[SOLVED] Lab 4: Volcanoes, Plutons, and Igneous Rocks

[SOLVED] Lab 4: Volcanoes, Plutons, and Igneous Rocks

Lab 4: Volcanoes, Plutons, and Igneous Rocks
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 light-colored.
● The viscosity of lavas relates to their silica content. Felsic (or silica-rich) 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 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)
● Oxygen (O)
● Magnesium (Mg)
● Iron (Fe)
● Sodium (Na)
● Aluminum (Al)
● Calcium (Ca)
● 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 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,
equidimensional grains
(Mg,Fe)2SiO4
Plagioclase
Usually White or Gray; 2
cleavages at 90°; elongate
grains; striations
Calcium Plagioclase: CaAl2Si2O8
;
Sodium Plagioclase: NaAlSi3O8
Pyroxene
Black, greenish black, or
brownish black; rather dull
luster; blocky grains; poor
cleavage, 2 planes at 90°
Complex Ca-Mg-Fe-Al silicates
Amphibole
Black with shiny, splintery
appearance; two cleavages at
60° and 120°; elongate grains
Complex hydrous Na-Ca-Mg-Fe-Al
silicates
Biotite
Shiny, black flexible sheets;
one perfect cleavage
K(Mg,Fe)3AlSi3O10
(OH)2
Orthoclase
Usually white or pink; 2
cleavages at 90°;
equidimensional grains
KAlSi3O8
Muscovite
Shiny, silvery flexible sheets;
one perfect cleavage
KAl3AlSi3O10
(OH)2
Quartz
Colorless to gray; glassy with
conchoidal fracture; irregular
grains in intrusive rocks;
equidimensional phenocrysts
in extrusive rocks
Quartz SiO2
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 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 mid-range (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-rich) minerals that crystallize at comparatively
low (e.g., 600°C) temperatures.
1. Use the following diagnostic properties to identify Mineral 6 (pictured below), a mineral
commonly found in igneous rocks.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
Luster
This mineral reflects light like glass. What is the luster of this mineral?
Streak
The streak of this mineral is white or gray.
Hardness
This mineral can scratch glass. What is the minimum hardness of this mineral?
Cleavage vs. Fracture
This mineral has two 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?
Now use these characteristics and Tables A1-3 to identify this mineral.
This mineral is ________.
2. Identify the other seven common igneous rock-forming minerals displayed on the following
page. 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).
a. Mineral M1 is _____________.
b. Mineral M2 is _____________.
c. Mineral M3 is _____________.
d. Mineral M4 is _____________.
e. Mineral M5 is _____________.
f. Mineral M7 is _____________.
g. Mineral M8 is _____________.
3. Examine the figure below with Minerals 1-8. For questions a-c, circle the correct answer.
a. The minerals on the top row all have relatively dark colors. What type of minerals
are these? Felsic Intermediate Mafic
b. The minerals on the bottom row all have relatively light colors, what type of
minerals are these? Felsic Intermediate Mafic
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 light minerals. What
type of mineral falls between mafic and felsic? Felsic Intermediate Mafic
4. Examine Table 4-1. What two elements are found in all eight of the minerals?
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 crystallize 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.
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:
5. 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?
a) Amphibole, Muscovite, Quartz
b) Olivine, Biotite, Muscovite
c) Olivine, Pyroxene, Ca-rich Plagioclase Feldspar
d) Pyroxene, Potassium Feldspar, Quartz
e) Amphibole, Na-rich Plagioclase Feldspar, Biotite
6. A rock is found to be composed almost entirely of quartz, but with a small amount of a
light-colored flaky mineral that breaks into thin sheets when you scratch it. What is the
mineral composition of this rock?
a) Quartz, Potassium Feldspar
b) Quartz, Pyroxene
c) Quartz, Muscovite
d) Quartz, Na-rich Plagioclase Feldspar
e) Muscovite, Biotite
7. A rock has quartz, orthoclase, and a black mineral. What is the black mineral most likely,
according to Bowen’s Reaction Series?
a) Biotite
b) Amphibole
c) Pyroxene
d) Muscovite
e) Quartz
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
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:
8. A rock that is composed of amphibole, biotite, and orthoclase can best be described as ____
(mafic, intermediate, felsic).
