Activity 1.5: Isostasy and Earth`s Global Topography

Names:________________ & _________________
Circle Section:
1=Wed or 2=Thurs
GEOS 100 Lab 1 - Based on AGI manual 9th edition
Observing and Measuring Earth Materials and Processes
This lab is worth ~2% of the course total. Use and hand in this lab handout and do not
tear pages from your lab manual to preserve it for later use.
Work in pairs (or in larger groups for some of the “experiments” ) and hand in one
lab for each pair, by the beginning of the following lab period.
The written part of the lab is based on the lab manual.
In addition, familiarize yourself with F300, the maps and diagrams on the walls, and
minerals and rock specimens. These are the materials that you will be dealing with during
the course (and the rest of your life). You will become aware of the geology of western
Canada and how the rocks and processes around Victoria are related to larger scale
processes over space and time.
Turn to Lab 1 of the lab manual and work your way through the readings and exercises.
Answer the questions from the manual in the corresponding spaces provided.
Activity 1-Nought: Observing Earth Materials and Testing Hypotheses
Read p 8-9 and examine specimens of Chalcopyrite. How do you determine if
this is a pure substance (elements cannot be chemically separated into simpler
Chalcopyrite is a pure element. True / False.
Why? _________________________________________________________
Chalcopyrite is a compound. True / False.
What elements does it contain and why for each element?
B.element_________ why_________________________________________
Activity 1.1 Basketball Model of Earth
Label in the correct relative position the spheres of the Earth on the following
page using the scale provided, wherein the position of the Inner Core at 1196 km is
represented by a circle at 22.3 mm radius. The ratio (scale factor) of km in the real Earth
per mm in your diagram is given by 6371 km per 119 mm.
1. Write the numerical value of this ratio here after converting both numbers to
meters:__________ m/m (2 points)
2. What is the fractional value of each radius in the table, if the basketball’s radius is 1.00
for the top of the Crust? If you do this correctly, the top of the crust should be the surface
of the basketball and the ocean and atmosphere sit above that. (10 points)
Hint: the thickness of each layer is the difference between it’s top and it’s base. The
fractional radius needs to count the total thickness of all layers below that level. We want
the top of the crust to be the total radius, the oceans and atmosphere sit above that >1.000
UM Lithosphere
UM Asthenosphere
Lower Mantle
Outer Core
Inner Core
Distance to top/base (km)
Thickness of layer Fraction of Earth Radius
Activity 1.2: Remote Sensing and Exploring the Earth for Copper
A. Read about MODIS on p.10-11 and Fig 1.8 then analyse Figure 1.9 on p 12.of the
photo and MODIS image of Mt. Etna.
1. How many volcanic vents (A) are active in the Oct.30, 2002 photo of Mt.Etna as taken
from the space station? # of.vents:_________and what types of materials are present in
the plume? ___________and ___________. How can you tell? ________________ (4)
2. How far has the plume travelled in the MODIS image of Oct 28, 2002? ________km
From the trajectory and location of the plume, where ash particles form deposits?
_____________________________ and __________________________________ (3).
3. From the images, how did this eruption affect the
atmosphere:_________________________________________________________ and
B. Analyse figure 1.10 using the information in figure 1.8 and p 10-11. Bear in mind that
the images do not directly sense Cu, Ag or Au, but just the response of the soils and their
minerals to the bands being detected.
1. Examine figure 1.13 showing ASTER images of Chile’s Escondida open pit copper
mines. Choose the best location for a new open pit from A, B or C _____________ and
explain your choice. _____________________________________________________
2. Suggest a scientific approach or program of activities to test this recommendation?
Activity 1.3: Measuring Earth Materials and relationships (use the units and charts
for conversions on p. x & xi)
A. converted units: (6 points)
1. 10 mi. = _______ kilometres
2. 1 ft. =
_______ metres
3. 16 km = _______ metres
4. 25 m =
_______ centimetres
5. 25.4 mL = _______ cm3
6. 1.3 L =
_______ cm3
Draw (3)
B. 1 cm line segment
C. 1 cm2 square
D. 1 cm3 cube
E. Work out a method, do the experiment, then explain in words how to measure the
density of water using a graduated cylinder. Write a formula to succinctly summarize
your method, giving your answer. The symbol for density is the greek letter rho.  (4)
The reference density for distilled water varies with temperature and atmospheric
pressure. The measured values of these today are: T°C _______, P mm Hg ______, P atm
______ and Density from the CRC Handbook table = ___________.
1. Method:___________________________________________________________
2. Calculate your own density for deionized water. Density is a derived unit of mass
divided by volume. Show your calculations (3).
3. Compare your measured value to the expected value for water under lab
conditions today. Express this as a % error given by:
100 x (Your value – True value)
True value
If your %error is greater than 5% find out what you did wrong and repeat your
F. Determine the density of clay. Describe your method. clay = _______ g/cm3
G. Review your work and conclusions from E and F above:
1. Why does the clay sink in water? (1)
2. Try to float the clay and describe or draw how this can be done. (2)
3. Explain the physics of how can the clay be made to float? (2)
H. Compare the densities of Earth’s spheres to liquid water at S.T.P. (25°C, 1atm): (4)
1. Use the composition of air at 78% Nitrogen and 22% Oxygen and 1 mole of gas
occupies 22.4 litres at STP and Air g/mol = (28 g/mol N2 x 0.78) + (32 g/mol O2 x 0.22)
atmosphere =
2. For the density of the lithosphere use the table on p.27 and the histogram on p.19 fig
1.14 to estimate the proportions of granitic and basaltic crust (4)
lithosphere = (fraction of basalt area x basalt ) + (fraction of granite area x granite )
Weighted Average lithosphere =
I. Complete the table for materials and densities for each of Earth’s “spheres” from the
lab manual or from your calculations above (atmosphere, cryosphere, hydrosphere,
lithosphere, mantle, core etc.) and explain why they occur in this order. Give 3 significant
figures. (10)
Atmosphere (N2 + O2 )
Crust (Granite + Basalt)
UM Lithosphere
Lower Mantle
Outer Core
Inner Core
Density (g/cm3)
Explanation for order:
Activity 1.4 Density, Gravity, and Isostasy
A.1. Sketch a wooden block labelling its dimensions and calculate its density. Recall
Volume = (Length x Width x Height) (measure to nearest tenth of a centimetre). (4)
A.2 Calculate the density of a wood block.  = mass (g) / volume (cm3) (2)
B. Sketch the wooden block measuring the location of the wetted line (plimsoll line on a
boat). The total mass of the lighter wood block only displaces a volume of water equal to
the wetted part. This demonstrates the principle of buoyancy. (3)
