LESSON 1
Geologic Time
Analyzing the Rock and Fossil Records
Key Concept
What do you think? Read the three statements below and decide
whether you agree or disagree with them. Place an A in the Before
column if you agree with the statement or a D if you disagree. After
you’ve read this lesson, reread the statements to see if you have
changed your mind.
Before
Statement
• How can rocks and
fossils be used to
determine the order of
past events?
After
lder rocks are always located below younger
O
rocks.
Relative age means that scientists are relatively
sure of the age.
Fossils are pieces of dead organisms.
Mark the Text
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What is the basis for understanding
Earth’s past?
Today, scientists know that Earth’s history started about 4.6
billion years ago. But early studies and ideas of geology and
Earth history did not consider that Earth is very old. Scientists
described what they saw in terms of what they experienced in
their everyday lives. For example, some early scientists
explained Earth’s history as a series of disastrous events that
occurred over short periods of time. They called this idea
catastrophism (kuh TAS truh fih zum). An important idea was
set forth during the late 1700s that caused scientists to change
their view of Earth’s history.
Principle of Uniformitarianism In the 1700s, James Hutton
rejected catastrophism. Hutton was a naturalist and farmer in
Scotland. He noticed that the landscape on his farm changed
over the years. Hutton thought that the processes responsible
for changing the landscape on his farm could also change
Earth’s surface. For example, Hutton thought that erosion
caused by streams could also wear down mountains. He realized
that these processes would take a long time to change Earth’s
surface. Hutton proposed that Earth was much older than a few
thousand years.
Reading Essentials
Identify Main Ideas
Highlight each head and
the details that support it.
Use the highlighted
information to review the
lesson.
Scientific Vocabulary
geology
(noun) the science and
study of the planet Earth
Academic Vocabulary
processes
(noun) a series of actions
or operations that lead to
an end result
Geologic Time
1
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Academic Vocabulary
uniform
(adjective) staying the
same all the time
Scientific Vocabulary
glacier
(noun) a large mass of ice
that moves slowly over land
Hutton’s ideas became part of the principle of
uniformitarianism (yew nuh for muh TER ee uh nih zum). The
principle of uniformitarianism states that geologic processes
that occur today are similar to those that have occurred in the
past. According to this view, Earth’s surface is constantly being
reshaped in a steady, uniform manner. For example, when a
glacier melts, the rock material that is contained in the glacial
ice gets deposited. The physical characteristics of this material is
very distinct. The material is unsorted and contains many
different sizes of sediment. Many of the rocks have scratches on
them. This material can be recognized anywhere. Today, there
are rock layers that are made of the same type of rocks and
sediment. Because they are so similar in appearance,
uniformitarianism tells us that these rock layers were once
material that was deposited by glaciers.
Because of uniformitarianism, scientists can learn about
Earth’s past by studying the present. One way to do this is by
studying the order in which geologic events occurred using a
method called relative-age dating. This does not allow scientists
to determine the absolute age, or the actual age of the events.
But it gives scientists a clearer understanding about Earth’s
history.
What is relative-age dating?
formation
(noun) something that is
formed or created
Relative Age There is order in a rock formation just as there is
order in a pile of clothes. In many rock formations, the oldest
rocks are in the bottom layer and the youngest rocks are in the
top layer.
If you have brothers and sisters, you might describe your age
by saying, “I’m older than my sister and younger than my
brother.” This tells how your age relates to others in your family.
It is your relative age. In geology, relative age is the age of rocks
and geologic features compared with other rocks and geologic
features nearby.
2
Geologic Time
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Academic Vocabulary
Suppose you have just remembered where you left the
money that you have been looking for. It is in the pocket of the
pants you wore to the movies last Saturday. Now imagine that
the pants are in your pile of dirty laundry. How can you tell
where your money is? It turns out that there is some order to
your pile of clothes. Every time you add clothes to the pile, you
place them on top. The clothes from last Saturday are on the
bottom. That is where your money is!
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The Principles of Relative-Age Dating Long ago, a man
named Nicolaus Steno developed a set of principles to compare
the ages of rock layers. Called Steno’s principles, they help
geologists organize rocks according to their relative ages. They
are superposition, original horizontality, lateral continuity, and
cross-cutting relationships. Modern geologists have added a
fifth principle, the principle of inclusions. So today, there are
5 principles used in relative-age dating. Let’s review.
• Your
pile of dirty clothes demonstrates the first principle of
relative-age dating—superposition. Superposition is the
principle that in undisturbed rock layers, the oldest rocks are
on the bottom. Forces do sometimes disturb rock layers after
they are deposited. But if no disturbance takes place, each
layer of rocks is younger than the layer below it.
