252 CHAPTER 8 Geologic Time ( 232 chapter 7 Fires Within: Igneous Activity figure 7.31 Seattle, Washington, with Mount Rainier in the background. (Photo by Ken Straiton/Corbis) ) Until recently the dominant view of Western societies was that humans possess the wherewithal to subdue volcanoes and other types of catastrophic natural hazards. Today it is apparent that volcanoes are not only very destructive but unpredictable as well. With this awareness, a new attitude is developing—"How do we live with volcanoes?" Volcanic Hazards Volcanoes produce a wide variety of potential hazards that can kill people and wildlife, as well as destroy property (figure 7.32). Perhaps the greatest threats to life are pyroclastic flows. These hot mixtures of gas, ash, and pumice that sometimes exceed 800°C race down the flanks of volcanoes, giving people little chance to escape. Lahars, which can occur even when a volcano is quiet, are perhaps the next most dangerous volcanic hazard (figure 7.33). These mixtures of volcanic debris and water can flow for tens of kilometers down steep volcanic slopes at speeds that may exceed 100 kilometers (60 miles) per hour. Lahars pose a potential threat to many communities downstream from glacier-clad volcanoes such as Mount Rainier. Other potentially destructive mass-wasting events include the rapid collapse of the volcano's summit or flank. Other obvious hazards include explosive eruptions that can endanger people and property hundreds of miles from a Eruption cloud Prevailing wind Ash fall 2009 steam and ash cloud ( figure 7.33 Soufriere Hills volcano on the Caribbean island of Montserrat has been active since 1995. A pyroclastic flow destroyed the airport and the capital city, Plymouth. About two thirds of the population have left the island. (NASA Photo) ) ( Lava dome collapse Pyroclastic flow ) ( Fumaroles Lava flow Lahar (mud or debris flow) figure 7.3 2 Simplified drawing showing a wide variety of natural hazards associated with volcanoes. (After U.S. Geological Survey) ) Acid rain Pyroclastic flow Jf^- Eruption column Bombs Collapse of flank Lava dome ( The Chapter in Review 233 figure 7.34 Monitoring South Sister Volcano, Cascade Range, Oregon. This geologist is measuring the degree of infla tion of the volcano's surface for potential eruptive activity. ) volcano. During the past 15 years at least 80 commercial jets have been damaged by inadvertently flying into clouds of volcanic ash (Figure 7.33). One of these was a near crash that occurred in 1989 when a Boeing 747, with more than 300 passengers aboard, encountered an ash cloud from Alaska's Redoubt volcano. All four engines stalled after they became clogged with ash. Fortunately, the engines were restarted at the last minute and the aircraft managed to land safely in Anchorage. Monitoring Volcanic Activity Today a number of volcano-monitoring techniques are employed, with most of them aimed at detec.

1. 252 CHAPTER 8 Geologic Time
(
232
chapter 7
Fires Within: Igneous Activity
figure
7.31
Seattle, Washington, with Mount Rainier in the background.
(Photo by Ken Straiton/Corbis)
)
Until recently the dominant view of Western societies was that
humans possess the wherewithal to subdue volcanoes and other
types of catastrophic natural hazards. Today it is apparent that
volcanoes are not only very destructive but unpredictable as
well. With this awareness, a new attitude is developing—"How
do we live with volcanoes?"
Volcanic Hazards
Volcanoes produce a wide variety of potential hazards that can
kill people and wildlife, as well as destroy property (figure
7.32). Perhaps the greatest threats to life are pyroclastic flows.
These hot mixtures of gas, ash, and pumice that sometimes
exceed 800°C race down the flanks of volcanoes, giving people
little chance to escape.
Lahars, which can occur even when a volcano is quiet, are
perhaps the next most dangerous volcanic hazard (figure 7.33).
These mixtures of volcanic debris and water can flow for tens of
kilometers down steep volcanic slopes at speeds that may
exceed 100 kilometers (60 miles) per hour. Lahars pose a
potential threat to many communities downstream from glacier-
clad volcanoes such as Mount Rainier. Other potentially
destructive mass-wasting events include the rapid collapse of
the volcano's summit or flank.