1. What is the total height or thickness? H(block) =__________cm
2. Measure the wetted part.
3. Measure the remaining dry part.
C. Given that the portion of the wood block’s height (and mass) that is below the
equilibrium line is equal to the total height of the wood block times the ratio of the
density of wood to water, write an isostasy equation for height of wood below
equilibrium line. (5)
Hb =
2.) Use this equation and the measured values for your block’s density, height above and
total height to predict the height below. Compare this calculated value to the actual
measured value and express this as a %error for your calculated value compared to the
real wet block.
% Error = 100 X {calculated-real}/real = ______________ %
D. Write an isostasy equation for height of wood above equilibrium line. While we can
readily observe both the upper and lower parts of wood blocks, it is not so easy for
icebergs (ask Captain Smith!) or mountain ranges (Ask Mr. Airy or Mr. Pratt!)
(Hint: Htotal = Ha + Hb Do a substitution using the other heights & rearrange the
equation in C-1 to solve for Ha .) (5)
Ha =
E. Using the density of sea ice as 0/.917 g/cm3 and that of sea water as 1.025 g/cm3,
1.) Repeat the Hb = height below calculation from part C, for the iceberg to predict what
proportion of the iceberg is hidden below sea level. Compare this to the total height of the
iceberg pictured in figure 1.13b and give your % error for Hb. (5)
2.) As for part 1, solve for Ha = the iceberg above water, using the equation from part D
to predict what proportion of the iceberg shows above the water. Compare this to the total
height of the iceberg pictured in figure 1.13b and give your % error Ha. (5)
3. Icebergs are not perfect blocks. Due to this irregular shape, the cross sectional area is
probably a better estimate of mass and buoyancy than height alone. Repeat part 1 and 2
above but this time accurately count the squares and use cross sectional area instead of
height. (count whole squares and fractional squares) (4)
Area below =
Area above =
How do these results compare to the estimates from the heights alone? (4)
For part a
% error for Area below = ________
For part b
% for Area above
= _________
4. Notice the tilted ledge above the waterline in figure 1.13b and discuss what will
happen to the position of the old plimsoll line (waterline like on a boat) as the top of
iceberg melts and why? (2)
5. Charles Dutton compared raised beaches along continental margins to that old tilted
ledge on the iceberg photo. Compare eroded continents to melted icebergs. Many sections
of coastlines have wave cut platforms that are considerably above modern sea level.
According to Dutton p.26 and again on p 29 Activity 1.5-F, ancient shorelines have
become elevated because… (2)
Activity 1.5: Isostasy and Earth’s Global Topography
A. My basalt sample: mass _______ g , volume ______
 basalt = _________g/cm3 (3)
Average density of basalt (yours plus 9 in table p.27) =___________ g/cm3
B. My granite sample: mass ______ g , volume ______  granite = _________ g/cm3 (3)
Average density of granite =___________ g/cm3 (1)
C. Isostasy calculations to explain Earth’s Topography (high continents and mountains
versus low ocean basins and deep sea trenches)
1. For 5 km thick seafloor basaltic crust in mantle calculate: H basalt above =
km (3)
2. For 30 km thick continental granite in mantle calculate: H granite above =
km (3)
3. Net elevation difference Continental Crust above mantle minus Oceanic crust above
mantle: (H granite above - H basalt above) =
km or ____________ metres. (2)
4. How close are your calculated isostasy H above results compared to the actual
topographic height differences between the average continental land elevation and
average ocean basin depth in terms of distances __________ (metres) and percent errors
__________ %. Assume the topographic averages to be the true value for this error
calculation. (3)
5. What can you infer from this about the validity of our simple 3 rock Earth (only granite
or basalt on peridotite) and 1 dimensional isostatic thickness model for calculating the
outside shape of the Earth? (Basically, if your answers agree to a few %, this is a pretty
reasonable simplification!) (2 points)
D. Reflect on your work using isostasy calculations to account for the observations either
on land and sea or from space, to account for the observation that Earth has a bimodal
global topography (2 predominant but different elevations; e.g. land and mountains are
high and while seas are deep.) How valid is isostatic theory? What kind of rocks must
underlie the land versus the sea? (3)
E. Compare a mountain with the iceberg in figure 1.16 in terms of the changes that affect
them, the driving forces which move them and the time scales for these changes and
processes. How are these forces or agents of change, changes and time scales similar or
different. (6)
F. Use the data provided in the manual p20-29 and your calculations, to make an
inference concerning the Pratt (variable density) versus Airy (variable thickness)
hypotheses for crustal elevation and isostasy’s driving forces. Which model best explains
continents and continental mountain ranges? Which model best explains the elevation
differences between mid ocean ridges and ocean basins? Is one model more right than the
other or does the real Earth require that we understand and use both ideas to account for
different features? Feel free to make and label your own sketches here. (4)