• The
second principle of relative-age dating is called original
horizontality. According to the principle of original
horizontality, most rock-forming materials are deposited in
horizontal layers. Sometimes rock layers are deformed or
disturbed after they form. For example, the layers might
be tilted or folded. When you see rocks that are tilted,
remember that all layers were originally deposited
horizontally.
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• Lateral
continuity is the principle that sediments are
deposited in large, flat sheets. The sheets, or layers, continue
in all lateral directions until they thin out or until they meet
a barrier. This principle is called the principle of lateral
continuity. For example, a river might erode the layers, but
the order of the layers does not change. This principle helps
geologists recognize the same rock layers on either side of a
large river or canyon.
• Sometimes
forces within Earth cause rock formations to
break, or fracture. When rocks move along a fracture line,
the fracture is called a fault. According to the principle of
cross-cutting relationships, if one geologic feature cuts
across another feature, the feature that it cuts across is older.
Imagine several rock layers that have been intruded by an
igneous intrusion. Then the layers and the intrusion are cut
by a fault. What are the relative ages? You can conclude that
the rock layers are the oldest, followed by the intrusion and
then the fault. The fault is the youngest because it cuts both
the rock layers and the intrusion.
Reading Essentials
Scientific Vocabulary
disturb
(verb) to change the
position or order of
something
Academic Vocabulary
horizontal
(adjective) flat and level
Word Origin
lateral
from Latin lateralis, means
“belonging to the side”
Science Use v. Common Use
intrusion
Science Use the entry of
magma into or between
rock formations; the
igneous rock formed
Common Use the act of
intruding
Geologic Time
3
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Scientific Vocabulary
magma
(noun) molten rock inside
Earth
• Sometimes,
when rocks form, they contain pieces of other
rocks. This can happen when part of an existing rock breaks
off and falls into soft sediment or flowing magma. When the
sediment or the magma becomes rock, the broken piece of
rock becomes a part of it. A piece of an older rock that
becomes part of a new rock is called an inclusion. According
to the principle of inclusions, if one rock contains pieces of
another rock, the rock containing the pieces is younger than
the pieces.
What can the fossil record tell us
about Earth’s history?
Word Origin
fossil
from Latin fossils, means
“dug up”
Scientific Vocabulary
ancient
(adjective) very old
Word Origin
from Latin extinctus,
means “dying out”
The Fossil Record While studying the fossils in rock layers,
early geologists recognized three things. First, fossils found in
the oldest layers of rock are of relatively simple organisms.
Fossils of more complex organisms appear in younger layers of
rock. Second, there were a few times when many types of fossils
that appear in one rock layer did not appear in the younger rock
layers right above it. These sudden disappearances in the fossil
record of many types of organisms mark mass extinctions—
times when many species on Earth died or became extinct within
a relatively short period of time. And finally, fossils and the rocks
they are within can be used to determine what the environment
of an area was like long ago.
Ancient Environments Scientists use the principle of
uniformitarianism to learn about ancient organisms and the
environments in which they lived. They compare fossils of
ancient organisms with organisms living today. For example, a
trilobite fossil and a horseshoe crab look alike. Horseshoe crabs
today live in shallow water on the ocean floor. Partly because
trilobite fossils look like horseshoe crabs, scientists infer that
trilobites also lived in shallow ocean water.
4
Geologic Time
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extinct
Did you know that rocks themselves can contain clues about
their relative ages? Some of the most obvious clues found in
rocks are fossils. Fossils are the preserved remains or evidence
of ancient living things. When many people think of fossils, they
think of dinosaur fossils. Many dinosaurs were large animals.
Their large bones remained after they died. Not all fossils are
large enough for you to see. Some fossils can be seen only by
using a microscope. These tiny fossils are called microfossils.
Some microfossils are about the size of a speck of dust. Yet, they
can all be used to help scientists interpret Earth’s history.
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Shallow Seas Today, Earth’s continents are mostly above sea
level. Many times in the past, the sea level rose and flooded
Earth’s continents. For example, a shallow ocean covered much
of North America about 450 million years ago. Fossils of
organisms that lived in that shallow ocean help scientists
reconstruct what the seafloor looked like in the past.
Past Climates Fossils also help scientists learn about past
climate changes. For example, fossils of ferns and other tropical
plants indicate that much of Earth was very warm 100 million
years ago. Tropical swamps and forests covered much of the
land. Dinosaurs lived on Earth during this period.