2. Other obvious hazards include explosive eruptions that can
endanger people and property hundreds of miles from a
Eruption cloud
Prevailing wind
Ash fall
2009 steam and ash cloud
(
figure
7.33
Soufriere Hills volcano on the Caribbean island of Montserrat
has been active since 1995. A pyroclastic flow destroyed the
airport and the capital city, Plymouth. About two thirds of the
population have left the island. (NASA Photo)
) (
Lava dome collapse
Pyroclastic flow

3. ) (
Fumaroles
Lava flow
Lahar (mud or debris flow)
figure
7.3
2
Simplified drawing showing a wide variety of natural hazards
associated with volcanoes. (After U.S. Geological Survey)
)
Acid rain
Pyroclastic flow
Jf^- Eruption column
Bombs Collapse of flank Lava dome
(
The Chapter in Review
233
figure
7.34
Monitoring South Sister Volcano, Cascade Range, Oregon. This
geologist is measuring the degree of infla
tion of the volcano's surface for potential eruptive activity.
)

4. volcano. During the past 15 years at least 80 commercial jets
have been damaged by inadvertently flying into clouds of
volcanic ash (Figure 7.33). One of these was a near crash that
occurred in 1989 when a Boeing 747, with more than 300
passengers aboard, encountered an ash cloud from Alaska's
Redoubt volcano. All four engines stalled after they became
clogged with ash. Fortunately, the engines were restarted at the
last minute and the aircraft managed to land safely in
Anchorage.
Monitoring Volcanic Activity
Today a number of volcano-monitoring techniques are
employed, with most of them aimed at detecting the movement
of magma from a subterranean reservoir (typically several
kilometers deep) toward the surface. The four most noticeable
changes in a volcanic landscape caused by the migration of
magma are (1) changes in the pattern of volcanic earthquakes;
(2) expansion of a near-surface magma chamber, which leads to
inflation of the volcano; (3) changes in the amount and /or
composition of the gases that are released from a volcano; and
(4) an increase in ground temperature caused by the
emplacement of new magma.
Almost a third of all volcanoes that have erupted in historic
times are now monitored using seismographs, instruments that
detect earthquake tremors. In general, a sharp increase in
seismic unrest followed by a period of relative quiet has been
shown to be a precursor for many volcanic eruptions. However,
some large volcanic structures have exhibited lengthy periods of
seismic unrest. For example, Rabaul Caldera in New Guinea
recorded a strong increase in seismicity in 1981. This activity
lasted 13 years and finally culminated with an eruption in 1994.
Occasionally, a large earthquake triggers a volcanic eruption, or
at least disturbs the volcano's plumbing. Kilauea, for example,
began to erupt after the Kalapana earthquake of 1977.
The roof of a volcano may rise as new magma accumulates in its
interior—a phenomena that precedes many volcanic eruptions.

5. Because the accessibility of many volcanoes is limited, remote
sensing devices, including lasers, Doppler radar, and Earth
orbiting satellites, are often used to determine whether or not a
volcano is swelling. The recent discovery of ground doming at
Three Sisters volcanoes in Oregon was first detected using radar
images obtained from satellites (figure 7.34).
Volcanologists also frequently monitor the gases that are
released from volcanoes in an effort to detect even minor
changes in their amount and /or composition. Some volcanoes
show an increase in sulfur dioxide (S02) emissions months or
years prior to an eruption. On the other hand, a few days prior
to the 1991 eruption of Mount Pinatubo, emissions of carbon
dioxide (C02) dropped dramatically.
The development of remote sensing devices has greatly
increased our ability to monitor volcanoes. These instruments
and techniques are particularly useful for monitoring eruptions
in progress. Photographic images and infrared (heat) sensors
can detect lava flows and volcanic columns rising from a
volcano. Furthermore, satellites can detect ground deformation
as well as monitor S02 emissions.
The overriding goal of all monitoring is to discover precursors
that may warn of an imminent eruption. This is accomplished by
first diagnosing the current condition of a volcano and then
using this baseline data to predict its future behavior. Stated
another way, a volcano must be observed over an extended
period to recognize significant changes from its "resting state."
THE CHAPTER IN REVIEW
• The primary factors that determine the nature of volcanic

