Lab: Exploring Density and Isostasy
Learning Objectives:
 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.
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 observe how two blocks of wood of
equal size, but different thickness, float in water. You will then compare your observations with
how oceanic and continental crust behave as they float on the mantle. As an additional part of
the activity, you will observe what happens when an ice cube is placed on a block of wood. The
weight of the ice will press the block lower into the water, and you can observe the block slowly
rising as the ice melts. What will this simulate?
Materials Required:
 3 rock samples (shale, granite, basalt)
 Graduated cylinder (100 mL)


Triple beam balance
Water
Hypothesis: Hypothesize about which sample will have the greatest density given your background
knowledge.
Objective: In this investigation, you will explore how to determine the density of crustal rocks, and
relate your results to the average density of the earth’s crust and principal of isostasy.
Procedures: Part I
1. 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.
2. 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)
3. 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.
4. 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.
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 in Data Table I, 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
Shale
Granite
Basalt
Mass (g)
Volume (cm3)
Density (g/cm3)