CHAPTER 18 Geologic Time
458
The two types of dates used by geologists to interpret Earth
history are (1) relative dates, which put events in their proper
sequence of formation, and (2) numerical dates, which pinpoint
the time in years when an event occurred.
Relative dates can be established using the law of superposition
(in an undeformed sequence of sedimentary rocks or surfacedeposited igneous rocks, each bed is older than the one above
and younger than the one below), the principle of original
horizontality (most layers are deposited in a horizontal position),
the principle of cross-cutting relationships (when a fault or
intrusion cuts through another rock, the fault or intrusion is
younger than the rocks cut through), and inclusions (the rock
mass containing the inclusion is younger than the rock that
provided the inclusion).
Unconformities are gaps in the rock record. Each represents a
long period during which deposition ceased, erosion removed
previously formed rocks, and then deposition resumed. The
three basic types of unconformities are angular unconformities
(tilted or folded sedimentary rocks that are overlain by younger,
more flat-lying strata), disconformities (the strata on either side
of the unconformity are essentially parallel), and nonconformities
(where a break separates older metamorphic or intrusive igneous
rocks from younger sedimentary strata).
Correlation, matching rocks of similar age in different areas,
is used to develop a geologic time scale that applies to the
entire Earth.
Fossils are the remains or traces of prehistoric life. The special
conditions that favor preservation are rapid burial and the
possession of hard parts such as shells, bones, or teeth.
Fossils are used to correlate sedimentary rocks from different
regions by using the rocks’ distinctive fossil content and applying
the principle of fossil succession. It is based on the work of William
Smith in the late 1700s and states that fossil organisms succeed
one another in a definite and determinable order; therefore any
time period can be recognized by its fossil content. Index fossils,
those that are widespread geographically and are limited to a
short span of geologic time, provide an important tool for
matching rocks of the same age.
Each atom has a nucleus containing protons (positively
charged particles) and neutrons (neutral particles). Orbiting
the nucleus are negatively charged electrons. The atomic number
of an atom is the number of protons in the nucleus. The mass
number is the number of protons plus the number of neutrons
in an atom’ s nucleus. Isotopes are variants of the same atom but
with a different number of neutrons and hence a different
mass number.
Radioactivity is the spontaneous breaking apart (decay) of
certain unstable atomic nuclei. Three common types of radioactive decay are (1) emission of alpha particles from the nucleus,
(2) emission of beta particles (or electrons) from the nucleus,
and (3) capture of electrons by the nucleus.
An unstable radioactive isotope, called the parent, will
decay and form stable daughter products. The length of time
for half of the nuclei of a radioactive isotope to decay is called
the half-life of the isotope. If the half-life of the isotope is
known, and the parent/daughter ratio can be measured, the age
of a sample can be calculated. An accurate radiometric date
can only be obtained if the mineral containing the radioactive
isotope remained a closed system during the entire period
since its formation.
The geologic time scale divides Earth’ s history into units of
varying magnitude. It is commonly presented in chart form,
with the oldest time and event at the bottom and the youngest
at the top. The principal subdivisions of the geologic time scale,
called eons, include the Archean, Proterozoic (together, these
two eons are commonly referred to as the Precambrian), and,
beginning about 542 million years ago, the Phanerozoic. The
Phanerozoic (meaning “visible life”) eon is divided into the
following eras: Paleozoic (“ancient life”), Mesozoic (“middle life”),
and Cenozoic (“recent life”).
A significant problem in assigning numerical dates to units of
time is that not all rocks can be radiometrically dated. A sedimentary rock may contain particles of many ages that have been
weathered from different rocks that formed at various times.
One way geologists assign numerical dates to sedimentary rocks
is to relate them to datable igneous masses, such as dikes and
volcanic ash beds.
C H A P T E R
E I G H T E E N
Geologic Time
in Review
From this kind of evidence, geologists
estimate that a part of the Morrison
Formation was deposited about 160
million years ago, as indicated by the ash
bed. Further, they conclude that the
Tertiary period began after the intrusion
of the dike, 66 million years ago. This is
one example of literally thousands that
illustrate how datable materials are used to
bracket the various episodes in Earth history within specific time periods. It shows
the necessity of combining laboratory
dating methods with field observations
of rocks.
C O N C E P T C H E C K 1 8 . 7
Briefly explain why it is often difficult
to assign a reliable numerical date to
a sample of sedimentary rock.