Millions of years later, the swamps and forests were gone. In
some of these areas, coarse grasses grew. Huge sheets of ice
formed as the climate cooled. This ice spread over parts of North
America, Europe, and Asia. Organisms that adapted to the cold
climate survived. Fossils of organisms, such as the woolly
mammoth, help scientists learn about this cold time in
Earth’s history.
Academic Vocabulary
indicate
(verb) to be a sign of; to
point to
Scientific Vocabulary
adapt
(verb) to change in order
to succeed in a new
situation
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Reread the statements at the beginning of the lesson. Fill in
the After column with an A if you agree with the statement or a
D if you disagree. Did you change your mind?
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Geologic Time
5
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LESSON 2
Geologic Time
Building a Time Line
Key Concept
• How do geologists
organize Earth’s history?
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before
column if you agree with the statement or a D if you disagree. After
you’ve read this lesson, reread the statements to see if you have
changed your mind.
Before
Statement
After
arth’s rock layers contain a complete record of
E
all of Earth history.
The units of time in the geologic time scale are
the same length of time.
Study Coach
Ask Questions As you
read, write a question
about any topic you don’t
understand. When you
finish reading the lesson,
discuss your question with
your teacher or another
student.
Academic Vocabulary
(verb) to make visible;
uncover
Scientific Vocabulary
tilted
(adjective) not perfectly
horizontal or vertical
6
Geologic Time
After rocks form, they are sometimes uplifted and exposed
at Earth’s surface. As soon as rocks are exposed, wind and rain
start to weather and erode them. Sometimes a lot of material is
removed from the rock record. These eroded areas represent a
gap in the rock record.
Unconformities Often, new rock layers are deposited on top
of old, eroded rock layers. When this happens, an unconformity
(un kun FOR muh tee) forms. An unconformity is a surface
where rock has eroded away, producing a break, or gap, in the
rock record. An unconformity is not a hole or a space in the
rock. It is a surface on a layer of eroded rocks with younger
rocks on top. An unconformity represents a gap in time. It could
represent a few hundred years, a million years, or even billions
of years.
There are three major types of unconformities. In a
disconformity, younger sedimentary layers are deposited on top of
older, horizontal sedimentary layers that have been eroded. In an
angular unconformity, sedimentary layers are deposited on top
of tilted or folded sedimentary layers that have been eroded. In a
nonconformity, younger sedimentary layers are deposited on
older igneous or metamorphic rock layers that have been eroded.
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expose
What happens when the rock and
fossil records are not complete?
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How can we fill gaps in the rock
record?
Rock layers contain clues about Earth. Geologists use these
clues to build a record of Earth’s geologic history. Many times the
rock record is incomplete. For example, unconformities create
gaps in the geologic record. Geologists fill in the gaps in the rock
record by matching rock layers and fossils from separate locations.
Correlation Matching rocks and fossils from separate locations
is called correlation (kor uh LAY shun). Sometimes, geologists
can connect rock layers simply by walking along rock formations
and looking for similarities. Other times, soil might cover the
rocks, or rocks might be eroded. When this happens, geologists
correlate rocks by matching exposed rock layers in different
locations. Geologists have used correlation to develop historical
records all over the world.
Some correlated rock formations are within a few hundred
kilometers of one another, such as those in some national parks.
They are correlated based on similarities in rock type, structure,
and fossil evidence. If scientists want to learn the relative ages of
rock formations that are very far apart or on different
continents, they often use fossils. If two or more rock formations
contain fossils of about the same age, scientists can infer that
the formations are also about the same age.
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Not all fossils are useful in determining the relative ages of
rock formations. Fossils of species that lived on Earth for
hundreds of millions of years are not helpful. They represent
time spans that are too long.
Academic Vocabulary
correlate
(verb) to connect things,
facts, or ideas where one
thing depends on or
affects the other
Scientific Vocabulary
evidence
(noun) information that
shows
Academic Vocabulary
infer
(verb) to conclude from
facts
Trilobites are among the most useful fossils for determing
relative ages. Certain trilobite species existed for only a short time
in many different areas on Earth. The fossils of these trilobites are
index fossils. Index fossils represent species that existed on Earth
for a short length of time, were abundant, and inhabited many
locations. When an index fossil is found in rock layers at different
locations, geologists can infer that the layers are of similar age.
How do major geologic events help
build Earth’s time line?
Most geologic events that occur on Earth, leave behind some
sort of evidence. Volcanic ash records a volcanic eruption, and
gouged and scarred rock surfaces record the passing of glaciers.
Geologists use such features to mark specific moments in
Earth’s history.
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Geologic Time
7
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Key Beds A large meteorite strike, volcanic eruption, or other
major event can leave a unique layer in the rock record. Because
these layers are easy to recognize, they can be used to correlate
rock formations in different regions where layers are exposed.
