changing color of the phenocryst from its
interior (gray) to its exterior (white).
a) Identify the mineral of the largest
phenocrysts (A).
plagioclase
b) According to Bowen’s Reaction
Series does the mineral which forms
the large phenocrysts (magnified
continuous
image shown in the left figure above)
crystallize as a discontinuous or
continuous reaction series?
interior: more Ca-rich
c) How would the composition in the
mineral vary from its interior (more gray
color) to its exterior (more white color)?
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exterior:
[ Select ]
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d) What is the composition of the
groundmass?
[ Select ]
e) Where did the final stage of cooling
occur?
[ Select ]
Answer 1:
plagioclase
Answer 2:
continuous
Answer 3:
more Ca-rich
Answer 4:
more Na-rich
Answer 5:
intermediate
Answer 6:
on the earth's surface
Question 16
Not yet graded / 1.5 pts
Describe the formation history of the rock from question 15. Address the
tectonic setting, source rock of the magma, and physical conditions under
which it formed.
Your Answer:
The formation history of the rock from question 15 is a result of the twostage cooling. One part of the rock formed submicroscopic small crystals,
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meaning it cooled quickly and extrusively. The other part has large
crystals which means the rock cooled slowly and intrusively.
Question 17
0.2 / 0.2 pts
Olivine commonly occurs as a phenocryst in basalt, which is a mafic
volcanic rock (pictured above, note the large, green phenocrysts). What
other mineral do you think is a common phenocryst in basalt? **Hint: think
about when phenocrysts form relative to the whole rock and the
crystallization/melting temperature of the respective mineral
pyroxene
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calcium-rich plagioclase feldspar
amphibole
biotite
quartz
0.25 / 0.25 pts
Question 18
Determine the composition and texture of the two rocks below.
rock
composition
R1
feslic
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texture
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[ Select ]
[ Select ]
R6
fine-grained
Answer 1:
feslic
Answer 2:
coarse-grained
Answer 3:
felsic
Answer 4:
fine-grained
Question 19
0.1 / 0.1 pts
Note that the composition of the rocks in question 18 should be the same
(if not, go back!). If this implies that each rock formed from a similar melt,
then why do these rocks have different textures?
R6 is more weathered than R1.
These two rocks cooled at different rates. R6 cooled more slowly than R1.
These two rocks cooled at different rates. R1 cooled more slowly than R6.
R6 crystallized underground, while R1 crystallized on the surface.
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Volcanic Textures
Volcanic eruptions are complex, dynamic events that subject cooling lava
to a wide range of special conditions. Watch some of this video of a
volcanic eruption
(https://www.youtube.com/watch?v=NGcbNn4Vk1w)
to see brand new extrusive rocks being made (to watch, you'll need to
follow the link to YouTube). The variable conditions of formation of
volcanic rocks create unique textures, which include:
(3) Glassy: When magmas cool very rapidly at the Earth’s surface
there frequently is not sufficient time for atoms to combine, and
minerals do not nucleate and form crystals. These hardened
magmas are noncrystalline solids and have a glassy texture.
(4) Vesicular: Some magmas may contain large amounts of
dissolved gas that expands as gas bubbles as the magma rises. If
extruded magma cools rapidly, some gas bubbles may not be able
to escape. These trapped bubbles form holes (vesicles) which form
a porous (spongy), or vesicular texture.
(5) Fragmental: Some magma is explosively ejected into the
atmosphere during volcanic eruptions. This magma usually solidifies
as ash or larger volcanic fragments before falling back to the
surface. Igneous rocks that form by consolidation of this fragmental
debris typically have a pyroclastic or fragmental texture. A
pyroclastic flow is a common process to form fragmental volcanic
rocks (See below video).
Dome collapse and pyroclastic flow at Unzen Volc…
Volc…
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Identifying Igneous Rocks
An igneous rock may be classified by comparing its textural and
mineralogical properties with the Igneous Rock Identification Chart (Table
B-1). Be aware that igneous rock types are texturally and mineralogically
gradational. Therefore, the subdivisions between rock types are
somewhat arbitrary.
Table B-1: Igneous Rock Identification Chart. *Some igneous rocks
have a porphyritic texture, which means it has minerals of at least
two distinctive sizes. If a rock is predominantly fine-grained and
mafic, it is a basalt. If phenocrysts (the larger mineral grains) are
present in the fine-grained matrix, this rock is called a porphyritic
basalt.
Use the Igneous Rock Identification Chart (Table B-1) and the
photographs below to name rock specimens R1-R10.
Question 20
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What is the composition of R1? felsic
Some of the minerals you may recognize to help you determine its composition,
such as the pink mineral and glassy mineral.
What is the texture of R1? coarse-grained
R1 is granite .
Answer 1:
felsic
Answer 2:
coarse-grained
Answer 3:
granite
Question 21
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What is the composition of R2? mafic
What is the texture of R2? coarse-grained
R2 is gabbro .
Answer 1:
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mafic
Answer 2:
coarse-grained
Answer 3:
gabbro
Question 22
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What is the composition of R3? mafic
What is the texture of R3? porphyritic
Notice that this sample has green phenocrysts surrounded by a matrix of darker
minerals.
