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GEOL 106 – LIFE IN THE AGE OF DINOSAURS
LAB 4: DENSITY, ISOSTASY, AND PLATE TECTONICS
Introduction:
Your aware that there is a difference between the weight of a rock and the weight of an
equal-sized chunk of bone or wood. This is because of density, or mass/volume. Rock is
much more dense than either wood or bone. But you may not realize that rock densities
are not all the same. In fact, relatively small differences in the density of common rock
types found in the continental crust, ocean crust, and mantle determine the depth of the
ocean basins, the height of mountain ranges, and prevent the continental crust from being
recycled. Today we will explore the density differences between common Earth
materials and simulate the effects of erosion on the topography of a mountain range in
order to grasp the concept of isostasy. Simply put, isostasy is similar to floating: the
crust and upper mantle are floating above the slushy asthenosphere. Loading of the crust
by folding or deposition of sediment leads to isostatic depression, or downwarping, while
removal of sediment or rock causes rebound or upwarping.
Learning Goals:
To appreciate the variability and importance of density in Earth materials
To understand the relationships between density, isostasy, and topography
To gain a working understanding of plate tectonics
Upon completion of this lab, students will be able to:
Calculate the density of a rock (or piece of wood) by measuring mass and volume
Describe the relative densities of continental crust, oceanic crust, and the mantle
Explain why continental crust cannot be returned to the mantle
Predict which of 2 wood blocks will float higher above the surface in a tank of water
Describe the effects of erosion on topography
Explain why mountains have roots
Construct and interpret x/y plots of data collected by the class
PART I: DENSITY OF DIFFERENT ROCK TYPES
Although the earth has many different kinds of rocks, some are more common than
others. These are considered the main constituents of the crust. In order to get a feel for
the range of densities in the rocks that make up the crust and upper mantle, weigh and
measure 1 sample each of granite, basalt, and peridotite. Fill in the table below and
write your data on the board so we can gather class measurements and find averages. Use
the electronic balance to weigh the samples, and use water displacement to measure the
volume. (Remember that 1 mL = 1 cm3). To get density, then, all you have to do is
divide the sample mass by its volume.
Basalt
Granite
Peridotite
Sample mass (g)
Original water
volume (ml)
Volume of water
and rock (ml)
Volume of rock
(cm3)
Density of rock
(g/cm3)
Class average
density (g/cm3)
Number 1-3,
from least-most
dense
Which
component of
the earth?*
* your choices are continental crust, oceanic crust, and mantle
In your own words, explain why ocean basins (which have a floor of ocean crust) are
areas of low elevation compared to the continents. In other words, why does basaltic
ocean crust for low spots on the surface of the Earth?
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PART II: MOUNTAIN BUILDING AND EROSION
It is a somewhat puzzling fact that we observe piles of sediment, up to several kilometers
deep, that was entirely deposited in a shallow ocean environment. In this exercise we
simulate a coastal mountain range, such as the Appalachians, and associated coastal
margin sedimentation. We will simulate this system using wooden blocks to represent
the crust and water to represent the higher-density mantle on which the crust floats.
Step 1: Select three thick wooden blocks (be sure to get 3 different types of wood) and
approximately 10 thinner wooden plates and determine their densities. The density of the
blocks is variable, but the density of all plates should be the same. Therefore you only
need to determine the density of one plate. To do this, weigh the block or plate and note
the number written on it. Then measure the length, width, and height of the block or
plate and record these measurements in the chart below. Calculate the volume (length x
width x height) in cm3. Divide the mass by the volume (as in part 1) to compute the
density.
Step 2: How high does each block float? Stake out on of the clear plastic basins that is
about three-quarters full of water. Float each block and the plate, recording the height of
the wood above and below the water line.
Block 1
Sample mass (g)
Sample length (cm)
Sample width (cm)
Sample height (cm)
Volume (cm3)
Density (g/cm3)
Height above water (cm)
Height below water (cm)
X*
Density wood/
Density water
Y*
H below water/
H total
Block 2
Block 3
Plate
Step 3: Now calculate a couple of ratios and add them to your chart (above). First,
divide the density of the wood by the density of water (the density of water is 1g/cm3).
Then divide the height below water by the height above water for each block or plate.
Express your values to 2 decimal places! Rounding will not do.
Step 4: Graph your results on a piece of graph paper, placing values from the row
labeled X on the x-axis (horizontal axis) and values from the row labeled Y on the y-axis
(vertical axis). Incorporate 3 additional data points from other teams who used different
types of wooden blocks.
Based on your graph, describe the relationship between the density ratios and the height
ratios.
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Suppose that the wooden blocks were floating in a tub of honey instead of water. What
would you expect to see, and why?
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Step 5: Return to your basin or water. Choose two of your thick blocks and float them
next to each other. This is the crust! Now create a mountain range by piling the plates on
top of one of the blocks to simulate thickened continental crust. This might be a bit
tricky, but try to pile them as high as you can (see Figure 1 below).
Step 6: Measure the thickness of the two piles. Then measure the elevation of the
mountains (pile of plates) and the plains (single block) and the depth of the water beneath
the two blocks. Record your results below.
Step 7: Simulate erosion by moving plates from the mountain range onto the plains.
After each step measure the thickness of the piles, the elevation of the mountains and the
plains, and the depth of water beneath each. Continue until the mountains and plains are
about the same height. Erosion now stops, because there is no relief left!
Step 8: Graph the elevation of the mountains and the plains after each erosion step. To
do this, put time (erosion step) on the x-axis and height on the y-axis. Now add the total
thickness of the mountain crust to your graph. You will end up with 3 lines, one for the
elevation of the mountains, one for the elevation of the plains, and one for the total
thickness of the mountain crust.
Erosion
Plates
Thickness
Elevation
Depth
Water Level
Block 1
Block 2
Figure 1: An object will float when the mass of the displaced fluid (water in this case) equals the
total mass of the object.
Start
Number of plates
(mountains)
Number of plates
(plains)
Thickness of mountain
crust (cm)
Mountain Elevation
(cm above water)
Mountain depth
(cm below water)
Thickness of plains crust
(cm)
Plains Elevation
(cm above water)
Plains depth
(cm below water)
Step 1
Step 2
Step 3
Step 4
Step 5
Consider the two graphs that you have made, one of density vs. height ratios and one of
elevation of the mountains, the plains, and the total thickness of the mountain crust over
time. In your own words, explain why regions of high elevation (mountains!) have
roots (thick crust that extends far into the mantle below).
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