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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.
GLY 108 – Plate Tectonics: The Active Earth
Lab 7: Plate Tectonics/Earth History
Objective:
To understand and identify how the theory of plate tectonics is expressed in the
Earth’s topographic features.
Introduction:
Plate tectonics is probably the most revolutionary concept of geology and has
become the central framework to view all of earth’s processes. It is a relatively new
theory that gained acceptance in the 1960s because it explains and ties together so many
processes on and within the earth. Supporting evidence for this theory is found in:
•
The fit of the continents, particularly South America and Africa: If you consider
the continental margins, the shallow offshore regions underlain by continental
crust, as part of the true continents then the fit is even closer.
•
Similarity in rock types across continents: For example, you can find similar
evidence of glaciers, including glacial sediments and landforms in both South
America and Africa. There is also parallel evidence of past mountain building in
North America, the British Isles, and Scandinavia. However, landforms and
mountains are not found on the sea floor between these continents.
•
Fossil evidence: The best evidence again comes from the Southern Hemisphere,
where the fossil Lystrosaurus is found on southern continents. This animal did not
swim well enough to cross-oceans, so the continents must have been joined at one
time. Even better fossil evidence is the Glossopteris plant, found on southern
continents. This plant required a similar climate on all continents, and that climate
is not found there today.
http://fossil.wikia.com/wiki/Lystrosaurus
•
Age of the Seafloor: Advances in technology made age dating of the seafloor
possible, and a marked trend was found that corresponds to the mid-ocean ridges,
the sites of volcanism. The age of the seafloor gets progressively younger toward
the ridges. Older seafloor occurs around trenches where earthquakes and
volcanism seem to indicate that the crust is sinking, or subducting, back into the
interior of the earth.
•
Paleomagnetic studies: These studies show that the earth’s magnetic field has
varied throughout geologic history. Rocks with iron will crystallize with the iron
crystals pointing to magnetic north. Studies of old oceanic rocks show how the
earth’s magnetic polarity has switched over time. Further, there are symmetrical
records of past reversals in polarity on both sides of the mid-ocean ridges.
The conceptual framework of plate tectonics is that the earth’s crust is broken into
plates that move in relation to one another. These plates are slowly drifting across the
surface of the globe, driven by convection currents within the mantle deep below. Their
shifting accounts for the major geologic activity that occurs at plate borders; this activity
includes the creation of oceans, continents, mountains, volcanoes, and earthquakes.
The rigid oceanic and continental crusts and the uppermost part of the mantle, jointly
called the lithosphere, are broken into seven large plates and 11 or more smaller ones.
Continental plates are thicker than oceanic plates because of differences in the
thickness of their crust. These plates slide around on the asthenosphere, the soft but solid,
putty-like mobile rock that makes up the lower part of the upper mantle. The plates are
100 to 350 kilometers thick and move at a rate of 1 to 12 centimeters per year creating
continental drift.
The plates may separate, slide past one another, or collide and in this process form the
following:
• Rift zones, where plates are separating and moving away from each other on
continents.
• Mid-ocean ridges, where plates are separating and moving away from each other
in oceans.
• Transformation of fault boundaries, where plates are sliding past each other,
such as in the San Andreas Fault.
• Trench, earthquake, volcano, and mountain ranges, where an oceanic plate is
colliding with a continental plate. As the oceanic plate sinks, or subducts, it forms
a trench. Earthquakes occur in conjunction with this movement. As a plate sinks
far enough to partially melt, volcanoes form. The pressure of the two plates
colliding form mountain ranges.
• Trench, earthquake, volcano, and island arcs, where two oceanic plates
converge: Here, the difference is that the denser oceanic plate does the
subducting, and the mountains and volcanoes that form tend to produce island
arcs, such as the Aleutians and the Philippine Islands.
• High mountain ranges, earthquakes, and suture zone, where two continental
plates collide: No subduction occurs here because continental plates are not dense
enough to subduct. The two plates collide and there is “no way to go but up,” as
tall mountains form. Earthquakes accompany this pressure, and the suture zone
marks the joining line of the two continents. The best modern-day example is the
Himalayan Mountains.
Today we have a good understanding of how the plates move and how such
movements relate to earthquake activity. Most movement occurs along narrow zones
between plates; this is where the results of plate-tectonic forces are most evident.
There are four types of plate boundaries:
•
Divergent boundaries – occur along spreading centers where plates are moving
away from each other and new crust is created by magma pushing up from the
mantle as in the Mid- Atlantic Ridge.
•
Convergent boundaries – can take several forms. They may involve two
continental plates, two oceanic plates or an oceanic plate and a continental plate.
If one plate sinks below another plate, this is called subduction. There are several
good examples of subduction, where an oceanic plate dips under a continental plate. The
oceanic Nazca plate dips under the S. American continental plate. Volcanism, earthquake
activity and ocean trenches are often associated with these subduction zones.
When two oceanic plates meet, one subducts below the other, and typically forms an
ocean trench like the formation of the Mariana’s Trench where the Pacific plate
converges with the Philippine plate.
When two continental plates meet head-on the crust tends to buckle and is pushed
upward or sideways. The collision of India into Asia 50 million years ago caused the
Eurasian Plate to crumple up and override the Indian Plate. Over millions of years this
collision pushed the Himalayas and the Tibetan Plateau up to their present heights.
•
Transform boundaries – where crust is neither produced nor destroyed as the
plates slide horizontally past each other. Most transform faults are found on the
ocean floor. They commonly offset the active spreading ridges, producing
zigzagged plate margins. However, a few occur on land such as the San Andreas
Fault zone in California.
•
Plate boundary zones – are broad belts in which boundaries are not well defined
and the effects of plate interaction are unclear. In some regions, the boundaries
are not well defined because the plate-movement deformation occurring there
extends over a broad belt called a plate-boundary zone. One of these zones marks
the Mediterranean-Alpine region between the Eurasian and African Plates within
which several smaller fragments of plates, microplates, have been recognized.
Plate-boundary zones involve at least two large plates and one or more
microplates caught up between them. They tend to have complicated geological
structures and earthquake patterns.
en.wikipedia.org/wiki/Plate_tectonics
The figure shows the major plate boundaries. The arrows indicate the type of plate
boundary that is present.
References: How Does Earth Work? Physical Geology and Process of Science, 2/E Gary
Smith and Aurora Pun, Prentice Hall, 2009. and Historical Geology, 6th Edition, Reed
Wicander and James S. Monroe, Brooks/Cole, 2009.
Questions:
Please answer the following questions using the lab packet or the Internet.
1. What type of plate boundary is under the Red Sea?
2. How are oceanic ridges identified?
3. What types of plate boundaries are associated with oceanic trenches?
4. The major mountains of the world are associated with what plate boundaries?
5. Where are the locations of at least three major transform boundaries?
6. What are some similarities and differences between the Caribbean and Scotia
plates?
7. Is it possible to have a tectonic plate completely surrounded by convergent
boundaries? Why or why not? Does one exist?
8. How is the Juan de Fuca plate related to the volcanism of northern California,
Oregon, and Washington?
9. What will happen to Africa in the future? What will it look like?
10. Imagine that you are looking at pictures sent back of a newly discovered planet.
You are specifically looking for evidence of plate tectonics. What topographic
features would you look for, and why?

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