9. 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).
10. An igneous rock that formed from a magma with a very low silica concentration is most likely
____ (mafic, intermediate, felsic).
11. In contrast, an igneous rock that formed from a magma with a very high silica concentration
is most likely ____ (mafic, intermediate, 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).
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
12. Identify the crystal size and mode of origin of the rocks below:
A. Crystal Size: Mode of Origin:
B. Crystal Size: Mode of Origin:
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.
13.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
a. The rock above contains many large (2-5cm across) phenocrysts that are salmon
pink. Which mineral is this?
(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?
(options: amphibole, biotite, muscovite, olivine, orthoclase, plagioclase, pyroxene,
quartz)
c. Is the grain size of the groundmass fine-grained or coarse-grained?
d. Is this an intrusive or extrusive rock?
14.
Identify the phenocryst (A) and groundmass (B) in the rock above (shown in the figure on
the left). The phenocryst has been magnified in 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).
b. According to Bowen’s Reaction Series does the mineral which forms the large
phenocrysts (magnified image shown in the left figure above) crystallize as a
discontinuous or continuous reaction series?
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
c. How would the composition in the mineral vary from its interior (more gray color) to
its exterior (more white color)?
d. What is the composition of the groundmass?
e. Where did the final stage of cooling occur?
15. Describe the formation history of the rock from question 14. Address the chemical and
physical conditions under which it formed.
16. 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
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
17. Compare the two rocks below:
a. What is the composition of each rock?
b. What is the texture of each rock?
18. 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?
a) R6 is more weathered than R1.
b) These two rocks cooled at different rates. R6 cooled more slowly than R1.
c) These two rocks cooled at different rates. R1 cooled more slowly than R6.
d) R6 crystallized underground, while R1 crystallized on the surface.
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 to see brand
new extrusive rocks being made. 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.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
(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 linked video).
C. 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.
Use the Igneous Rock Identification Chart (Table B-1) and the photographs below to
name rock specimens R1-R10.
TableB-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.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
19.
a. What is the composition of R1 (some of the minerals you may recognize to help you
determine its composition, such as the pink mineral and glassy mineral)?
Mafic Intermediate Felsic
b. What is the texture of R1? Fine-grained Coarse-grained Glassy
c. R1 is ___________.
20.
a. What is the composition of R2? Felsic Intermediate Mafic
b. What is the texture of R2? Vesicular Fine-grained Coarse-grained
c. R2 is __________.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
21.
a. What is the composition of R3? Felsic Intermediate Mafic
b. Notice that this sample has green phenocrysts surrounded by a matrix of darker
minerals. What is this texture? Vesicular Porphyritic Coarse-grained
c. R3 is __________.
22.
a. What is the composition of R4? Felsic Intermediate Mafic
b. What is the texture of R4? Glassy Porphyritic Coarse-grained
c. R4 is __________.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
23.
a. What is the composition of R5? Felsic Intermediate Mafic
b. What is the texture of R5? Glassy Vesicular Fine-grained
c. R5 is __________.
24.
a. What is the composition of R6? Felsic Intermediate Mafic
b. What is the texture of R6? Glassy Coarse-grained Fine-grained
c. R6 is __________.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
25.
a. What is the composition of R7? Felsic Intermediate Mafic
b. What is the texture of R7? Porphyritic Coarse-grained Fine-grained
c. R7 is __________.
26.
a. What is the composition of R8? Felsic Intermediate Mafic
b. What is the texture of R8? Porphyritic Vesicular Fine-grained
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
c. R8 is __________.
27.
a. What is the composition of R9? Felsic Intermediate Mafic
b. What is the texture of R9? **HINT** R9 was formed during a volcanic eruption.
Glassy Vesicular Fragmental
c. R9 is __________.
28.
a. What is the composition of R10? Felsic Intermediate Mafic
b. What is the texture of R10? Glassy Vesicular Fine-grained
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
c. R10 is __________.
d. 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 (R10 vs R8) would be composed of lower density minerals?
e. Based on its overall composition, which sample (R10 vs R8) would tend to have a lower
overall density?