6. eruptions include the magma's temperature, its composition, and
the amount of dissolved gases it contains. As lava cools, it
begins to congeal; and as viscosity increases, its mobility
decreases. The viscosity of magma is directly related to its
silica content. Rhyolitic lava, with its high silica content, is
very viscous and forms short, thick flows. Basaltic lava, with a
lower silica content, is more fluid and may travel a long
distance before congealing. Dissolved gases provide the force
that propels molten rock from the vent of a volcano.
; The materials associated with a volcanic eruption include lava
flows (pahoehoe and aa flows for basaltic lavas); gases
(primarily in the form of water vapor); and pyroclastic material
(pulverized rock and lava fragments blown from the volcano's
vent, which include ash, pumice, lapilli, cinders, blocks, and
bombs).
Dating with Radioactivity 249
250 CHAPTER 8 Geologic Time
(
Of
) (
Rock unit A
) (
ft*
) (
N9
e
Rock unit B
) (
DID YOU KNOW?
Numerical dates of the fossil record show that life began in the
ocean approximately
3.8

7. billion years ago.
was once covered by a shallow sea, because that is where clams
live today. Also, by using what we know of living or
ganisms, we can conclude that fossil animals with thick shells
capable of withstanding pounding and surging waves must have
inhabited shorel
ines. On the other hand, animals with thin, delicate shells
probably indicate deep, calm offshore wa
ters. Hence, by looking closely at the types of fossils, the ap
proximate position of an ancient shoreline may be identified.
Further, fossils can indicate
the former temperature of the water. Certain present-day corals
require warm and shal
low tropical seas like those around Florida and the Bahamas.
When similar corals are found in ancient limestones, they indi
cate that a Florida-like marine environment m
ust have existed when they were alive. These are just a few
examples of how fossils can help unravel the complex story of
Earth history.
LA
Dl
O ACTIVITY
■
* Deciphering Earth History slffllgE Radiometric
Dating
In addition to establishing re
lative dates by using the princi
ples
described in the preceding sections, it is also possible to obtain
reliable numerical dates for events in the geologic
)Age ranges of some fossil groups
a
past. We know that Earth is about 4.6 billion years old and that
the dinosaurs became extinct about 65.5 million years ago.

8. Dates that are expressed in millions and billions of years truly
stretch our imagination because our personal calendars involve
time measured in hours, weeks, and years. Nevertheless, the
vast expanse of geologic time is a reality, and it is radiometric
dating that allows us to measure it accurately. In this section,
you will learn about radioactivity and its application in
radiometric dating.
Reviewing Basic Atomic Structure
s o
figure 8.13 Overlapping ranges of fossils help date rocks more
exactly than using a single fossil
Recall from Chapter 1 that each atom has a nucleus containing
protons and neutrons and that the nucleus is orbited by
electrons. Electrons have a negative electrical charge, and
protons have a positive charge. A neutron is actually a proton
and an electron combined, so it has no charge (it is neutral).
The atomic number (the element's identifying number) is the
number of protons in the nucleus. Every element has a different
number of protons in the nucleus and thus a different atomic
number (hydrogen ■ 1, oxygen = 8, uranium = 92, etc.). Atoms
of the same element always have the same number of protons,
so the atomic number is constant.
Practically all (99.9 percent) of an atom's mass is found in the
nucleus, indicating that electrons have practically no mass at
all. By adding together the number of protons and neutrons in
the nucleus, the mass number of the atom is determined. The
number of neutrons in the nucleus can vary. These variants,
called isotopes, have different mass numbers.
To summarize with an example, uranium's nucleus always has
92 protons, so its atomic number always is 92. But its neutron
population varies, so uranium has three isotopes: uranium-234
(number of protons + neutrons = 234), uranium-235, and
uranium-238. All three isotopes are mixed in nature. They look
the same and behave the same in chemical reactions.

