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MS 20 Introduction To Oceanography
Isostasy
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Record all data in the appropriate metric units (centimeters, grams, etc.). Remember to
use significant figure rules and to indicate appropriate units (if the scale reads 13.4 g,
your answer is not 13.4, but 13.4 g (or 13.4 grams).
A.
Demonstrating Archimedes’ Principle
Pine Block
Oak Block
Mass of block <grams>
Weight of water in pan <grams>
Displaced volume <cm3 or mL>
(volume of water in pan)
Block thickness <cm>
Length of side <cm>
Vblock (equation 1) <cm3>
Rhole <cm>
(½ diameter)
Vhole (equation 2) <cm3>
Vb - h (equation 3) <cm3>
ρwood (equation 4) <g/cm3>
MS 20 Laboratory
Revised on 6/17/2017
Density, Specific Gravity, Archimedes and Isostasy
Page 1 of 7
Archimedes' Principle states that a floating body will displace its own weight of fluid. Are
the masses of the blocks the same as the masses of water each displaces? If there is a
difference, what are the likely sources of error?
B.
Modeling the Continental and Oceanic Lithosphere
Pine Block
Oak Block
Depth of water <cm>
Volume of water <cm3 or mL>
Mass of water <g>
Mass of block <g>
Combined masses <g>
(water column + wood block)
Depth of water / no. blocks <g>
Mass of water / no. blocks <g>
Again, according to Archimedes' Principle the pressure (or the total weight) acting on the
bottom of the tank (or at some depth in the asthenosphere) should not change as more
floating masses are added; i.e., the combined masses of each of the wood blocks and
the water columns beneath them should be the same as the total mass of the open
water. Is this true here? Again, try to account for likely sources of error.
MS 20 Laboratory
Revised on 6/17/2017
Density, Specific Gravity, Archimedes and Isostasy
Page 2 of 7
Figure 6 shows our model of the earth's crust using two different types of wood of to
represent the two different types of crust. Assume you shaved some wood off the top of
the pine block (representing continental erosion). What would happen to the pine block?
Explain.
Depth of water under two pine blocks <cm>
Volume of water column <cm3 or mL>
Mass of water column <g>
Total mass of two pine blocks <g>
Combined masses of blocks and water column <g>
MS 20 Laboratory
Revised on 6/17/2017
Density, Specific Gravity, Archimedes and Isostasy
Page 3 of 7
Is the total mass acting on the bottom of the aquarium approximately the same as in the
previous calculations? Again, try to account for likely sources of error?
MS 20 Laboratory
Revised on 6/17/2017
Density, Specific Gravity, Archimedes and Isostasy
Page 4 of 7
Gravity and time, aided by various processes of physical and chemical weathering,
removes rock from higher continental elevations and transports it to lower elevations,
ultimately to the ocean floor. For every meter of rock removed from a mountain range,
would you expect the elevation to decrease by one meter? Explain.
What happens to the oceanic crust as water, ice and wind continuously deliver and
deposit continental sediments? Explain.
C.
Density and Specific Gravity of Rocks
GRANITE
BASALT
PERIDOTITE
Mass in air
Mass in water
Massair - Masswater
Rock volume (equation 5)
Rock density (equation 4)
MS 20 Laboratory
Revised on 6/17/2017
Density, Specific Gravity, Archimedes and Isostasy
Page 5 of 7
Can you think of another way to measure the volume of the rock specimens?
D.
Determination of total
asthenosphere
weight under ocean crust
5.0 x 105 cm
x
A. Depth of water
10 x 105 cm
exerted at a
x
Water density
x
Basalt density
x
1.0 cm
x
Peridotite density
1.0 cm
1.0 cm
=
Width (W)
x
Length (L)
x
C. Depth of mantle
1.0 cm
fixed depth within the
Length (L)
x
B. Depth of ocean crust
135 x 105 cm
mass
1.0 cm
=
Width (W)
x
Length (L)
1.0 cm
=
Width (W)
Total weight under ocean (A + B + C)
=
weight under continental crust
30 x 105 cm
x
D. Depth of crust
120 x 105 cm
x
Granite density
x
Length (L)
x
E. Depth of mantle
1.0 cm
x
Peridotite density
1.0 cm
1.0 cm
=
Width (W)
x
Length (L)
1.0 cm
=
Width (W)
Total weight under continent (D + E)
=
weight under mountains
55 x 105 cm
x
F. Depth of crust
100 x 105 cm
G. Depth of mantle
x
Granite density
x
x
Length (L)
x
Peridotite density
1.0 cm
1.0 cm
Length (L)
1.0 cm
Width (W)
x
1.0 cm
Revised on 6/17/2017
=
Width (W)
Total weight under continental mountains (F + G)
MS 20 Laboratory
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Density, Specific Gravity, Archimedes and Isostasy
=
Page 6 of 7
During ice ages slightly cooler temperatures at mid-to-high latitudes cause less snow to
melt in spring and summer than accumulates in winter. Over many years this ice piles up
to form continental glaciers that can exceed 2 kilometers in thickness. Glacial ice has a
density of about 0.9 g/cm3, just below that of water, and about 1/3 that of granite. What
would be the effect of a 2000 meter thick ice sheet on the continent? What would
happen when the ice melted?
The total weight under the continents should precisely equal the weight under the ocean
basin. The calculations, although close, don't really match. What does this tell us about
our simple three-rock model? Explain.
The last ice age ended in northern Europe and in North America approximately 10,000
years ago, yet there is excellent evidence that the Scandinavian peninsula (which
includes Norway, Sweden and Finland) is still uplifting rapidly. From this information,
what can we conclude about the physical properties of the asthenosphere?
MS 20 Laboratory
Revised on 6/17/2017
Density, Specific Gravity, Archimedes and Isostasy
Page 7 of 7