Laboratory Manual for Introductory Geology 4e

1.3 Units for Geologic Measurement
Before we begin to examine the components of the Earth System scientifically, it is
important to be familiar with the units of measurement used to quantify them. We
can then examine the challenges of scale that geologists face when studying the
Earth and the atoms of which it is made.
1.3.1 Units of Length and Distance
People have struggled for thousands of years to describe size in a precise way with
widely accepted standard units of measurement. Scientists everywhere and people
in nearly all countries except the United States use the metric system to measure
length and distance. The largest metric unit of length is the kilometer (km),
which is divided into smaller units: 1 km 5 1,000 meters (m); 1 m 5 100 centimeters
(cm); 1 cm 5 10 millimeters (mm). Metric units differ from each other by
a factor of 10, making it very easy to convert one unit into another. For example,
5 km 5 5,000 m 5 500,000 cm 5 5,000,000 mm. Similarly, 5 mm 5 0.5 cm 5 0.005
m 5 0.000005 km.
The United States uses the U.S. customary system (called the English system
until Great Britain adopted the metric system) to describe distance. Distances
are given in miles (mi), yards (yd), feet (ft), and inches (in), where 1 mile 5
5,280 feet; 1 yard 5 3 feet; and 1 foot 5 12 inches. As scientists, we use metric
units in this book, but when appropriate, U.S. customary equivalents are also
given (in parentheses).
Appendix 1.1, at the end of this chapter, provides basic conversions between U.S.
customary and metric units.
1.3.2 Other Dimensions, Other Units
Length and distance are just two of the dimensions of the Earth that you will
examine during this course. Other units are used to describe other aspects of the
Earth, its history, and its materials: units of time, velocity, temperature, mass, and
density.
Time is usually measured in seconds (s), minutes (min), hours (h), days (d),
years (yr), centuries (hundreds of years), and millennia (thousands of years). A
year is the amount of time it takes for the Earth to complete one orbit around
the Sun. Because the Earth is very old, geologists have to use larger units of
time: a thousand years ago (abbreviated Ka, for kilo-annum), a million years ago
(Ma, for mega-annum), and a billion years ago (Ga, for giga-annum). The formation
of the Earth 4,560,000,000 years ago can thus be expressed as 4.56 Ga,
or 4,560 Ma.
Velocity, or the rate of change of the position of an object, is described by units
of distance divided by units of time, such as meters per second (m/s), feet per second
(ft/s), kilometers per hour (km/h), or miles per hour (mph). You will learn
later that the velocity at which geologic materials move ranges from extremely slow
(mm/yr) to extremely fast (km/s).
Temperature is a measure of how hot an object is relative to a standard. It
is measured in degrees Celsius (°C) in the metric system and degrees Fahrenheit
(°F) in the U.S. customary system. The reference standards in both systems
are the freezing and boiling points of water: 0°C and 100°C or 32°F and 212°F,
respectively. Note that there are 180 Fahrenheit degrees between freezing and
boiling, but only 100 Celsius degrees. A change of 1°C is thus 1.8 times larger
12 CHAPTER 1 SETTING THE STAGE FOR LEARNING ABOUT THE EARTH