9. Radioactivity
The forces that bind protons and neutrons together in the
nucleus are usually strong. However, in some isotopes, the
nuclei are unstable because the forces binding protons and
neutrons together are not strong enough. As a result, the nuclei
spontaneously break apart (decay), a process called
radioactivity. What happens when unstable nuclei break apart?
(
IP
8
) (
(
Pb
2
'
_Pb
2t
)Three common types of radioactive decay are illustrated in
figure s. 14 and are summarized as follows:
1. Alpha particles (a particles) may be emitted from the nucleus.
An alpha particle consists of 2 protons and 2 neutrons.
Consequently, the emission of an alpha particle means (a) the
mass number of the isotope is reduced by 4, and (b) the atomic
number is decreased by 2.
2. When a beta particle (B particle), or electron, is given off
from a nucleus, the mass number remains unchanged, because
electrons have practically no mass. However, because the
electron has come from a neutron (remember, a neutron is a
combination of a proton and an electron), the nucleus contains
one more proton than before. Therefore, the atomic number
increases by 1.
3. Sometimes an electron is captured by the nucleus. The
electron combines with a proton and forms an additional
neutron. As in the last example, the mass number remains
unchanged. However, because the nucleus now contains one less

10. proton, the atomic number decreases by 1.
An unstable (radioactive) isotope is referred to as the parent.
The isotopes resulting from the decay of the parent
U" ■
238 236 234 232 230 228 226 224 222 220 218 216 214 212 210
208 206
Alpha emission Beta emission
Th2: ■
Pa234
Th230
.Ra22
BPo2'
Bp
Po2
BP«)
Per™. ^
V
92 91 90 89 88 87 86 85 84 83 82
252 CHAPTER 8 Geologic Time
(
Atomic number: 2 fewer
Atomic mass: 4 fewer
) (
Unstable parent nucleus
) (

11. Daughter nucleus
: -
Neutron
Alpha particle emission
) (
Daughter Atomic number: nucleus 1 more
).+
B. Beta Emission
Unstable parent nucleus
(
Unstable parent nucleus
Atomic mass: no change
Beta (electron) emission
)Proton *6*^'*s A. Alpha Emission
Neutron
Electron »
H &
Proton
Daughter Atomic number:
nucleus 1 fewer
, Atomic mass:
£ no change
C. Electron Capture
Common types of radioactive decay. Notice that in each case,
the number of protons (atomic number) in the nucleus changes,
thus producing a different element.
FIGURE 8.15 The most common isotope of uranium (U-238) is
an example of a radioactive decay series. Before the stable end
product (Pb-206) is reached, many different isotopes are
produced as intermediate steps.

12. are the daughter products, figure 8. is provides an example of
radioactive decay. When the radioactive parent, uranium-238
(atomic number 92, mass number 238), decays, it follows a
number of steps, emitting eight alpha particles and six beta
particles before finally becoming the stable daughter product
lead-206 (atomic number 82, mass number 206).
Certainly among the most important results of the discovery of
radioactivity is that it provides a reliable method of calculating
the ages of rocks and minerals that contain particular
radioactive isotopes. The procedure is called radiometric dating.
Why is radiometric dating reliable? The rates of decay for many
isotopes have been precisely measured and do not vary under
the physical conditions that exist in Earth's outer layers.
Therefore, each radioactive isotope used for dating has been
decaying at a fixed rate ever since the formation of the rocks in
which it occurs, and the products of decay have been
accumulating at a corresponding rate. For example, when
uranium is incorporated into a mineral that crystallizes from
magma, there is no lead (the stable daughter product) from
previous decay. The radiometric "clock" starts at this point. As
the uranium in this newly formed mineral disintegrates, atoms
of the daughter product are trapped and measurable amounts of
lead eventually accumulate.
Half-Life
The time required for one half of the nuclei in a sample to decay
is called the half-life of the isotope. Half-life is a common way
of expressing the rate of radioactive disintegration. figure 8.16
illustrates what occurs when a radioactive parent decays
directly into its stable daughter product. When the
Dating with Radioactivity 25 I
1
40
o

13. 30
c
2
i
a
20
(
Radioactive
Stable Daughter
Currently Accepted
Parent
Product
Half-Life Values
Uranium-238
Lead-206
4.5 billion years
Uranium-235
Lead-207
713 million years
Thorium-232
Lead-208
14.1 billion years
Rubidium-87
Strontium-87
47.0 billion years
Potassium-40
Argon-40
1.3 billion years
Table 8.1
Radioactive isotopes frequently used in radiometric dating
) (
50
100 atoms of parent isotope
) (
25 atoms of parent 75 atoms of daughter product
) (