29. Watch the two videos linked 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.
Sample R8 immersed in water
Sample R10 immersed in water
Based on these two videos, which sample has a lower density?
30. 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?
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 (an active volcano in the Cascade volcanic arc, located between Seattle and Portland,
OR) with Kilauea (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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Lab 4: Volcanoes, Plutons, and Igneous Rocks
Figure 4-2 Kilauea erupting effusively in 2018 .
1
Figure 4-3 Mount Saint Helens erupting explosively in 1980 .
2
1
https://media.npr.org/assets/img/2018/06/07/ap_18158142427935_wide-8f829d9188c83409350b9460abc
564c21b93df19-s800-c85.jpg
2 https://www.theatlantic.com/photo/2015/05/the-eruption-of-mount-st-helens-in-1980/393557/
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
31. Based on its plate tectonic setting, Mount Saint Helens likely erupts…
a) Basalt (more mafic than andesite)
b) Andesite (more mafic than rhyolite; more felsic than basalt)
c) Rhyolite (more felsic than andesite)
32. Based on its plate tectonic setting, Kilauea likely erupts…
a) Basalt (more mafic than andesite)
b) Andesite (more mafic than rhyolite; more felsic than basalt)
c) Rhyolite (more felsic than andesite)
Observe the following two videos of lava eruptions at Kilauea and at Mount Saint Helens.
Kilauea 2018 lava flow
Mount Saint Helens historic 1980 eruption
33. Based off of these two videos, complete the following sentences:
a. Mount Saint Helens displayed ________ eruption.
Options: explosive, non-explosive
b. Mount Saint Helens produced ________, which affected cities hundred of miles away.
Options: numerous lahars, widespread fires, a large ash plume
c. Kilauea displace _________ eruption.
Options: explosive, non-explosive
d. Lava from the Kilauea eruption ________.
Options: cooled immediately and did not travel far, flowed over the landscape
towards the ocean
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.
34. 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).
a. 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.
**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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Lab 4: Volcanoes, Plutons, and Igneous Rocks
b. 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.
c. Use “Google Search” on smart phone or computer to convert % slope to degrees (°).
35. 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.
a. 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 ruler bar.
b. 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.
Remember that 5280 feet are in 1 mile.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
c. Use “Google Search” on your smartphone or computer to convert % slope to degrees
(°).
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
36. We can use these measurements to make the following generalization:
a) Volcanoes that are composed of more mafic lava are also steeper
b) Volcanoes that are composed of more felsic lava are also steeper
c) Lava composition has no bearing on the morphology of a volcano
Next we will explore some of the lava characteristics that contribute to these differences.
37. Think about Bowen’s Reaction Series and the general temperatures at which
compositionally different magmas can crystallize. Which of the following is true?
a) The lava from Mount Saint Helens will crystallize at a colder temperature than
Mauna Loa.
b) The lava from Mauna Loa will crystallize at a colder temperature than Mount
Saint Helens.
c) There is no relationship between the crystallization temperature of a lava and its
composition.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
Viscosity is a material property that describes a liquid’s resistance to flow. For example, honey
is more viscous than water. Viscosity is also dependent on temperature (so, for example, hot
honey is less viscous than cold honey).
38. Based on your observations from the above videos, and your knowledge of plate tectonic
settings, do you think basaltic lava is more or less viscous than andesitic lava?
39. How does the viscosity of a lava affect the steepness of a volcano slope? Select all answers
that apply.
a) The viscosity of lava has no impact on slope steepness.
b) Lavas with higher viscosity will pile up on themselves, leading to steeper slopes.
c) Lavas with lower viscosity will spread out and flow farther distances, leading to less
steep slopes.
d) Lavas with lower viscosity will cool more quickly, leading to steeper slopes.
e) 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.
Figure 4-4 This aerial image shows the 15 km that the eruption of Mt. St. Helens affected.
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Lab 4: Volcanoes, Plutons, and Igneous Rocks
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.
40. Would you expect a similar long-range impact from the eruption of Mauna Loa or Kilauea,
yes or no?
Below is an image from the map of plate tectonics we used in Lab 2.
41. 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?
27


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