R3 is basalt .
Answer 1:
mafic
Answer 2:
porphyritic
Answer 3:
basalt
Question 23
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What is the composition of R4? intermediate
What is the texture of R4? coarse-grained
R4 is diorite .
Answer 1:
intermediate
Answer 2:
coarse-grained
Answer 3:
diorite
Question 24
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What is the composition of R5? felsic
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What is the texture of R5? glassy
R5 is obsidian .
Answer 1:
felsic
Answer 2:
glassy
Answer 3:
obsidian
Question 25
0.5 / 0.5 pts
What is the composition of R6? felsic
What is the texture of R6? fine-grained
R6 is rhyolite .
Answer 1:
felsic
Answer 2:
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fine-grained
Answer 3:
rhyolite
Question 26
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What is the composition of R7? intermediate
What is the texture of R7? porphyritic
R7 is andesite .
Answer 1:
intermediate
Answer 2:
porphyritic
Answer 3:
andesite
Question 27
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What is the composition of R8? mafic
What is the texture of R8? vesicular
R8 is scoria .
Answer 1:
mafic
Answer 2:
vesicular
Answer 3:
scoria
Question 28
0.5 / 0.5 pts
What is the composition of R9? felsic
What is the texture of R9? fragmental
**HINT** R9 was formed during a volcanic eruption.
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R9 is tuff/volcanic breccia .
Answer 1:
felsic
Answer 2:
fragmental
Answer 3:
tuff/volcanic breccia
Question 29
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What is the composition of R10? felsic
What is the texture of R10? vesicular
R10 is pumice .
Field geologists, and yourselves during this lab, will often differentiate R8
from R10 based on color. The lighter the color, the more felsic the
composition. Based on this relationship, which sample would be
composed of lower density minerals? R10
Based on its overall composition, which sample would tend to have
a lower overall density? R10
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Answer 1:
felsic
Answer 2:
vesicular
Answer 3:
pumice
Answer 4:
R10
Answer 5:
R10
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Question 30
Watch the two videos below. Samples R8 and R10 are immersed in
water. Watch what happens to each sample as they are immersed in
water. **Note: the bubbles escaping from the samples are air bubbles,
and do not reflect the gas content of the original lava.
R8
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R10
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Based on these two videos, which sample has a lower density?
R8 has lower density.
R10 has lower density.
Question 31
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Samples R8 and R10 both represent volcanic igneous rocks, whose
textures reflect the presence of gas in the lavas from which they formed.
Based on composition and the above density experiments, which lava do
you think contained more gas?
The lava from which R8 formed contained more gas.
The lava from which R10 formed contained more gas.
D. Volcano Comparison
In this section, we will think about how differences in parent magma
composition can lead to volcanic environments with different
characteristics. We will compare Mount Saint Helens (Figure 4-2; an
active volcano in the Cascade volcanic arc, located between Seattle and
Portland, OR) with Kilauea (Figure 4-3; an active volcano in Hawaii,
formed by the mantle hot spot beneath the Pacific Plate).
At both volcanoes, lava sometimes erupts onto the earth surface. Past
volcanic rock flows are stacked on top of each other, forming volcanic
mountains.
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Figure 4-2: Mount Saint Helens erupting explosively in 1980.1
Figure 4-3: Kilauea erupting effusively in 2018.2
1
https://www.theatlantic.com/photo/2015/05/the-eruption-of-mount-st-helens-in-1980/393557/
(https://www.theatlantic.com/photo/2015/05/the-eruption-of-mount-st-helens-in-1980/393557/)
2
https://media.npr.org/assets/img/2018/06/07/ap_18158142427935_wide-
8f829d9188c83409350b9460abc564c21b93df19-s800-c85.jpg
(https://media.npr.org/assets/img/2018/06/07/ap_18158142427935_wide8f829d9188c83409350b9460abc564c21b93df19-s800-c85.jpg)
Question 32
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Based on its plate tectonic setting, Mount Saint Helens likely erupts…
basalt (more mafic than andesite)
andesite (more mafic than rhyolite; more felsic than basalt)
rhyolite (more felsic than andesite)
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Question 33
Based on its plate tectonic setting, Kilauea likely erupts...
basalt (more mafic than andesite)
andesite (more mafic than rhyolite; more felsic than basalt)
rhyolite (more felsic than andesite)
0.2 / 0.2 pts
Question 34
Observe the following two videos of lava eruptions at Kilauea and at
Mount Saint Helens.
Kilauea 2018 lava flow
(https://www.youtube.com/watch?
v=mSTEXd_mcLQ)
(https://www.youtube.com/watch?v=mSTEXd_mcLQ)
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Mount Saint Helens historic 1980 eruption
(https://www.youtube.com/watch?v=AYla6q3is6w)
(https://www.youtube.com/watch?v=AYla6q3is6w)
Based off of these two videos, complete the following sentences.
Mount Saint Helens displayed an explosive eruption.
Mount Saint Helens produced
[ Select ]
, which
affected cities hundred of miles away.
Kilauea displayed
[ Select ]
eruption.
