BrodieBaileTopographicMapsLab
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Colorado State University, Fort Collins *
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Course
121
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Geology
Date
Apr 3, 2024
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Name __________Brodie_Baile______ GEOL 121 Lab Section _____________ Topographic Maps
Lab
Why Use a Map? With the click of a mouse, Google Earth
TM
and NASA World Wind provide satellite images of any point on the planet. These and other tools allow us to build models of the landscape, draw topographic profiles, measure straight-line distances and the length of meandering streams, and estimate slope steepness. Why are geologists interested in maps? We use them as the fundamental tool for communicating information about the distribution of rock units and landforms. If you ever go to a national park, you will find topographic and geologic maps of the park area prominently displayed at the visitor center. In fact, you can even download an electronic version of the map showing your neighborhood from the US Geological Survey website
. Figure 1.1.
Topographic map of the Colorado State University campus at Fort Collins. Reading a Topographic Map In the United States, topographic quadrangle maps
are produced by the United States Geological Survey (USGS). A quadrangle is a rectangular area of Earth bounded by north-south and east-west lines. Usually the quadrangle is named after a prominent geographic feature (e.g., a mountain or a town) in the map area. Quadrangles use many symbols for various natural and artificial features of the landscape; these symbols are explained in a booklet published by the USGS and available online at https://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf. Each topo map includes the following features: 1
GEOL 121 – Topographic Maps Lab Map name and date: On a topographic quadrangle, the name is printed in the lower right corner of the map, and under it is the date when the map was compiled. Note that the name is also printed in the upper right corner. Scale: This explains how large an area the map covers. Specifically, scale is the ratio of a linear distance on the map to the corresponding distance on the surface of the earth. For example, if your map scale is 1:50,000, it means that 1 inch on the map equals 50,000 inches on the face of the earth, or 1 centimeter on the map equals 50,000 centimeter on the face of the earth. There are four ways to express scale. •
As a ratio: 1:50,000 •
As a fraction: 1/50,000 •
Verbally: 1 inch equals 50,000 inches (or 1" = ~ 4167') •
Graphically: Using lines marked in kilometers, meters, miles, or feet (scale bars) Note that if you reduce or enlarge a map, the original fractional scale will no longer be valid, but a graphical scale will still apply. Contour lines: Elevation (or altitude) is the vertical distance between a given point and a reference elevation. In most cases, the reference is sea level. Because a map is a flat sheet, some method is needed to show different elevations on the map. Topographic maps show elevation by using contour lines
, which connect points of equal elevation. At every point on the 100-foot contour line, for example, the elevation is 100 ft. If you walked along that line, you would not go either uphill or downhill. The contour interval
refers to the vertical difference in elevation between adjacent contour lines. The contour interval for a given map is usually specified at the bottom of the map along with the scale. Every 4th or 5th contour line is shown as a heavier line and labeled with its elevation. This is called an index contour
. Some rules for contour lines are listed below and illustrated in Fig. 1.2. •
Contour lines never divide or split. •
Contour lines never simply end; they either close or intersect the edge of the map. •
A contour line must represent one and only one elevation. •
A contour line can never intersect another contour line. •
The contour interval must remain constant within a given area.
•
Closely spaced contour lines indicate steep slopes, and widely spaced contour lines show gentle slopes. •
When a contour line crosses a stream, it forms a V-shape that points upstream. 2
GEOL 121 – Topographic Maps Lab Figure 1.2.
This map has three index contours, a hill with a top elevation between 110 and 120 meters, and a stream flowing south. Note how the contours trace V-shapes as they cross the stream. The elevation of point Q is 100 m; point R, 110 m; and point S, 65 m. Part I: Visualizing Topography using Contour Lines Overview In this part of the exercise, you learn to read topographic maps by manipulating topography using an AR Sandbox and studying an example of a topographic map from Colorado. Learning Objectives •
Visualize 3-dimensional topography using 2-dimensional contours lines Investigating Topography In this section of the lab you will use the Augmented Reality (AR) Sandbox to investigate how contour lines reflect the topography of a land surface. Take a minute to play around with the sandbox. Move the sand around to create different features in the landscape. Notice how the contour lines appear as you change the landscape. 1. Flatten out the sand in the sand box. Notice how the contour lines appear. Now, create a steep slope in the sand. How did the contour lines change when you created the slope? The lines are spaced out more, less evidence of elevation changes 2. Create a hill in the sand. Draw the shape of the contour lines below. 3. Create a valley in the sand. Draw the shape of the contour lines. 3
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GEOL 121 – Topographic Maps Lab 4. Reconstruct the map below as best you can using the AR Sandbox. Imagine there are people standing at each of the indicated points. Who can see who in this scenario? Explain your answer. Assume a contour interval of 20 feet, answer, the people can use binoculars, and there is no vegetation. B can see everyone, as they are the most elevated A would have a good chance of seeing everyone too, but depending how short b is, b might be excluded from a’s visuals Both d and c would not be able to see each other or b because of the elevation between them, depending how short a is, a might be excluded from c and d’s visuals 5. In the diagram below, match the numbered contour lines on the left to corresponding landform on the right. 1=b 2=e 3=d 4=c 5=f 4
GEOL 121 – Topographic Maps Lab 6=a Part II: Using Topographic Maps to Investigate Geologic Questions 5
GEOL 121 – Topographic Maps Lab Overview
On May 18, 1980, Lawetlat’la (Mt. St. Helens) in the state of Washington exploded in a cloud of ash, plus lava and mud flows. What had been a beautiful symmetrical snow-covered mountain with heavily forested slopes became a startling landscape of ash, mud, and downed trees surrounding a broken, irregular peak. The power of the initial blast was directed upward and laterally, snapping off trees for miles in the blast zone. In the years since 1980, many people – geologists, biologists, environmentalists – have been observing and studying how the landscape recovers after a major volcanic eruption. Screen shot from the Mt. St. Helens WebCam on February 8, 2023. Learning objectives •
Describe the shape of Lawetlat’la (Mt. St. Helens) before and after the eruption.
