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Warm Up
Atoms with the same atomic number but different
mass numbers are called ____.
a. ions
c. neutrons
b. protons
d. isotopes
2) Radiocarbon dating could be used to date which of
the following?
a. 60,000-year-old metamorphic rock
b. 60,000-year-old mammoth bone
c. 15-million-year-old lava flow
d. 65-million-year-old meteorite
3) Radioactivity is produced when unstable nuclei
____.
a. break apart
c. bond together
b. become cooler d. expand
Answers: 1) d. 2) b. 3) a.
1)
The Geologic Time
Scale
Chapter 12, Section 4
Geologic Time Scale
Based on their interpretation of the rock
record, geologists have divided Earth’s
4.56-billion-year history into units that
represent specific amounts of time
 Geologic Time Scale – the division of
Earth history into blocks of time—eons,
eras, periods, and epochs; the time scale
was created using relative dating
principles
 Since the time scale was created in the
19th Century, when radiometric dating was
unavailable, it wasn’t until the 20th
Century that radiometric dates were
incorporated

Structure of the Time Scale
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Eons represent the greatest expanse of time
Eons are divided into eras
Each era is subdivided into periods
Periods are divided into still smaller units called
epochs
The eon that began about 540 million years ago
is the Phanerozoic, Greek for “visible life”
There are three eras within the Phanerozoic: the
Paleozoic (“ancient life”), the Mesozoic (“middle
life”), and the Cenozoic (“recent life”)
Each era is subdivided into periods, each of which
is characterized by a somewhat less profound
change in life forms as compared with the eras
The periods of the Cenozoic are divided into still
smaller units called epochs
The epochs of other periods are not usually
referred to by specific names
Concept Check
 What
is the geologic time scale
divided into?
 Eons, eras, periods, and epochs
Precambrian Time
The detail of the geologic time scale does
not begin until the start of the Cambrian
Period (540 million years ago)
 The 4 billion years prior to the Cambrian
are divided into eons, and generally called
the Precambrian
 The Precambrian represents about 88% of
Earth’s history, but little is known about it
 During Precambrian time, there were
fewer life forms; these life forms are more
difficult to identify and the rocks have
been disturbed often

Precambrian Fossils
Difficulties With the Geologic Time Scale

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For a radiometric date to be useful, all minerals in
the rock must have formed at about the same time
Radioactive isotopes can be used to determine when
minerals in an igneous rock crystallized and when
pressure and heat made new minerals in
metamorphic rock
A sedimentary rock may contain particles that
contain radioactive isotopes, but these particles are
not the same age as the rock in which they occur
The sediments that are eventually cemented
together into a sedimentary rock have been
weathered from older rocks
The age of a particular mineral in a metamorphic
rock does not necessarily represent the time when
the rock first formed; the date may indicate when
the rock was metamorphosed
Absolute Dating of Sedimentary Rocks
Concept Check
 How
can geologists overcome the
problem of sedimentary rocks and
dating the time units of the geologic
time scale?
 Geologists can date igneous rocks,
then determine how the igneous
rocks relate to nearby sedimentary
rocks.
Dendrochronology
Assignment
 Read
Chapter 12, Section 4
(pg. 352-355)
 Do Chapter 12 Assessment #1-33
(pg. 259-260)
 For Section 4: Do #’s 5 & 11
 Study for Chapter 12 Quiz!