Page 33 - Laboratory Manual in Physical Geology

The water displacement procedure for determining WATER DISPLACEMENT METHOD
the volume of a rock is illustrated in FIGURE 1.9 . First place FOR DETERMINING VOLUME

water in the bottom of a graduated cylinder. Choose a OF A ROCK SAMPLE
graduated cylinder into which the rock will fit easily, and
add enough water to be able to totally immerse the rock. It
is also helpful to use a dropper or wash bottle to raise the mL Written mL or ml mL
volume of water (before adding the rock) up to an exact 10 10
graduation mark (5.0 mL mark in FIGURE 1.9A ). Record
this starting volume of water. Then carefully slide the rock 9 9
sample down into the same graduated cylinder and record
this ending level of the water (7.8 mL mark in FIGURE 1.9B ).
Subtract the starting volume of water from the ending 8 8 7.8 mL
volume of water, to obtain the displaced volume of water
3

(2.8 mL, which is the same as 2.8 cm ). This volume of 7 7

displaced water is also the volume of the rock sample. C. Volume
of water is
2.8 mL
Mass 6 6
Earth materials do not just take up space (volume). They
5.0 mL
also have a mass of atoms that can be weighed. You will 5 5
use a gram balance to measure the mass of materials
(by determining their weight under the pull of Earth’s 4 4
gravity). The gram (g) is the basic unit of mass in the
metric system, but instruments used to measure grams
vary from triple-beam balances to spring scales to digital 3 Rock 3
balances (see page xiii at the front of the manual) . Consult sample
with your laboratory instructor or other students to be 2 2
sure that you understand how to read the gram balance
provided in your laboratory.
1 1
Determining Rates
Geologists make many comparisons. You may find
yourself comparing similar kinds of objects (a so-called
A. Starting volume B. Ending volume
apples-to-apples comparison) in one case, but different of water of water
kinds of objects in another case (a so-called apples-to-
oranges comparison). The same is true when comparing
PROCEDURES
measurements of things (quantitative data). You may
find yourself recording one kind of data in one unit of A. Place water in the bottom of a graduated
measure, but a second kind of data in another unit of cylinder. Add enough water to be able to totally
measure. If the two measures are of the same class, such immerse the rock sample. It is also helpful to
as two lengths or two masses, then you can simplify your use a dropper bottle or wash bottle and bring
the volume of water (before adding the rock
comparison (from apples-to-oranges to apples-to-apples)
sample) up to an exact graduation mark like the
by converting the different units of measure to one kind 5.0 mL mark above. Record this starting volume
of unit. For example, if one distance is measured in of water.
miles and another in kilometers (an apples-to-oranges
comparison), then simply convert the miles measurement B. Carefully slide the rock sample down into the
same graduated cylinder, and record the ending
to kilometers so both distances are in kilometers (and make
volume of the water (7.8 mL in the above
a simpler apples-to-apples comparison). Conversion tables example).
are provided on page xii at the front of the manual for
this purpose. C. Subtract the starting volume of water from
What if you want to compare measures of different the ending volume of water to obtain the
displaced volume of water. In the above
classes, such as how long objects are (units of length)
example: 7.8 mL – 5.0 mL = 2.8 mL (2.8 mL is
compared to their mass (units of weight)? You are “stuck” the same as 2.8 cm ). This volume of displaced
3
with an apples-to-oranges comparison, so you must water is the volume of the rock sample.
determine a rate. Rate is a mathematical expression of
how much an amount determined in one unit of measure FIGURE 1.9 Procedure for determining volume of a rock
varies “per” (divided by) an amount determined in a sample by water displacement.
different unit of measure.

18 ■ L ABOR ATORY 1