•
Draw two topographic profiles across the volcano and determine their vertical exaggeration.
•
Use your profile to estimate the dimensions of the material removed by the eruption and calculate the volume. •
Compare your result with published values and identify sources of error in your work A Brief Chronology of the Eruption In the month of March 1980, gas-rich magma rose beneath the volcano, causing the ground to rise ~300 feet (like blowing up a balloon). Small earthquakes were detected beneath the mountain, as 6
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GEOL 121 – Topographic Maps Lab magma began moving toward the surface. Older eruptions here had produced silicic lavas, so geologists warned that there was a high likelihood of an explosive eruption. At 8:30 AM, on May 18, two strong earthquakes caused by movement of magma triggered a large landslide. This in turn released the pressure on the magma and set off the eruption. The side of the mountain burst horizontally in a lateral blast of volcanic ash, and almost simultaneously, the top blew up. There was an enormous force to the blast (~25 megaton H bomb). The mountain's symmetry was destroyed, with a large crater forming at the top, and trees were knocked over for large distances. The ash reached 16 km up into the atmosphere and darkened skies as far away as western Montana. Ash was carried by the jet stream all the way to the eastern part of the United States. Hot ash melted the snow on the mountain, and the mixture of ash and water produced destructive mudflows that extended miles from the mountain. In the subsequent years, Lawetlat’la (Mt. St. Helens) has occasionally produced small earthquakes and ash eruptions, but nothing comparable to 1980. Within the crater, a small bulge has developed, called a lava dome, which indicates the mountain is still active, only biding its time. Instructions There are two topographic maps of Lawetlat’la (Mt. St. Helens) that accompany this exercise (separate file). The first map shows the arrangement of contour lines around Lawetlat’la before the eruption, while the second map shows contours after the eruption. 1. Study the first map of Lawetlat’la (before the eruption), and examine the contour lines closely. Describe the shape of the volcano before it erupted, including its outline, general topography, symmetry, and the slopes of its sides. The shape is very conical, looks a lot like a mountain with a gradual slope. 2. Now study the second map of Lawetlat’la (after the eruption), and examine the contour lines closely. Describe the shape of the volcano after it erupted, including its outline, general topography, symmetry, and the slopes of its sides. The outline in the prime side is very similar, The mountain now has a chunk taken out of it on the other side though, The eruption must have moved material away from the point of the mountain down the side closest to the eruption site. 3. These maps show many glaciers surrounding the peak, some of which are labeled with their names. Which of these named glaciers were totally destroyed by the eruption, as shown by comparing the before and after maps? Which glaciers survived the eruption but were cut off from their source at the top of the mountain? 7
GEOL 121 – Topographic Maps Lab The glaciers that were destroyed are loowit, lesch, and the wishbone glaciers The glaciers that survived are toutle, talus, swift, dryer, shoestring, ape, nelson, and the forsyth glaciers . You will now use topographic profiles to estimate the amount of rock that was removed from the volcano during the eruption. What is a Topographic Profile? A topographic profile is an illustration of the shape of the ground surface between two points. If a topographic map provides a top view (i.e., what a bird would see if looking straight down), a topographic profile provides a side view (i.e., what you might see along the edge of a cliff). Applications of topographic profiles include engineering projects such as roads and pipelines and scientific programs such as studying landforms or hydrology of an area. Geologists use topographic profiles as a basis for geologic profiles, which indicate the position and orientation of subsurface rock layers. Figure 2.1.
Topographic profile across Antelope Peak. Screenshot from Google Earth. Constructing a Topographic Profile A topographic profile
is a cross-section (vertical slice) that illustrates the topography between two points. The profile traces the variations in elevation along the line between the points. To construct a profile, follow the steps below. The process is illustrated in Fig. 2.2. 1. Draw a line across a topographic map connecting the two end points of the profile line. 2. Mark the places where the line intersects contour lines. 8
GEOL 121 – Topographic Maps Lab 3.
