Pacific NW Rocks & Minerals
Lab 3: Topographic Maps
Name: ________________________________________________ Date: ________________
A topographic map is a two-dimensional (flat) representation (model) of a three-dimensional land
surface (landscape). It shows landforms (hills, valleys, slopes, coastlines, gullies) and their relief
(difference in elevation) by using contour lines to represent elevations of hills and valleys. The contour
lines are the distinguishing features of a topographic map. They are what make a topographic map
different from the more familiar planimetric map, such as a highway map, which has no contour lines
and does not show relief of the land. Yet topographic maps still have many of the features of the
planimetric maps (roads, buildings, streams, etc.).
Busch, (2011) AGI
Part 1: Understanding Topographic Maps (5 pts)
1. Label the elevation of the contours on the map below. Watch out for depressions with
repeated contours!
Contour Interval = 20 m
2. Use interpolation and
extrapolation to estimate and
label elevations of all points
below that are not labeled (see
Figure 9.10 for help). Then add
contour lines using a contour
interval of 100 meters. Notice
how the 0-meter and 100meter contour lines have
already been drawn. Sea level
elevation = 0 m. Remember
the rule of V's!
Estimate the elevation of
location X by interpolating
between the contour lines.
X = ____250_______ m
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3. Using the topographic map below, construct a topographic profile from A to A’. Grey shaded
areas are rivers. Refer to the Rules for Contour Lines above.
On the profile you just created
a. What is the horizontal scale in meters/inch?
1,000___________ m/inch
b. What is the vertical scale in meters/inch
(round to the nearest 100)?
100____________ m/inch
c. What is the vertical exaggeration?
10 : 1
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Part 2: Topography of Mt. Saint Helens (5 pts)
This contains material adapted from Richard Abbot (Appalachian State University, Department of
Geology) and from the USGS Volcanoes! 1997 Teacher packet.
In the illustration at the
right is a vertical crosssection (from A to A’) of
Earth’s crust and mantle
from west of the Cascadia
subduction zone to east
of Mt. St. Helens and a
map showing this region
and the location of the
vertical cross-section.
1. What is the distance
between the Volcano and
the Cascadia subduction
zone fault? 360 km ______
2. What is the depth of the
Juan de Fuca plate beneath
Mt. St. Helens? 85 km_____
The cross-section (above the map) show the relations between the convergent plate boundary and
volcanism. In the case of Mount St. Helens, like the other stratovolcanoes of the Cascades, the
production of the andesitic/rhyolitic magma is ultimately related to the subduction of the Juan de Fuca
plate beneath the western edge of the North America plate. Oceanic crust of the Juan de Fuca plate
and sedimentary material are being shoved downward, toward the east, underneath North America.
This down-going material undergoes metamorphism as it becomes exposed to progressively higher
pressures and temperatures in the interior of the earth beneath North America. Temperatures at
depths of the top of the Juan de Fuca plate directly below Mt. St. Helens can be as high as 1,200° C, hot
enough to cause some partial melting of the metamorphosed basalt of the subducting oceanic crust.
The metamorphism also releases volatile (gas) components such as H2O, CO2, and SO2. The relatively
small amounts of basaltic magma and rather large amounts of volatile gasses migrate upward,
ultimately encountering the base of the overlying continental crust. Here, the basaltic magma and
volatile gasses contribute to partial melting of the base of the continental crust. The magma thus
formed is andesitic and rhyolitic, reflecting the composition of the lower part of the continental crust.
Generally, only very small amounts of the basaltic magma ever reach the surface. The greater part of
the basaltic magma becomes mixed, sort of homogenized, with the andesitic/rhyolitic magma. This
andesitic/rhyolitic magma readily dissolves, quite literally soaks up, the volatile components. Under the
very high pressures at such depths near the base of the continental crust these dissolved gasses are
effectively trapped in the magma. However, the buoyant, gas-charged magma tends to work its way
upward toward the surface (intrusion). Most of the andesitic/rhyolitic magma cools and crystallizes
before it ever reaches the surface, forming sometimes vast plutonic (intrusive) bodies of
granodiorite/granite. Only comparatively small amounts of the gas-charged andesitic/rhyolitic magma
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ever reach the surface, but when this happens - LOOK OUT! The rapid formation of gas bubbles in the
magma and their explosive expansion can have catastrophic effects.
3. Stratovolcanoes are typically circular in map view. The radius of Mount St. Helens is about 6 km at
the base, and the elevation of the base is about 1 km above sea level. Before the eruption of May,
1980, the elevation at the top of Mount St. Helens was about 3 km.
Using this information, and modeling the stratovolcano as a simple cone-shape, estimate the volume of
volcanic material in Mount St. Helens, in cubic kilometers (km3). To remind you, the volume of a cone
is given by the following formula, Volume = (1/3)*pi*r2*h, where pi = ~ 3.14, r = radius of the cone, and
h = height of the cone. Volume = _________________
(1/3) X (3.14) X (62) X 2 = ~75 km3
The next questions refer to these maps of Mt. St. Helens from before and from after the eruption. This
map is adapted by the USGS from Brugman and Post, 1981 http://pubs.er.usgs.gov/publication/cir850D
4. What is the slope and angle of the slope of the surface of Mount St. Helens along the thick black line
in the "before" map above? The length of the thick black line is 2,000 m. Recall that slope = rise/run.
Convert this from a fraction to a percent:
1,100/2,000 = 0.55 = 55%
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5. Shield Volcanoes have slopes that range from 9 % along their lower slopes to 18 % along their higher
slopes. How does the slope you calculated in step 3 compare with shield volcanoes?
Mt. St. Helens is steeper than the average shield volcano.
6. Why are these slopes different?
Shield volcanoes are built from lava that has a lower Silica content, so the viscosity is lower than that
of stratovolcanoes.
