Axia College Material
Appendix H
Week Four Lab Report: Volcanic Products
Answer the lab questions for this week and summarize the lab experience using this form.
Full Name
Date
Carefully read pages 69-86 in Geoscience Laboratory.
Complete this week’s lab by filling in your responses to the questions from Geoscience
Laboratory. Select answers are provided for you in red font to assist you with your lab work.
Although you are only required to respond to the questions in this worksheet, you are encouraged
to answer others from the text on your own.
Lab Questions
4.8.
A. What do you suppose was the shape (in horizontal plan view) of the magma body
that pushed up the layers of sedimentary rocks that now comprise Green Mountain—
circular, amoeboid (irregular), or linear (elongated)?
Answer: circular
B. Judging from sketches in Figure 4.5 of the Lab book, which type of plutonic igneous
rock body do you suppose is buried beneath the layers of sedimentary rocks at
Green Mountain?
4.9.
Granite comprising Stone Mountain crystallized from magma tens of kilometers
within the Earth, but uplift and erosion has since laid this pluton bare. Assuming that
erosion has only slightly modified the original shape of the more durable granite body,
which of the igneous rock bodies in Figure 4.5 does Stone Mountain most closely
resemble?
4.10.
The lip of the crater has been breached by a lava flow, and a deep furrow marks its path.
In what map direction did that lava move, southeastward or northwestward? (Notice the
arrow along the left margin of the page indicating north.)
Answer: southeastward
4.11.
Judging from the sizes of the before-and-after craters atop Mount St. Helens (Figs. 4.8
and 4.9 of the Lab book), which volcanic event appears to have been the more explosive,
the last one before 1980, or that which occurred in 1980?
Given the size of the crater, it would appear that the eruption in 1980 was more explosive than
the previous eruption. The 1980 eruption destroyed the dome and over 1100 feet of the top of
the volcano.
4.14.
Applying the rule—the most complete form crystallizes first, and the least complete form
crystallizes last—list the four minerals in Figure 4.11 of the Lab book (W, X, Y, and Z) in
the order in which they crystallized. (Caution: The stack of cards in Figure 4.10 is 3dimensional, with one card upon another; whereas minerals in the thin-section in Figure
4.11 have essentially no depth. View this thin section as 2-dimensional, with minerals
side-by-side.
Answer: X, Z, Y, W
4.15.
If the granite in Figure 4.11 were melted in the laboratory, what would be the order in
which W, X, Y, and Z would melt? Hint: Imagine a pat of butter and a lump of sugar in a
sauce pan. Add heat. Which melts first? Maintain the heat until both have melted, then
turn off the heat. Which solidifies (crystallizes) first?
W, Y, Z, X
4.22.
(Refer to Figure 4.16. of the Lab book) Reflecting on the preceding paragraph, in what
way would you expect the chemical composition of an igneous rock crystallizing at X to
differ from that crystallizing at Y?
Answer: Rock at X would be richer in iron, magnesium, and calcium. Rock at Y would be
richer in silicon anions, aluminum, potassium, and sodium.
4.23.
In what textural way would you expect igneous rock crystallizing at Y to differ from that
crystallizing at Z? Hint: Which rock crystallizes more quickly?
Rocks at Y would have a coarser texture (phaneritic) and rocks at Z would have a finer texture
(aphanitic).
4.25.
Where would you most likely find obsidian (volcanic glass), at W or at Z? Hint: See
obsidian’s compositional position in Table 4.1 on page 81 of the Lab book.
Z
Lab Summary
Address the following questions in a 100- to 200-word summary:




Summarize the general principles and purpose of the lab.
Explain how this lab helped you better understand the topics and concepts addressed this
week.
Describe what you found challenging about this lab.
Describe what you found interesting about this lab.
Write your summary here: