Lab: Exploring Density and Isostasy
Learning Objectives:
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To determine the density of several common rocks and other materials and relate these to isostasy.
To demonstrate the principle of isostasy by creating a simple model to illustrate how the Earth’s crust
floats on the mantle.
Background Information:
Seismic studies show that the Earth is made up of three principal layers. The crust represents a thin, solid
outer layer covering the Earth’s surface, and varying in thickness from place to place. Parts of the crust that make
up the sea floor (oceanic crust) are generally thinner and range from 4 to 7 kilometers in thickness. The crust under
the continents (continental crust) is typically about 35 kilometers in thickness, but can be as much as 70 kilometers
thick in some mountainous regions. Oceanic rocks also have a greater density than continental rocks. The Earth’s
crust can be deformed by the processes of folding, faulting, and uplift, and also by changes in the weight of the
crust.
Beneath the solid Earth’s crust is a thick layer of rock called the mantle. The mantle extends to a depth of
about 2,870 kilometers and makes up about 80% of the Earth’s volume and almost two-thirds of the Earth’s mass.
The rocks of the mantle are more dense than those of the Earth’s crust, and can actually flow like a thick liquid
under the mantle’s higher temperature and pressure conditions. Because the crustal rocks are less dense, the solid
crust actually floats on the denser mantle.
Below the mantle lies the Earth’s core. This very dense core has a radius of about 3,500 kilometers and is
comprised of a liquid outer core and a solid inner core. The core represents nearly one-third of the Earth’s mass
and 19% of its volume.
In this dynamic Earth system, the floating crust exerts a downward force on the denser mantle, while the
mantle also exerts an upward force on the crust. Just as large, thick logs float higher in water than smaller logs, the
thicker parts of the crust found in mountainous regions float higher on the mantle than the thinner crustal areas of
lower elevation. These thicker crustal areas also extend deeper into the supporting mantle material below. The
state of balance which exists between the lower-density crust and the higher-density mantle is called isostasy.
Under the concept of isostasy, when weight is added to an area of the crust, the weight increases the force
with which the crust presses on the mantle. The crust in this area will sink until a balance of forces is again
reached. When weight is removed from the crust, that area rises. These up and down movements of the crust to
reach isostasy are called isostatic adjustments.
One example of isostatic adjustment takes place as crustal material is eroded from mountain ranges over
thousands of years. The erosion of rock material can significantly reduce the height of a mountain range and the
weight of the crust. As the weight of the crust is reduced, the area will rise. Another example can be found in areas
where rivers flow into larger bodies of water. Carrying large amounts of silt, sand, and gravel, rivers deposit this
material on the lake or ocean floor as they enter these larger bodies. The added weight of this material causes the
floor to sink deeper into the mantle. During glacial periods, the crustal rocks were covered by many tons of ice. In
this instance, the weight of the ice caused the crust to sink. As the ice melted over time, the weight on the crustal
rocks was reduced causing the land to rebound and float higher on the mantle.
These and many other examples demonstrate how isostatic adjustment can account for considerable
crustal movement. As erosion lowers the summit of mountains, the crust will rise in response to the reduced
weight. Similarly, as the mountains wear down, the weight of the eroded sediments deposited on nearby
continental margins will cause the crust to subside.
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Pre-Lab Questions:
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Draw a scaled diagram of the Earth labeling the three principal layers and the thickness of each. Include
all sub-layers of the three principal layers.
Compare the density of crustal rocks to the density of mantle. What effect does this difference in density
cause?
What is isostasy and how is it related to relationship of the crust and mantle?
Define isostatic adjustments and give an example.
Why should the rock sample in part I be completely dry before making measurements?
What information is needed to calculate the density of a rock and how should this information be
measured?
What is the formula and unit for density?
What materials are needed to complete Part I? Make a materials list in your journal.
This lab consists of two different sections Part I, which is “cookbook”, and Part II, which is inquiry. In
your journals, create a data table for part I of the lab to record data. An example follows. Label your data
table appropriately and include all units of measurement.
Overview:
In this two-part investigation, the role of density in isostasy is explored by first comparing the densities of
three basic rock samples. The three rocks represent typical crustal rocks at the Earth’s surface. You will determine
the density of each rock and relate your results to the concept of isostasy. You will make use of a 100 mL
graduated cylinder and a laboratory balance in this exercise. The rock samples should fit easily into the graduated
cylinder. However, if necessary, you may break the rocks into smaller pieces to fit the cylinder. Make sure the
rock is dry before making measurements.
In the second part of the investigation, you will be designing your own lab model using all the tools and
materials at your disposal in order to model principles of density and isostasy. In the model you design, you should
be able to
 observe how oceanic and continental crust behave as they float on the mantle.
 model subduction due to density and “slab pull”
 model how weathering, erosion and deposition might affect density and isostasy.
 model how glaciation affects density and isostasy.
There will be a list of materials following part A that you will have at your disposal in order to build your model. If
you can think of anything else you need for your model, please let me know, and I will try to come up with it.
Part I
Materials Required:
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3 rock samples (shale, granite, basalt)
Graduated cylinder (100 mL)
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Triple beam balance
Water
Hypothesis: Hypothesize about which sample will have the greatest density given your background knowledge.
Procedures: Part I
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Obtain the samples of shale, granite, and basalt. Check to make sure each sample will fit into the graduated cylinder. Using
a laboratory balance, measure the mass of each sample and record the information in Data Table 1 in your journal.
Fill the graduated cylinder with water to at least 30mL level. Carefully note this starting water level in the cylinder to the
nearest milliliter. Tilt the graduated cylinder and carefully slide your first rock sample into the cylinder. The sample will
displace the water in the cylinder to a new level. Subtract the starting level from this new level to determine the sample’s
volume, and record in Data Table 1. (Note: one mL = one cm3)
Carefully remove your first sample, and repeat procedure B for the remaining two samples. Note: You may dump the water
out as you do this, and then just refill it for the other samples.
Now, find the density of each rock sample by dividing the mass by the volume. Record your results in Data Table 1 in your
journal.
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Questions (Part I)
1. Which rock sample has the greatest density?
2. Which rock sample has the least density?
3. Did these results surprise you? If so, why? Make sure you make your conclusion in your journal, and that it
reflects your hypothesis.
4. Shale, granite, and basalt represent three of the most common rocks in the Earth’s crust. Based on your
results, what is their average density?
5. Researchers have determined the average density of the Earth to be about 5.5g/cm3. How does the average
density of your three rock samples compare to this figure?
6. With your understanding of the principal of isostasy, if the three rock samples were of equal volume, which
of the samples would float highest if resting on the mantle. Why?
7. Which rock sample (of equal volume) would extend deepest into the supporting mantle? Why?
8. Which has the highest density, the Earth’s crust as represented by the rock samples in this investigation, or
the entire Earth?
9. How does the answer to Question #8 support the idea that the Earth has a lower-density crust floating on a
higher-density mantle?
Data Table I
Rock Sample
Mass (g)
Volume (cm3)
Density (g/cm3)
Shale
Granite
Basalt
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Part II: Inquiry into Density and Isostasy
Remember, In the second part of the investigation, you will be designing your own lab model using all the tools and
materials at your disposal in order to model principles of density and isostasy. In the model you design, you should be able to:
A. observe how oceanic and continental crust behave as they float on the mantle.
B. model subduction due to density and “slab pull”
C. model how weathering, erosion and deposition might affect density and isostasy
D. model how glaciation affects density and isostasy
Here is the list of materials that you will have at your disposal in order to build your model. If you can think of anything else you
need for your model, please let me know, and I will try to come up with it.
Materials Available:
Water
Graduated cylinders
Wood blocks of different sizes
Metal washers
Water trays
Ice
timer
zip loc baggies of sand
granite, basalt, and shale pieces
metric ruler
zip loc baggies of pebbles
Triple beam balance/electronic balance
Anything else you can think of, that I might have
Part II Analysis Questions:
1. Sketch and label your model showing what each piece simulated, and how the continental and oceanic crust, “float” on
the mantle. Make sure you list the parts of your model, and describe how each part simulated parts of the Earth
2. How would you rate the model your group made of the “slab-pull” situation? How would you change it, given other
materials?
3. How did you model weathering, erosion and deposition in your model? How did if affect density and isostasy?
4. Given the results of your investigation, what conclusions can you make about the relative densities of water, wood,
metal washers, sand, pebbles…(and any other materials you may have used in your investigation)? Build a data table to
display your conclusions regarding relative densities.
5. How can isostasy be used to explain the uneven surface of the Earth’s crust? How is the Earth’s crust able to stay
balanced on the mantle? Use the various models you built to help answer the question!
6. How would you change your model of glaciation’s affect on isostasy, to help solve problems that you may have
encountered?
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