7. One way to examine the effects of the eruption is to construct "before" and "after" topographic
profiles of the volcano.
a. Construct “before” and “after” topographic profiles on the grids that are placed along the A-A’
profile lines. The elevation contours are in meters.
b. On your copy of the post-eruption profile use colored pencils to denote the following regions, (1)
where there has been no change, (2) where material has been removed, and (3) where material has
been added (deposited). Create a legend that lists what color you used for each of the three regions.
8. The amount of material (6.5 km3) that was removed from Mount St. Helens represents what
percentage of the original volcano?
6.5 km3 / 75 km3 = 0.0867 = 8.67%
9. What is the vertical exaggeration of your profiles? 1,000 m / 1,000 m = 1, therefore 1 X (no vertical
exaggeration)
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We will use some basic geologic mapping principles to complete this geologic mapping lab.
Generally, geologists can see how bodies of rock or sediment are positioned three dimensionally where
they crop out (stick out of the ground as an outcrop) at Earth’s surface. They record this outcrop data
on flat (two-dimensional) geologic maps using different colors and symbols to represent the locations
of different layers or formations of rock and their directions and angles of tilting or other deformation.
This map data are used to infer the geometry of geologic structures.
Structural geology symbols and abbreviations used on geologic maps.
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RULES FOR INTERPRETING GEOLOGIC MAPS
1. Anticlines have their oldest beds in the center, and their limbs (sides) dip away from the fold
axis.
2. Synclines have their youngest beds in the center, and their limbs (sides) dip toward the fold
axis.
3. Plunging anticlines plunge toward the nose (closed end) of the V-shaped outcrop belt.
4. Plunging synclines plunge toward the open end of the V-shaped outcrop belt.
5. Streams cut “V” shapes into tilted beds and formation contacts that point in the direction of
dip (except in rare cases when the slope of the stream is greater than the dip of the beds and
formation contacts).
6. Streams cut “V” shapes into horizontal beds and formation contacts that point upstream. The
formation contacts are parallel to topographic contour lines, and the stream drainage system
developed on horizontal and/or unstratified formations has a dendritic pattern that resembles
the branching of a tree.
7. Vertical beds do not “V” where streams cut across them.
8. The upthrown blocks of faults tend to be eroded more (down to older beds) than downthrown
blocks.
9. Contacts migrate downdip upon erosion.
10. True dip angles can only be seen in cross section if the cross section is perpendicular to the
fault or to the strike of the beds.
The strike and dip symbol is used on maps to indicate the attitude (position relative to horizontal) of
rock layers. The long line segment in the symbol indicates strike (a compass direction), and the short
line segment points in the compass direction
of dip (always perpendicular to strike). A
number beside the symbol (if given)
indicates the angle of dip (tilting). Compass
directions can be expressed as bearings in
quadrant or azimuth form. Azimuth bearings
should be expressed as three digits in order
to distinguish them from two-digit dip
angles. Strike can be expressed as a bearing in either direction.
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Part I (2 pts). This is a copy of the geologic map from the ppt presentation.
(1) Add symbols to show the attitude (strike and dip) of the
formations.
(2) Notice the abrupt change in dip at the contact between
the Blue Fm and Yellow Fm Figure 10.1. This contact could be
at least two different kinds of geologic structures. What are
they?
__unconformity___________________
__fault___________________
Part II (3 pts). For each map below, record the strike of the
formations and their contacts. Then use the Rule of Vs to
determine the dip direction and sketch formation contacts on
the geologic cross section. Cut a V-shaped notch in a 3 X 5
card. Hold the card vertically and look on the edge. See Vs at
different dips of the card.
1. Strike: north-south
2. Strike: northwest
1. Dip: Vertical
2. Dip: southwest
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Part III (5 pts). Three Paper Models; Cut and fold them as noted on each model.
Model 1: This model shows Ordovician (O), Silurian (S), Devonian (D), Mississippian (M), Pennsylvanian
(IP), and Permian (P) formations striking due north and dipping 24° to the west. Provided are a
complete geologic map (the top of the diagram) and three of the four vertical cross sections (south,
east, and west sides of the block diagram).
Finalize the vertical cross section on the north side of the block so it shows the formations and their
dips. On the map, draw a strike and dip symbol on the Mississippian sandstone that dips 24 degrees to
the west.
Model 2: The geologic map is complete, but only two of the cross sections are available. Letters A–G
are ages from oldest (A) to youngest (G).
Complete the north and east sides of the block. Notice that the rock units define a fold. This fold is an
anticline, because the strata are convex upward and the oldest formation (A) is in the center of the
fold. It is symmetric (non-plunging), because its axis is horizontal. On the geologic map, draw strike and
dip symbols to indicate the attitudes of formation E at points I, II, III, and IV. Also draw the proper
symbol on the map (top of model) along the axis of the fold.
How do the strikes at all four locations compare with each other?
They are all north-south.
How does the dip direction at points I and II compare with the dip direction at points III and IV? In your
answer, include the dip direction at all four points.
The dips are I and II dip to the West and the dips at III and IV dip to the East. They dip in opposite
directions.
Model 3: Complete the remaining two-and-a-half sides of this model, using as guides the geologic map
on top of the block and the one-and-a-half completed sides. On the map, draw strike and dip symbols
showing the orientation of formation C at points I, II, III, and IV. Also draw the proper symbol along the
axis of the fold.
How do the strikes of all four locations compare with each other?
They are all north-south
How does the dip direction (of formation C) at points I and II compare with the dip direction at points III
and IV? Include the dip direction at all four points in your answer.
The dips are I and II dip to the East and the dips at III and IV dip to the West. They dip in opposite
directions.
Is this fold plunging or non-plunging? __non-plunging____
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