Project the elevation of each intersection to the correct level on the graph.
4.
Connect the points on the graph with a smooth curve. If two consecutive points lie at the same elevation, bring the profile line slightly above or slightly below that elevation in between the two points. Figure 2.2.
Procedure for drawing a topographic profile. 4. Use the graph page (separate file) to draw a topographic profile along a line from point A to point A’ on the first topographic map (Lawetlat’la before the eruption). 5. Now draw another topographic profile along line B-B’ on the second map (Lawetlat’la after the eruption), using the same graph. A and B are the same point and A’ and B’ are the same point. So when you graph the second profile, start from the south end (at B’), and the profile should match the first one up to the 2500-meter contour. 9
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GEOL 121 – Topographic Maps Lab 6. If 1 cm = 150 m on the Y-axis of the profile graph, what is the fractional vertical scale of your profiles? Hint: change 150 meters to centimeters. 1:15000 cm 7. If 3.5 cm = 1 km on the map, what is the fractional horizontal scale of the profiles? Hint: change 1 km to centimeters, then divide both sides of the equation by 3.5. Round your answer to the nearest 1000. 1 cm = 100000/3.5 = 29000 cm 8. Topographic profiles are often drawn with a vertical scale that is much larger than the map scale. Geologists draw profiles this way in order to bring out detail in the topography. This is known as vertical exaggeration, and can be calculated by dividing the fractional vertical scale of the profile by the fractional horizontal scale of the profile. What is the vertical exaggeration of your topographic profile? Hint: divide the denominator of the horizontal scale by the denominator
of the vertical scale. You can remember this as dividing the larger number by the smaller
number, so that your answer is always greater than 1. Round your answer to the nearest whole number 29000/15000= 2x Compare Topographic Profiles Look again at your two topographic profiles. In this section, you will estimate how much material was removed from the top of the mountain by the explosion and eruption. Assume that the upper part of the volcano (the part that was blown away) was a perfect cone. You will draw a horizontal line across the profile to represent the base of the cone. Of course, the part of the mountain removed by the eruption was not a perfect cone, so you will have to choose the best position for the base line. In the sketch below, the black line represents the original profile of Mt. St. Helens, and the red line shows its profile after the eruption. 10
GEOL 121 – Topographic Maps Lab Representative sketch of the topographic profiles and possible choices for the base of the conical section removed by the eruption. If you use the highest point of the after profile as the base of the cone (blue line), this would correctly account for the area colored blue, which was removed by the eruptions. However, it would omit rock below the blue line and above the red line, so that the calculated volume would be too small. But if you use the lowest point of the after profile as the base of the cone (yellow line), this would imply that everything shaded blue and
yellow was removed, including all rock between the red and yellow lines. Because these lower rocks were not removed, the calculated volume would be too large. You must choose a location for the base of the cone such that the amount that is included but should not be
is approximately the same as the amount that is not included but should be
! Representative sketch of the topographic profiles and one possible choice for the base of the cone. Vertically ruled area ≈
horizontally ruled area. Measure h
using the scale on the Y-axis and d
using the scale on the X-axis.
10. Draw a horizontal line across your topographic profile to represent the base of the cone-shaped mass of rock removed by the eruption. Measure the diameter d
from side to side on the "before" profile, using the scale printed at the bottom of the graph. What is the approximate value of
d
for the cone on your profile? 3.7 km 11
h
d
GEOL 121 – Topographic Maps Lab 11. Given the diameter d
you found in the previous question, what is the radius of the cone? Hint: d
= 2
r. 1.85 km 12. What is the approximate height h
of the cone? Measure the height from the highest point of the mountaintop on the "before" profile to the base line that you drew for the cone.
.9 km 13. Make sure that r
and h
are expressed in the same units (both meters or both km). Use the formula for the volume of a cone: V =
1/3
(
π
r
2
h).
What is your calculated volume for the cone that represents material removed by the eruption? Show your work. V = 1/3(3.14*1.85^2*.9)= 3.226 km*.24= .774 mi 14. Open the fact sheet on Mt. St. Helens http://pubs.usgs.gov/fs/2000/fs036-00/
, which summarizes the 1980 eruption. Compare numbers on the fact sheet for the volume of mountain removed with the value you found in the previous question. You will have to convert between metric and English units. Is your calculated volume greater than, less than, or about the same as the number given? . .67 mi, Our calculated number is greater by .1 15. What are some possible sources of error in your calculation? Possibly Rounding, Different data points, Human error, and Different diameter or heights used Part V: Reflection 1. What aspect(s) of this lab were easiest for you in this lab? 12
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GEOL 121 – Topographic Maps Lab Calculations, I can be good at math. I also can be good at interpreting data. 2. What aspect(s) of this lab were hardest for you in this lab? Figuring out different aspects of the lab and where they apply within the lab. 3. What questions do you still have about topographic maps or contours? What is your favorite part of topographic maps?
13