14. 50 atoms of parent 50 atoms of daughter product
13 atoms of parent 87 atoms of daughter
product
•••••••••
**••••••»*
«•»»•••••*
) (
••••••••••
,
)6 atoms of parent 94 atoms of daughter product
10
••••••••••
T
2 3
Number of half-lives
FIGURE 8.16 The radioactive decay curve shows change that is
exponential. Half of the radioactive parent remains after one
half-life. After a second half-life, one-quarter of the parent
remains, and so forth.
quantities of parent and daughter are equal (ratio 1:1), we know
that one half-life has transpired. When one-quarter of the
original parent atoms remain and three-quarters have decayed to
the daughter product, the parent/daughter ratio is 1:3, and we
know that two half-lives have passed. After three half-lives, the
ratio of parent atoms to daughter atoms is 1:7 (one parent atom
for every seven daughter atoms).
If the half-life of a radioactive isotope is known and the parent-
daughter ratio can be determined, the age of the sample can be
calculated. For example, assume that the half-life of a
hypothetical unstable isotope is 1 million years and the parent-
slaughter ratio in a sample is 1:15. Such a ratio indicates that

15. four half-lives have passed and that the sample must be 4
million years old.
Radiometric Dating
Notice that the percentage of radioactive atoms that decay
during one half-life is always the same: 50 percent. However,
the actual number of atoms that decay with the passing of each
half-life continually decreases. As the percentage of radioactive
parent atoms declines, the proportion of stable daughter atoms
rises, with the increase in daughter atoms just matching the drop
in parent atoms. This fact is the key to radiometric dating.
Of the many radioactive isotopes that exist in nature, five have
proved particularly important in providing radiometric ages for
ancient rocks (Table 8.1). Rubidium-87, uranium-238, and
uranium-235 are used for dating rocks that are millions of years
old, but potassium-40 is more versatile. Although the half-life
of potassium-40 is 1.3 billion years, analytical techniques make
possible the detection of tiny amounts of its stable daughter
product, argon-40, in some rocks that are younger than 100,000
years. Another important reason for its frequent use is that
potassium is abundant in many common minerals, particularly
micas and feldspars.
It is important to realize that an accurate radiometric date can
be obtained only if the mineral remained a closed system during
the entire period since its formation. A correct date is not
possible unless there was neither the addition nor loss of parent
or daughter isotopes. This is not always the case. In fact, an
important limitation of the potassium-argon method arises from
the fact that argon is a gas, and it may leak from minerals,
throwing off measurements.
Remember that although the basic principle of radiometric
dating is simple, the actual procedure is quite complex. The
analysis that determines the quantities of parent and daughter
must be painstakingly precise. In addition, some radioactive
materials do not decay directly into the stable daughter product.
As you saw in Figure 8.15 uranium-238 produces 13
intermediate unstable daughter products before the fourteenth

16. and final daughter product, the stable isotope lead-206, is
produced.
Dating with Carbon-14
To date very recent events, carbon-14 is used. Carbon-14 is the
radioactive isotope of carbon. The process is often called
radiocarbon dating. Because the half-life of carbon-14 is only
5,730 years, it can be used for dating events from the historic
past as well as those from very recent geologic history. In some
cases, carbon-14 can be used to date events as
far back as 75,000 years.
DID YOU KNOW?
One common precaution against sources of error in radiometric
dating is the use of cross checks. This simply involves
subjecting a sample to two different methods. If the two dates
agree, the likelihood is high that the date is reliable. If an
appreciable difference is found, other cross checks must be
employed to determine which, if either, is correct.
Carbon-14 is continuously produced in the upper atmosphere as
a consequence of cosmic-ray bombardment. Cosmic rays, which
are high-energy particles, shatter the nuclei of gas atoms,
releasing neutrons. Some of the neutrons are absorbed by
nitrogen atoms (atomic number 7), causing their nuclei to emit a
proton. As a result, the atomic number decreases by 1 (to 6),
and a different element, carbon-14, is created (figure 8.i7a).
This isotope of carbon quickly becomes incorporated into
carbon dioxide, which circulates in the atmosphere and is
absorbed by living matter. As a result, all organisms contain a
small amount of carbon-14, including yourself.
While an organism is alive, the decaying radiocarbon is
continually replaced, and the proportions of carbon-14 and
carbon-12 remain constant. Carbon-12 is the stable and most
common isotope of carbon. However, when any plant or animal
dies, the amount of carbon-14 gradually decreases as it decays
to nitrogen-14 by beta emission (figure 8.17b). By comparing

