Lab report 8

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Lab 8: virtual field trip: Please complete everything within the time frame, if you have any questions please feel free to ask me.

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GLY 108 – Plate Tectonics: The Active Earth
Lab 8: Virtual Field Trip
Today’s virtual field trip takes us to Crown Butte, Montana!
(Figure modified after Rod Benson)
Go to the website below:
http://formontana.net/cb.html
Answer the following questions, on each page:
Page 1
1) Which laccolith did western artist C.M. Russell include on some of his paintings?
Page 2
2) In which part of the state are laccoliths common?
Reference: Rod Benson, Earth Science Teacher, Helena High School, 2013.
3) What is the purpose of the diagrams shown on this page?
4) Laccoliths are a type of “plutonic” (a.k.a. intrusive) formation. What does this mean?
5) Laccoliths are formed beneath the surface. Explain why they now stand above the
surrounding prairie?
Page 3
6) Why did the Nature Conservancy purchase Crown Butte?
7) When did Crown Butte and the other laccoliths in this part of the state form?
8) How did the magma get from the volcanic center to the locations where the laccoliths
formed?
Page 4
9) Look at the map. As you travel from Helena to Great Falls along I-15 you are passing
through the guts of an ancient volcano. Which two small towns is the ancient volcano
located between?
10) How does the size of the Crown Butte Laccolith compare to that of the Shaw Butte
Laccolith?
11) Use the scale to estimate how far it is from Crown Butte to the closest edge of the
ancient volcano.
Page 5
12) What is unusual about the dikes of central Montana’s laccoliths?
Reference: Rod Benson, Earth Science Teacher, Helena High School, 2013.
13) What is the specific name of the sandstone that was forced upward by the laccolith,
and where else in Montana can it be seen?
Page 6
14) This aerial photo shows the entire butte, including part of the dike that supplied
magma to the laccolith is shown in the photo. If you were to travel from the butte to find
the source of the magma, which direction would you travel?
15) The dashed line of the photo shows one way to hike onto the butte. Which side of the
butte is dominated by cliffs?
Page 7
16) What is the geologic name for the ridges shown in this photo?
17) How are ridges like the ones shown in the top photo represented on the map?
18) What would be shown in this photo if it had been taken 75 million years ago?
Page 8
19) As the magma froze (cooled and became rock) why did it crack to form the columns
shown in this photo?
Page 9
20) How were most the mountains in central Montana formed?
Reference: Rod Benson, Earth Science Teacher, Helena High School, 2013.
Page 10
21) Explain why Crown Butte made up of distinct layers as shown in this photo.
22) Magma is a mixture of minerals. Why may they separate as the molten material
cools?
Page 11
23) Explain why the mineral augite formed crystals before the other minerals started to
solidify?
24) Besides the fact that the magma was cooling slowly, why did the augite (black
mineral) form nice big crystals as it froze?
25) What caused the magma to begin to cool so quickly that the other minerals did not
form large crystals?
26) Look at the bottom photo, which includes the pencil. How did these crystals of
augite, which were once embedded in the rock, become separate from it?
Reference: Rod Benson, Earth Science Teacher, Helena High School, 2013.

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