Lava from the Kilauea eruption flowed over the landscape toward the
ocean.
Answer 1:
an explosive
Answer 2:
a large ash plume
Answer 3:
a non-explosive
Answer 4:
flowed over the landscape toward the ocean.
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Now let’s think about how the properties of these different lavas
result in different types of mountains. First we will compare the
slopes of the two volcanoes.
Question 35
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Use the topographic map of Mount St. Helens below to answer the
following questions. Note that one inch on the map equals 1859 feet on
the earth, and one mile is equal to 5,280 feet. Elevation contours are
given in feet above sea level. Measurements in inches are shown by the
ruler line on the map (from 0 to 5 inches).
Calculate the steepest portion of the slope in feet/mile along the SW flank
of Mount Saint Helens (measured between 0-3 inches on the ruler). The
closed contour at the crater rim is 8000 feet. 2370 ±100 feet/mile
**Hint: slope = rise/run. Find the change in elevation (in feet) using the contour
lines, then divide by the change in horizontal distance using the ruler bar and
conversions.**
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Convert the above slope (feet/mile) measurement into a percent slope.
You need to ensure that the distance units are the same. Rise (in feet)/
run (in feet) x 100. 45%
Use "Google Search" on smart phone or computer to convert percent
slope to degrees. 24.2
Answer 1:
2370 ±100 feet/mile
Answer 2:
45%
Answer 3:
24.2
Question 36
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Use the topographic map of the Mauna Loa summit, Hawaii below to
answer the following questions. Remember that Mauna Loa and
Kilauea formed in the same way. Note that the representative fraction is
1:24,000 and the contour interval is 40 feet. We have added a scale bar
that denotes 1 mile distance along the east flank of the summit. An
index map showing the area defined in the topographic map of Mauna
Loa's summit is shown below.
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Calculate the slope in feet/mile along the east summit ridge of Mauna
Loa.
**Hint: slope = rise/run. Find the change in elevation (in feet) using the
contour lines, then divide by the change in horizontal distance using the
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ruler bar.
600 feet/mile
**Hint: slope = rise/run. Find the change in elevation (in feet) using the contour
lines, then divide by the change in horizontal distance using the ruler bar.**
Convert the above slope (feet/mile) measurement into a percent slope.
You need to ensure that the distance units are the same. Rise (in feet)/
run (in feet) x 100. ~11.4%
*Remember that 5280 feet are in 1 mile.
Use "Google Search" on smart phone or computer to convert percent
slope to degrees. 6.5 degrees
Answer 1:
600 feet/mile
Answer 2:
~11.4%
Answer 3:
6.5 degrees
Question 37
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The above measurements of Mount Saint Helens and Mauna Loa can be
used to make the following generalization.
Volcanoes that are composed of more felsic lava are also steeper
Volcanoes that are composed of more mafic lava are also steeper
Lava composition has no bearing on the morphology of a volcano
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Next we will explore some of the lava characteristics that
contribute to these differences.
Question 38
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Think about Bowen’s Reaction Series and the general temperatures at
which compositionally different magmas can crystallize. Which of the
following is true?
The lava from Mount Saint Helens will crystallize at a colder temperature
than Mauna Loa.
The lava from Mauna Loa will crystallize at a colder temperature than
Mount Saint Helens.
There is no relationship between the crystallization temperature of a lava
and its composition.
Question 39
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Based on your observations from the above videos, and your knowledge
of plate tectonic settings, basaltic lava is less viscous than andesitic lava.
Answer 1:
less
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Question 40
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How does the viscosity of a lava affect the steepness of a volcano slope?
Select all answers that apply.
The viscosity of lava has no impact on slope steepness.
Lavas with higher viscosity will pile up on themselves, leading to steeper
slopes.
Lavas with lower viscosity will spread out and flow farther distances,
leading to less steep slopes.
Lavas with lower viscosity will cool more quickly, leading to steeper slopes.
Lavas with higher viscosity will cool more slowly, leading to less steep
slopes.
The explosive eruption of Mt. St. Helens in 1980 produced a blast wave
that extended northward, flattening trees in its wake. As shown in the
Google Earth image below, the effects of that blast can still be seen today,
extending over 15 km to the north of the volcano.
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Figure 4-4. This aerial image shows the 15 km that the eruption of Mt. St. Helens
affected.
While this blast was devastating to those in the immediate vicinity of Mt.
St. Helens, the eruption had impacts on those living much farther from the
volcano.
Question 41
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Would you expect a similar long-range impact from the eruption of
volcanoes like Mauna Loa or Kilauea?
Yes
No
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Question 42
Not yet graded / 1.6 pts
Below is an image from the map of plate tectonics we used in Lab 2.
Based on the tectonic setting of the Aleutian Islands (the islands
extending off the coast of Alaska, shown in the black box), do you think
eruptions of the volcanoes there would have long-range impacts like Mt.
St. Helens? Why or why not?
Your Answer:
Based on the tectonic setting of the Aleutian Islands, I think that the
eruptions of the volcanoes would be more similar to Mt. Saint Helens
because they are both stratovolcanoes.
Quiz Score: 11.9 out of 15
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