17. the proportions of carbon-14 and carbon-12 in a sample,
radiocarbon dates can be determined.
Although carbon-14 is useful in dating only the last small
fraction of geologic time, it has become a valuable tool for
anthropologists, archaeologists, and historians, as well as for
geologists who study very recent Earth history. In fact, the
development of radiocarbon dating was considered so important
that the chemist who discovered this application, Willard F.
Libby, received a Nobel prize.
Importance of Radiometric Dating
Radiometric dating methods have produced literally thousands
of dates for events in Earth history. Rocks exceeding 3.5 billion
years in age are found on all of the continents. Earth's oldest
rocks (so far) are gneisses from northern Canada near Great
Slave Lake that have been dated at 4.03 billion years (b.y).
Rocks from western Greenland have been
DID YOU KNOW?
Dating with carbon-14 is useful to archeologists and historians
as well as geologists. For example, University of Arizona
researchers used carbon-14 to determine the age of the Dead Sea
Scrolls, considered among the great archeological discoveries of
the twentieth century. Parchment from the scrolls dates between
150 B.c. and 5 B.C. Portions of the scrolls contain dates that
match 'those determined by the carbon-14 measurements.
dated at 3.7 to 3.8 b.y., and rocks nearly as old are found in the
Minnesota River Valley and northern Michigan (3.5 to 3.7 b.y.),
in southern Africa (3.4 to 3.5 b.y.), and in western Australia
(3.4 to 3.6 b.y.). It is important to point out that these ancient
rocks are not from any sort of "primordial crust" but originated
as lava flows, igneous intrusions, and sediments deposited in
shallow water—an indication that Earth history began before
these rocks formed. Even older mineral grains have been dated.

18. Tiny crystals of the mineral zircon having radiometric ages as
old as 4.3 b.y. have been found in younger sedimentary rocks in
western Australia. The source rocks for these tiny durable
grains either no longer exist or have not yet been found.
Radiometric dating has vindicated the ideas of Hutton, Charles
Darwin, and others who inferred that geologic time must be
immense. Indeed, modern dating methods have proved that there
has been enough time for the processes we observe to have
accomplished tremendous tasks.
Deciphering Earth History Geologic Time Scale
(
Nitrogen-14 atomic number 7 atomic mass 14
Neutron capture
A.
Production of carbon-14
Carbon-14 atomic number 6 atomic mass 14
Proton emission
) (
Neutron
Proton
(-) Beta (electron) emission
Carbon-14
B. Decay of carbon-14
Nitrogen-14
figure 8.17
A.
Production and
B.
decay of carbon-14. These sketches repre
sent the nuclei of the respective atoms.
)Geologists have divided the whole of geologic history into

19. units of varying magnitude. Together, they comprise the
geologic time scale of Earth history (figure 8.18). The major
units of the time scale were delineated during the nineteenth
century, principally by scientists in Western Europe and Great
Britain. Because radiometric dating was unavailable at that
time, the entire time scale was created using methods of relative
dating. It was only in the twentieth century that radiometric
dating permitted numerical dates to be added.
Structure of the Time Scale
The geologic time scale divides the 4.6-billion-year history of
Earth into many different units and provides a meaningful time
frame within which the events of the geologic past are arranged.
As shown in Figure 8.18, eons represent the greatest expanses
of time. The eon that began about 542 million years ago is the
Phanerozoic, a term derived from Greek words meaning "visible
life." It is an appropriate description because the rocks and
deposits of the Phanerozoic eon contain abundant fossils that
document major evolutionary trends.