1
458
The two types of dates used by geologists to interpret Earth
history are (1) relative dates, which put events in their proper
sequence of formation, and (2) numerical dates, which pinpoint
the time in years when an event occurred.
Relative dates can be established using the law of superposition
(in an undeformed sequence of sedimentary rocks or surfacedeposited igneous rocks, each bed is older than the one above
and younger than the one below), the principle of original
horizontality (most layers are deposited in a horizontal position),
the principle of cross-cutting relationships (when a fault or
intrusion cuts through another rock, the fault or intrusion is
younger than the rocks cut through), and inclusions (the rock
mass containing the inclusion is younger than the rock that
provided the inclusion).
Unconformities are gaps in the rock record. Each represents a
long period during which deposition ceased, erosion removed
previously formed rocks, and then deposition resumed. The
three basic types of unconformities are angular unconformities
(tilted or folded sedimentary rocks that are overlain by younger,
more flat-lying strata), disconformities (the strata on either side
of the unconformity are essentially parallel), and nonconformities
(where a break separates older metamorphic or intrusive igneous
rocks from younger sedimentary strata).
Correlation, matching rocks of similar age in different areas,
is used to develop a geologic time scale that applies to the
entire Earth.
Fossils are the remains or traces of prehistoric life. The special
conditions that favor preservation are rapid burial and the
possession of hard parts such as shells, bones, or teeth.
Fossils are used to correlate sedimentary rocks from different
regions by using the rocks’ distinctive fossil content and applying
the principle of fossil succession. It is based on the work of William
Smith in the late 1700s and states that fossil organisms succeed
one another in a definite and determinable order; therefore any
time period can be recognized by its fossil content. Index fossils,
those that are widespread geographically and are limited to a
short span of geologic time, provide an important tool for
matching rocks of the same age.
Each atom has a nucleus containing protons (positively
charged particles) and neutrons (neutral particles). Orbiting
the nucleus are negatively charged electrons. The atomic number
of an atom is the number of protons in the nucleus. The mass
number is the number of protons plus the number of neutrons
in an atom’ s nucleus. Isotopes are variants of the same atom but
with a different number of neutrons and hence a different
mass number.
Radioactivity is the spontaneous breaking apart (decay) of
certain unstable atomic nuclei. Three common types of radioactive decay are (1) emission of alpha particles from the nucleus,
(2) emission of beta particles (or electrons) from the nucleus,
and (3) capture of electrons by the nucleus.
An unstable radioactive isotope, called the parent, will
decay and form stable daughter products. The length of time
for half of the nuclei of a radioactive isotope to decay is called
the half-life of the isotope. If the half-life of the isotope is
known, and the parent/daughter ratio can be measured, the age
of a sample can be calculated. An accurate radiometric date
can only be obtained if the mineral containing the radioactive
isotope remained a closed system during the entire period
since its formation.
The geologic time scale divides Earth’ s history into units of
varying magnitude. It is commonly presented in chart form,
with the oldest time and event at the bottom and the youngest
at the top. The principal subdivisions of the geologic time scale,
called eons, include the Archean, Proterozoic (together, these
two eons are commonly referred to as the Precambrian), and,
beginning about 542 million years ago, the Phanerozoic. The
Phanerozoic (meaning “visible life”) eon is divided into the
following eras: Paleozoic (“ancient life”), Mesozoic (“middle life”),
and Cenozoic (“recent life”).
A significant problem in assigning numerical dates to units of
time is that not all rocks can be radiometrically dated. A sedimentary rock may contain particles of many ages that have been
weathered from different rocks that formed at various times.
One way geologists assign numerical dates to sedimentary rocks
is to relate them to datable igneous masses, such as dikes and
volcanic ash beds.
C H A P T E R
E I G H T E E N
Geologic Time
in Review
From this kind of evidence, geologists
estimate that a part of the Morrison
Formation was deposited about 160
million years ago, as indicated by the ash
bed. Further, they conclude that the
Tertiary period began after the intrusion
of the dike, 66 million years ago. This is
one example of literally thousands that
illustrate how datable materials are used to
bracket the various episodes in Earth history within specific time periods. It shows
the necessity of combining laboratory
dating methods with field observations
of rocks.
C O N C E P T C H E C K 1 8 . 7
Briefly explain why it is often difficult
to assign a reliable numerical date to
a sample of sedimentary rock.
1