Page 50 - Laboratory Manual in Physical Geology

A CTIVIT Y 1.5 Density, Gravity, and Isostasy

Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. Obtain one of the wood blocks provided at your
table. Determine the density of the wood block
3
1r wood 2 in g>cm . Show your calculations.

B. Float the same wood block in a bowl of water (like FIGURE 1.10A ) and mark the equilibrium line (waterline).

1. Measure and record H
block
(total height of the wood block) in cm: _______________ cm

2. Measure and record H (height of the
below
wood block that is submerged below the water line) in cm: _______________ cm
3. Measure and record H (height of

above
the wood block that is above the water line) in cm: _______________ cm

C. Write an isostasy equation (mathematical model) that expresses how the density of

the wood block 1r wood 2 compared to the density of the water 1r water 2 is related

to the height of the wood block that floats below the equilibrium line 1H below 2.
[ Hint: Recall that the wood block achieves isostatic equilibrium (motionless
balanced floating) when it displaces a volume of water that has the same mass
as the entire wood block. For example, if the wood block is 80% as dense as the
water, then only 80% of the wood block will be below the equilibrium line
(water line). Therefore, the portion of the wood block’s height that is below the

equilibrium line 1H below 2 is equal to the total height of the wood block 1H block 2

times the ratio of the density of the wood block 1r wood 2 to the density of

water 1r water 2.
D. Change your answer in Part C to an equation (mathematical model) that expresses

how the density of the wood block 1r wood 2 compared to the density of the water

1r water 2 is related to the height of the wood block that floats above the equilibrium

line 1H above 2.
3
3

E. The density of water ice (in icebergs) is 0.917 g>cm . The average density of (salty) ocean water is 1.025 g>cm .

1. Use your isostasy equation for (H below ) (Question C)

to calculate how much of an iceberg is submerged below
sea level. Show your work.
2. Use your isostasy equation for (H above ) (Question D)

to calculate how much of an iceberg is exposed above
sea level. Show your work.

3. Notice the graph paper grid overlay on the picture of an iceberg in
FIGURE 1.10B . Use this grid to determine and record the cross-sectional

area of this iceberg that is below sea level and the cross-sectional area
that is above sea level (by adding together all of the whole boxes and
fractions of boxes that overlay the root of the iceberg or the exposed
top of the iceberg). Use this data to calculate what proportion of the
iceberg is below sea level (the equilibrium line) and what proportion is
above sea level. How do your results compare to your calculations in
Questions E1 and E2?
4. What will happen as the top of the iceberg melts?

F. REFLECT & DISCUSS Clarence Dutton proposed his isostasy hypothesis to explain how some ancient shorelines have been
elevated to where they now occur on the slopes of adjacent mountains. Use your understanding of isostasy and icebergs to
explain how this may happen.
35