Rocks or sediment layers that are used as a marker to correlate
rock layers are called key beds. Using the principle of
superposition, geologists know that the layers above a key bed
are younger than the layers below it.
Scientific Vocabulary
distinctive
(adjective) different in a
way that is easy to notice
Major Geologic Events Not all of Earth’s major events leave
distinctive key beds, but they do leave evidence in the rock record.
Rocks around the world contain evidence that vast mountains were
uplifted and eroded, and that oceans flooded the land and retreated
many times. Geologists use the type of rock, the order of rock
layers, and the fossils within as evidence for these major events.
Using a combination of the principles of relative-age dating,
the fossil record, and evidence of major events in Earth’s history,
geologists can determine the relative order of events. In doing
so, a time scale of Earth’s long past can be constructed.
What is the geologic time scale?
Science Use v. Common Use
scale
Academic Vocabulary
duration
(noun) the length of time
something continues
8
Geologic Time
Eons are the longest units of geologic time. Earth’s current
eon, the Phanerozoic (fan er oh ZOH ihk) Eon, began 541 million
years ago. Eons are subdivided into smaller units of time called
eras. We live in the Cenozoic Era, which began 66 million years
ago. Eras are subdivided into periods. Earth’s current period is
called the Quaternary Period, and began 2.6 million years ago.
Periods are subdivided into epochs (EH pocks). The Holocene
Epoch began 11,600 years ago.
Notice that the units of time are not equal in duration. For
example, the Paleozoic Era is longer than the Mesozoic and
Cenozoic eras combined. Why? Hundreds of years ago, as geologists
began developing the geologic time scale, they chose boundaries
between time units based on what they observed in Earth’s rock
layers. Different layers contained different fossils. Geologists used
changes in the fossil record to mark divisions in geologic time.
Because the lengths of time between changes were different, the
geologic time scale is divided into unequal units of time.
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Science Use a series of
marks or points at known
intervals
Common Use an
instrument used for
measuring the weight of
an object
To organize events in your life, you use different units of
time, such as days, weeks, months, and years. Geologists
organize Earth’s past in a similar way. They developed a model
of Earth’s history from its origin 4.6 billion years ago to the
present called the geologic time scale. The scale, shown on the
next page, divides the 4.6 billion years of Earth’s history into
time units. These units are thousands and millions of years
long—much longer than the units you use to organize your life.
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The Geologic Time Scale Geologists developed the geologic
time scale to help them correlate rock units across countries and
continents, and to have a standard model and vocabulary for
describing geologic time.
The geologic time scale is a work in progress. The
boundaries, or lengths in time, can change as scientists make
new discoveries.
mya
66.0
251.9
Paleozoic
Era
Period
Quaternary*
Neogene*
mya
2.6
Period
Quaternary*
23.0
66.0
Cretaceous
Mesozoic
(adjective) well
established; widely
recognized as acceptable
Epoch
Holocene*
Pleistocene*
Pliocene
Paleogene
541.0
Proterozoic
standard
Neogene
Era
Cenozoic
Mesozoic
Cenozoic
Phanerozoic
Eon
Academic Vocabulary
mya
0.01
2.6
5.3
Miocene
145.0
Jurassic
23.0
201.3
Oligocene
Triassic
251.9
33.9
Permian
Carboniferous
Paleozoic
Devonian
Silurian
Archean
Eocene
419.2
443.8
Ordovician
485.4
56.0
Paleocene
Cambrian
66.0
541.0
4000
Ediacaran
635
Hadean
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2500
358.9
Paleogene
298.9
4600
Reading
Essentials
RE_E150_212A_NGMSS-BANK
*Not to scale
Geologic Time
9
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Academic Vocabulary
comprehend
(verb) to understand the
nature, significance, or
meaning of
Relating Time Scales You have learned that Earth is 4.6 billion
years old. This very large number can be difficult to understand.
How truly long is 4,600,000,000 years? It is difficult to comprehend
time that extends so far into the past unless you can relate it to
your own experience on a scale that is familiar to you.
Geologists often make comparisons using relative time to
help understand this very large age. For example, sometimes
they place all of Earth’s notable events into the course of one
year. Sometimes they place Earth’s long history into the course
of one 24-hour day. Whatever relative-time comparison used, the
idea that Earth is truly this old is fantastic any way you think
about it.
Reread the statements at the beginning of the lesson. Fill in
the After column with an A if you agree with the statement or a
D if you disagree. Did you change your mind?
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10
Geologic Time
Reading Essentials
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